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Haemodynamic and coronary vascular responses after β-adrenoceptor blockade in the anaesthetised dog: a comparison of tolamolol with practolol and propranolol
EUROPEAN JOURNAL OF PHARMACOLOGY 26 (1974) 96-107.
HAEMODYNAMIC BLOCKADE
AND CORONARY
VASCULAR
IN THE ANAESTHETISED PRACTOLOL Kenneth
NORTH-HOLLAND PUBLISHING COMPANY
RESPONSES
DOG: A COMPARISON
AFTER
/3-ADRENOCEI’TOR
OF TOLAMOLOL
WITH
AND PROPRANOLOL
R. ADAM and Susan M. BOYLJZS
Central Research, Pfizer Limited, Sandwich, Kent, England
Received 17 August 1973
Accepted 30 November 1973
K.R. ADAM and S.M. BOYLES, Haemodynamic
and coronary vascular responses after p-adrenoceptor blockade in the anaesthetised dog: a comparison of tolamolol with practolol and propranolol, European J. Pharmacol. 26 (1974)
96-107. In anaesthetised dogs, the fl-adrenoceptor blocking agent tolamolol had approximately ten times the activity of practolol and twice that of propranolol in antagonising the myocardial responses to injected isoprenaline, adrenaline and noradrenaline and to stimulation of the left stellate ganglion. Tolamolol, like practolol, was cardioselective; in amounts which blocked myocardial responses to exogenous catecholamines and to sympathetic stimulation, it did not block peripheral vasodilatation caused by isoprenaline, and isoprenaline and adrenaline continued to cause coronary vasodilatation. In contrast, propranolol blocked myocardial, peripheral vascular and coronary vascular responses to isoprenaline to similar degrees. These results imply that the coronary vascular P-adrenoceptors resemble those of the peripheral vasculature. The coronary dilatation due to noradrenaline and stellate ganglion stimulation was generally abolished after fl-adrenoceptor blockade and in some experiments was converted to a vasoconstriction. &Adrenoceptor blockade did not affect peak reactive hyperaemia following coronary artery occlusion. Cardioselective p-adrenoceptor blockade Coronary vascular adrenoceptors
1. Introduction The purpose of the current investigation in anaesthetised dogs was twofold. Firstly, the cardiovascular effects oftolamolol((*)-4-[2(2-hydroxy-3-o-tolyloxypropylamino)-ethoxy] -benzamide, UK-6558 (Aug stein et al., 1974)) were examined. Tolamolol is a /3-adrenoceptor blocking agent which lacks direct myocardial depressant activity (Augstein et al., 1974), does not show intrinsic sympathomimetic activity (Adam et al., 1974), and is also cardioselective. The latter has been shown both in man, where myocardial/peripheral vascular selectivity was demonstrated (Briant et al., 1973) and in rodents, where tolamolol was found to be selective for myocardial as opposed to bronchial /3-adrenoceptors (Adam et al., 1974). Secondly, the nature of coronary vascular /3-adrenoceptors was further investigated by compar-
Stellate ganglion stimulation Propranolol
Tolamolol Practolol
ing the responses of the coronary vessels and of the peripheral vasculature to sympathetic stimuli following blockade of inotropic and chronotropic effects by tolamolol, practolol and the non-selective padrenoceptor blocking agent, propranolol. Practolol blocks myocardial /%adrenoceptors in amounts which have no effect on the /3-adrenoceptors of the peripheral vasculature (Dunlop and Shanks, 1968). The latter receptors have been classified in the same group @,) as those of bronchiolar smooth muscle, and contrasted with those (PI) in the myocardium (Lands et al., 1967). The nature of the /3adrenoceptors in the coronary vessels remains however, controversial. It has been found in previous investigations (Adam et al., 1970; Ross and Jorgensen, 1970) that the /3-adrenoceptors in the myocardium of the dog can be blocked by practolol independently of those of the coronary vasculature, thus allowing the
K.R. Adam, S.M. Boyles, ~-Adrenoceptor blockade by tolamolol
direct effects of sympathetic stimulation and of catecholamines to be observed in these vessels free of the autoregulatory changes in coronary blood flow accompanying inotropic and chronotropic effects. Such observations support the view that the 13-adrenoceptors of coronary vessels resemble those of the peripheral vasculature. Nevertheless, a substantial body of evidence from in vitro studies (Baron et al., 1972; Drew and Levy, 1972) indicates that the coronary /3-adrenoceptors fall into the/3~ -category.
2. Materials and methods
18 male beagle dogs (weight range 11-18 kg) anaesthetised with sodium pentobarbitone (30 mg/kg i.v.) were used in this study. Body temperature, monitored by an oesophageal probe, was maintained at 37°C by means of a thermostatically controlled blanket. Pressures were measured from catheters passed into the aortic arch from the right femoral artery, into the left ventride via the left carotid artery and in 14 of the dogs femoral arterial pressure was also measured via the right femoral artery. The left ventricular pressure pulse was differentiated electronically to obtain the rate of rise of pressure in the left ventricle (dp/dt), as a measure of contractility and was also used to trigger an instantaneous rate meter (Devices Instruments Ltd.). The diastolic portion of the left ventricular pressure was amplified to facilitate identification of end diastole. In dogs in which femoral arterial pressure was measured, blood flow was recorded in the contralateral femoral artery using a 2.5 mm electromagnetic flow probe (Biotronix Laboratory Inc.). Immediately distal to the flow probe, a hydraulic cuff occlusive device was positioned on the artery in order to obtain a zero flow reference. In 12 of these dogs, a fine polyethylene catheter was inserted into the femoral artery from a muscular branch for close-arterial injection into the femoral vasculature. Resistance of the vascular bed was continuously monitored by electronic division of the femoral arterial pressure (an approximation to the pressure gradient across the femoral vasculature) by flow in the artery. A left thoracotomy was performed in the fourth intercostal space; ventilation was maintained with room air through a cuffed endotracheal tube using a
97
Palmer respiration pump. The left stellate ganglion was dissected free of surrounding tissue, its connections with the sympathetic chain crushed and shielded platinum wire electrodes placed on it through a stab incision in the chest wall. The pericardium was incised laterally and sutured into a cradle to support the heart, and the left atrium was deflected. A length of the left circumflex coronary artery about 1 cm long and free of branches was cleared of surrounding tissue and a 2 mm electromagnetic flow probe (Biotronix Laboratory Inc.) applied with a cuff occluder distal to it. The ligatures restraining the pericardium and atrium were then released, allowing the heart to assume its original position. The flow probes and flow meters (S.E. Laboratories Ltd.) used in these experiments were calibrated using excised dog arteries of suitable size through which isotonic saline was passed at known flow rates. The resistance of the coronary vascular bed supplied by the left circumflex artery was continuously monitored by electronic division of the aortic pressure (which approximates closely to the pressure drop across the coronary vascular bed) by the phasic coronary flow. Recorded parameters were displayed on a multichannel recorder (Devices Instruments Ltd.). Haemodynamic and coronary vascular responses were obtained to isoprenaline (0.4/.tg/kg), adrenaline (1.0 /.tg/kg) and noradrenaline (1.0 /~g/kg) injected into the right femoral vein and 30 sec periods of submaximal stimulation of the left stellate ganglion ( 3 - 8 V; 0.5 msec pulse width; 10 Hz). Coronary flow and resistance were measured at end-diastole, this being the point in the cardiac cycle at which myocardial compression has least effect on coronary blood flow, and values were calculated from the mean of readings taken at two cardiac cycles during control periods and at the point of maximal myocardial response after i.v. catecholamine or ganglion stimulation. Femoral vascular responses were also obtained to closearterial injection of isoprenaline (0.02/ag/kg), adrenaline (0.025/ag/kg) and noradrenaline (0.025/ag/kg). All interventions were carried out at 10-15 min intervals, during which time the measured parameters returned to control levels. The three ~-blocking agents under investigation were administered i.v. by infusfon, at a rate of 2 ml/min for 10 min, with an interval between successive doses of approximately 1.5 hr. The direct effects
(4)
110 ,+ 0.5 111 ± 3.8
117 ,+ 3.3 105 ,+ 8.3
114 ,+ 5.9 101 ,+ 5.3
(9)
113 ,+ 4.6 107 ,+ 4 . 2
(4)
Control 112 ,+ 4.8 P r o p r a n o l o l 0.2 m g / k g 100 ,+ 7.4
Control Practolo1 20 m g / k g
(5)
Control P r a c t o l o l 1.0 m g / k g
(5)
Control T o l a m o l o l 2.0 m g / k g
Control T o l a m o l o l 0.1 m g / k g
Mean a o r t i c pressure (mmHg)
143 ± 7.2 126 ,+ 9.3
128 ± 3.6 120 -+ 3.7 (p < 0 . 0 5 )
144 ,+ 6.1 124 ,+ 3.6 60 < 0 . 0 5 )
132 ± 6.5 123 ,+ 7.8
136 ,+ 4 . 0 115 ± 4.3 (9) (p < 0 . 0 1 )
Heart rate (beats/min)
2 4 5 0 -+ 4 9 7 1800 ± 280
3 3 0 0 ,+ 704 3125 ,+ 6 9 0
3 4 0 0 -+ 4 1 0 3 2 2 0 ,+ 5 1 6
2 9 4 0 ,+ 289 2 9 4 0 ,+ 276
2911 ,+ 277 2911 ± 309 (9)
Left v e n t r i c u l a r (dp/dt)max ( m m Hg/sec)
29.3 -+ 2.3 25.7 ,+ 3.1
26.3 ,+ 3.2 25.1 ,+ 3.7
29.4 ± 2.5 25.0 +- 2.0
28.3 ,+ 3.2 27.4 ± 3.7
28.0 -+ 2 A 2 3 . 2 ,+ 2.0 (9)
End-diastolic coronary flow (ml/min)
354 -+ 21 356 +- 4 0
335 ,+ 33 359 -+ 4 7
318 ± 20 343 ,+ 20
348 ,+ 54 310 ± 4 7
342 ,+ 56 378 ± 4 9 (5)
4 9 . 3 -+ 9.3 4 1 . 9 ,+ 7.9
4 4 . 8 ,+ 2.8 4 6 . 3 ,+ 5.0
4 2 . 6 ± 8.2 39.7 ± 7.3
37.7 ,+ 6.1 32.1 ± 2.5
4 7 . 0 +- 6.7 38.0 -+ 6.0 (5)
End-diastolic Mean f e m o r a l coronary resistance flow ( d y n c m "5 sec ×103) ( m l / m i n )
211 -+ 34 235 ,+ 58
159 ± 13 150 -+ 19
203 ,+ 4 4 213 ,+ 4 4
244 ,+ 30 238 ,+ 22
192 ,+ 18 224 ± 28 (5)
Mean f e m o r a l resistance ( d y n c m -s secXl 03)
Table 1 Direct cardiovascular effects o f t o l a m o l o l , p r a c t o l o l a n d p r o p r a n o l o l in a n a e s t h e t i s e d , o p e n - c h e s t d o g s . V a l u e s are m e a n -+ S.E.M. S i g n i f i c a n t d i f f e r e n c e s refer to d i f f e r e n c e f r o m original c o n t r o l value. F i g u r e s in p a r e n t h e s e s i n d i c a t e n u m b e r s o f e x p e r i m e n t s .
~O
K.R. Adam, S.M. Boyles, &Adrenoceptorblockade by tolamolol of these agents on the recorded parameters were measured 10 min after the infusions were ended and then responses were again obtained to catecholamines and to ganglion stimulation. The control response time to peak contractility after the i.v. administration of catecholamines was used as a reference point for measuring coronary responses after abolition of the contractile response by /3-adrenoceptor blockade. In addition, the peak reactive hyperaemic response of blood flow and vascular resistance was measured after 15 sec periods of complete occlusion of the circumflex coronary artery and of the femoral artery before and after/3-adrenoceptor blockade. All results are expressed as the mean + S.E.M. and, where applicable, were tested for significance using Student's t-test. Drugs used were (---)-isoprenaline hydrochloride (Sigma), (-)-noradrenaline bitartrate (Koch-Light), (-)-adrenaline chloride (Parke-Davis), (-+)-tolamolol hydrochloride, (+)-practolol hydrochloride (I.C.I.), (+)-propranolol hydrochloride (I.C.I.) and phentolamine mesylate (Ciba). All drugs were dissolved in isotonic saline, and doses are expressed in terms of the salts.
3. Results
3.1. Direct cardiovascular effects of tolamolol, practolol and propranolol The direct effects of tolamolol, practolol and propranolol are shown in table 1. At dose levels showing comparable activity in blocking the cardiac effects of isoprenaline, tolamolol (0.1 mg/kg), practolol (1.0 mg/kg) and propranolol (0.2 mg/kg) had no significant effects on the maximal rate of rise of pressure in the left ventricle (dp/dt)max, but substantially reduced the heart rate. Only slight and non-significant changes occurred in coronary blood flow and resistance. There was an overall tendency for the mean aortic pressure to decrease after each agent, but the extent of this varied markedly between dogs. Higher doses of tolamolol and practolol, 2.0 and 20 mg/kg respectively, also had no marked effects on the cardiovascular parameters measured. Although the mean heart rate for the 5 dogs receiving 2.0 mg/kg tolamolol was higher than the overall mean heart rate after 0.1 mg/kg, in the former group of 5
99
dogs, the mean heart rate fell to only 122 -+ 5.2 beats/min after 0.1 mg/kg. 3.2. Effects o f tolamolol, practolol and propranolol on cardiovascular responses to exogenous cateehoL amines and to left stellate ganglion stimulation 3.2.1. Isoprenaline Responses to isoprenaline before and after the three antagonists are shown in fig. 1. Tolamolol (0.1 mg/kg), practolol (1.0 mg/kg) and propranolol (0.2 mg/kg) all reduced the positive inotropic and chronotropic effects of isoprenaline to a similar extent (64-75% and 83-87%). Propranolol, in addition, blocked the isoprenaline-induced decreases in mean aortic pressure (75%) and in femoral vascular resistance (62%), but in contrast, tolamolol and practolol did not reduce the responses of these parameters. The effects of both tolamolol and practolol on myocardial responses differed to a highly significant extent (p < 0.001) from their effects on peripheral vasodilator responses to isoprenaline. The dose range over which tolamolol and practolol retain this selective antagonism of cardiac responses to isoprenaline is indicated in fig. 1, where responses to isoprenaline are shown after the doses of these two agents were increased 20 fold. Tolamolol (2.0 mg/kg) caused slight blockade of isoprenaline vasodepression, although the level of antagonism was still considerably less than blockade of the cardiac effects (p < 0.01). The fall in femoral vascular resistance to isoprenaline was also reduced, although its antagonism was still significantly less than that of the inotropic effect (p < 0.05). Practolol (20 mg/kg) did not antagonise the peripheral vascular response to isoprenaline. The coronary vascular response to isoprenaline consisted of a fall in end-diastolic coronary resistance accompanied by only a slight rise in blood flow, since the aortic pressure was also reduced. Propranolol ( 0 2 mg/kg) substantially blocked this response;the fall in coronary resistance was antagonised by 60%, which was not significantly different from the antagonism of either the myocardial or peripheral vascular responses to isoprenaline. Equivalent blockade of myocardial /~-adrenoceptors with tolamolol (0.1 mg/kg) resulted in only a 27% reduction in the isoprenalineinduced fall in coronary resistance. The effects of
K.R. Adam, S.M. Boyles, ~-Adrenoceptor blockade by tolamolol
100
16,ooor /-t Left Ventricular | d_pp max (ramHg/sec) 12'000r A
dt
80%
+ 4000 r 01120 F
A Heart Rate (beats/min)
80 -t-40 t 0 t-
A Mean Aortic
-20
Pressure (ramHg)
°EL
40
60 +4o r
A End-diastolic CoronarY201. Blood Flow (rots/rain) ok
~
1
End-diastolic CoronarY20~ ~ A Vascular Resistance1<~o140~
12OlA Mean Femoral Artery 80L Flow (mls/min) + 401/
OL-
FI_F"
A Mean Femoral Vascular 0 r Resistance (°/o) 20r 60 -80 •
•
•
•
•
0.1 2.0mg/kg 1.0 20 mg/kg 0.2 mg/kg TOLAMOLOL PRACTOLOL PROPRANOLOL Fig. 1. Changes in left ventricular (dp/dt)max, heart rate, mean aortic blood pressure and end-diastolic coronary blood flow and resistance in response to isoprenaline (0.4 tag/kg i.v.) and in mean femoral artery blood flow and resistance to close-arterial injection of isoprenaline (0.02 ;tg/kg). The columns represent mean +- S.E.M. of the control responses to isoprenaline [ ] and responses after tolamolol (0.1 and 2.0 mg/kg) U • practolol (1.0 and 2.0 mg/kg) i~;~;, and propranolol (0.2 mg/kg) "';'~ Values are the mean of 4 - 9 observations.
K.R. Adam, S.M. Boyles, fi-Adrenoceptor blockade by tolamolol
practolol (1.0 mg/kg) were qualitatively similar; the fall in coronary resistance to isoprenaline was antagonised by 12%. With both drugs, blockade of the coronary response was considerably less than that of the cardiac responses (p < 0.001) and differed only slightly from that of the femoral vascular responses to isoprenaline. Tolamolol at the higher dose of 2.0 mg/kg almost completely abolished the myocardial response to isoprenaline and the degree of antagonism of the fall in coronary resistance was increased to 62%. This was still significantly less than blockade of the inotropic and chronotropic responses at this dose (p < 0.01,p 0.05 respectively), but did not differ significantly from blockade of the femoral vascular and blood pressure responses.. The higher dose of practolol (20 mg/kg), which also virtually abolished the myocardial response to isoprenaline, increased blockade of the fall in coronary resistance to isoprenaline to 37%, which was still significantly less than the antagonism of the myocardial response at this dose (p < 0.01). The effect of practolol (20 mg/kg) on the coronary resistance response to isoprenaline was significantly greater than its effect on the femoral vascular and blood pressure responses (p < 0.05, p < 0.01 respectively).
101
tered these responses to a significant extent. Three dogs received phentolamine (5 mg/kg) subsequent to the second dose of tolamolol, which converted the femoral vasoconstriction to a fall in resistance of 7.9 -+ 2.4%. The coronary vascular response to adrenaline (1.0 #g/kg), consisted of a similar fall in resistance to that caused by isoprenaline (0.4/ag/kg), accompanied by a relatively greater increase in blood flow due to the raised arterial pressure. The rise in coronary blood flow was reduced to a similar extent after blockade of cardiac/3-adrenoceptors with all three agents. The fall in coronary resistance was markedly reduced by propranolol and, to a much lesser extent, by tolamolol (0.1 mg/kg) and practolol (1.0 mg/kg) (/9 < 0.02;p < 0.05 respectively). Increasing the doses of tolamolol and practolol to 2.0 and 20 mg/kg respectively completely abolished the inotropic response to adrenaline. After practolol, the adrenaline-induced coronary vasodilatation was further reduced, whilst after tolamolol the fall in resistance was converted to a slight rise in four of the five dogs. Phentolamine (5 mg/kg i.v.) administered to these four dogs, reversed the pressor effect of adrenaline to a fall in mean aortic pressure of -15.5 +2.3 mm Hg and also reversed the rise in coronary resistance of 7.76 -+ 7.24% to a fall of 14.0 -+ 7.0%.
3.2.2. A d r e n a l i n e
Responses to adrenaline before and after tolamolol, practoloi and propranolol are shown in fig. 2. At the initial doses used (tolamolol 0.1 mg/kg, practolol 1.0 mg/kg and propranolol 0.2 mg/kg), all three agents substantially blocked the adrenaline-induced increase in left ventricular (dp/dt)max. (Adrenaline had variable effects on the heart rate, and these are not included in the results). Tolamolol and practolol had very little effect on the pressor response to adrenaline, but after propranolol, this response was significantly potentiated (p <0.01). Similarly, there was very little alteration in the rise in femoral vascular resistance to close-arterial adrenaline after either tolamolol or practolol, but propranolol caused significant potentiation (p < 0.01) of the vasoconstriction. Increasing the doses of tolamolol and practolol to 2.0 and 20 mg/kg respectively had little further effect on the adrenaline-induced rise in mean aortic pressure and whilst there was some potentiation of the increase in femoral vascular resistance, neither agent al-
3.2. 3. Noradrenaline
Responses to noradrenaline are presented in fig. 3. The inotropic response to noradrenaline was substantially blocked by the initial doses of tolamolol (0.1 mg/kg) and practolol (1.0 mg/kg) and by propranolol (0.2 mg/kg). (Noradrenaline had variable chronotropic effects which have not been included in the results). None of the agents, at any of the doses used, had any effect on the pressor response to noradrenaline or on the increase in femoral resistance elicited by close-arterial noradrenaline. An apparent potentiation of the femoral vasoconstriction after propranolol was not significant. The coronary dilatation in response to noradrenaline was less than that observed after the same dose of adrenaline although their inotropic effects were very similar. The fall in coronary resistance and the increase in coronary blood flow in response to noradrenaline were reduced to a greater extent by blockade of cardiac /3-adrenoceptors with tolamolol (0.1
K.R. Adam, S.M. Boyles, #.Adrenoceptor blockade by tolamolol
102
'6"°°°F
Left Ventricular
12.OOO~-
dd~trmax(turnFIg/sec) 8OOO I+ 4000[0 L. /~ MeanAortic Pressure (ramFig)
~
.
60 40 +20 0 t-
80I
A End-diastolic Coronary 60 Blood Flow (mls/min) 40 +20 0
T
O tA End-diastolic Coronary 20 j Vascular Resistance (~o)40 t -60 L 80
A Mean FernoralArtery Flow (mls/min) A
40~ -8oL
800r Mean Femoral Vascular I 6001Resistance (~o) 4001/
+ 2OO~orO.I 2.Omg/kg TOLAMOLOL
I.O 2Omg/kg O'2rng/kg PRACTOLOL PROPRANOLOL
Fig, 2. Changes in left ventricular (dp/dt)ma x, mean aortic blood pressure and end-diastolic coronary blood flow and resistance in response to adrenaline (1.0 /ag/kg i.v.) and in mean femoral artery blood flow and resistance to close-arterial injection of adrenaline (0.025 #g/kg). The columns represent mean -+ S.E.M. of the control responses to adrenaline 1--] and responses after tolamolol (0.1 and 2.0 mg/kg) • , practolol (1.0 and 20 mg/kg) :~: and propranolol (0.2 mg/kg) ~,~. Values are the mean of 4 - 5 observations.
K.R. Adam, S.M, Boyles, #-Adrenoceptor blockade by tolamolol
A Left Ventricular d_p_max (ram Hg) dt
A Mean Aortic Pressure (mmHg)
16,ooor
, .ooo I
i
40
103
L
FL
T
+20 Or-
End-diastolic Coronary : 0 ~ A Blood FIow(mls/min) +20t - ~
T
T
.
0 L-
6
VascularEnd-diast°licCoronarY20[ Resistance (~o)40 k / -60L-
A Mean Femoral Artery 0 r" Flow (mls/min) -40 L 600F A Mean FemoralVoscular / o 400~ Resistonce(7o) +200~
. F:::i:i:t!:ii:il
OL
0.1 2.0 mg/kg 1.0 20 mg/kg 0.2 mglkg TOLAMOLOL PRACTOLOL PROPRANOLOL Fig. 3. Changes in left ventricular (dp/dt)ma x, mean aortic blood pressure and end-diastolic coronary blood flow and resistance in response to noradrenaline (1.0 pg/kg i.v.) and in mean femoral artery blood flow and resistance to close-arterial injection of noradrenaline (0.025 pg/kg). The columns represent mean ~ S.E.M. of the control responses to noradrenaline [] and responses after tolamol (0.1 and 2.0 mg/kg) m , practolol (1.0 and 20 mg/kg) ::: and propranolol (0.2 mg/kg) ,~-.'.Values are the mean of 4 - 5 observations.
mg/kg), practolol (1.0 mg/kg) and propranolol (0.2 mg/kg) than were the equivalent responses to adrenaline. The inotropic effect o f noradrenaline was completely abolished after tolamolol (2.0 mg/kg) and practolol 20 mg/kg). In all five dogs which received this dose of tolamolol, the rise in coronary flow induced by noradrenaline was blocked and the fall in coronary resistance was converted to an increase. Four o f these dogs subsequently received phentol-
amine at a dose o f 5 mg/kg i.v., which reduced the noradrenaline-induced rise in mean aortic pressure from 39.0 + 8.6 to 6.0 -+ 0.7 m m Hg, completely abolished the rises in femoral vascular resistance and also reduced the rise in coronary resistance of 21.0 -+ 2.16 to 2.5 -+ 1.8%. Practolol (20 mg/kg) reversed the effect o f noradrenaline on the coronary resistance in two dogs, although the mean response in the group o f four experiments was a fall in resistance.
104
K.R. Adam, S.M. Boyles, #-Adrenoceptor blockade by tolamolol +8000~
Left Ventricular J do 4000~ ~t max. (ram Hg/sec) oL A Mean Aortic Pressur. (mmNg)
i. T j..,.,
jT
+40[ 20 t
o
+40 rA End-diastolic Coronary 20[Blood Fl"ow (mls/min) OL
/
-20 L A End diastolic Coronary+ 2 oL __ T °r Vascular Resistanc'(') 20~ j-t F
,.T.~
~r
~
U
-40 t0-1 2.0mg/k9 TOLAMOLOL
1.0 20rng/k9 0.2mg/k9 PRACTOLOL PROPRANOLOL
Fig. 4. Changesin left ventricular (dp/dt)max, meanaortic blood pressureand end-diastoliccoronary blood flow and resistancein response to stimulation of the left stellate ganglion (3-8 V; 0.5 msec pulse width; l0 Hz). The columns represent mean +- S.E.M. of the control responses to stimulation [] and responses after tolamolol (0.1 and 2.0 mg/kg) ~ , practolol (1.0 and 20 mg/kg) i:i' and propranolol (0.2 mg/kg) "~.".Values are the mean of 4-5 observations.
3.2.4. Stimulation of the left stellate ganglion Responses to stimulation of the left stellate ganglion are shown in fig. 4. (Stimulation of this ganglion has a positive inotropic action, with no effect on the heart rate). The increases in left ventricular (dp/dt)max were substantially blocked by tolamolol (0.1 mg/kg), practolol (1.0 mg/kg) and by propranolol (0.2 mg/kg). The raised coronary blood flow and the fall in coronary vascular resistance which occurred were also very much reduced, and in some experiments were reversed, after blockade of cardiac /3-adrenoceptors. At the higher dose levels, tolamolol (2.0 mg/kg) reversed the coronary dilator response in four of five dogs, as did practolol (20 mg/kg) in three of four experiments. In one of the three dogs in the tolamolol group in which stellate ganglion stimulation increased coronary resistance phentolamine (5 mg/kg) abolished the vasoconstriction. This dose of phentolamine
had no effect on the response to stimulation in the other two experiments.
3.3. Effect of ~-adrenoceptor blockade on reactive hyperaemia Blood flow and vascular resistance were measured at the point of peak reduction in resistance during the hyperaemic response immediately following a 15 sec period of complete occlusion of the left circumflex coronary artery or of the left femoral artery. The responses consisted of decreases of approximately 70 and 50% from pre-occlusion levels in coronary and femoral vascular resistance respectively, accompanied by marked increases in flow through the arteries; there were no other haemodynamic effects. Hyperaemic responses to arterial occlusion were entirely unaffected by fl-adrenoceptor blockade with any of the agents used.
K.R. Adam, S.M. Boyles, #-Adrenoceptor blockade by tolamolol
4. Discussion
In anaesthetised dogs, tolamolol was approximately ten times as potent as practolol and twice as potent as propranolol in antagonising isoprenaline-induced increases in heart rate and rate of rise of left ventricular pressure (dp/dt)max. The results with practolol and propranolol are in agreement with those of Dunlop and Shanks (1968) who found propranolol to be at least three to four times as active as practolol in blocking the myocardial effects of isoprenaline. Tolamolol is also considerably more potent than practolol in conscious dogs, against both isoprenaline and exercise-induced tachycardia (Adam et al., 1973) and is more potent than practolol against isoprenaline in man (Briant et al., 1973). Significant selectivity for myocardial as opposed to peripheral vascular /3-adrenoceptors was apparent with both tolamolol and practolol, when either isoprenaline or adrenaline was used as the agonist. At dose levels sufficient to cause 65-87% blockade of the myocardial responses to isoprenaline, neither agent had any effect on the isoprenaline-induced reductions in mean blood pressure or in femoral vascular resistance. When the initial doses of tolamolol and practo1ol were increased by a factor of 20, partial blockade of the peripheral vascular response to isoprenaline was apparent after the higher dose of tolamolol, but not after the higher dose of practolol. In contrast, /3-receptor blockade with propranolol was entirely non-selective: antagonism of the isoprenaline-induced reductions in blood pressure and in coronary and femoral vascular resistances, as well as that of the positive inotropic and chronotropic effects all lay between 60 and 85%. Presumably, the potentiation by propranolol of the rises in mean aortic pressure and in femoral vascular resistance in response to exogenous adrenaline, was the result of or-receptor stimulation, facilitated by blockade of/3-receptors in the peripheral vasculature. This did not occur after equivalent cardiac/3-blockade with tolamolol or practolol. Tolamolol also displayed significant cardioselectivity in man when blockade of the cardiac and peripheral vasodilator effects of isoprenaline were compared in normal human subjects (Briant et al., 1973). The selectivity of tolamolol for myocardial/3-receptors as opposed to those of the trachea or lung in rodents has also been demonstrated, although estimates of the de-
105
gree of selectivity, when compared with practolol, varied with the preparation and species employed (Adam et al., 1974). Just as practolol has markedly less effect than propranolol in increasing airways resistance in asthmatic patients (Macdonald and McNeill, 1969), recent clinical data suggest that tolamolol also may be well tolerated by asthmatic patients (Aronow et al., 1973). It has been suggested that the use of a/3-blocking agent with sympathomimetic activity may have advantages of safety in asthmatic patients, and may also involve less cardiac risk. However, /3-blocking drugs with such activity have been known to precipitate heart failure (Fitzgerald, 1969). Furthermore, the rationale for the use of such drugs in angina has been questioned, and the study in man of an agent similar to practolol but lacking its sympathomimetic activity has been advocated (Fitzgerald, 1972). Tolamolol appears to be such an agent. The effect of/3-blocking agents on the ability of the heart to respond to hypoxia with a reactive hyperaemia is relevant to the use of these drug in angina. None of the/3-blockers used in the present study had any effect on the peak fall in coronary vascular resistance following a brief period o f coronary artery occlusion. Pauly et al. (1973) demonstrated a reduction in total reactive hyperaemia volume in the dog after propranolol due to the reduced myocardial metabolic activity, although in an earlier study the peak flow reached during the hyperaemic response was unchanged by propranolol (Pauly and Bittar, 1971). The effects of/3-blockade on the coronary vascular responses to sympathetic stimuli are difficult to interpret, since the coronary blood flow is not only closely controlled by metabolic autoregulation, but is also affected by the aortic blood pressure, compression of the vessels due to the contracting myocardium and by ~- and /3-adrenoceptor activity influencing vascular constriction and dilatation respectively. Coronary vasodilatation produced by stimulation of the left stellate ganglion and by injected noradrenaline was markedly reduced after tolamolol, practolol and propranolol, and in some dogs, was reversed to a coronary vasoconstriction. Phentolamine, at a dose which blocked the pressor response to i.v. noradrenaline, effectively antagonised the coronary constriction occurring in response to exogenous noradrenaline, but
106
K.R. Adam, S.M. Boyles, #.Adrenoceptor blockade by tolamolol
only antagonised that due to stellate ganglion stimulation in one of three dogs. This could be due to the relative difficulty encountered in totally blocking the effects of endogenous, compared with exogenous, transmitters. Thus, the direct effect of noradrenaline is unmasked as a vasoconstriction via stimulation of coronary vascular a-receptors after/3-blockade has obviated cardiac metabolic autoregulation; this occurs after cardioselective or non-selective /3-blockade. These results confirm those obtained in an earlier study (Adam et al., 1970), in which a higher dose of practolol was used, resulting in complete abolition of the inotropic response to left stellate ganglion stimulation; coronary constriction was then consistently observed. In the present experiments, the myocardial response to ganglion stimulation was not completely abolished by the dose of practolol used, and coronary vasoconstriction did not always occur. The effect of cardioselective /3-blockade on the coronary vascular/3-receptors may become therapeutically relevant during the response to circulating adrenaline, the levels of which are elevated during angina of effort (Raab, 1941). The results of the present study clearly show that, in anaesthetised dogs, the cardioselective /3-blockers tolamolol and practolol cause less reduction of the coronary vasodilator response to injected adrenaline and isoprenaline than does propranolol, at doses producing equivalent blockade of myocardial responses. Adrenaline produced coronary vasoconstriction only under conditions of complete abolition of the myocardial response, with some additional peripheral vascular /3-blockade. These conditions were produced in this study only by an excessive dose of tolamolol. This observation suggests that adrenaline has a direct action on both/3- and a-receptors in the coronary vascular bed, the action on a-receptors becoming apparent only when there is some degree of blockade of vascular/3-receptors, and is thus in agreement with results obtained with practolol by Adam et al. (1970)and Mark et al. (1972). Propranolol did not cause adrenaline to constrict the coronary vessels, as might have been expected from other published results (Mark et al., 1972), presumably because the myocardial response was not completely abolished by the dose of propranolol used. In contrast, in the femoral vascular bed, the predominant effect of adrenaline was vaso. constriction which was potentiated after /3-blockade
with propranolol. These observations are consistent with the suggestion made by Mark et al. (1972) of the paucity of a-receptors in the coronary vasculature, compared with vessels in skin and skeletal muscle. After cardiac/3-blockade with tolamolol or practolol, the isoprenaline-induced coronary vasodilatation was only slightly reduced, although the myocardial responses were almost abolished. This is in agreement with the observations of McRaven et al. (1971). When the myocardial response to isoprenaline was completely abolished by the higher dose of tolamolol, the antagonism of the coronary and femoral vascular responses were almost identical, indicating that in the complete absence of metabolic autoregulation, the coronary vascular bed behaves very similarly to that of the femoral vascular bed to/3-receptor stimulation. These results add further support to the view that the coronary vascular /3-receptors resemble the/32-receptors of the peripheral vasculature, rather than the myocardial/31 -receptors. A similar conclusion has also been reached from the results of several recent studies in the dog and cat, in which practolol was used to abolish the effects of metabolic autoregulation on the coronary vessels. As a result of these studies, coronary vascular /3-receptors have been ascribed to the /32-peripheral vascular sub-group (Parratt and Wadsworth, 1970; Mark et al., 1972) or shown to be different from myocardial /31-receptors (Ross and Jgrgensen, 1970; Bussman et al., 1970; Adam et al., !970). In contrast, evidence in favour of a similarity between coronary vascular and myorcardial /3-receptors has been presented, mainly from the results of in vitro experiments using isolated coronary arteries (Baron et al., 1972; Drew and Levy, 1972) or isolated heart preparations (Somani et al., 1970). Baron et al. (1972) considered it unlikely that coronary vascular /3-receptors should acquire a sensitivity to practolol in vitro which they do not possess in vivo. It is possible that the selectivity of practolol shown in vivo is due to a difference in accessibility of various /3-receptor sites to the compound; when the coronary arteries are isolated, practolol may be more easily able to make contact with the/3-receptors. Alternatively, since the action of adrenaline on isolated coronary arteries has been shown to be dependent on the size of the artery used (Zuberbuhler and Bohr, 1965), there may be a different distribution or sensitivity of/3-receptors in large and small coronary arteries which may account
K.R. Adam, S. M. Boyles, #-Adrenoceptor btockade by tolamolol for the discrepancy b e t w e e n in vivo and in vitro resuits.
Acknowledgements The authors wish to thank Dr. Valerie A. Alabaster for her invaluable help in preparing the manuscript.
References Adam, K.R., S.M. Boyles and P.C. Schofield, 1970, Cardioselective a-adrenoceptor blockade and the coronary circulation, Brit. J. Pharmacol. 40,534. Adam, K.R., L.G. Pullman and P.C. Schofield, 1973, Brit. J. Pharmacol. 49,560. Adam, K.R., J.R.C. Baird, R.A. Burges and J. LinneU, 1974, European J. Pharmacol. 25,170. Axonow, W.S., P.R. Harding, W.H. Nelson, J.S. Vangrow, L.L. Johnson and M. Khursheed, 1973, Treatment of arrhythmias with tolamidol*, Clin. Pharmacol. Therap. 13, 856. (*Correct name is tolamolol). Augstein, J., D.A. Cox, A.L. Ham, P.R. Leeming and M. Snarey, 1973, a-Adrenoceptor blocking agents. I. Cardioselective 1-aryloxy-3-(aryloxyalkylamine)-propan-2-ols, J. Med. Chem. 16, 1245. Baron, G.D., R.N. Speden and D.F. Bohr, 1972, a-Adrenergic receptors in coronary and skeletal muscle arteries, Amer. J. Physiol. 223,878. Briant, R.H., C.T. DoUery, T. Fenyvesi and C.F. George, 1973, Assessment of selective a-adrenoceptor blockade in man, Brit. J. Pharmacol. 49,106. Bussmann, W.D., M. Rauh and H.P. Krayenbuehl, 1970, Coronary and haemodynamic effects of myocardio-selectire/~-receptor blockade by ICI 50172 in the closed-chest dog, Amer. Heart J. 79,347. Drew, G.M. and G.P. Levy, 1972, Characterization of the coronary vascular a-adrenoceptor in the pig, Brit. J. Pharmacol. 46,348.
107
Dunlop, D. and R.G. Shanks, 1968, Selective blockade of adrenoceptive a-receptors in the heart, Brit. J. Pharmacol. 32,201. Fitzgerald, J.D., 1969, Perspectives in adrenergic a-receptor blockade, Clin. Pharmacol. Therap. 10,292. Fitzgerald, J.D., 1972, Cardioselective a-adrenergic blockade, Proc. Roy. Soc. Med. 65,761. Lands, A.M., A. Arnold, J. McAuliff, F. Luduena and T. Brown, 1967, Differentiation of receptor systems activated by sympathomimetic amines, Nature 214,597. Macdonald, A.G. and R.S. McNeill, 1969, A comparison of the effect on airway resistance of a new a-blocking drug, ICI 50172 and propranolol, Brit. J. Anaesth. 40,508. Mark, A.L., F.M. Abboud, P.G. Schmid, D.D. Heistad and H.E. Mayer, 1972, Differences in direct effects of adrenergic stimuli on coronary, cutaneous and muscular vessels, J. Clin. Invest. 51,279. McRaven, D.R., A.L. Mark, F.M. Abboud and H.E. Mayer, 1971, Responses of coronary vessels to adrenergic stimuli, J. Clin. Invest. 50,773. Parratt, J.R. and R.M. Wadsworth, 1970, The effect of 'selective' a-adrenoceptor blocking drugs on the myocardial circulation, Brit. J. Pharmacol. 39,296. Pauly, T.J. and N. Bittar, 1971, Myocardial reactive hyperaemia responses in the dog after a-receptor block with propranolol, Card. Res. 5,440. Pauly, T.J., W.C. Zarnstofff and N. Bittar, 1973, Myocardial metabolic activity as a determinant of reactive hyperaemia responses in the dog heart, Card. Res. 7, 90. Raab, W., 1941, Abnormal suprarenal discharges in angina pectoris and their control by X-ray therapy, J. Clin. Endocrinol. 1,977. Ross, G. and C.R. Jorgensen, 1970, Effects of a cardio-selective/~adrenergic blocking agent on the heart and coronary circulation, Card. Res. 4,148. Somani, P., A.R. Laddu and H.F. Hardman, 1970, Nutritional circulation in the heart. III. Effect of isoproterenol and a-adrenergic blockade on myocardial haemodynamics and rubidium-86 extraction in the isolated supported heart preparation, J. Pharmacol. Exptl. Therap. 175,577. Zuberbuhler, R.C. and D.F. Bohr, 1965, Responses of coronary smooth muscle to catecholamines, Circulation Res. 16,431.
HAEMODYNAMIC BLOCKADE
AND CORONARY
VASCULAR
IN THE ANAESTHETISED PRACTOLOL Kenneth
NORTH-HOLLAND PUBLISHING COMPANY
RESPONSES
DOG: A COMPARISON
AFTER
/3-ADRENOCEI’TOR
OF TOLAMOLOL
WITH
AND PROPRANOLOL
R. ADAM and Susan M. BOYLJZS
Central Research, Pfizer Limited, Sandwich, Kent, England
Received 17 August 1973
Accepted 30 November 1973
K.R. ADAM and S.M. BOYLES, Haemodynamic
and coronary vascular responses after p-adrenoceptor blockade in the anaesthetised dog: a comparison of tolamolol with practolol and propranolol, European J. Pharmacol. 26 (1974)
96-107. In anaesthetised dogs, the fl-adrenoceptor blocking agent tolamolol had approximately ten times the activity of practolol and twice that of propranolol in antagonising the myocardial responses to injected isoprenaline, adrenaline and noradrenaline and to stimulation of the left stellate ganglion. Tolamolol, like practolol, was cardioselective; in amounts which blocked myocardial responses to exogenous catecholamines and to sympathetic stimulation, it did not block peripheral vasodilatation caused by isoprenaline, and isoprenaline and adrenaline continued to cause coronary vasodilatation. In contrast, propranolol blocked myocardial, peripheral vascular and coronary vascular responses to isoprenaline to similar degrees. These results imply that the coronary vascular P-adrenoceptors resemble those of the peripheral vasculature. The coronary dilatation due to noradrenaline and stellate ganglion stimulation was generally abolished after fl-adrenoceptor blockade and in some experiments was converted to a vasoconstriction. &Adrenoceptor blockade did not affect peak reactive hyperaemia following coronary artery occlusion. Cardioselective p-adrenoceptor blockade Coronary vascular adrenoceptors
1. Introduction The purpose of the current investigation in anaesthetised dogs was twofold. Firstly, the cardiovascular effects oftolamolol((*)-4-[2(2-hydroxy-3-o-tolyloxypropylamino)-ethoxy] -benzamide, UK-6558 (Aug stein et al., 1974)) were examined. Tolamolol is a /3-adrenoceptor blocking agent which lacks direct myocardial depressant activity (Augstein et al., 1974), does not show intrinsic sympathomimetic activity (Adam et al., 1974), and is also cardioselective. The latter has been shown both in man, where myocardial/peripheral vascular selectivity was demonstrated (Briant et al., 1973) and in rodents, where tolamolol was found to be selective for myocardial as opposed to bronchial /3-adrenoceptors (Adam et al., 1974). Secondly, the nature of coronary vascular /3-adrenoceptors was further investigated by compar-
Stellate ganglion stimulation Propranolol
Tolamolol Practolol
ing the responses of the coronary vessels and of the peripheral vasculature to sympathetic stimuli following blockade of inotropic and chronotropic effects by tolamolol, practolol and the non-selective padrenoceptor blocking agent, propranolol. Practolol blocks myocardial /%adrenoceptors in amounts which have no effect on the /3-adrenoceptors of the peripheral vasculature (Dunlop and Shanks, 1968). The latter receptors have been classified in the same group @,) as those of bronchiolar smooth muscle, and contrasted with those (PI) in the myocardium (Lands et al., 1967). The nature of the /3adrenoceptors in the coronary vessels remains however, controversial. It has been found in previous investigations (Adam et al., 1970; Ross and Jorgensen, 1970) that the /3-adrenoceptors in the myocardium of the dog can be blocked by practolol independently of those of the coronary vasculature, thus allowing the
K.R. Adam, S.M. Boyles, ~-Adrenoceptor blockade by tolamolol
direct effects of sympathetic stimulation and of catecholamines to be observed in these vessels free of the autoregulatory changes in coronary blood flow accompanying inotropic and chronotropic effects. Such observations support the view that the 13-adrenoceptors of coronary vessels resemble those of the peripheral vasculature. Nevertheless, a substantial body of evidence from in vitro studies (Baron et al., 1972; Drew and Levy, 1972) indicates that the coronary /3-adrenoceptors fall into the/3~ -category.
2. Materials and methods
18 male beagle dogs (weight range 11-18 kg) anaesthetised with sodium pentobarbitone (30 mg/kg i.v.) were used in this study. Body temperature, monitored by an oesophageal probe, was maintained at 37°C by means of a thermostatically controlled blanket. Pressures were measured from catheters passed into the aortic arch from the right femoral artery, into the left ventride via the left carotid artery and in 14 of the dogs femoral arterial pressure was also measured via the right femoral artery. The left ventricular pressure pulse was differentiated electronically to obtain the rate of rise of pressure in the left ventricle (dp/dt), as a measure of contractility and was also used to trigger an instantaneous rate meter (Devices Instruments Ltd.). The diastolic portion of the left ventricular pressure was amplified to facilitate identification of end diastole. In dogs in which femoral arterial pressure was measured, blood flow was recorded in the contralateral femoral artery using a 2.5 mm electromagnetic flow probe (Biotronix Laboratory Inc.). Immediately distal to the flow probe, a hydraulic cuff occlusive device was positioned on the artery in order to obtain a zero flow reference. In 12 of these dogs, a fine polyethylene catheter was inserted into the femoral artery from a muscular branch for close-arterial injection into the femoral vasculature. Resistance of the vascular bed was continuously monitored by electronic division of the femoral arterial pressure (an approximation to the pressure gradient across the femoral vasculature) by flow in the artery. A left thoracotomy was performed in the fourth intercostal space; ventilation was maintained with room air through a cuffed endotracheal tube using a
97
Palmer respiration pump. The left stellate ganglion was dissected free of surrounding tissue, its connections with the sympathetic chain crushed and shielded platinum wire electrodes placed on it through a stab incision in the chest wall. The pericardium was incised laterally and sutured into a cradle to support the heart, and the left atrium was deflected. A length of the left circumflex coronary artery about 1 cm long and free of branches was cleared of surrounding tissue and a 2 mm electromagnetic flow probe (Biotronix Laboratory Inc.) applied with a cuff occluder distal to it. The ligatures restraining the pericardium and atrium were then released, allowing the heart to assume its original position. The flow probes and flow meters (S.E. Laboratories Ltd.) used in these experiments were calibrated using excised dog arteries of suitable size through which isotonic saline was passed at known flow rates. The resistance of the coronary vascular bed supplied by the left circumflex artery was continuously monitored by electronic division of the aortic pressure (which approximates closely to the pressure drop across the coronary vascular bed) by the phasic coronary flow. Recorded parameters were displayed on a multichannel recorder (Devices Instruments Ltd.). Haemodynamic and coronary vascular responses were obtained to isoprenaline (0.4/.tg/kg), adrenaline (1.0 /.tg/kg) and noradrenaline (1.0 /~g/kg) injected into the right femoral vein and 30 sec periods of submaximal stimulation of the left stellate ganglion ( 3 - 8 V; 0.5 msec pulse width; 10 Hz). Coronary flow and resistance were measured at end-diastole, this being the point in the cardiac cycle at which myocardial compression has least effect on coronary blood flow, and values were calculated from the mean of readings taken at two cardiac cycles during control periods and at the point of maximal myocardial response after i.v. catecholamine or ganglion stimulation. Femoral vascular responses were also obtained to closearterial injection of isoprenaline (0.02/ag/kg), adrenaline (0.025/ag/kg) and noradrenaline (0.025/ag/kg). All interventions were carried out at 10-15 min intervals, during which time the measured parameters returned to control levels. The three ~-blocking agents under investigation were administered i.v. by infusfon, at a rate of 2 ml/min for 10 min, with an interval between successive doses of approximately 1.5 hr. The direct effects
(4)
110 ,+ 0.5 111 ± 3.8
117 ,+ 3.3 105 ,+ 8.3
114 ,+ 5.9 101 ,+ 5.3
(9)
113 ,+ 4.6 107 ,+ 4 . 2
(4)
Control 112 ,+ 4.8 P r o p r a n o l o l 0.2 m g / k g 100 ,+ 7.4
Control Practolo1 20 m g / k g
(5)
Control P r a c t o l o l 1.0 m g / k g
(5)
Control T o l a m o l o l 2.0 m g / k g
Control T o l a m o l o l 0.1 m g / k g
Mean a o r t i c pressure (mmHg)
143 ± 7.2 126 ,+ 9.3
128 ± 3.6 120 -+ 3.7 (p < 0 . 0 5 )
144 ,+ 6.1 124 ,+ 3.6 60 < 0 . 0 5 )
132 ± 6.5 123 ,+ 7.8
136 ,+ 4 . 0 115 ± 4.3 (9) (p < 0 . 0 1 )
Heart rate (beats/min)
2 4 5 0 -+ 4 9 7 1800 ± 280
3 3 0 0 ,+ 704 3125 ,+ 6 9 0
3 4 0 0 -+ 4 1 0 3 2 2 0 ,+ 5 1 6
2 9 4 0 ,+ 289 2 9 4 0 ,+ 276
2911 ,+ 277 2911 ± 309 (9)
Left v e n t r i c u l a r (dp/dt)max ( m m Hg/sec)
29.3 -+ 2.3 25.7 ,+ 3.1
26.3 ,+ 3.2 25.1 ,+ 3.7
29.4 ± 2.5 25.0 +- 2.0
28.3 ,+ 3.2 27.4 ± 3.7
28.0 -+ 2 A 2 3 . 2 ,+ 2.0 (9)
End-diastolic coronary flow (ml/min)
354 -+ 21 356 +- 4 0
335 ,+ 33 359 -+ 4 7
318 ± 20 343 ,+ 20
348 ,+ 54 310 ± 4 7
342 ,+ 56 378 ± 4 9 (5)
4 9 . 3 -+ 9.3 4 1 . 9 ,+ 7.9
4 4 . 8 ,+ 2.8 4 6 . 3 ,+ 5.0
4 2 . 6 ± 8.2 39.7 ± 7.3
37.7 ,+ 6.1 32.1 ± 2.5
4 7 . 0 +- 6.7 38.0 -+ 6.0 (5)
End-diastolic Mean f e m o r a l coronary resistance flow ( d y n c m "5 sec ×103) ( m l / m i n )
211 -+ 34 235 ,+ 58
159 ± 13 150 -+ 19
203 ,+ 4 4 213 ,+ 4 4
244 ,+ 30 238 ,+ 22
192 ,+ 18 224 ± 28 (5)
Mean f e m o r a l resistance ( d y n c m -s secXl 03)
Table 1 Direct cardiovascular effects o f t o l a m o l o l , p r a c t o l o l a n d p r o p r a n o l o l in a n a e s t h e t i s e d , o p e n - c h e s t d o g s . V a l u e s are m e a n -+ S.E.M. S i g n i f i c a n t d i f f e r e n c e s refer to d i f f e r e n c e f r o m original c o n t r o l value. F i g u r e s in p a r e n t h e s e s i n d i c a t e n u m b e r s o f e x p e r i m e n t s .
~O
K.R. Adam, S.M. Boyles, &Adrenoceptorblockade by tolamolol of these agents on the recorded parameters were measured 10 min after the infusions were ended and then responses were again obtained to catecholamines and to ganglion stimulation. The control response time to peak contractility after the i.v. administration of catecholamines was used as a reference point for measuring coronary responses after abolition of the contractile response by /3-adrenoceptor blockade. In addition, the peak reactive hyperaemic response of blood flow and vascular resistance was measured after 15 sec periods of complete occlusion of the circumflex coronary artery and of the femoral artery before and after/3-adrenoceptor blockade. All results are expressed as the mean + S.E.M. and, where applicable, were tested for significance using Student's t-test. Drugs used were (---)-isoprenaline hydrochloride (Sigma), (-)-noradrenaline bitartrate (Koch-Light), (-)-adrenaline chloride (Parke-Davis), (-+)-tolamolol hydrochloride, (+)-practolol hydrochloride (I.C.I.), (+)-propranolol hydrochloride (I.C.I.) and phentolamine mesylate (Ciba). All drugs were dissolved in isotonic saline, and doses are expressed in terms of the salts.
3. Results
3.1. Direct cardiovascular effects of tolamolol, practolol and propranolol The direct effects of tolamolol, practolol and propranolol are shown in table 1. At dose levels showing comparable activity in blocking the cardiac effects of isoprenaline, tolamolol (0.1 mg/kg), practolol (1.0 mg/kg) and propranolol (0.2 mg/kg) had no significant effects on the maximal rate of rise of pressure in the left ventricle (dp/dt)max, but substantially reduced the heart rate. Only slight and non-significant changes occurred in coronary blood flow and resistance. There was an overall tendency for the mean aortic pressure to decrease after each agent, but the extent of this varied markedly between dogs. Higher doses of tolamolol and practolol, 2.0 and 20 mg/kg respectively, also had no marked effects on the cardiovascular parameters measured. Although the mean heart rate for the 5 dogs receiving 2.0 mg/kg tolamolol was higher than the overall mean heart rate after 0.1 mg/kg, in the former group of 5
99
dogs, the mean heart rate fell to only 122 -+ 5.2 beats/min after 0.1 mg/kg. 3.2. Effects o f tolamolol, practolol and propranolol on cardiovascular responses to exogenous cateehoL amines and to left stellate ganglion stimulation 3.2.1. Isoprenaline Responses to isoprenaline before and after the three antagonists are shown in fig. 1. Tolamolol (0.1 mg/kg), practolol (1.0 mg/kg) and propranolol (0.2 mg/kg) all reduced the positive inotropic and chronotropic effects of isoprenaline to a similar extent (64-75% and 83-87%). Propranolol, in addition, blocked the isoprenaline-induced decreases in mean aortic pressure (75%) and in femoral vascular resistance (62%), but in contrast, tolamolol and practolol did not reduce the responses of these parameters. The effects of both tolamolol and practolol on myocardial responses differed to a highly significant extent (p < 0.001) from their effects on peripheral vasodilator responses to isoprenaline. The dose range over which tolamolol and practolol retain this selective antagonism of cardiac responses to isoprenaline is indicated in fig. 1, where responses to isoprenaline are shown after the doses of these two agents were increased 20 fold. Tolamolol (2.0 mg/kg) caused slight blockade of isoprenaline vasodepression, although the level of antagonism was still considerably less than blockade of the cardiac effects (p < 0.01). The fall in femoral vascular resistance to isoprenaline was also reduced, although its antagonism was still significantly less than that of the inotropic effect (p < 0.05). Practolol (20 mg/kg) did not antagonise the peripheral vascular response to isoprenaline. The coronary vascular response to isoprenaline consisted of a fall in end-diastolic coronary resistance accompanied by only a slight rise in blood flow, since the aortic pressure was also reduced. Propranolol ( 0 2 mg/kg) substantially blocked this response;the fall in coronary resistance was antagonised by 60%, which was not significantly different from the antagonism of either the myocardial or peripheral vascular responses to isoprenaline. Equivalent blockade of myocardial /~-adrenoceptors with tolamolol (0.1 mg/kg) resulted in only a 27% reduction in the isoprenalineinduced fall in coronary resistance. The effects of
K.R. Adam, S.M. Boyles, ~-Adrenoceptor blockade by tolamolol
100
16,ooor /-t Left Ventricular | d_pp max (ramHg/sec) 12'000r A
dt
80%
+ 4000 r 01120 F
A Heart Rate (beats/min)
80 -t-40 t 0 t-
A Mean Aortic
-20
Pressure (ramHg)
°EL
40
60 +4o r
A End-diastolic CoronarY201. Blood Flow (rots/rain) ok
~
1
End-diastolic CoronarY20~ ~ A Vascular Resistance1<~o140~
12OlA Mean Femoral Artery 80L Flow (mls/min) + 401/
OL-
FI_F"
A Mean Femoral Vascular 0 r Resistance (°/o) 20r 60 -80 •
•
•
•
•
0.1 2.0mg/kg 1.0 20 mg/kg 0.2 mg/kg TOLAMOLOL PRACTOLOL PROPRANOLOL Fig. 1. Changes in left ventricular (dp/dt)max, heart rate, mean aortic blood pressure and end-diastolic coronary blood flow and resistance in response to isoprenaline (0.4 tag/kg i.v.) and in mean femoral artery blood flow and resistance to close-arterial injection of isoprenaline (0.02 ;tg/kg). The columns represent mean +- S.E.M. of the control responses to isoprenaline [ ] and responses after tolamolol (0.1 and 2.0 mg/kg) U • practolol (1.0 and 2.0 mg/kg) i~;~;, and propranolol (0.2 mg/kg) "';'~ Values are the mean of 4 - 9 observations.
K.R. Adam, S.M. Boyles, fi-Adrenoceptor blockade by tolamolol
practolol (1.0 mg/kg) were qualitatively similar; the fall in coronary resistance to isoprenaline was antagonised by 12%. With both drugs, blockade of the coronary response was considerably less than that of the cardiac responses (p < 0.001) and differed only slightly from that of the femoral vascular responses to isoprenaline. Tolamolol at the higher dose of 2.0 mg/kg almost completely abolished the myocardial response to isoprenaline and the degree of antagonism of the fall in coronary resistance was increased to 62%. This was still significantly less than blockade of the inotropic and chronotropic responses at this dose (p < 0.01,p 0.05 respectively), but did not differ significantly from blockade of the femoral vascular and blood pressure responses.. The higher dose of practolol (20 mg/kg), which also virtually abolished the myocardial response to isoprenaline, increased blockade of the fall in coronary resistance to isoprenaline to 37%, which was still significantly less than the antagonism of the myocardial response at this dose (p < 0.01). The effect of practolol (20 mg/kg) on the coronary resistance response to isoprenaline was significantly greater than its effect on the femoral vascular and blood pressure responses (p < 0.05, p < 0.01 respectively).
101
tered these responses to a significant extent. Three dogs received phentolamine (5 mg/kg) subsequent to the second dose of tolamolol, which converted the femoral vasoconstriction to a fall in resistance of 7.9 -+ 2.4%. The coronary vascular response to adrenaline (1.0 #g/kg), consisted of a similar fall in resistance to that caused by isoprenaline (0.4/ag/kg), accompanied by a relatively greater increase in blood flow due to the raised arterial pressure. The rise in coronary blood flow was reduced to a similar extent after blockade of cardiac/3-adrenoceptors with all three agents. The fall in coronary resistance was markedly reduced by propranolol and, to a much lesser extent, by tolamolol (0.1 mg/kg) and practolol (1.0 mg/kg) (/9 < 0.02;p < 0.05 respectively). Increasing the doses of tolamolol and practolol to 2.0 and 20 mg/kg respectively completely abolished the inotropic response to adrenaline. After practolol, the adrenaline-induced coronary vasodilatation was further reduced, whilst after tolamolol the fall in resistance was converted to a slight rise in four of the five dogs. Phentolamine (5 mg/kg i.v.) administered to these four dogs, reversed the pressor effect of adrenaline to a fall in mean aortic pressure of -15.5 +2.3 mm Hg and also reversed the rise in coronary resistance of 7.76 -+ 7.24% to a fall of 14.0 -+ 7.0%.
3.2.2. A d r e n a l i n e
Responses to adrenaline before and after tolamolol, practoloi and propranolol are shown in fig. 2. At the initial doses used (tolamolol 0.1 mg/kg, practolol 1.0 mg/kg and propranolol 0.2 mg/kg), all three agents substantially blocked the adrenaline-induced increase in left ventricular (dp/dt)max. (Adrenaline had variable effects on the heart rate, and these are not included in the results). Tolamolol and practolol had very little effect on the pressor response to adrenaline, but after propranolol, this response was significantly potentiated (p <0.01). Similarly, there was very little alteration in the rise in femoral vascular resistance to close-arterial adrenaline after either tolamolol or practolol, but propranolol caused significant potentiation (p < 0.01) of the vasoconstriction. Increasing the doses of tolamolol and practolol to 2.0 and 20 mg/kg respectively had little further effect on the adrenaline-induced rise in mean aortic pressure and whilst there was some potentiation of the increase in femoral vascular resistance, neither agent al-
3.2. 3. Noradrenaline
Responses to noradrenaline are presented in fig. 3. The inotropic response to noradrenaline was substantially blocked by the initial doses of tolamolol (0.1 mg/kg) and practolol (1.0 mg/kg) and by propranolol (0.2 mg/kg). (Noradrenaline had variable chronotropic effects which have not been included in the results). None of the agents, at any of the doses used, had any effect on the pressor response to noradrenaline or on the increase in femoral resistance elicited by close-arterial noradrenaline. An apparent potentiation of the femoral vasoconstriction after propranolol was not significant. The coronary dilatation in response to noradrenaline was less than that observed after the same dose of adrenaline although their inotropic effects were very similar. The fall in coronary resistance and the increase in coronary blood flow in response to noradrenaline were reduced to a greater extent by blockade of cardiac /3-adrenoceptors with tolamolol (0.1
K.R. Adam, S.M. Boyles, #.Adrenoceptor blockade by tolamolol
102
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Left Ventricular
12.OOO~-
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I.O 2Omg/kg O'2rng/kg PRACTOLOL PROPRANOLOL
Fig, 2. Changes in left ventricular (dp/dt)ma x, mean aortic blood pressure and end-diastolic coronary blood flow and resistance in response to adrenaline (1.0 /ag/kg i.v.) and in mean femoral artery blood flow and resistance to close-arterial injection of adrenaline (0.025 #g/kg). The columns represent mean -+ S.E.M. of the control responses to adrenaline 1--] and responses after tolamolol (0.1 and 2.0 mg/kg) • , practolol (1.0 and 20 mg/kg) :~: and propranolol (0.2 mg/kg) ~,~. Values are the mean of 4 - 5 observations.
K.R. Adam, S.M, Boyles, #-Adrenoceptor blockade by tolamolol
A Left Ventricular d_p_max (ram Hg) dt
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0.1 2.0 mg/kg 1.0 20 mg/kg 0.2 mglkg TOLAMOLOL PRACTOLOL PROPRANOLOL Fig. 3. Changes in left ventricular (dp/dt)ma x, mean aortic blood pressure and end-diastolic coronary blood flow and resistance in response to noradrenaline (1.0 pg/kg i.v.) and in mean femoral artery blood flow and resistance to close-arterial injection of noradrenaline (0.025 pg/kg). The columns represent mean ~ S.E.M. of the control responses to noradrenaline [] and responses after tolamol (0.1 and 2.0 mg/kg) m , practolol (1.0 and 20 mg/kg) ::: and propranolol (0.2 mg/kg) ,~-.'.Values are the mean of 4 - 5 observations.
mg/kg), practolol (1.0 mg/kg) and propranolol (0.2 mg/kg) than were the equivalent responses to adrenaline. The inotropic effect o f noradrenaline was completely abolished after tolamolol (2.0 mg/kg) and practolol 20 mg/kg). In all five dogs which received this dose of tolamolol, the rise in coronary flow induced by noradrenaline was blocked and the fall in coronary resistance was converted to an increase. Four o f these dogs subsequently received phentol-
amine at a dose o f 5 mg/kg i.v., which reduced the noradrenaline-induced rise in mean aortic pressure from 39.0 + 8.6 to 6.0 -+ 0.7 m m Hg, completely abolished the rises in femoral vascular resistance and also reduced the rise in coronary resistance of 21.0 -+ 2.16 to 2.5 -+ 1.8%. Practolol (20 mg/kg) reversed the effect o f noradrenaline on the coronary resistance in two dogs, although the mean response in the group o f four experiments was a fall in resistance.
104
K.R. Adam, S.M. Boyles, #-Adrenoceptor blockade by tolamolol +8000~
Left Ventricular J do 4000~ ~t max. (ram Hg/sec) oL A Mean Aortic Pressur. (mmNg)
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Fig. 4. Changesin left ventricular (dp/dt)max, meanaortic blood pressureand end-diastoliccoronary blood flow and resistancein response to stimulation of the left stellate ganglion (3-8 V; 0.5 msec pulse width; l0 Hz). The columns represent mean +- S.E.M. of the control responses to stimulation [] and responses after tolamolol (0.1 and 2.0 mg/kg) ~ , practolol (1.0 and 20 mg/kg) i:i' and propranolol (0.2 mg/kg) "~.".Values are the mean of 4-5 observations.
3.2.4. Stimulation of the left stellate ganglion Responses to stimulation of the left stellate ganglion are shown in fig. 4. (Stimulation of this ganglion has a positive inotropic action, with no effect on the heart rate). The increases in left ventricular (dp/dt)max were substantially blocked by tolamolol (0.1 mg/kg), practolol (1.0 mg/kg) and by propranolol (0.2 mg/kg). The raised coronary blood flow and the fall in coronary vascular resistance which occurred were also very much reduced, and in some experiments were reversed, after blockade of cardiac /3-adrenoceptors. At the higher dose levels, tolamolol (2.0 mg/kg) reversed the coronary dilator response in four of five dogs, as did practolol (20 mg/kg) in three of four experiments. In one of the three dogs in the tolamolol group in which stellate ganglion stimulation increased coronary resistance phentolamine (5 mg/kg) abolished the vasoconstriction. This dose of phentolamine
had no effect on the response to stimulation in the other two experiments.
3.3. Effect of ~-adrenoceptor blockade on reactive hyperaemia Blood flow and vascular resistance were measured at the point of peak reduction in resistance during the hyperaemic response immediately following a 15 sec period of complete occlusion of the left circumflex coronary artery or of the left femoral artery. The responses consisted of decreases of approximately 70 and 50% from pre-occlusion levels in coronary and femoral vascular resistance respectively, accompanied by marked increases in flow through the arteries; there were no other haemodynamic effects. Hyperaemic responses to arterial occlusion were entirely unaffected by fl-adrenoceptor blockade with any of the agents used.
K.R. Adam, S.M. Boyles, #-Adrenoceptor blockade by tolamolol
4. Discussion
In anaesthetised dogs, tolamolol was approximately ten times as potent as practolol and twice as potent as propranolol in antagonising isoprenaline-induced increases in heart rate and rate of rise of left ventricular pressure (dp/dt)max. The results with practolol and propranolol are in agreement with those of Dunlop and Shanks (1968) who found propranolol to be at least three to four times as active as practolol in blocking the myocardial effects of isoprenaline. Tolamolol is also considerably more potent than practolol in conscious dogs, against both isoprenaline and exercise-induced tachycardia (Adam et al., 1973) and is more potent than practolol against isoprenaline in man (Briant et al., 1973). Significant selectivity for myocardial as opposed to peripheral vascular /3-adrenoceptors was apparent with both tolamolol and practolol, when either isoprenaline or adrenaline was used as the agonist. At dose levels sufficient to cause 65-87% blockade of the myocardial responses to isoprenaline, neither agent had any effect on the isoprenaline-induced reductions in mean blood pressure or in femoral vascular resistance. When the initial doses of tolamolol and practo1ol were increased by a factor of 20, partial blockade of the peripheral vascular response to isoprenaline was apparent after the higher dose of tolamolol, but not after the higher dose of practolol. In contrast, /3-receptor blockade with propranolol was entirely non-selective: antagonism of the isoprenaline-induced reductions in blood pressure and in coronary and femoral vascular resistances, as well as that of the positive inotropic and chronotropic effects all lay between 60 and 85%. Presumably, the potentiation by propranolol of the rises in mean aortic pressure and in femoral vascular resistance in response to exogenous adrenaline, was the result of or-receptor stimulation, facilitated by blockade of/3-receptors in the peripheral vasculature. This did not occur after equivalent cardiac/3-blockade with tolamolol or practolol. Tolamolol also displayed significant cardioselectivity in man when blockade of the cardiac and peripheral vasodilator effects of isoprenaline were compared in normal human subjects (Briant et al., 1973). The selectivity of tolamolol for myocardial/3-receptors as opposed to those of the trachea or lung in rodents has also been demonstrated, although estimates of the de-
105
gree of selectivity, when compared with practolol, varied with the preparation and species employed (Adam et al., 1974). Just as practolol has markedly less effect than propranolol in increasing airways resistance in asthmatic patients (Macdonald and McNeill, 1969), recent clinical data suggest that tolamolol also may be well tolerated by asthmatic patients (Aronow et al., 1973). It has been suggested that the use of a/3-blocking agent with sympathomimetic activity may have advantages of safety in asthmatic patients, and may also involve less cardiac risk. However, /3-blocking drugs with such activity have been known to precipitate heart failure (Fitzgerald, 1969). Furthermore, the rationale for the use of such drugs in angina has been questioned, and the study in man of an agent similar to practolol but lacking its sympathomimetic activity has been advocated (Fitzgerald, 1972). Tolamolol appears to be such an agent. The effect of/3-blocking agents on the ability of the heart to respond to hypoxia with a reactive hyperaemia is relevant to the use of these drug in angina. None of the/3-blockers used in the present study had any effect on the peak fall in coronary vascular resistance following a brief period o f coronary artery occlusion. Pauly et al. (1973) demonstrated a reduction in total reactive hyperaemia volume in the dog after propranolol due to the reduced myocardial metabolic activity, although in an earlier study the peak flow reached during the hyperaemic response was unchanged by propranolol (Pauly and Bittar, 1971). The effects of/3-blockade on the coronary vascular responses to sympathetic stimuli are difficult to interpret, since the coronary blood flow is not only closely controlled by metabolic autoregulation, but is also affected by the aortic blood pressure, compression of the vessels due to the contracting myocardium and by ~- and /3-adrenoceptor activity influencing vascular constriction and dilatation respectively. Coronary vasodilatation produced by stimulation of the left stellate ganglion and by injected noradrenaline was markedly reduced after tolamolol, practolol and propranolol, and in some dogs, was reversed to a coronary vasoconstriction. Phentolamine, at a dose which blocked the pressor response to i.v. noradrenaline, effectively antagonised the coronary constriction occurring in response to exogenous noradrenaline, but
106
K.R. Adam, S.M. Boyles, #.Adrenoceptor blockade by tolamolol
only antagonised that due to stellate ganglion stimulation in one of three dogs. This could be due to the relative difficulty encountered in totally blocking the effects of endogenous, compared with exogenous, transmitters. Thus, the direct effect of noradrenaline is unmasked as a vasoconstriction via stimulation of coronary vascular a-receptors after/3-blockade has obviated cardiac metabolic autoregulation; this occurs after cardioselective or non-selective /3-blockade. These results confirm those obtained in an earlier study (Adam et al., 1970), in which a higher dose of practolol was used, resulting in complete abolition of the inotropic response to left stellate ganglion stimulation; coronary constriction was then consistently observed. In the present experiments, the myocardial response to ganglion stimulation was not completely abolished by the dose of practolol used, and coronary vasoconstriction did not always occur. The effect of cardioselective /3-blockade on the coronary vascular/3-receptors may become therapeutically relevant during the response to circulating adrenaline, the levels of which are elevated during angina of effort (Raab, 1941). The results of the present study clearly show that, in anaesthetised dogs, the cardioselective /3-blockers tolamolol and practolol cause less reduction of the coronary vasodilator response to injected adrenaline and isoprenaline than does propranolol, at doses producing equivalent blockade of myocardial responses. Adrenaline produced coronary vasoconstriction only under conditions of complete abolition of the myocardial response, with some additional peripheral vascular /3-blockade. These conditions were produced in this study only by an excessive dose of tolamolol. This observation suggests that adrenaline has a direct action on both/3- and a-receptors in the coronary vascular bed, the action on a-receptors becoming apparent only when there is some degree of blockade of vascular/3-receptors, and is thus in agreement with results obtained with practolol by Adam et al. (1970)and Mark et al. (1972). Propranolol did not cause adrenaline to constrict the coronary vessels, as might have been expected from other published results (Mark et al., 1972), presumably because the myocardial response was not completely abolished by the dose of propranolol used. In contrast, in the femoral vascular bed, the predominant effect of adrenaline was vaso. constriction which was potentiated after /3-blockade
with propranolol. These observations are consistent with the suggestion made by Mark et al. (1972) of the paucity of a-receptors in the coronary vasculature, compared with vessels in skin and skeletal muscle. After cardiac/3-blockade with tolamolol or practolol, the isoprenaline-induced coronary vasodilatation was only slightly reduced, although the myocardial responses were almost abolished. This is in agreement with the observations of McRaven et al. (1971). When the myocardial response to isoprenaline was completely abolished by the higher dose of tolamolol, the antagonism of the coronary and femoral vascular responses were almost identical, indicating that in the complete absence of metabolic autoregulation, the coronary vascular bed behaves very similarly to that of the femoral vascular bed to/3-receptor stimulation. These results add further support to the view that the coronary vascular /3-receptors resemble the/32-receptors of the peripheral vasculature, rather than the myocardial/31 -receptors. A similar conclusion has also been reached from the results of several recent studies in the dog and cat, in which practolol was used to abolish the effects of metabolic autoregulation on the coronary vessels. As a result of these studies, coronary vascular /3-receptors have been ascribed to the /32-peripheral vascular sub-group (Parratt and Wadsworth, 1970; Mark et al., 1972) or shown to be different from myocardial /31-receptors (Ross and Jgrgensen, 1970; Bussman et al., 1970; Adam et al., !970). In contrast, evidence in favour of a similarity between coronary vascular and myorcardial /3-receptors has been presented, mainly from the results of in vitro experiments using isolated coronary arteries (Baron et al., 1972; Drew and Levy, 1972) or isolated heart preparations (Somani et al., 1970). Baron et al. (1972) considered it unlikely that coronary vascular /3-receptors should acquire a sensitivity to practolol in vitro which they do not possess in vivo. It is possible that the selectivity of practolol shown in vivo is due to a difference in accessibility of various /3-receptor sites to the compound; when the coronary arteries are isolated, practolol may be more easily able to make contact with the/3-receptors. Alternatively, since the action of adrenaline on isolated coronary arteries has been shown to be dependent on the size of the artery used (Zuberbuhler and Bohr, 1965), there may be a different distribution or sensitivity of/3-receptors in large and small coronary arteries which may account
K.R. Adam, S. M. Boyles, #-Adrenoceptor btockade by tolamolol for the discrepancy b e t w e e n in vivo and in vitro resuits.
Acknowledgements The authors wish to thank Dr. Valerie A. Alabaster for her invaluable help in preparing the manuscript.
References Adam, K.R., S.M. Boyles and P.C. Schofield, 1970, Cardioselective a-adrenoceptor blockade and the coronary circulation, Brit. J. Pharmacol. 40,534. Adam, K.R., L.G. Pullman and P.C. Schofield, 1973, Brit. J. Pharmacol. 49,560. Adam, K.R., J.R.C. Baird, R.A. Burges and J. LinneU, 1974, European J. Pharmacol. 25,170. Axonow, W.S., P.R. Harding, W.H. Nelson, J.S. Vangrow, L.L. Johnson and M. Khursheed, 1973, Treatment of arrhythmias with tolamidol*, Clin. Pharmacol. Therap. 13, 856. (*Correct name is tolamolol). Augstein, J., D.A. Cox, A.L. Ham, P.R. Leeming and M. Snarey, 1973, a-Adrenoceptor blocking agents. I. Cardioselective 1-aryloxy-3-(aryloxyalkylamine)-propan-2-ols, J. Med. Chem. 16, 1245. Baron, G.D., R.N. Speden and D.F. Bohr, 1972, a-Adrenergic receptors in coronary and skeletal muscle arteries, Amer. J. Physiol. 223,878. Briant, R.H., C.T. DoUery, T. Fenyvesi and C.F. George, 1973, Assessment of selective a-adrenoceptor blockade in man, Brit. J. Pharmacol. 49,106. Bussmann, W.D., M. Rauh and H.P. Krayenbuehl, 1970, Coronary and haemodynamic effects of myocardio-selectire/~-receptor blockade by ICI 50172 in the closed-chest dog, Amer. Heart J. 79,347. Drew, G.M. and G.P. Levy, 1972, Characterization of the coronary vascular a-adrenoceptor in the pig, Brit. J. Pharmacol. 46,348.
107
Dunlop, D. and R.G. Shanks, 1968, Selective blockade of adrenoceptive a-receptors in the heart, Brit. J. Pharmacol. 32,201. Fitzgerald, J.D., 1969, Perspectives in adrenergic a-receptor blockade, Clin. Pharmacol. Therap. 10,292. Fitzgerald, J.D., 1972, Cardioselective a-adrenergic blockade, Proc. Roy. Soc. Med. 65,761. Lands, A.M., A. Arnold, J. McAuliff, F. Luduena and T. Brown, 1967, Differentiation of receptor systems activated by sympathomimetic amines, Nature 214,597. Macdonald, A.G. and R.S. McNeill, 1969, A comparison of the effect on airway resistance of a new a-blocking drug, ICI 50172 and propranolol, Brit. J. Anaesth. 40,508. Mark, A.L., F.M. Abboud, P.G. Schmid, D.D. Heistad and H.E. Mayer, 1972, Differences in direct effects of adrenergic stimuli on coronary, cutaneous and muscular vessels, J. Clin. Invest. 51,279. McRaven, D.R., A.L. Mark, F.M. Abboud and H.E. Mayer, 1971, Responses of coronary vessels to adrenergic stimuli, J. Clin. Invest. 50,773. Parratt, J.R. and R.M. Wadsworth, 1970, The effect of 'selective' a-adrenoceptor blocking drugs on the myocardial circulation, Brit. J. Pharmacol. 39,296. Pauly, T.J. and N. Bittar, 1971, Myocardial reactive hyperaemia responses in the dog after a-receptor block with propranolol, Card. Res. 5,440. Pauly, T.J., W.C. Zarnstofff and N. Bittar, 1973, Myocardial metabolic activity as a determinant of reactive hyperaemia responses in the dog heart, Card. Res. 7, 90. Raab, W., 1941, Abnormal suprarenal discharges in angina pectoris and their control by X-ray therapy, J. Clin. Endocrinol. 1,977. Ross, G. and C.R. Jorgensen, 1970, Effects of a cardio-selective/~adrenergic blocking agent on the heart and coronary circulation, Card. Res. 4,148. Somani, P., A.R. Laddu and H.F. Hardman, 1970, Nutritional circulation in the heart. III. Effect of isoproterenol and a-adrenergic blockade on myocardial haemodynamics and rubidium-86 extraction in the isolated supported heart preparation, J. Pharmacol. Exptl. Therap. 175,577. Zuberbuhler, R.C. and D.F. Bohr, 1965, Responses of coronary smooth muscle to catecholamines, Circulation Res. 16,431.