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Effect of dehydraemetine on myocardial circulation and contractility in vivo
EUROPEAN JOURNAL OF PHARMACOLOGY 23 (1973) 6-12. NORTH-HOLLAND PUBLISHING COMPANY
EFFECT
OF DEHYDROEMETINE
CIRCULATION
ON MYOCARDIAL
AND CONTRACTILITY
IN VIVO
L.A. SALAKO* and A.O. DUROTOYE** Department of Pharmacology, University o f lbadan, lbadan, Nigeria
Accepted 26 February 1973
Received 8 August 1972
L.A. SALAKO and A.O. DUROTOYE, Effect of dehydroemetine on myocardial circulation and contractility in vivo, European J. Pharmacol. 23 (1973) 6-12. The effects of dehydroemetine on myocardial haemodynamics and contractility were studied in anaesthetised cats. Doses of 3 and 5 mg/kg caused a fall in blood pressure, heart rate, myocardial blood flow, 'cardiac effort index', left ventricular systolic pressure and maximum rate of rise of left ventricular pressure, without affecting myocardial vascular resistance and heat production. These doses, as well as a 1-mg/kg dose, also caused electrocardiographic changes suggestive of disturbed generation and spread of electrical activity of the heart. None of the effects of dehydroemetine was prevented by previous administration of propranolol or atropine or by vagotomy. It is concluded that the only primary effect of the drug on the heart is a depression of the myocardium and that any observed reduction in coronary blood flow is secondary to this.
Myocardial haemodynamics Myocardial contractility
Myocardial heat production Cardiac effort index
1. Introduction Dehydroemetine is a recently introduced (Brossi et al., 1959) synthetic analogue o f emetine, the naturally occurring amoebicidal alkaloid. Clinically, it has been shown to be at least as effective as emetine, and appears to be better tolerated by patients, in the treatment o f acute intestinal and systemic amoebiasis (Herrero et al., 1960; Salem and Abd-Rabbo, 1964; Powell et al., 1967). Recent reports have', nevertheless, shown that, as with emetine, adverse effects on the cardiovascular system are encountered in human subjects during the administration o f dehydro* All communications to be addressed to: Dr. L.A. Salako, Department of Pharmacology, University of Ibadan, Ibadan, Nigeria. **Present address: Department of Physiology, University of Ife, Ile-lfe, Nigeria.
Dehydroemetine Blood pressure
emetine. These include hypotension, tac.hycardia, premature beats, cardiac syncope and electrocardiographic (ECG) abnormalities (Gonz~lez de Coss[o, 1960; Dempsey and Salem, 1966; Lister, 1968). When injected into anaesthetised rats and cats, hypotension accompanied by a normal or reduced heart rate is observed (Herrero et al., 1960; Salako and Durotoye, 1971), and this hypotensive effect has been said to be largely due to a direct vasodilatory action of the drug on peripheral blood vessels (Salako and Durotoye, 1971). However, in the isolated rabbit and guinea-pig heart and atria, dehydroemetine causes a reduction in the rate and force of contraction, suggesting a depressant effect on the myocardium (Durotoye and Salako, 1971, 1972). The purpose o f this investigation was to study the action o f dehydroemetine on the heart in vivo by determining its effects on cardiac haemodynamics, and relating these to its other effects on the cardiovascular system.
L.A. Salako,A. O. Durotoye, Dehydroemetineand the heart
7
counting the oscillations of the mercury column in the manometer.
2. Materials and methods 2.1. Gene~l
2.3. Left ventricular contratility
Cats of both sexes weighing between 1.8 and 2.5 kg were used. They were anaesthetized with a mixture of chloralose, 70 mg/kg, and sodium pentobarbitone, 5 mg/kg, injected into a forearm vein. The animals were artificially ventilated using a constant pressure pump (Palmer's Ideal) delivering room air at 20 strokes/rain and 45 ml/stroke. 2.2. Myocardial haemodynamics, heart rate and ECG
blood
pressure,
Effects on myocardial haemodynamics were studied in 10 cats using the heat clearance technique of Grayson (Grayson and Mendel, 1961; Grayson and Parratt, 1966; Grayson, 1967). Standard equipment was used except that an electronic 'thermocouple driver' (Grayson et al., 1971) was included to eliminate the drift in current strength that occurred with battery-operated equipment. The operational procedure was similar in detail to tha~; described by earlier workers where the heated thermocouple was embedded in the muscle of the left ventricle and the 'cold junction' was inserted in the abdominal aorta. Myocardial blood flow, expressed as the thermal conductivity increment (AK) and myocardial 'corrected temperature' which estimates myocardial heat production were calculated as described by Grayson (1967). Myocardial vascular resistance was calculated from mean arterial pressure divided by myocardial blood flow as suggested by Grayson and Parratt (1966), and 'cardiac effort index' taken to be a measure of the oxygen consumption of the myocardium was calculated from the product of heart rate and mean arterial pressure (Gerolo et al., 1959; Feinberg et al., 1962). In these experiments arterial blood pressure was measured from a carotid artery using a mercury manometer and recording on smoked paper. ECG was recorded from standard limb lead II using a Cambridge Transrite III electrocardiograph and heart rate was measured either from the ECG record or by
Left ventricular pressure was measured in 8 separate experiments by inserting a needle through the ventricular wall into the cavity of the left ventricle. The needle was connected to a Statham P23 pressure transducer via polythene tubing, and the pressure recorded on a 2-channel Schwarzer physiograph (Fritz Schwarzer GmbH, Munchen, W. Germany). After some practice it was possible to position the needle so that its opening was not occluded at any phase of the cardiac cycle. The left ventricular enddiastolic pressure (LVEDP) and systolic pressure (LVSP) were monitored continuously. Contractility indices were obtained by measurement of the maximum rate of rise developed pressure (dP/dtmax) in mmHg/sec and the time to peak systolic pressure (t-PSP) in sec, from pressure waves recorded with fast paper speed, dP/dtmax was calculated from the pressure wave by drawing a tangent to the ascending limb of the wave at its maximum slope and estimating the gradient geometrically. The value of dP/dtmax before giving the drug was compared with the value when the drug-induced effect on LVSP was maximal to obtain the effect of dehydroemetine on this variable. Dehydroemetine dihydrochloride (Roche) was administered into the left femoral vein in single injections of 1,3 or 5 mg/kg, the three doses being given in a random order. An interval of 30 min was allowed after the 1 and 3 mg/kg doses but after a 5 mg/kg dose, the interval allowed before the next dose was 1 hr. These intervals were found in preliminary experiments to be adequate for the return of the meastired variables to steady state levels which, ih all expe(iments, were within -+ 15% limits of the value before the first dose was given. The 3 doses were tested only once in any particular experiment. Doses of dehydroemetine are stated in terms of the salt and results are expressed as means -+ standard errors of the means (S.E.M). Significance of the effects of the different doses of the drug were evaluated using Student's paired t-test.
8
L.A. Salako, A.O. Durotoye, Dehydroemetine and the heart
3. R e s u l t s
resistance a n d m e t a b o l i c h e a t p r o d u c t i o n were n o t
3.1. Effect o f dehydroemetine on myocardial haemodynamics, blood pressure, heart rate and ECG
significantly a f f e c t e d at a n y o f t h e 3 doses tested. All 3 doses o f d e h y d r o e m e t i n e p r o d u c e d E C G a b n o r m a l -
T h e results o f e x p e r i m e n t s o n 10 cats in w h i c h t h e s e variables were all m o n i t o r e d at t h e same t i m e are s u m m a r i z e d in table 1. T h e y i n c l u d e d 7 ' u n t r e a t e d ' a n i m a l s a n d 3 p r e v i o u s l y v a g o t o m i z e d a n d injected w i t h 1.0 m g / k g p r o p a n o l o l , i.v. T h e results
ities w h i c h were o f i m m e d i a t e o n s e t b u t w h i c h varied in degree a n d d u r a t i o n w i t h the dose given. With 1 a n d 3 m g / k g doses, t h e a b n o r m a l i t i e s c o n s i s t e d o f d e e p e n i n g o f t h e S wave, increase in t h e h e i g h t o f t h e R wave a n d increased a m p l i t i t u d e a n d slope o f the T wave. These effects were reversible b u t t h e y always o u t l a s t e d t h e e f f e c t o n b l o o d pressure, h e a r t rate,
f r o m all 10 cats h a v e b e e n p o o l e d t o g e t h e r for analysis b e c a u s e t h e r e did n o t a p p e a r t o be a n y d i f f e r e n c e in t h e r e s p o n s e o f t h e ' t r e a t e d ' a n d ' u n t r e a t e d ' animals t o d e h y d r o e m e t i n e . B l o o d pressure, h e a r t rate, m y o c a r d i a l b l o o d flow a n d ' c a r d i a c e f f o r t i n d e x ' were significantly reduced by 3 and 5 mg/kg but not by 1 m g / k g d e h y d r o e m e t i n e . T h e o n s e t o f t h e e f f e c t was i m m e d i a t e a n d t h e effects l a s t e d u p t o 10 m i n (3 m g / kg) or 3 0 m i n ( 5 m g / k g ) , b e f o r e t h e variables r e t u r n e d to n o r m a l or n e a r n o r m a l levels. M y o c a r d i a l vascular
m y o c a r d i a l b l o o d flow a n d ' c a r d i a c e f f e c t i n d e x ' , norreal E C G b e i n g r e s t o r e d 1 0 - 2 0 m i n a f t e r giving a 3 - m g / k g dose c o m p a r e d w i t h 5 - 1 0 rain r e c o v e r y t i m e for t h e o t h e r variables. With a 5-mg/kg dose, m o r e severe a l t e r a t i o n s in t h e E C G c h a r a c t e r i z e d b y sinus b r a d y c a r d i a , p a r o x y s m a l atrial a n d v e n t r i c u l a r t a c h y cardia a n d v a r y i n g degrees o f a t r i o v e n t r i c u l a r a n d int r a v e n t r i c u l a r b l o c k were o b s e r v e d ( f i g . l ) . T h e s e effects were also reversible. P e a k e f f e c t was r e a c h e d a n d m a i n t a i n e d b e t w e e n 5 a n d 10 m i n f o l l o w e d b y grad-
Table 1 Cardiovascular and cardiac haemodynamic effects of dehydroemetine in 10 cats. After-drug values represent maximum response. All values are given as mean ± S.E.M. Significance of the differences has been evaluated using Student's paired t-test. Measured variable
Dose of dehydroemetine (mg/kg)
Control value
Arterial blood pressure (AP) (mmHg)
1 3 5
105 -+ 8.4 95 ± 6.4 96 ± 7.2
101 -+ 7.5 78 ± 7.0 51 ± 8.1
-
4 ± 3.0 17 ± 5.0 45 ± 5.9
n.s. p < 0.01 p < 0.001
Heart Rate (HR) (beats per min)
1 3 5
182 ± 6.6 174 ± 7.6 171±7.6
182 ± 7.3 165 ± 8.9 144±8.0
-
0 ± 3.0 9 ± 3.5 27+4.9
n.s. p < 0.05 p<0.001
Myocardial blood flow, (Ak) (c.g.s.units X 10-'~)
1 3 5
4.82 ± 0.6 4.36 ± 0.6 3.60 ± 0.3
4.50 ± 0.5 3.18 ± 0.3 1.65 ± 0.2
- 0.32 ± 0.2 - 1.18 ± 0.3 - 1.95 -+ 0.2
n.s. p < 0.005 p < 0.001
Myocardial vascular resistance (AP/AK)
1 3 5
2.2 ± 0.9 2.2 ± 0.5 2.7 ± 0.5
2.2 ± 0.7 2.5 ± 0.8 3.1 ± 0.8
0 ± 0.4 + 0.3 ± 0.5 + 0.4 ± 0.5
n.s. n.s. n.s.
Metabolic heat production as 'corrected temperature' (°C)
1 3 5
0.46 ± 0.06 0.44 ± 0.05 0.42 ± 0.05
0.41 ± 0.06 0.38 ± 0.04 0.34 ± 0.05
- 0.05 ± 0.03 - 0.06 ± 0.03 - 0.08 ± 0.04
n.s. n.s. n.s.
Cardiac effort index (AP × HR × 10 -3)
1 3 5
- 0.73 ± 0.5 - 3.66 ± 0.7 - 9.08 ± 1.2
n.s. p < 0.001 p < 0.001
19.11 ± 1.6 16.53 ± 1.4 16.42 ± 1.4
After-drug value
18.38 ± 1.2 12.87 ± 1.4 7.34 ± 1.4
Difference
L.A. Salako, A.O. Durotoye, Dehydroemetine and the heart
;~
; ~:;~
~;~;:
~ ; ~ ~:
:~;~
:~:~;~
~ ~i~~ ' ~ i ~ ! ~ ~i~!~ !~i:~i:~
3 m~g
5 mg~g
Fig.1. Effects of 1, 3 and 5 mg/kg (i.v.) doses of dehydroemetine on the ECG of the anaesthetized cat (Standard limb lead II). Left hand panels show control while right hand panels show recovery tracings. The drug effects are shown in the middle panels. ual recovery but normal ECG was not restored until after 4 5 - 6 0 min. The ECG changes were essentially similar to those previously described in detail in the rat (Salako and Durotoye, 1972).
3.2. Effect o f dehydroemetine on left ventricular contractility In 6 cats, both 3 and 5 mg/kg doses of dehydroemetine caused significant reductions in LVSP and dP/dtmax but the effect produced by the 5 mg/kg dose was very much greater than that produced by 3 mg/kg (table 2). With the 3-mg/kg dose there was no measurable change in LVEDP and t-PSP. The 5-medkg dose caused a substantial increase in t-PSP and a small elevation of LVEDP (fig.2). With both doses, onset of the effect was immediate; peak effect was reached within 2 min after a 3-mg/kg dose and within 10 min after a 5-mg/kg dose, and return to pretreatment levels was complete within 5 and 30 min respectively. In 2 additional experiments in which the cats were vagotomized and injected with 1-mg/kg propranolol, i.v., before administering dehydroemetine, results similar to those described above were obtained.
Fig.2. Effect of dehydroemetine, 1, 3 and 5 mg/kg (top, middie and bottom panels respectively) on left ventricular enddiastolic and systolic pressure. Drug given at arrow. Vertical calibration in mm Hg. Horizontal calibration: 30 sec = 30 seconds; 0.5 = 0.5 seconds.
10
L.A. Salako, A.O. Durotoye, Dehydroemetine and the heart
Table 2 Effect of d e h y d r o e m e t i n e on left ventricular systolic pressure (LVSP) and left ventricular d P / d t m a x . All values are m e a n s ± S.E.M. of 6 experiments. Dose of dehydroemetine (mg/kg)
3 5
LVSP (mmHg)
d P / d t m a x (mmHg/sec)
Before dehydroemetine
After dehydroemetine
Before dehydroemetine
After dehydroemetine
90 ± 4.6 92±5.4
80 ± 5.5 * 36±6.3'*
2180 -+ 222 2032-+210
1636 ± 208 * 272± 64"*
* p < 0.05. ** p < 0.001 (using S t u d e n t ' s paired t-test).
4. Discussion These results show that doses of 3 and 5 mg/kg dehydroemetine caused significant reduction in myocardial blood flow whereas a dose of 1 mg/kg did not. A decrease in myocardial blood flow could result from a fall in perfusion pressure, a reduction in 'nutritive' demand due to decrease in contractility and work of the heart, or a direct vasoconstrictor action of the drug. Since myocardial vascular resistance was unaffected by any of the 3 doses, the results do not suggest a coronary vasoconstrictor effect of the drug. This conclusion is further strengthened by the fact that at none of the 3 dose levels did ECG show an ischaemic pattern. However, the 2 doses which caused a reduction in myocardial blood flow also produced a fall in blood pressure and in myocardial contractility (as shown by the fall in LVSP and dP/dtmax with or without increase in LVEDP and t-PSP). Since a fall in blood pressure would decrease coronary perfusion pressure and a fall in myocardial contractility would decrease the 'nutritive' demand of the myocardium for blood flow, the results suggest that the reduction in myocardial blood flow might be secondary to diminished perfusion pressure and myocardial contractility. The absence of a significant effect on metabolic heat production by any of the 3 doses tested suggests that in these acute experiments, the observed effect on myocardial contractility is probably not due to inhibition of myocardial metabolism. In contrast, all 3 doses produced changes in the ECG. The increased amplitude and slope of the t-wave caused by 1and 3-mg/kg doses suggest accelerated repolarisatio/~
while the abnormalities observed with 5 mg/kg are evidence of pace-maker depression and blockade of spread of electrical activity within the myocardium. It therefore seems likely, from the effects of the drug on ECG, that in addition to its effect on contractility, dehydroemetine also has an important effect on the electrochemical activity of the heart. Studies on the isolated guinea-pig atria have produced evidence suggesting that dehydroemetine might influence the transmembrane diffusion of potassium and that the effects of the drug on spontaneous atrial activity could be secondary to this action (Durotoye and Salako, 1972). This would imply that the mechanical effect of the drug results from an electrochemical action on the myocardial cells. The present results are not inconsistent with this hypothesis although more direct evidence would be needed to make more definite conclusions on the existence of a cause and effect relationship between the electrochemical and mechanical effects of the drug. The observations that 1 mg/kg dehydroemetine, which has no effect on contractility, has an effect on ECG and that the effects of 3 and 5 mg/kg doses of dehydroemetine on ECG outlast the effects of corresponding doses on contractility do not contradict the hypothesis, since a certain degree of electrochemical disturbance would be expected before the production of mechanical abnormalities. Both 3 and 5 mg/kg produced a significant effect on 'cardiac effort index' a qualitative measurement of oxygen consumption. This effect on estimated oxygen consumption might be secondary to the reduction in contractility induced by these two doses since effect on oxygen consumption has been shown in sev-
L.A. Salako, A. 0. Durotoye, Dehydroemetine and the heart
eral instances to parallel that on myocardial contractility (Parratt and Wadsworth, 1970; Parratt and Winslow, 1971). In conclusion, these results show that whereas 1, 3 and 5 mg/kg doses of dehydroemetine produced changes in the electrical activity o f the heart, only 3 and 5 mg/kg consistently produced changes in systemic blood pressure, LVSP and myocardial blood flow. The reduction in blood flow is probably due to the combination o f a fall in perfusion pressure and in 'nutritive' demand secondary to decreased myocardial contractility rather than to an increase in coronary vascular resistance. It is also suggested that reduced myocardial contractility is unrelated to any inhibition of myocardial metabolism but could be secondary to electrochemical changes. It was previously shown by Salako and Durotoye (1971), and confirmed in this study, that the effects of dehydroemetine on blood pressure and heart rate were unaffected by vagotomy, atropine or propranolol, and a similar resistance of the effects on ECG and myocardial contractility to vagotomy and propranolol was demonstrated in this study. These would suggest that the observed effects o f dehydroemetine are probably direct on the heart and not due to vagal stimulation or inhibition o f sympathetic drive. It was suggested in an earlier study (Salako and Durotoye, 1971) that the fall in blood pressure produced by dehydroemetine was due, at least in part, to direct peripheral vasodilation by the drug. The observation in this study that both 3 and 5 mg/kg dehydroemetine, which lowered the blood pressure, also reduced myocardial contractility and heart rate suggests that depression of the myocardium contributes to the hypotensive effect. If hypotension were due solely to periphal vasodilatation then a compensatory increase in cardiac rate and contractility would be expected. It is of interest to compare our results with the results of a similar study by Bianchi et al. (1965) on the natural alkaloid, emetine. These workers found that emetine caused a reduction in myocardial contractility as well as a reduction in coronary circulation in the anaesthetised dog. Reduced coronary blood flow was however produced only by doses of emetine which caused severe hypotension and ventricular fibrillation. Bianchi et al. (1965) concluded that the only primary effect o f emetine on the heart was a depression o f myocardial contractility, any observed
11
reduction in blood flow being secondary to this - a conclusion similar to that reached in the present paper with dehydroemetine.
Acknowledgements We are grateful to Dr. 1. Lenox-Smith of Roche Products Ltd., Hefts, England, for a generous gift of dehydroemetine.
References Bianchi, A., V. de Marino, G.R. de Vleeschhouwer and A. Marino, 1965, Effects of emetine on heart, coronary circulation and blood pressure (research in vitro on the guinea-pig heart and coronary flow; research in vivo on the dog heart, coronary flow and blood pressure), Arch. Intern. Pharmacodyn. 156,238. Brossi, A., M. Baumann, L.H. ChopardoDit-Jean, J. Wursch, F. Schneider and O. Schnider, 1959, Syntheseversuche in der Emetine-Reihe. 4. Mitteilung. Racemisches 2-Dehydroemetin, Helv. Chim. Acta 42, 772. Dempsey, J.J. and H.H. Salem, 1966, An enzymatic electrocardiographic study on toxicity of dehydroemetine, Brit. Heart J. 28,505. Durotoye, A.O. and L.A. Salako, 1971, Effect of dehydroemetine on isolated preparations of rabbit and guinea-pig heart and atria, Life Sci. 10, 623. Durotoye, A.O. and L.A. Salako, 1972, The effect of dehydroemetine on isolated guinea-pig atria, Brit. J. Pharmacol. 44,723. Feinberg, H., L.N. Katz and E. Boyd, 1962, Determinants of coronary flow and myocardial oxygen consumption, Amer. J. Physiol. 202, 45. Gerolo, M., H. Feinberg and L.N. Katz, 1959, Coronary flow and oxygen consumption of the myocardium in conditions of hypocapnia, Atti. Soc. Ital. Cardiol. 21, 58. Gonz~ilez de Cossi'o, A., 1960, Electrocardiographic changes under therapy with Ro 1-9334, a synthetic raeemic 2-dehydroemetine, Rev. Inst. Med. Trop. Sao Paulo 2, 313. Grayson, J., 1967, Heat exchange methods in the assessment of blood flow and heat production in solid organs, Gastroenterology 52, 391. Grayson, J. and D. Mendel, 1961, Myocardial blood flow in the rabbit, Amer. J. Physiol. 200, 968. Grayson, J. and J.R. Parratt, 1966, A species comparison of the effects of changing perfusion pressure on blood flow and heat production in the myocardium, J. Physiol., London 187,465. Grayson, J., R.L. Coulson and B. Winchester, 1971, Internal calorimetry assessment of myocarial blood flow and heat production, J. Appl. Physiol. 30, 251. Herrero, J., A. Brossi, M. Faust and J.R. Frey, 1960, Preliminary experimental and clinical results with a new syn-
12
L.A. Salako, A.O. Durotoye, Dehydroemetine and the heart
thetic emetine-like compound, Ann. Biochem. Exptl. Med. 20, 475. Lister, D.J., 1968, Delayed myocardial intoxication following the administration of dehydroemetine hydrochloride, J. Trop. Med. Hyg. 71,219. Parratt, J.R. and R.M. Wadsworth, 1970, The effect of catecholamine infusions on myocardial blood flow, metabolic heat production and on general haemodynamics, before and after alprenolol (H56/28) in anaesthetised cats, Brit. J. Pharmacol. 38, 554.. Parratt, J.R. and E. Winslow, 1971, Cardiovascular pharmacology of quazodine (MJ-1988), with particular reference to effects on myocardial blood flow and metabolic heat production, Brit. J. Pharmacol. 42, 193.
Powell, S.J., A.J. Wilmot, I.N. MacLeod and R. Elsdon-Dew, 1967, A comparative trial of dehydroemetine and emetine hydrochloride in identical dosage in amoebic liver abscess, Ann. Trop. Med. Parasit. 61,26. Salako, L.A. and A.O. Durotoye, 1971, Observations on the hypotensive response to dehydroemetine, European J. Pharmacol. 14,200. Salako, L.A. and A.O. Durotoye, 1972, The electrocardiogram in acute dehydroemetine intoxication, Cardiovascular Res. 6, 150. Salem, H.H. and H. Abd-Rabbo, 1964, Dehydroemetine in acute amoebiasis, Trans. Roy. Soc. Trop. Med. Hyg. 58, 539.
EFFECT
OF DEHYDROEMETINE
CIRCULATION
ON MYOCARDIAL
AND CONTRACTILITY
IN VIVO
L.A. SALAKO* and A.O. DUROTOYE** Department of Pharmacology, University o f lbadan, lbadan, Nigeria
Accepted 26 February 1973
Received 8 August 1972
L.A. SALAKO and A.O. DUROTOYE, Effect of dehydroemetine on myocardial circulation and contractility in vivo, European J. Pharmacol. 23 (1973) 6-12. The effects of dehydroemetine on myocardial haemodynamics and contractility were studied in anaesthetised cats. Doses of 3 and 5 mg/kg caused a fall in blood pressure, heart rate, myocardial blood flow, 'cardiac effort index', left ventricular systolic pressure and maximum rate of rise of left ventricular pressure, without affecting myocardial vascular resistance and heat production. These doses, as well as a 1-mg/kg dose, also caused electrocardiographic changes suggestive of disturbed generation and spread of electrical activity of the heart. None of the effects of dehydroemetine was prevented by previous administration of propranolol or atropine or by vagotomy. It is concluded that the only primary effect of the drug on the heart is a depression of the myocardium and that any observed reduction in coronary blood flow is secondary to this.
Myocardial haemodynamics Myocardial contractility
Myocardial heat production Cardiac effort index
1. Introduction Dehydroemetine is a recently introduced (Brossi et al., 1959) synthetic analogue o f emetine, the naturally occurring amoebicidal alkaloid. Clinically, it has been shown to be at least as effective as emetine, and appears to be better tolerated by patients, in the treatment o f acute intestinal and systemic amoebiasis (Herrero et al., 1960; Salem and Abd-Rabbo, 1964; Powell et al., 1967). Recent reports have', nevertheless, shown that, as with emetine, adverse effects on the cardiovascular system are encountered in human subjects during the administration o f dehydro* All communications to be addressed to: Dr. L.A. Salako, Department of Pharmacology, University of Ibadan, Ibadan, Nigeria. **Present address: Department of Physiology, University of Ife, Ile-lfe, Nigeria.
Dehydroemetine Blood pressure
emetine. These include hypotension, tac.hycardia, premature beats, cardiac syncope and electrocardiographic (ECG) abnormalities (Gonz~lez de Coss[o, 1960; Dempsey and Salem, 1966; Lister, 1968). When injected into anaesthetised rats and cats, hypotension accompanied by a normal or reduced heart rate is observed (Herrero et al., 1960; Salako and Durotoye, 1971), and this hypotensive effect has been said to be largely due to a direct vasodilatory action of the drug on peripheral blood vessels (Salako and Durotoye, 1971). However, in the isolated rabbit and guinea-pig heart and atria, dehydroemetine causes a reduction in the rate and force of contraction, suggesting a depressant effect on the myocardium (Durotoye and Salako, 1971, 1972). The purpose o f this investigation was to study the action o f dehydroemetine on the heart in vivo by determining its effects on cardiac haemodynamics, and relating these to its other effects on the cardiovascular system.
L.A. Salako,A. O. Durotoye, Dehydroemetineand the heart
7
counting the oscillations of the mercury column in the manometer.
2. Materials and methods 2.1. Gene~l
2.3. Left ventricular contratility
Cats of both sexes weighing between 1.8 and 2.5 kg were used. They were anaesthetized with a mixture of chloralose, 70 mg/kg, and sodium pentobarbitone, 5 mg/kg, injected into a forearm vein. The animals were artificially ventilated using a constant pressure pump (Palmer's Ideal) delivering room air at 20 strokes/rain and 45 ml/stroke. 2.2. Myocardial haemodynamics, heart rate and ECG
blood
pressure,
Effects on myocardial haemodynamics were studied in 10 cats using the heat clearance technique of Grayson (Grayson and Mendel, 1961; Grayson and Parratt, 1966; Grayson, 1967). Standard equipment was used except that an electronic 'thermocouple driver' (Grayson et al., 1971) was included to eliminate the drift in current strength that occurred with battery-operated equipment. The operational procedure was similar in detail to tha~; described by earlier workers where the heated thermocouple was embedded in the muscle of the left ventricle and the 'cold junction' was inserted in the abdominal aorta. Myocardial blood flow, expressed as the thermal conductivity increment (AK) and myocardial 'corrected temperature' which estimates myocardial heat production were calculated as described by Grayson (1967). Myocardial vascular resistance was calculated from mean arterial pressure divided by myocardial blood flow as suggested by Grayson and Parratt (1966), and 'cardiac effort index' taken to be a measure of the oxygen consumption of the myocardium was calculated from the product of heart rate and mean arterial pressure (Gerolo et al., 1959; Feinberg et al., 1962). In these experiments arterial blood pressure was measured from a carotid artery using a mercury manometer and recording on smoked paper. ECG was recorded from standard limb lead II using a Cambridge Transrite III electrocardiograph and heart rate was measured either from the ECG record or by
Left ventricular pressure was measured in 8 separate experiments by inserting a needle through the ventricular wall into the cavity of the left ventricle. The needle was connected to a Statham P23 pressure transducer via polythene tubing, and the pressure recorded on a 2-channel Schwarzer physiograph (Fritz Schwarzer GmbH, Munchen, W. Germany). After some practice it was possible to position the needle so that its opening was not occluded at any phase of the cardiac cycle. The left ventricular enddiastolic pressure (LVEDP) and systolic pressure (LVSP) were monitored continuously. Contractility indices were obtained by measurement of the maximum rate of rise developed pressure (dP/dtmax) in mmHg/sec and the time to peak systolic pressure (t-PSP) in sec, from pressure waves recorded with fast paper speed, dP/dtmax was calculated from the pressure wave by drawing a tangent to the ascending limb of the wave at its maximum slope and estimating the gradient geometrically. The value of dP/dtmax before giving the drug was compared with the value when the drug-induced effect on LVSP was maximal to obtain the effect of dehydroemetine on this variable. Dehydroemetine dihydrochloride (Roche) was administered into the left femoral vein in single injections of 1,3 or 5 mg/kg, the three doses being given in a random order. An interval of 30 min was allowed after the 1 and 3 mg/kg doses but after a 5 mg/kg dose, the interval allowed before the next dose was 1 hr. These intervals were found in preliminary experiments to be adequate for the return of the meastired variables to steady state levels which, ih all expe(iments, were within -+ 15% limits of the value before the first dose was given. The 3 doses were tested only once in any particular experiment. Doses of dehydroemetine are stated in terms of the salt and results are expressed as means -+ standard errors of the means (S.E.M). Significance of the effects of the different doses of the drug were evaluated using Student's paired t-test.
8
L.A. Salako, A.O. Durotoye, Dehydroemetine and the heart
3. R e s u l t s
resistance a n d m e t a b o l i c h e a t p r o d u c t i o n were n o t
3.1. Effect o f dehydroemetine on myocardial haemodynamics, blood pressure, heart rate and ECG
significantly a f f e c t e d at a n y o f t h e 3 doses tested. All 3 doses o f d e h y d r o e m e t i n e p r o d u c e d E C G a b n o r m a l -
T h e results o f e x p e r i m e n t s o n 10 cats in w h i c h t h e s e variables were all m o n i t o r e d at t h e same t i m e are s u m m a r i z e d in table 1. T h e y i n c l u d e d 7 ' u n t r e a t e d ' a n i m a l s a n d 3 p r e v i o u s l y v a g o t o m i z e d a n d injected w i t h 1.0 m g / k g p r o p a n o l o l , i.v. T h e results
ities w h i c h were o f i m m e d i a t e o n s e t b u t w h i c h varied in degree a n d d u r a t i o n w i t h the dose given. With 1 a n d 3 m g / k g doses, t h e a b n o r m a l i t i e s c o n s i s t e d o f d e e p e n i n g o f t h e S wave, increase in t h e h e i g h t o f t h e R wave a n d increased a m p l i t i t u d e a n d slope o f the T wave. These effects were reversible b u t t h e y always o u t l a s t e d t h e e f f e c t o n b l o o d pressure, h e a r t rate,
f r o m all 10 cats h a v e b e e n p o o l e d t o g e t h e r for analysis b e c a u s e t h e r e did n o t a p p e a r t o be a n y d i f f e r e n c e in t h e r e s p o n s e o f t h e ' t r e a t e d ' a n d ' u n t r e a t e d ' animals t o d e h y d r o e m e t i n e . B l o o d pressure, h e a r t rate, m y o c a r d i a l b l o o d flow a n d ' c a r d i a c e f f o r t i n d e x ' were significantly reduced by 3 and 5 mg/kg but not by 1 m g / k g d e h y d r o e m e t i n e . T h e o n s e t o f t h e e f f e c t was i m m e d i a t e a n d t h e effects l a s t e d u p t o 10 m i n (3 m g / kg) or 3 0 m i n ( 5 m g / k g ) , b e f o r e t h e variables r e t u r n e d to n o r m a l or n e a r n o r m a l levels. M y o c a r d i a l vascular
m y o c a r d i a l b l o o d flow a n d ' c a r d i a c e f f e c t i n d e x ' , norreal E C G b e i n g r e s t o r e d 1 0 - 2 0 m i n a f t e r giving a 3 - m g / k g dose c o m p a r e d w i t h 5 - 1 0 rain r e c o v e r y t i m e for t h e o t h e r variables. With a 5-mg/kg dose, m o r e severe a l t e r a t i o n s in t h e E C G c h a r a c t e r i z e d b y sinus b r a d y c a r d i a , p a r o x y s m a l atrial a n d v e n t r i c u l a r t a c h y cardia a n d v a r y i n g degrees o f a t r i o v e n t r i c u l a r a n d int r a v e n t r i c u l a r b l o c k were o b s e r v e d ( f i g . l ) . T h e s e effects were also reversible. P e a k e f f e c t was r e a c h e d a n d m a i n t a i n e d b e t w e e n 5 a n d 10 m i n f o l l o w e d b y grad-
Table 1 Cardiovascular and cardiac haemodynamic effects of dehydroemetine in 10 cats. After-drug values represent maximum response. All values are given as mean ± S.E.M. Significance of the differences has been evaluated using Student's paired t-test. Measured variable
Dose of dehydroemetine (mg/kg)
Control value
Arterial blood pressure (AP) (mmHg)
1 3 5
105 -+ 8.4 95 ± 6.4 96 ± 7.2
101 -+ 7.5 78 ± 7.0 51 ± 8.1
-
4 ± 3.0 17 ± 5.0 45 ± 5.9
n.s. p < 0.01 p < 0.001
Heart Rate (HR) (beats per min)
1 3 5
182 ± 6.6 174 ± 7.6 171±7.6
182 ± 7.3 165 ± 8.9 144±8.0
-
0 ± 3.0 9 ± 3.5 27+4.9
n.s. p < 0.05 p<0.001
Myocardial blood flow, (Ak) (c.g.s.units X 10-'~)
1 3 5
4.82 ± 0.6 4.36 ± 0.6 3.60 ± 0.3
4.50 ± 0.5 3.18 ± 0.3 1.65 ± 0.2
- 0.32 ± 0.2 - 1.18 ± 0.3 - 1.95 -+ 0.2
n.s. p < 0.005 p < 0.001
Myocardial vascular resistance (AP/AK)
1 3 5
2.2 ± 0.9 2.2 ± 0.5 2.7 ± 0.5
2.2 ± 0.7 2.5 ± 0.8 3.1 ± 0.8
0 ± 0.4 + 0.3 ± 0.5 + 0.4 ± 0.5
n.s. n.s. n.s.
Metabolic heat production as 'corrected temperature' (°C)
1 3 5
0.46 ± 0.06 0.44 ± 0.05 0.42 ± 0.05
0.41 ± 0.06 0.38 ± 0.04 0.34 ± 0.05
- 0.05 ± 0.03 - 0.06 ± 0.03 - 0.08 ± 0.04
n.s. n.s. n.s.
Cardiac effort index (AP × HR × 10 -3)
1 3 5
- 0.73 ± 0.5 - 3.66 ± 0.7 - 9.08 ± 1.2
n.s. p < 0.001 p < 0.001
19.11 ± 1.6 16.53 ± 1.4 16.42 ± 1.4
After-drug value
18.38 ± 1.2 12.87 ± 1.4 7.34 ± 1.4
Difference
L.A. Salako, A.O. Durotoye, Dehydroemetine and the heart
;~
; ~:;~
~;~;:
~ ; ~ ~:
:~;~
:~:~;~
~ ~i~~ ' ~ i ~ ! ~ ~i~!~ !~i:~i:~
3 m~g
5 mg~g
Fig.1. Effects of 1, 3 and 5 mg/kg (i.v.) doses of dehydroemetine on the ECG of the anaesthetized cat (Standard limb lead II). Left hand panels show control while right hand panels show recovery tracings. The drug effects are shown in the middle panels. ual recovery but normal ECG was not restored until after 4 5 - 6 0 min. The ECG changes were essentially similar to those previously described in detail in the rat (Salako and Durotoye, 1972).
3.2. Effect o f dehydroemetine on left ventricular contractility In 6 cats, both 3 and 5 mg/kg doses of dehydroemetine caused significant reductions in LVSP and dP/dtmax but the effect produced by the 5 mg/kg dose was very much greater than that produced by 3 mg/kg (table 2). With the 3-mg/kg dose there was no measurable change in LVEDP and t-PSP. The 5-medkg dose caused a substantial increase in t-PSP and a small elevation of LVEDP (fig.2). With both doses, onset of the effect was immediate; peak effect was reached within 2 min after a 3-mg/kg dose and within 10 min after a 5-mg/kg dose, and return to pretreatment levels was complete within 5 and 30 min respectively. In 2 additional experiments in which the cats were vagotomized and injected with 1-mg/kg propranolol, i.v., before administering dehydroemetine, results similar to those described above were obtained.
Fig.2. Effect of dehydroemetine, 1, 3 and 5 mg/kg (top, middie and bottom panels respectively) on left ventricular enddiastolic and systolic pressure. Drug given at arrow. Vertical calibration in mm Hg. Horizontal calibration: 30 sec = 30 seconds; 0.5 = 0.5 seconds.
10
L.A. Salako, A.O. Durotoye, Dehydroemetine and the heart
Table 2 Effect of d e h y d r o e m e t i n e on left ventricular systolic pressure (LVSP) and left ventricular d P / d t m a x . All values are m e a n s ± S.E.M. of 6 experiments. Dose of dehydroemetine (mg/kg)
3 5
LVSP (mmHg)
d P / d t m a x (mmHg/sec)
Before dehydroemetine
After dehydroemetine
Before dehydroemetine
After dehydroemetine
90 ± 4.6 92±5.4
80 ± 5.5 * 36±6.3'*
2180 -+ 222 2032-+210
1636 ± 208 * 272± 64"*
* p < 0.05. ** p < 0.001 (using S t u d e n t ' s paired t-test).
4. Discussion These results show that doses of 3 and 5 mg/kg dehydroemetine caused significant reduction in myocardial blood flow whereas a dose of 1 mg/kg did not. A decrease in myocardial blood flow could result from a fall in perfusion pressure, a reduction in 'nutritive' demand due to decrease in contractility and work of the heart, or a direct vasoconstrictor action of the drug. Since myocardial vascular resistance was unaffected by any of the 3 doses, the results do not suggest a coronary vasoconstrictor effect of the drug. This conclusion is further strengthened by the fact that at none of the 3 dose levels did ECG show an ischaemic pattern. However, the 2 doses which caused a reduction in myocardial blood flow also produced a fall in blood pressure and in myocardial contractility (as shown by the fall in LVSP and dP/dtmax with or without increase in LVEDP and t-PSP). Since a fall in blood pressure would decrease coronary perfusion pressure and a fall in myocardial contractility would decrease the 'nutritive' demand of the myocardium for blood flow, the results suggest that the reduction in myocardial blood flow might be secondary to diminished perfusion pressure and myocardial contractility. The absence of a significant effect on metabolic heat production by any of the 3 doses tested suggests that in these acute experiments, the observed effect on myocardial contractility is probably not due to inhibition of myocardial metabolism. In contrast, all 3 doses produced changes in the ECG. The increased amplitude and slope of the t-wave caused by 1and 3-mg/kg doses suggest accelerated repolarisatio/~
while the abnormalities observed with 5 mg/kg are evidence of pace-maker depression and blockade of spread of electrical activity within the myocardium. It therefore seems likely, from the effects of the drug on ECG, that in addition to its effect on contractility, dehydroemetine also has an important effect on the electrochemical activity of the heart. Studies on the isolated guinea-pig atria have produced evidence suggesting that dehydroemetine might influence the transmembrane diffusion of potassium and that the effects of the drug on spontaneous atrial activity could be secondary to this action (Durotoye and Salako, 1972). This would imply that the mechanical effect of the drug results from an electrochemical action on the myocardial cells. The present results are not inconsistent with this hypothesis although more direct evidence would be needed to make more definite conclusions on the existence of a cause and effect relationship between the electrochemical and mechanical effects of the drug. The observations that 1 mg/kg dehydroemetine, which has no effect on contractility, has an effect on ECG and that the effects of 3 and 5 mg/kg doses of dehydroemetine on ECG outlast the effects of corresponding doses on contractility do not contradict the hypothesis, since a certain degree of electrochemical disturbance would be expected before the production of mechanical abnormalities. Both 3 and 5 mg/kg produced a significant effect on 'cardiac effort index' a qualitative measurement of oxygen consumption. This effect on estimated oxygen consumption might be secondary to the reduction in contractility induced by these two doses since effect on oxygen consumption has been shown in sev-
L.A. Salako, A. 0. Durotoye, Dehydroemetine and the heart
eral instances to parallel that on myocardial contractility (Parratt and Wadsworth, 1970; Parratt and Winslow, 1971). In conclusion, these results show that whereas 1, 3 and 5 mg/kg doses of dehydroemetine produced changes in the electrical activity o f the heart, only 3 and 5 mg/kg consistently produced changes in systemic blood pressure, LVSP and myocardial blood flow. The reduction in blood flow is probably due to the combination o f a fall in perfusion pressure and in 'nutritive' demand secondary to decreased myocardial contractility rather than to an increase in coronary vascular resistance. It is also suggested that reduced myocardial contractility is unrelated to any inhibition of myocardial metabolism but could be secondary to electrochemical changes. It was previously shown by Salako and Durotoye (1971), and confirmed in this study, that the effects of dehydroemetine on blood pressure and heart rate were unaffected by vagotomy, atropine or propranolol, and a similar resistance of the effects on ECG and myocardial contractility to vagotomy and propranolol was demonstrated in this study. These would suggest that the observed effects o f dehydroemetine are probably direct on the heart and not due to vagal stimulation or inhibition o f sympathetic drive. It was suggested in an earlier study (Salako and Durotoye, 1971) that the fall in blood pressure produced by dehydroemetine was due, at least in part, to direct peripheral vasodilation by the drug. The observation in this study that both 3 and 5 mg/kg dehydroemetine, which lowered the blood pressure, also reduced myocardial contractility and heart rate suggests that depression of the myocardium contributes to the hypotensive effect. If hypotension were due solely to periphal vasodilatation then a compensatory increase in cardiac rate and contractility would be expected. It is of interest to compare our results with the results of a similar study by Bianchi et al. (1965) on the natural alkaloid, emetine. These workers found that emetine caused a reduction in myocardial contractility as well as a reduction in coronary circulation in the anaesthetised dog. Reduced coronary blood flow was however produced only by doses of emetine which caused severe hypotension and ventricular fibrillation. Bianchi et al. (1965) concluded that the only primary effect o f emetine on the heart was a depression o f myocardial contractility, any observed
11
reduction in blood flow being secondary to this - a conclusion similar to that reached in the present paper with dehydroemetine.
Acknowledgements We are grateful to Dr. 1. Lenox-Smith of Roche Products Ltd., Hefts, England, for a generous gift of dehydroemetine.
References Bianchi, A., V. de Marino, G.R. de Vleeschhouwer and A. Marino, 1965, Effects of emetine on heart, coronary circulation and blood pressure (research in vitro on the guinea-pig heart and coronary flow; research in vivo on the dog heart, coronary flow and blood pressure), Arch. Intern. Pharmacodyn. 156,238. Brossi, A., M. Baumann, L.H. ChopardoDit-Jean, J. Wursch, F. Schneider and O. Schnider, 1959, Syntheseversuche in der Emetine-Reihe. 4. Mitteilung. Racemisches 2-Dehydroemetin, Helv. Chim. Acta 42, 772. Dempsey, J.J. and H.H. Salem, 1966, An enzymatic electrocardiographic study on toxicity of dehydroemetine, Brit. Heart J. 28,505. Durotoye, A.O. and L.A. Salako, 1971, Effect of dehydroemetine on isolated preparations of rabbit and guinea-pig heart and atria, Life Sci. 10, 623. Durotoye, A.O. and L.A. Salako, 1972, The effect of dehydroemetine on isolated guinea-pig atria, Brit. J. Pharmacol. 44,723. Feinberg, H., L.N. Katz and E. Boyd, 1962, Determinants of coronary flow and myocardial oxygen consumption, Amer. J. Physiol. 202, 45. Gerolo, M., H. Feinberg and L.N. Katz, 1959, Coronary flow and oxygen consumption of the myocardium in conditions of hypocapnia, Atti. Soc. Ital. Cardiol. 21, 58. Gonz~ilez de Cossi'o, A., 1960, Electrocardiographic changes under therapy with Ro 1-9334, a synthetic raeemic 2-dehydroemetine, Rev. Inst. Med. Trop. Sao Paulo 2, 313. Grayson, J., 1967, Heat exchange methods in the assessment of blood flow and heat production in solid organs, Gastroenterology 52, 391. Grayson, J. and D. Mendel, 1961, Myocardial blood flow in the rabbit, Amer. J. Physiol. 200, 968. Grayson, J. and J.R. Parratt, 1966, A species comparison of the effects of changing perfusion pressure on blood flow and heat production in the myocardium, J. Physiol., London 187,465. Grayson, J., R.L. Coulson and B. Winchester, 1971, Internal calorimetry assessment of myocarial blood flow and heat production, J. Appl. Physiol. 30, 251. Herrero, J., A. Brossi, M. Faust and J.R. Frey, 1960, Preliminary experimental and clinical results with a new syn-
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L.A. Salako, A.O. Durotoye, Dehydroemetine and the heart
thetic emetine-like compound, Ann. Biochem. Exptl. Med. 20, 475. Lister, D.J., 1968, Delayed myocardial intoxication following the administration of dehydroemetine hydrochloride, J. Trop. Med. Hyg. 71,219. Parratt, J.R. and R.M. Wadsworth, 1970, The effect of catecholamine infusions on myocardial blood flow, metabolic heat production and on general haemodynamics, before and after alprenolol (H56/28) in anaesthetised cats, Brit. J. Pharmacol. 38, 554.. Parratt, J.R. and E. Winslow, 1971, Cardiovascular pharmacology of quazodine (MJ-1988), with particular reference to effects on myocardial blood flow and metabolic heat production, Brit. J. Pharmacol. 42, 193.
Powell, S.J., A.J. Wilmot, I.N. MacLeod and R. Elsdon-Dew, 1967, A comparative trial of dehydroemetine and emetine hydrochloride in identical dosage in amoebic liver abscess, Ann. Trop. Med. Parasit. 61,26. Salako, L.A. and A.O. Durotoye, 1971, Observations on the hypotensive response to dehydroemetine, European J. Pharmacol. 14,200. Salako, L.A. and A.O. Durotoye, 1972, The electrocardiogram in acute dehydroemetine intoxication, Cardiovascular Res. 6, 150. Salem, H.H. and H. Abd-Rabbo, 1964, Dehydroemetine in acute amoebiasis, Trans. Roy. Soc. Trop. Med. Hyg. 58, 539.