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Comparative Central and Peripheral Antihypertensive Mechanisms of Urapidil and Prazosin
Comparative Central and Peripheral Antihypertensive Mechanisms of Urapidil and Prazosin
MICHAEL J. BRODY, Ph.D. RANDY L. WEBB, Ph.D. MICHAEL L. MANGIAPANE, Ph.D. JAMES P. PORTER, Ph.D. ANN C. BONHAM, M.S. ANGELO J. TRAPANI, Ph.D. Iowa City, Iowa
From the Department of Pharmacology and Cardiovascular Center, University of Iowa College of Medicine, Iowa City, Iowa. This work was supported in part by Grants HLB-14388, HLB-07121, HL 0905, HL 06101, 5-T32-GM07069, MH 15172 and by Marion Laboratories. Requests for reprints should be addressed to Dr. M.J. Brody, Department of Pharmacology, University of Iowa, College of Medicine, Iowa City, Iowa 52242.
74
October 5, 1984
The central effects of urapidil were investigated in conscious rats with sinoaortic denervation. Intraventricular urapidil administration (40 to 100 #Lg) produced a transient depressor response followed by a pressor response coupled with tachycardia. In comparison, intraventricular prazosin administration {2.5 to 5.0 #Lg) produced only a prolonged depressor effect. The effect of intravenously administered urapidil (3 mg/kg) on arterial pressure, heart rate, and mesenteric, renal, and hindquarter resistances was then compared with that of prazosin (0.5 mg/kg) in conscious rats with sinoaortic denervation, instrumented with pulsed Doppler flow probes. Both agents caused similar significant decreases in arterial pressure and vascular resistances, but urapidil decreased renal resistance significantly more than did prazosin. Prazosin increased heart rate, whereas no change was found with urapidil. Prazosin blocked the pressor and regional constrictor effects of intravenously administered norepinephrine more effectively than urapidil. Interference with the arterial pressure-maintaining activity of the sympathetic nervous system is a highly efficacious means of controlling high arterial pressure. With the development of knowledge about specific adrenergic receptor subtypes, it is possible to understand the mechanisms by which agents that interfere with sympathetic transmission exert their antihypertensive effect. For example, it is now well established that clonidine and alpha-methyldopa have prominent central actions that are produced by activation of alpha 2 adrenergic receptors. Agents with the capacity to interfere with the overall pressor effects of adrenergic receptor stimulation were used in the early years of antihypertension therapy; however, their severe side effects, such as orthostatic hypotension, significantly limited their continued use. Much more specific control of arterial pressure was obtained with a new class of alpha 1 adrenergic receptor antagonists, the prototype of which is prazosin. This agent lowers arterial pressure and produces relatively little tachycardia and a low enough frequency of orthostasis to be well tolerated by subjects with hypertension [1]. The antihypertensive agent urapidil also exerts alpha 1 adrenergic receptor antagonism. However, its pharmacology appears to differ from that of prazosin in several important respects [2]. This agent has been reported to combine, in the same molecule, alpha 1 adrenergic receptor antagonist activity and alpha 2 adrenergic receptor agonist effects. It has also been proposed that urapidil lowers arterial pressure in part by a direct action on the central nervous system. It is interesting to note that
The American Journal of Medicine
NEW CONCEPTS IN HYPERTENSION THERAPY SYMPOSIUM-BRODY ET AL
prazosin also exerts a central inhibitory effect on sympathetic discharge; however, the mechanism of this action is not yet understood [3). Studies on potential central mechanisms of action of antihypertensive agents have almost always employed anesthetized animals. Two potential problems limit such studies. First, the failure to exert a centrally-mediated antihypertensive action in an anesthetized preparation does not rule out the possibility that the agent might be active in a preparation examined in the conscious state. Conversely, the presence of a centrally-mediated antihypertensive effect might be peculiar to the anesthetized preparation in which reflex adjustments for a decrease in blood pressure are minimal. To evaluate the possibility that prazosin and urapidil might exhibit central actions, studies were carried out in conscious rats in which all baroreceptor reflexes were removed by chronic denervation of the sinoaortic nerves. In addition to studies on potential cehtral effects, comparisons were made between the peripheral hemodynamic effects and the peripheral alpha-adrenergic receptor blocking actions of the two agents.
METHODS Chronic Instrumentation. Male, Sprague-Dawley rats (250 to 350 g) were anesthetized with 0.1 ml/1 00 g of ketamine (100 mg/ml), given intramuscularly, to which 10 mg of acepromazine had been added. Supplemental anesthesia was given as required. Using aseptic procedures, a midline laparotomy was performed, and 3 to 4 mm lengths of the superior mesenteric artery, lower abdominal aorta below the left renal artery, and the right or left renal artery were carefully isolated under a dissecting microscope to avoid damage to nearby nerves. Miniaturized pulsed Doppler flow probes were sutured in place around each vessel. A complete description of the construction and implantation of these flow probes has already been described [4]. The wire leads were led beneath the skin to exit at the back of the neck where they were soldered to a connector plug that was fixed to the animals's skull with jeweler's screws and dental cement. Those animals in which drugs were administered into the lateral cerebral ventricle received a 23-gauge stainless steel lateral ventricular cannula that was stereotaxically positioned employing the following coordinates (in rnm) relative to the bregma: caudal 0.4; lateral 1.4 to 1.5; ventral 3.5 from top of the dura. A catheter (PE-1 0) was placed in the lower abdominal aorta via the left femoral artery for measurement of arterial pressure, and another catheter (PE-1 0) was placed in the lower abdominal vena cava via the right femoral vein for infusion of drugs. Catheters were exteriorized at the back of the neck. Animals were treated with 80,000 units penicillin intramuscularly (Fiocilin) and were allowed two to three days to recover from the surgery. On the day of the experiment, each rat was connected to a light-weight flexible spring that contained the flow-probe wire connectors and arterial and venous connector lines. The spring-guarded connector line was suspended from the top of the animal's home cage to allow freedom of movement during
the experiment. Regional blood flow was measured with a pulsed Doppler flowmeter (University of Iowa Bioengineering Facility). Mean arterial pressure wa~ electronically derived from a Century CP-01 pressure transducer, and heart rate was obtained with a Beckman 9857 B tachometer that was triggered from the arterial pressure pulse. Regional flows, arterial pressure, and heart rate were continuously recorded on a Beckman Dynograph. Baroreceptor Denervation. Three to five days prior to chronic instrumentation, all animals were subjected to sinoaortic baroreceptor deafferentation according to a modification of the method of Krieger [5]. Briefly, this involved aseptically exposing the right and left carotid sinuses and stripping all connective tissue and nerves from the internal, external, occipital, and thyroid arteries in the carotid bifurcation region. The exposed vessels were painted with 10 percent phenol in ethanol, taking care to avoid damaging the vagi and other nearby nerves. The right and left superior laryngeal nerves near the vagi were cut, as were the cervical sympathetic trunks and any distinct aortic depressor nerves that were located there. All sinoaortic denervated animals in these studies were allowed three to five days recovery before being subjected to chronic instrumentation with flow probes, catheters, and lateral ventricular cannulas. All animals were tested for absence of baroreflex control of heart rate. Each rat was given intravenous injections of phenylephrine hydrochloride (1 to 3 ~-tg/kg) to increase mean arterial pressure by approximately 50 mm Hg. For all studies, only sinoaortic denervated animals exhibiting a change in heart rate of < 15 beats/minute were considered adequately debuffered and selected for study. Protocols. In conscious rats with lateral ventricular, arterial, and venous cannulas, the effects of intraventricular administration of urapidil and prazosin were compared. The agents were administered in volumes of 5 to 10 ~-tl. A range of doses from 10 to 100 ~-tg was tested with urapidil. Data from 40 and 100 ~-tg doses are reported since lower doses exerted no significant effects. Doses of 2.5 and 5 ~-tg of prazosin were employed. These doses were selected because they were in the range of the approximate 6:1 potency difference between urapidil and prazosin determined from peripheral administration. In studies on regional hemodynamic effects and adrenergic blocking activity, the two agents were administered intravenously. Doses were determined from preliminary studies designed to identify approximately equidepressor doses. Studies were carried out using intravenous administration of 3 mg/kg of urapidil and 0.5 mg/kg of prazosin. These doses produced long-lasting hypotensive effects associated with changes in regional vascular resistances. Vascular responsiveness to the intravenous administration of norepinephrine was tested before and during the stable hypotensive phase after administration of urapidil and prazosin at the doses given. Norepinephrine was administered intravenously in bolus doses of 0.1, 0.3, and 1.0 p.g/kg. Data Analysis. Maximum responses were determined for each pharmacologic intervention and compared to control values for each parameter that had been obtained for the one-minute interval immediately before drug administration.
October 5, 1984
The American Journal of Medicine
75
NEW CONCEPTS IN HYPERTENSION THERAPY SYMPOSIUM-BRODY ET AL
Early Response
Late Response
40
20
AMAP
(mmHg) -20 -40
-60 40~g
100~g
2.5~g
5.0~g
0
Urapidil
•
Prazosin
40~g
100~g
Relative resistance in each vascular bed was calculated from the following formula: Mean arterial pressure/blood velocity (Doppler shift in kiloHertz). Changes in regional vascular resistance are expressed as a percent of control resistance. This is a valid determination of vascular resistance since the measured blood velocity is directly and linearly proportional to volume flow measured with electromagnetic flowmeters [4]. This proportionality assumes that the cross-sectional area of the vessel adjacent to the implanted flow probe, the velocity of sound through tissue, and the acoustical and geometrical coupling between the piezoelectric crystal and the vessel remain constant. RESULTS The effects of lateral ventricular administration of urapidil and prazosin in conscious rats with sinoaortic denervation are summarized in Figure 1. Urapidil produced a biphasic response consisting of a transient decrease in arterial pressure followed by a more sustained increase in pressure. The sustained pressor response was associated with mild tachycardia. In contrast, ventricular administration of prazosin produced a profound reduction in arterial pressure that lasted 20 to 30 minutes. No evidence of an initial or secondary pressor effect was observed. The hypotensive action of centrally administered prazosin was associated with mild bradycardia ( -15 ± 6 beats/minute for the 5 1-Lg dose). The rats showed no behavioral evidence of central nervous system depression with either agent. The regional hemodynamic effects of intravenous administration of urapidil and prazosin are summarized in Figure 2. Doses that produced approximately equivalent reductions in arterial pressure were compared. In these
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October 5, 1984
The American Journal of Medicine
2.5~g
5
O~g
Figure 1. Comparisons between the effects on arterial pressure of urapidil and prazosin administered by the cerebraventricular route.
experiments, arterial pressure averaged 138 ± 5 mm Hg prior to administration of the agents. Each agent produced marked hypotensive effects; however, several important differences between the overall hemodynamic consequences of the two agents were observed. Heart rate was not significantly altered with urapidil, whereas it was increased by approximately 50 beats/minute with prazosin. Since baroreceptor reflexes were removed in these animals by sinoaortic denervation, this increase in heart rate must be assumed to be the result of a direct effect on the myocardium, a centrally-mediated action, or the release of an endogenous chronotropic substance. We have not yet determined the mechanism of this effect. A potentially important difference between the regional hemodynamic effects of the two agents was uncovered. As expected, regional vascular resistances in the mesenteric, renal, and hindquarter circulations were all reduced with the two agents. Whereas the reductions were not significantly different between the agents with respect to the mesenteric and hindquarter resistances, urapidil produced a significantly greater decrease in renal vascular resistance than did prazosin. The intravenous administration of graded doses of norepinephrine produced graded increases in mean arterial pressure and the three regional vascular resistances. Following the administration of prazosin, the increases in arterial pressure and regional vascular resistances produced by norepinephrine were markedly attenuated (Figure 3). Additional differences between prazosin and urapidil were observed in the studies on cardiovascular responsiveness to norepinephrine. As seen in Figure 4, urapidil produced only minimal effects on arterial pressure and
NEW CONCEPTS IN HYPERTENSION THERAPY SYMPOSIUM-BRODY ET AL
0
75
-30
50
D
•
llHR (bpm)
llMAP (mmHg) -60
25
Urapidil - 3mg/kg. iv Prazosin - 0.5mg/kg. 1v
(5)
%fl
MR
-90
0
0
0
-25
%fl -25
-50
-50
0
%fl -25
HQR
RR
-50
Figure 2. Comparison of the maximum regional hemodynamic effects produced by urapidil and prazosin. MAP = mean arterial pressure; HR = heart rate; MR = mesenteric resistance; RR = renal resistance; HQR = hindquarter resistance.
0
Control Prazoson (0.5mg/kg)
e 60
/
AMAP
(mmHg)
30
-
/ ~ /
I
I
0
250
~ o/oA
MR
tl
I
0
)>
100o/ofl
RR 50 -
0
r:l'
/
~
0
/
50
/
%A
HQR 25
/
/
/
o/ /'
L~t====!:*:::j~[_J 0 1
/
/
75
150 . . - - - - - - - - - - ,
Figure 3. The effect of prazosin on the regional vasoconstrictor actions of norepinephrine (NE). Norepinephrine was administered intravenously before and during the peak hypotensive action of prazosin. MAP = mean arterial pressure; MR = mesenteric resistance; RR = renal resistance; HQR = hindquarter resistance.
125
/
03 NE (IJg/kg)
1.0
October 5, 1984
0
0.1
03
10
NE (IJg/kg)
The American Journal of Medicine
n
NEW CONCEPTS IN HYPERTENSION THERAPY SYMPOSIUM-BRODY ET AL
0 Control Urapidil (3.0mg/kg)
e 60
150
/
AMAP
(mmHg)
30
)>
-~ I
0
I
100
%A MR 50 r-
I
/ %A RR
50
0
r
/
/o
•
HQR
20~~ •
0
?/
/
0
I
I
0.3
10
L--....1....--..l.---'---...J
1.0
0.1
mesenteric, renal, and hindquarter resistances. Thus, in contrast to prazosin, which exerted marked adrenergic receptor blocking activity, the effects of urapidil on pressor and vasoconstrictor actions of norepinephrine were much less prominent.
COMMENTS In this study, central and peripheral actions of two antihypertensive agents known to produce peripheral alpha 1 adrenergic receptor blocking activity were compared. Significant differences between the pharmacologic agents prazosin and urapidil were found. In an effort to increase the possibility of observing central actions of the agents, two strategies were employed. First, baroreceptor reflexes were removed by chronic sinoaortic deafferentation. Second, the animals were studied in the conscious state in which potential confounding effects of anesthesia were not present. Under these conditions, quite different effects on the cardiovascular system were obtained from the selective central administration of the agents. Urapidil produced a small and transient depressor effect followed by a prolonged increase in arterial pressure. Prazosin produced only a marked decrease in arterial pressure that persisted for up to 30 minutes. These data suggest that the potential for prazosin to produce a centrally-mediated hypotensive effect is much greater than that observed with urapidil. With respect to central nervous system depression associated with cerebroventricular administration, no effects were observed. In our hands only the alpha2 agonists,
October 5, 1984
I
/
NE (IJg/kg)
78
I
0
%A
L..-..L-l_ __ll...__....&.I---J 0.3
~
40 . - - - - - - - - - - ,
/
~
0 1
/
'
0
100 . - - - - - - - - - - , 0
))
r
The American Journal of Medicine
NE (IJg/kg)
Figure 4. The effect of urapidil on the regional vasoconstrictor actions of norepinephrine (NE). Norepinephrine was administered intravenously before and during the peak hypotensive action of urapidil. MAP = mean arterial pressure; MR =mesenteric resistance. RR =renal resistance; HQR = hindquarter resistance.
such as clonidine, produce sedation upon central administration. In our laboratory, striking differences have been found among a number of antihypertensive agents believed to exert their effects in part or wholly by reducing central sympathetic outflow. Agents we have tested include the alpha 2 adrenergic receptor agonists clonidine and guanabenz and the alpha1 adrenergic receptor antagonists urapidil, indoramin, and prazosin. In the case of each of these agents, data obtained from anesthetized animals have indicated that the agents can reduce centrally derived sympathetic nerve activity [2,3,6-8]. When these agents were tested for their potential to reduce arterial pressure in unanesthetized, unrestrained animals, a number of unexpected findings appeared. Clonidine, the prototypic centrally acting antihypertensive agent, failed to lower arterial pressure when given by the cerebroventricular route to animals with intact baroreflexes. Despite the failure to lower arterial pressure with clonidine, these animals exhibited marked bradycardia. When clonidine was retested in animals with chronic sinoaortic denervation, it produced profound hypotension and bradycardia. Similar effects were obtained with guanabenz [9]. These data suggest that intact baroreflexes can qualitatively alter the nature of the arterial pressure response to a putative centrally acting agent. Urapidil, which possesses alpha2 adrenoceptor agonist activity, did not behave like clonidine when administered into the central nervous system. This agent produced only a transient hypotensive effect that was overridden by a more prolonged increase in arterial pressure. Prazosin, an
NEW CONCEPTS IN HYPERTENSION THERAPY SYMPOSIUM-BRODY ET AL
agent believed to have a relatively high specificity for alpha 1 adrenoceptors, produced a marked lowering of arterial pressure; however, unlike clonidine, it produced only a modest effect on heart rate. Finally, indoramin produced an entirely different cardiovascular response to central administration. The primary effect was an initial increase in arterial pressure that was followed by a very modest decrease in arterial pressure accompanied by prolonged bradycardia [1 0]. This remarkable diversity of cardiovascular effects, produced by central administration of agents known to reduce centrally-mediated sympathetic discharge, underscores the differences between studies carried out in anesthetized animals and those made under more normal physiologic conditions. Further investigation is needed to determine more precisely whether any of these agents depends upon central actions for its long-term antihypertensive effects. Experiments should be carried out in conscious animals treated long-term with antihypertensive agents and in which measurements of central sympathetic nerve discharge are made with the animals in the conscious state. Furthermore, we need to understand the precise mechanisms by which intact baroreceptors are able to produce qualitative differences in cardiovascular effects mediated through the central nervous system. From the present data and from other studies, there is no question that both urapidil and prazosin are potent antihypertensive agents when administered by the systemic route. This effect is shared by the related agent, indoramin [1 0]. The question we attempted to answer about this peripheral antihypertensive action is the extent to which it can be attributed to peripheral alpha-adrenergic receptor blockade. The ability of equidepressor doses of urapidil and prazosin to affect differentially the vasoconstrictor and pressor responses to norepinephrine leaves little question that prazosin is a more effective adrenergic receptor antagonist. In similar studies carried out with indoramin, we found that it resembled prazosin in exhibiting significant alpha adrenergic blockade at doses (20 times greater than prazosin) that produced a similar lowering of arterial pressure (1 0]. Thus, among these three antihypertensive agents, urapidil appears to exert the least peripheral adrenergic blockade, although it is equally effective in lowering arterial pressure. These data indicate the need for additional studies on the peripheral mechanism of action. Comparisons need to be made between the effects of these agents in vascular beds that have been denervated and pretreated with adrenergic receptor antagonists to determine the extent to which vasodilation produced by mechanisms other than adrenergic receptor blockade contributes to the vasodilator effects. In addition, comparisons need to be made of the relative abilities of these agents to interrupt vasoconstriction produced by neural activity compared to that produced by circulating catecholamines. In our previous studies with indoramin
Comparative Hemodynamic Effects Urapidil Relative Potency Arterial Pressure Resistance Mesenteric Hindquarter Renal Heart Rate Effect on Norepinephrine Central Effect on MAP
Prazosin
• • •• •• • t• • • •t • 6
Figure 5. Summary comparison of the hemodynamic effects of urapidil and prazosin. Relative potency indicates that a dose of urapidil six times greater than that of prazosin is needed to produce the same fall in arterial pressure.
[1 0], we found that this agent was equally effective in blocking the constrictor effects of exogenous norepinephrine and the vasoconstrictor effects produced by central activation of the sympathetic nervous system.
CONCLUSION The major differences between urapidil and prazosin observed in these studies carried out in unanesthetized, unrestrained rats are summarized in Figure 5. The agents are equally effective in lowering arterial pressure by peripheral mechanisms; however, urapidil is more active as a renal vasodilator. It remains to be determined from clinical studies whether this effect will translate into less sodium retention. The finding that prazosin produces tachycardia in the areflexic animal suggests that this agent has more potential to increase heart rate; however, tachycardia has not been reported to be a problem with either agent when used in the clinical setting. Using blockade of peripherally administered norepinephrine as an index of capacity to interfere with adrenergic receptors, urapidil appeared to be a much weaker alpha-adrenergic receptor antagonist. This finding needs to be extended to potential differences between neurogenically-mediated vasoconstriction before any projections about differences in potential to produce orthostasis can be made. Finally, prazosin appeared to be a much more efficacious centrally acting hypotensive agent. The contribution of central actions to the antihypertensive effects of any putative centrally acting agent requires much more study.
October 5, 1984
The American Journal of Medicine
79
NEW CONCEPTS IN HYPERTENSION THERAPY SYMPOSIUM-BROD Y ET AL
REFERENCES Scriabine A: Prazosin. In: Scriabine A, ed. Pharmacology of antihypertensive drugs. New York: Raven Press, 1980; 151160. 2. Schoetensack W, Bruckschen EG, Zech K: Urapidil. In: Scriabine A, ed. New drugs annual. Cardiovascular drugs, vol. 1. New York: Raven Press, 1983; 19-48. 3. McCall RB, Humphrey SJ: Evidence for a central depressor action of postsynaptic a 1-adrenergic receptor antagonists. J Auton Nerv Syst 1981; 3: 9-23. 4. Haywood JR, Schaffer RA, Fastenow C, et al: Regional blood flow measurement in the conscious rat with a pulsed Doppler flowmeter. Am J Physiol 1981; 241: H273-H278. 5. Krieger EM: Neurogenic hypertension in the rat. Circ Res 1964; 15:511-521. 6. Schmitt H, Schmitt H, Boissier JR, Giudicelli JF: Centrally medi1.
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The American Journal of Medicine
ated decrease in sympathetic tone induced by 2-(2,6dichlorophenylamine)-2-imidazoline (st 155, catapresan). Eur J Pharmacal 1967; 2: 147-148. 7. Baum T, Schropshire AT: Inhibition of spontaneous sympathetic nerve activity by the antihypertensive agent Wy-8678. Neuropharmacology 1970; 9: 503-506. 8. Baum T, Schropshire AT: Central and peripheral contribution to the antihypertensive action of indoramin. Eur J Pharmacal 1975; 32: 30-38. 9. Bonham AC, Trapani AJ, Portis LR, Brody MJ: Studies on the mechanism of the central antihypertensive effect of clonidine and guanabenz. J Hypertens 1984, in press. 10. Porter JP, Bohnam A, Mangiapane ML, Webb RL, Brody MJ: Cardiovascular effects of indoramin in the conscious rat. Circulation 1983; 68: 111-321.
MICHAEL J. BRODY, Ph.D. RANDY L. WEBB, Ph.D. MICHAEL L. MANGIAPANE, Ph.D. JAMES P. PORTER, Ph.D. ANN C. BONHAM, M.S. ANGELO J. TRAPANI, Ph.D. Iowa City, Iowa
From the Department of Pharmacology and Cardiovascular Center, University of Iowa College of Medicine, Iowa City, Iowa. This work was supported in part by Grants HLB-14388, HLB-07121, HL 0905, HL 06101, 5-T32-GM07069, MH 15172 and by Marion Laboratories. Requests for reprints should be addressed to Dr. M.J. Brody, Department of Pharmacology, University of Iowa, College of Medicine, Iowa City, Iowa 52242.
74
October 5, 1984
The central effects of urapidil were investigated in conscious rats with sinoaortic denervation. Intraventricular urapidil administration (40 to 100 #Lg) produced a transient depressor response followed by a pressor response coupled with tachycardia. In comparison, intraventricular prazosin administration {2.5 to 5.0 #Lg) produced only a prolonged depressor effect. The effect of intravenously administered urapidil (3 mg/kg) on arterial pressure, heart rate, and mesenteric, renal, and hindquarter resistances was then compared with that of prazosin (0.5 mg/kg) in conscious rats with sinoaortic denervation, instrumented with pulsed Doppler flow probes. Both agents caused similar significant decreases in arterial pressure and vascular resistances, but urapidil decreased renal resistance significantly more than did prazosin. Prazosin increased heart rate, whereas no change was found with urapidil. Prazosin blocked the pressor and regional constrictor effects of intravenously administered norepinephrine more effectively than urapidil. Interference with the arterial pressure-maintaining activity of the sympathetic nervous system is a highly efficacious means of controlling high arterial pressure. With the development of knowledge about specific adrenergic receptor subtypes, it is possible to understand the mechanisms by which agents that interfere with sympathetic transmission exert their antihypertensive effect. For example, it is now well established that clonidine and alpha-methyldopa have prominent central actions that are produced by activation of alpha 2 adrenergic receptors. Agents with the capacity to interfere with the overall pressor effects of adrenergic receptor stimulation were used in the early years of antihypertension therapy; however, their severe side effects, such as orthostatic hypotension, significantly limited their continued use. Much more specific control of arterial pressure was obtained with a new class of alpha 1 adrenergic receptor antagonists, the prototype of which is prazosin. This agent lowers arterial pressure and produces relatively little tachycardia and a low enough frequency of orthostasis to be well tolerated by subjects with hypertension [1]. The antihypertensive agent urapidil also exerts alpha 1 adrenergic receptor antagonism. However, its pharmacology appears to differ from that of prazosin in several important respects [2]. This agent has been reported to combine, in the same molecule, alpha 1 adrenergic receptor antagonist activity and alpha 2 adrenergic receptor agonist effects. It has also been proposed that urapidil lowers arterial pressure in part by a direct action on the central nervous system. It is interesting to note that
The American Journal of Medicine
NEW CONCEPTS IN HYPERTENSION THERAPY SYMPOSIUM-BRODY ET AL
prazosin also exerts a central inhibitory effect on sympathetic discharge; however, the mechanism of this action is not yet understood [3). Studies on potential central mechanisms of action of antihypertensive agents have almost always employed anesthetized animals. Two potential problems limit such studies. First, the failure to exert a centrally-mediated antihypertensive action in an anesthetized preparation does not rule out the possibility that the agent might be active in a preparation examined in the conscious state. Conversely, the presence of a centrally-mediated antihypertensive effect might be peculiar to the anesthetized preparation in which reflex adjustments for a decrease in blood pressure are minimal. To evaluate the possibility that prazosin and urapidil might exhibit central actions, studies were carried out in conscious rats in which all baroreceptor reflexes were removed by chronic denervation of the sinoaortic nerves. In addition to studies on potential cehtral effects, comparisons were made between the peripheral hemodynamic effects and the peripheral alpha-adrenergic receptor blocking actions of the two agents.
METHODS Chronic Instrumentation. Male, Sprague-Dawley rats (250 to 350 g) were anesthetized with 0.1 ml/1 00 g of ketamine (100 mg/ml), given intramuscularly, to which 10 mg of acepromazine had been added. Supplemental anesthesia was given as required. Using aseptic procedures, a midline laparotomy was performed, and 3 to 4 mm lengths of the superior mesenteric artery, lower abdominal aorta below the left renal artery, and the right or left renal artery were carefully isolated under a dissecting microscope to avoid damage to nearby nerves. Miniaturized pulsed Doppler flow probes were sutured in place around each vessel. A complete description of the construction and implantation of these flow probes has already been described [4]. The wire leads were led beneath the skin to exit at the back of the neck where they were soldered to a connector plug that was fixed to the animals's skull with jeweler's screws and dental cement. Those animals in which drugs were administered into the lateral cerebral ventricle received a 23-gauge stainless steel lateral ventricular cannula that was stereotaxically positioned employing the following coordinates (in rnm) relative to the bregma: caudal 0.4; lateral 1.4 to 1.5; ventral 3.5 from top of the dura. A catheter (PE-1 0) was placed in the lower abdominal aorta via the left femoral artery for measurement of arterial pressure, and another catheter (PE-1 0) was placed in the lower abdominal vena cava via the right femoral vein for infusion of drugs. Catheters were exteriorized at the back of the neck. Animals were treated with 80,000 units penicillin intramuscularly (Fiocilin) and were allowed two to three days to recover from the surgery. On the day of the experiment, each rat was connected to a light-weight flexible spring that contained the flow-probe wire connectors and arterial and venous connector lines. The spring-guarded connector line was suspended from the top of the animal's home cage to allow freedom of movement during
the experiment. Regional blood flow was measured with a pulsed Doppler flowmeter (University of Iowa Bioengineering Facility). Mean arterial pressure wa~ electronically derived from a Century CP-01 pressure transducer, and heart rate was obtained with a Beckman 9857 B tachometer that was triggered from the arterial pressure pulse. Regional flows, arterial pressure, and heart rate were continuously recorded on a Beckman Dynograph. Baroreceptor Denervation. Three to five days prior to chronic instrumentation, all animals were subjected to sinoaortic baroreceptor deafferentation according to a modification of the method of Krieger [5]. Briefly, this involved aseptically exposing the right and left carotid sinuses and stripping all connective tissue and nerves from the internal, external, occipital, and thyroid arteries in the carotid bifurcation region. The exposed vessels were painted with 10 percent phenol in ethanol, taking care to avoid damaging the vagi and other nearby nerves. The right and left superior laryngeal nerves near the vagi were cut, as were the cervical sympathetic trunks and any distinct aortic depressor nerves that were located there. All sinoaortic denervated animals in these studies were allowed three to five days recovery before being subjected to chronic instrumentation with flow probes, catheters, and lateral ventricular cannulas. All animals were tested for absence of baroreflex control of heart rate. Each rat was given intravenous injections of phenylephrine hydrochloride (1 to 3 ~-tg/kg) to increase mean arterial pressure by approximately 50 mm Hg. For all studies, only sinoaortic denervated animals exhibiting a change in heart rate of < 15 beats/minute were considered adequately debuffered and selected for study. Protocols. In conscious rats with lateral ventricular, arterial, and venous cannulas, the effects of intraventricular administration of urapidil and prazosin were compared. The agents were administered in volumes of 5 to 10 ~-tl. A range of doses from 10 to 100 ~-tg was tested with urapidil. Data from 40 and 100 ~-tg doses are reported since lower doses exerted no significant effects. Doses of 2.5 and 5 ~-tg of prazosin were employed. These doses were selected because they were in the range of the approximate 6:1 potency difference between urapidil and prazosin determined from peripheral administration. In studies on regional hemodynamic effects and adrenergic blocking activity, the two agents were administered intravenously. Doses were determined from preliminary studies designed to identify approximately equidepressor doses. Studies were carried out using intravenous administration of 3 mg/kg of urapidil and 0.5 mg/kg of prazosin. These doses produced long-lasting hypotensive effects associated with changes in regional vascular resistances. Vascular responsiveness to the intravenous administration of norepinephrine was tested before and during the stable hypotensive phase after administration of urapidil and prazosin at the doses given. Norepinephrine was administered intravenously in bolus doses of 0.1, 0.3, and 1.0 p.g/kg. Data Analysis. Maximum responses were determined for each pharmacologic intervention and compared to control values for each parameter that had been obtained for the one-minute interval immediately before drug administration.
October 5, 1984
The American Journal of Medicine
75
NEW CONCEPTS IN HYPERTENSION THERAPY SYMPOSIUM-BRODY ET AL
Early Response
Late Response
40
20
AMAP
(mmHg) -20 -40
-60 40~g
100~g
2.5~g
5.0~g
0
Urapidil
•
Prazosin
40~g
100~g
Relative resistance in each vascular bed was calculated from the following formula: Mean arterial pressure/blood velocity (Doppler shift in kiloHertz). Changes in regional vascular resistance are expressed as a percent of control resistance. This is a valid determination of vascular resistance since the measured blood velocity is directly and linearly proportional to volume flow measured with electromagnetic flowmeters [4]. This proportionality assumes that the cross-sectional area of the vessel adjacent to the implanted flow probe, the velocity of sound through tissue, and the acoustical and geometrical coupling between the piezoelectric crystal and the vessel remain constant. RESULTS The effects of lateral ventricular administration of urapidil and prazosin in conscious rats with sinoaortic denervation are summarized in Figure 1. Urapidil produced a biphasic response consisting of a transient decrease in arterial pressure followed by a more sustained increase in pressure. The sustained pressor response was associated with mild tachycardia. In contrast, ventricular administration of prazosin produced a profound reduction in arterial pressure that lasted 20 to 30 minutes. No evidence of an initial or secondary pressor effect was observed. The hypotensive action of centrally administered prazosin was associated with mild bradycardia ( -15 ± 6 beats/minute for the 5 1-Lg dose). The rats showed no behavioral evidence of central nervous system depression with either agent. The regional hemodynamic effects of intravenous administration of urapidil and prazosin are summarized in Figure 2. Doses that produced approximately equivalent reductions in arterial pressure were compared. In these
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2.5~g
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Figure 1. Comparisons between the effects on arterial pressure of urapidil and prazosin administered by the cerebraventricular route.
experiments, arterial pressure averaged 138 ± 5 mm Hg prior to administration of the agents. Each agent produced marked hypotensive effects; however, several important differences between the overall hemodynamic consequences of the two agents were observed. Heart rate was not significantly altered with urapidil, whereas it was increased by approximately 50 beats/minute with prazosin. Since baroreceptor reflexes were removed in these animals by sinoaortic denervation, this increase in heart rate must be assumed to be the result of a direct effect on the myocardium, a centrally-mediated action, or the release of an endogenous chronotropic substance. We have not yet determined the mechanism of this effect. A potentially important difference between the regional hemodynamic effects of the two agents was uncovered. As expected, regional vascular resistances in the mesenteric, renal, and hindquarter circulations were all reduced with the two agents. Whereas the reductions were not significantly different between the agents with respect to the mesenteric and hindquarter resistances, urapidil produced a significantly greater decrease in renal vascular resistance than did prazosin. The intravenous administration of graded doses of norepinephrine produced graded increases in mean arterial pressure and the three regional vascular resistances. Following the administration of prazosin, the increases in arterial pressure and regional vascular resistances produced by norepinephrine were markedly attenuated (Figure 3). Additional differences between prazosin and urapidil were observed in the studies on cardiovascular responsiveness to norepinephrine. As seen in Figure 4, urapidil produced only minimal effects on arterial pressure and
NEW CONCEPTS IN HYPERTENSION THERAPY SYMPOSIUM-BRODY ET AL
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mesenteric, renal, and hindquarter resistances. Thus, in contrast to prazosin, which exerted marked adrenergic receptor blocking activity, the effects of urapidil on pressor and vasoconstrictor actions of norepinephrine were much less prominent.
COMMENTS In this study, central and peripheral actions of two antihypertensive agents known to produce peripheral alpha 1 adrenergic receptor blocking activity were compared. Significant differences between the pharmacologic agents prazosin and urapidil were found. In an effort to increase the possibility of observing central actions of the agents, two strategies were employed. First, baroreceptor reflexes were removed by chronic sinoaortic deafferentation. Second, the animals were studied in the conscious state in which potential confounding effects of anesthesia were not present. Under these conditions, quite different effects on the cardiovascular system were obtained from the selective central administration of the agents. Urapidil produced a small and transient depressor effect followed by a prolonged increase in arterial pressure. Prazosin produced only a marked decrease in arterial pressure that persisted for up to 30 minutes. These data suggest that the potential for prazosin to produce a centrally-mediated hypotensive effect is much greater than that observed with urapidil. With respect to central nervous system depression associated with cerebroventricular administration, no effects were observed. In our hands only the alpha2 agonists,
October 5, 1984
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The American Journal of Medicine
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Figure 4. The effect of urapidil on the regional vasoconstrictor actions of norepinephrine (NE). Norepinephrine was administered intravenously before and during the peak hypotensive action of urapidil. MAP = mean arterial pressure; MR =mesenteric resistance. RR =renal resistance; HQR = hindquarter resistance.
such as clonidine, produce sedation upon central administration. In our laboratory, striking differences have been found among a number of antihypertensive agents believed to exert their effects in part or wholly by reducing central sympathetic outflow. Agents we have tested include the alpha 2 adrenergic receptor agonists clonidine and guanabenz and the alpha1 adrenergic receptor antagonists urapidil, indoramin, and prazosin. In the case of each of these agents, data obtained from anesthetized animals have indicated that the agents can reduce centrally derived sympathetic nerve activity [2,3,6-8]. When these agents were tested for their potential to reduce arterial pressure in unanesthetized, unrestrained animals, a number of unexpected findings appeared. Clonidine, the prototypic centrally acting antihypertensive agent, failed to lower arterial pressure when given by the cerebroventricular route to animals with intact baroreflexes. Despite the failure to lower arterial pressure with clonidine, these animals exhibited marked bradycardia. When clonidine was retested in animals with chronic sinoaortic denervation, it produced profound hypotension and bradycardia. Similar effects were obtained with guanabenz [9]. These data suggest that intact baroreflexes can qualitatively alter the nature of the arterial pressure response to a putative centrally acting agent. Urapidil, which possesses alpha2 adrenoceptor agonist activity, did not behave like clonidine when administered into the central nervous system. This agent produced only a transient hypotensive effect that was overridden by a more prolonged increase in arterial pressure. Prazosin, an
NEW CONCEPTS IN HYPERTENSION THERAPY SYMPOSIUM-BRODY ET AL
agent believed to have a relatively high specificity for alpha 1 adrenoceptors, produced a marked lowering of arterial pressure; however, unlike clonidine, it produced only a modest effect on heart rate. Finally, indoramin produced an entirely different cardiovascular response to central administration. The primary effect was an initial increase in arterial pressure that was followed by a very modest decrease in arterial pressure accompanied by prolonged bradycardia [1 0]. This remarkable diversity of cardiovascular effects, produced by central administration of agents known to reduce centrally-mediated sympathetic discharge, underscores the differences between studies carried out in anesthetized animals and those made under more normal physiologic conditions. Further investigation is needed to determine more precisely whether any of these agents depends upon central actions for its long-term antihypertensive effects. Experiments should be carried out in conscious animals treated long-term with antihypertensive agents and in which measurements of central sympathetic nerve discharge are made with the animals in the conscious state. Furthermore, we need to understand the precise mechanisms by which intact baroreceptors are able to produce qualitative differences in cardiovascular effects mediated through the central nervous system. From the present data and from other studies, there is no question that both urapidil and prazosin are potent antihypertensive agents when administered by the systemic route. This effect is shared by the related agent, indoramin [1 0]. The question we attempted to answer about this peripheral antihypertensive action is the extent to which it can be attributed to peripheral alpha-adrenergic receptor blockade. The ability of equidepressor doses of urapidil and prazosin to affect differentially the vasoconstrictor and pressor responses to norepinephrine leaves little question that prazosin is a more effective adrenergic receptor antagonist. In similar studies carried out with indoramin, we found that it resembled prazosin in exhibiting significant alpha adrenergic blockade at doses (20 times greater than prazosin) that produced a similar lowering of arterial pressure (1 0]. Thus, among these three antihypertensive agents, urapidil appears to exert the least peripheral adrenergic blockade, although it is equally effective in lowering arterial pressure. These data indicate the need for additional studies on the peripheral mechanism of action. Comparisons need to be made between the effects of these agents in vascular beds that have been denervated and pretreated with adrenergic receptor antagonists to determine the extent to which vasodilation produced by mechanisms other than adrenergic receptor blockade contributes to the vasodilator effects. In addition, comparisons need to be made of the relative abilities of these agents to interrupt vasoconstriction produced by neural activity compared to that produced by circulating catecholamines. In our previous studies with indoramin
Comparative Hemodynamic Effects Urapidil Relative Potency Arterial Pressure Resistance Mesenteric Hindquarter Renal Heart Rate Effect on Norepinephrine Central Effect on MAP
Prazosin
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Figure 5. Summary comparison of the hemodynamic effects of urapidil and prazosin. Relative potency indicates that a dose of urapidil six times greater than that of prazosin is needed to produce the same fall in arterial pressure.
[1 0], we found that this agent was equally effective in blocking the constrictor effects of exogenous norepinephrine and the vasoconstrictor effects produced by central activation of the sympathetic nervous system.
CONCLUSION The major differences between urapidil and prazosin observed in these studies carried out in unanesthetized, unrestrained rats are summarized in Figure 5. The agents are equally effective in lowering arterial pressure by peripheral mechanisms; however, urapidil is more active as a renal vasodilator. It remains to be determined from clinical studies whether this effect will translate into less sodium retention. The finding that prazosin produces tachycardia in the areflexic animal suggests that this agent has more potential to increase heart rate; however, tachycardia has not been reported to be a problem with either agent when used in the clinical setting. Using blockade of peripherally administered norepinephrine as an index of capacity to interfere with adrenergic receptors, urapidil appeared to be a much weaker alpha-adrenergic receptor antagonist. This finding needs to be extended to potential differences between neurogenically-mediated vasoconstriction before any projections about differences in potential to produce orthostasis can be made. Finally, prazosin appeared to be a much more efficacious centrally acting hypotensive agent. The contribution of central actions to the antihypertensive effects of any putative centrally acting agent requires much more study.
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REFERENCES Scriabine A: Prazosin. In: Scriabine A, ed. Pharmacology of antihypertensive drugs. New York: Raven Press, 1980; 151160. 2. Schoetensack W, Bruckschen EG, Zech K: Urapidil. In: Scriabine A, ed. New drugs annual. Cardiovascular drugs, vol. 1. New York: Raven Press, 1983; 19-48. 3. McCall RB, Humphrey SJ: Evidence for a central depressor action of postsynaptic a 1-adrenergic receptor antagonists. J Auton Nerv Syst 1981; 3: 9-23. 4. Haywood JR, Schaffer RA, Fastenow C, et al: Regional blood flow measurement in the conscious rat with a pulsed Doppler flowmeter. Am J Physiol 1981; 241: H273-H278. 5. Krieger EM: Neurogenic hypertension in the rat. Circ Res 1964; 15:511-521. 6. Schmitt H, Schmitt H, Boissier JR, Giudicelli JF: Centrally medi1.
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ated decrease in sympathetic tone induced by 2-(2,6dichlorophenylamine)-2-imidazoline (st 155, catapresan). Eur J Pharmacal 1967; 2: 147-148. 7. Baum T, Schropshire AT: Inhibition of spontaneous sympathetic nerve activity by the antihypertensive agent Wy-8678. Neuropharmacology 1970; 9: 503-506. 8. Baum T, Schropshire AT: Central and peripheral contribution to the antihypertensive action of indoramin. Eur J Pharmacal 1975; 32: 30-38. 9. Bonham AC, Trapani AJ, Portis LR, Brody MJ: Studies on the mechanism of the central antihypertensive effect of clonidine and guanabenz. J Hypertens 1984, in press. 10. Porter JP, Bohnam A, Mangiapane ML, Webb RL, Brody MJ: Cardiovascular effects of indoramin in the conscious rat. Circulation 1983; 68: 111-321.