Critical appraisal of secondary prevention after myocardial infarction. What have beta-blockers to offer in the experimental situation?

This contribution reviews the beneficial effects of beta-adrenoceptor blocking drugs in experimental myocardial ischaemia (infarction). These effects include the ability to reduce early, life-threatening ventricular arrhythmias, improvement in ultimate long-term survival from an ischaemic insult with or without subsequent reperfusion and a reduction in the extent and degree of myocardial (and microvascular) ischaemic damage. Most of the evidence for protection, however, comes from studies in which drugs have been administered before the onset of ischaemia. Much less is known about the effects of intervening with beta-blocking drugs after the commencement of the ongoing infarction process. Although beta-blockers have a great deal to offer in the experimental situation there remains the strong possibility that other approaches might be able to offer even more. One promising approach is the combination of a beta-blocking drug with a specific inhibitor of thromboxane synthesis.

The effects of ethanol and lead, alone and in combination, on the severity of arrhythmias induced by coronary artery occlusion, and by noradrenaline, in anaesthetised rats

The aim of this study was to investigate whether chronic (3 or 10 months) administration, via the drinking water, of lead (25 ppm) and/or ethanol (25% v/v) altered the susceptibility of the heart to arrhythmias induced either by coronary artery occlusion or noradrenaline infusion in pentobarbitone-anaesthetised male Sprague-Dawley rats. The cardiac effects of acute intravenous infusions of ethanol (17, 33 and 66 mg kg-1 min-1) were also measured. Chronic exposure to ethanol and/or lead in the drinking water had no marked effect on the severity of arrhythmias occurring within the initial 30 min of coronary artery occlusion. In control rats and in those administered ethanol and/or lead for 10 months, noradrenaline (16 micrograms kg-1 min-1 given IV 1 h post-occlusion for a 15-min period) induced a similar number of ectopic beats during the infusion period, although these arrhythmias persisted beyond the infusion period in treated animals only. There was a significant accumulation of lead in the bone but not in the blood of lead-treated rats. Blood ethanol concentrations varied considerably between animals, ranging from 0 to 319 mg %. Ethanol (66 mg kg-1 min-1) given acutely and yielding a blood concentration of 174 mg % had a slight antiarrhythmic effect in this model.

Stereospecific antiarrhythmic effect of opioid receptor antagonists in myocardial ischaemia

The effects of the stereoisomers of two different antagonists at opioid receptors were examined on the ventricular arrhythmias that result from acute coronary artery ligation in anaesthetized male rats. (-)-Mr 1452(but not the (+)-isomer, Mr 1453) reduced, in a dose-dependent manner, the number of ventricular ectopic beats and the incidence or duration of both ventricular tachycardia and fibrillation. (-)-WIN 44,441-3 (but not its (+)-isomer, WIN 44,441-2) had a similar protective effect in ischaemia. These results suggest that antagonism of the effects of endogenous opioid peptides at specific receptors results in reduced severity of arrhythmias in myocardial ischaemia.

The effects of the protease inhibitor, aprotinin, on the course of shock induced by endotoxin in cats

The administration of endotoxin derived from Escherichia coli to anaesthetized cats resulted, within the first 5 min, in an initial increase in right atrial pressure and a reduction in systemic arterial blood pressure. Over the next 2 h shock was characterized by a reduced cardiac output, tachycardia, reduced arterial pH and an increased level of lactate. The survival rate at the end of the 8 h experimental period was only 10%. The protease inhibitor aprotinin (Trasylol), given as a continuous intravenous infusion 1000 kallikrein inhibitor units (k.i.u.) kg-1h-1 together with a bolus of 40,000 k.i.u. kg-1, significantly inhibited the severity and incidence of the initial endotoxin response (increase in right atrial pressure and systemic hypotension), perhaps suggesting the direct or indirect involvement of kinins. Aprotinin did not reduce the delayed effects of endotoxin (sustained reduction in cardiac output, lacticacidosis), nor did it improve survival at 8 h. Indeed, there was some evidence that aprotinin exaggerated the delayed effects of endotoxin in this model.

The effects of chronic low lead treatment and hypertension on the severity of cardiac arrhythmias induced by coronary artery ligation in anesthetized rats

The aim of this study was to investigate whether chronic (1, 3, 6, 12, or 16 months) low lead (5 or 25 ppm), administered as lead acetate in the drinking water, commencing either after weaning or from conception, altered the susceptibility of the heart to arrhythmias induced by coronary artery ligation in pentobarbitone anesthetized male Sprague-Dawley rats. The cardiac effects of chronic (3 or 12 months) administration of lead (25 ppm) were also examined in spontaneously hypertensive rats. Treatment from weaning of normotensive rats with 5 or 25 ppm lead for periods of 1, 3, or 6 months had no statistically significant effect on the severity of ischemia-induced arrhythmias. Each of the groups treated from conception with 5 or 25 ppm lead for periods of 1, 12, or 16 months exhibited a higher incidence of ventricular tachycardia than the appropriate control group, but the difference was statistically significant only in the case of animals treated with 25 ppm lead for 12 months. The incidence of ventricular fibrillation was significantly higher in rats treated from conception for 16 months with 5, but not with 25 ppm lead. Spontaneously hypertensive rats treated with 25 ppm lead for 3 but not for 12 months had significantly more ectopic beats than control Sprague-Dawley rats. Lead treatment for the longer exposure periods only caused significant accumulation of lead in the blood whereas all lead treatments resulted in a marked accumulation in the bone. We conclude that chronic exposure to low concentrations of lead does not consistently alter susceptibility of the heart to ischemia-induced arrhythmias in anesthetized rats

AH23848, a thromboxane antagonist, suppresses ischaemia and reperfusion-induced arrhythmias in anaesthetized greyhounds

The effects of the thromboxane antagonist AH23848 (1 mg kg-1 i.v.) were examined in anaesthetized greyhounds prepared for occlusion of the left anterior descending coronary artery with subsequent reperfusion after 40 min of myocardial ischaemia. Pretreatment with AH23848 30 min before coronary artery occlusion reduced the number of ischaemia-induced extrasystoles to 339 +/- 111 compared with 736 +/- 153 in the control group. The incidence of ventricular fibrillation following reperfusion was also markedly reduced; 25% compared with 88% in the controls. Late intervention with AH23848, 25 min after the onset of myocardial ischaemia did not significantly alter the incidence of reperfusion-induced ventricular fibrillation; 70% of the control group died and 60% of those that received AH23848. The ischaemia-induced release of thromboxane A2 and prostacyclin was not altered by pretreatment with AH23848. The results suggest that blockade of thromboxane receptors before myocardial ischaemia is an effective antiarrhythmic strategy in this model.

Effects of a combination of metoprolol and dazmegrel on myocardial infarct size in rats

The effects of acute pretreatment with metoprolol, dazmegrel and a combination of these two drugs has been examined on myocardial infarct size in rats. Ischaemic damage was assessed 4 h after coronary artery occlusion in anaesthetized rats and after 48 h of ischaemia in conscious rats. Infarct size was measured histochemically (by using periodic-acid-Schiff diastase reaction for glycogen) and by standard histological examination (haematoxylin and eosin stain). There was some evidence of protection of the myocardium by metoprolol following 4 h of ischaemia (determined histologically) but this was not apparent 48 h after occlusion. When given alone, dazmegrel had no significant effects on infarct size assessed by either method. A clear reduction in the extent of glycogen depletion and histological damage was observed with the combination of metoprolol and dazmegrel 48 h after the onset of ischaemia. This protection was seen to occur in the horizontal plane of the heart, preventing the extension of the infarct towards the posterior wall of the left ventricle and showing some salvage of the epicardial surfaces.

The effects of metoprolol and dazmegrel, alone and in combination, on arrhythmias induced by coronary artery occlusion in conscious rats

The effects of metoprolol and the thromboxane synthetase inhibitor dazmegrel, alone and in combination, were examined in a model of coronary artery occlusion in conscious rats. In a dose (2 mg kg-1), intravenously, that resulted in a marked bradycardia (of 50-80 beats min -1) metoprolol did not influence the incidence or severity of the ventricular arrhythmias that occur in the first 20 min following occlusion, nor did it improve survival (assessed at both 20 min and 16 h). In a dose (5 mg kg-1), intravenously, that in another conscious rat model involving tissue hypoperfusion inhibited thromboxane production, dazmegrel also did not modify ischaemic arrhythmias or survival. In contrast, metoprolol and dazmegrel (2 mg kg-1 and 5 mg kg-1 i.v.) when given together prior to coronary artery occlusion, produced a significant reduction in mortality both at 20 min and 16 h (e.g. from 60-75% in the control, metoprolol alone and dazmegrel alone groups and only 25% in the combined-treatment group). This was due to a decrease in the incidence of terminal ventricular fibrillation. The results suggest that a combination of beta-adrenoceptor blocking drug with a drug that inhibits thromboxane synthesis may offer more protection against ischaemia-induced ventricular fibrillation than either drug used alone. They suggest a role for both catecholamines and thromboxane in the genesis of ischaemia-induced ventricular fibrillation.

Relationships between the severity of myocardial ischaemia, reperfusion-induced ventricular fibrillation, and the late administration of dazmegrel or nifedipine

We examined the effects of late administration of the thromboxane synthetase inhibitor dazmegrel (UK 38485) and the calcium channel blocker nifedipine in anaesthetised greyhounds subject to occlusion of the left anterior descending coronary artery with reperfusion after 40 min of ischaemia. Administration of dazmegrel, 3 mg/kg i.v., or nifedipine, 5 micrograms/kg + 1 microgram kg-1 min-1 i.v., 25 min after coronary artery occlusion failed to reduce the incidence of reperfusion-induced ventricular fibrillation (controls, 70%; dazmegrel, 50%; nifedipine, 70%; n = 10). Measurement of plasma prostanoid concentrations indicated that within 5 min of receiving dazmegrel there was a significant reduction in thromboxane B2 concentrations in the local coronary vein draining the ischaemic myocardium. The results suggest that the occurrence of reperfusion-induced ventricular fibrillation depends upon the severity of changes occurring during ischaemia. Analysis of various factors suggested that the number of ischaemia-induced arrhythmias, heart rate, and the magnitude of changes in local coronary venous PO2 may be important predictors of reperfusion-induced ventricular fibrillation.

The relationship between coronary artery occlusion-induced arrhythmias and myocardial cyclic nucleotide levels in the anaesthetized rat

The aims of this study were to determine whether a relationship exists between the occurrence of coronary artery occlusion-induced arrhythmias in the anaesthetized rat and the levels of cyclic AMP and cyclic GMP in both normal and ischaemic myocardium, and to assess whether such arrhythmias were modified by pretreatment with the phosphodiesterase inhibitors, quazodine and isobutyl methylxanthine (IBMX), or with the butyryl derivatives of cyclic AMP and cyclic GMP. At 5 min after coronary artery ligation (when only a few arrhythmias had occurred) both cyclic AMP and cyclic GMP levels were elevated in normal myocardium whereas in ischaemic tissue only cyclic AMP was raised. As the peak of the arrhythmic activity and after cessation of the arrhythmias, i.e. at 10 and 30 min post-ligation respectively, levels of both nucleotides had fallen in ischaemic although not in normal tissue. The severity of these occlusion-induced arrhythmias was exacerbated by pretreatment intravenously with quazodine, IBMX, dibutyryl cyclic AMP and dibutyryl cyclic GMP. Pretreatment with IBMX was also shown to elevate significantly both cyclic AMP and cyclic GMP content of left ventricular tissue before occlusion. None of the drug pretreatments markedly affected mean arterial blood pressure but heart rate was significantly increased following quazodine and IBMX administration. We conclude that in the pentobarbitone-anaesthetized rat the occurrence of occlusion-induced arrhythmias was not accompanied by a rise in cyclic nucleotide content of the ischaemic myocardium but agents which may elevate either myocardial cyclic AMP or cyclic GMP levels exacerbate such arrhythmias.

The effects of verapamil, prenylamine, flunarizine and cinnarizine on coronary artery occlusion-induced arrhythmias in anaesthetized rats

In male rats, anaesthetized with pentobarbitone, ligation of the main left coronary artery causes an early phase of ventricular arrhythmias which last about 30 min. In approximately 60% of control animals, ventricular fibrillation occurs but since spontaneous reversion to sinus rhythm may occur, mortality is of the order of 30%. When administered intravenously 15 min prior to ligation, verapamil (0.01 and 0.05 mg kg-1), prenylamine (0.5 mg kg-1), flunarizine (0.1, 0.25, 0.5 and 1.0 mg kg-1) and cinnarizine (0.25, 0.5 and 1.0 mg kg-1) protected against these arrhythmias. Higher doses of verapamil (0.1 and 0.5 mg kg-1), prenylamine (5 mg kg-1) and flunarizine (2.5 mg kg-1) did not afford a similar protection and mortality was increased to or above control values. Death was due in prenylamine-treated rats to atrioventricular block leading to asystole whereas in those administered verapamil or flunarizine it was a consequence of persistent ventricular fibrillation. Prior to ligation, a sustained fall in mean arterial blood pressure was observed only following the administration of the highest doses of prenylamine, flunarizine and cinnarizine. Heart rate was reduced by administration of only the highest dose of prenylamine. These studies show that although the four calcium antagonists studied, i.e. verapamil, prenylamine, flunarizine and cinnarizine do suppress ischaemia-induced arrhythmias, this protective effect may be limited to a narrow concentration range.

Aspects of the cardiovascular pharmacology of exaprolol

This study confirms that exaprolol is a potent beta-adrenoceptor antagonist, having a pA2 value of 9.8 for the inhibition of the inotropic and chronotropic responses of guinea-pig isolated atrial preparations to isoprenaline. In the anaesthetized cat, exaprolol blocks both the myocardial stimulatory and vasodilating effects of isoprenaline, suggesting that it is a non-selective antagonist at beta-adrenoceptors. Exaprolol also has direct electrophysiological effects on cardiac (Purkinje) tissue, reducing the rate of rise of phase 0 of the action potential. This direct action together with its marked blockade of beta-adrenoceptors may explain the drug's ability to markedly suppress the ischaemic ventricular arrhythmias that follow coronary artery occlusion in anaesthetized rats.

An investigation into the characteristics of reperfusion-induced arrhythmias in the anaesthetized rat and their susceptibility to antiarrhythmic agents

Reperfusion-induced arrhythmias were elicited in the pentobarbitone-anaesthetized rat by occlusion of the left main coronary artery and subsequent release. These arrhythmias were rapid in onset, occurring within 20 s after release of the ligature, and were of short duration (1-2 min). Their severity was dependent upon the duration of the preceding occlusion. A 5 or 15 min occlusion period produced the most severe arrhythmias on release, the incidence of ventricular fibrillation being 56 and 50% respectively. Evidence that reperfusion had occurred was provided by fluorescein dye distribution and intramyocardial temperature studies. The severity of reperfusion arrhythmias and mortality was unaffected by bilateral vagotomy, beta-adrenoceptor blockade by atenolol (2 mg kg-1 i.v.) or a combination of the two. The incidence of reperfusion-induced ventricular fibrillation was significantly reduced by Org 6001 (which blocks the fast inward sodium current), melperone (which acutely prolongs the cardiac action potential duration) and bepridil (which blocks both fast and slow inward currents). It was unaffected by nitroglycerine and the calcium antagonists verapamil, prenylamine and nifedipine. We have shown that reperfusion-induced cardiac arrhythmias can be consistently elicited in the anaesthetized rat and that they are particularly susceptible to drugs that can block the fast inward sodium current.

Effects of prolonged administration of oxprenolol on severity of ischaemic arrhythmias, enzyme leakage, infarct size, and intracellular cardiac muscle action potentials

We examined the effects of prolonged oral administration of oxprenolol (twice daily for 6 weeks) to male Sprague-Dawley rats. At two times (1 or 16-18 h) after the last oral dose, the rats were anaesthetised and subjected to acute coronary artery ligation, and the severity of the resulting arrhythmias was assessed. Ischaemic damage was measured histochemically (using frozen section analysis by toluidine blue dye in nitrobluetetrazolium ) and by myocardial enzyme release. Cardiac muscle (atria and papillary muscle) was also removed and the transmembrane action potentials recorded using conventional microelectrode techniques. When coronary artery ligation was performed 1 h after the last oral dose (at which time there was evidence of substantial myocardial beta 1-adrenoceptor blockade), there was significant reduction in the severity of early arrhythmias, but no evidence that the severity of ischaemic damage was reduced or that the intracellular cardiac action potentials were modified. No protection was observed when coronary artery ligation was carried out 16 h after the last oral dose of oxprenolol. These results support our previous studies with acutely administered beta-adrenoceptor blocking drugs that myocardial beta-adrenoceptor blockade is the main factor involved in the protection afforded by such drugs against early ischaemic arrhythmias and that other possible effects, such as membrane stabilisation and action potential prolongation, are relatively unimportant in this model.

Prostaglandins and thromboxane in the delayed phase of shock induced by Serratia marcescens endotoxin

The cardiovascular and metabolic effects of an endotoxin derived from Serratia marcescens were examined in anaesthetized, spontaneously-breathing cats. There was a marked initial elevation of right atrial pressure (the result of pulmonary vasoconstriction) and decreases in systemic arterial pressure and in arterial PO2. The 'delayed' effects of endotoxin shock in this species (1-8 h) consisted of a reduced cardiac output and decreased urinary excretion. Blood pressure and myocardial contractility (assessed from measurement of left ventricular (LV) dP/dt and LV end-diastolic pressure) were maintained throughout this phase. There was evidence of a metabolic (lactic) acidosis largely compensated by hyperventilation. Plasma levels (both arterial and mixed venous blood samples) of prostaglandin (PG)E2, PGF2 alpha, 6-keto PGF1 alpha and thromboxane (TX)B2 were measured by radioimmunoassay techniques. Endotoxin administration caused substantial increases in the plasma levels of all four derivatives of arachidonic acid, especially between 1 and 6 h. Separation of the endotoxin-treated cats into survivors and non-survivors showed that the non-survivors had significantly higher circulating levels of PGE2, TXB2 and PGF2 alpha. It is suggested that TXB2 and PGF2 alpha might contribute to some of the detrimental effects of endotoxin (e.g. pulmonary, mesenteric, renal vasoconstriction; platelet aggregation with resulting organ failure) and that prostacyclin may be beneficial in endotoxin shock in this species.

Failure of drugs that selectively inhibit thromboxane synthesis to modify endotoxin shock in conscious rats

The effects of two thromboxane synthetase inhibitors ( dazoxiben and UK 38485) were investigated on the cardiovascular and metabolic effects of Escherichia coli endotoxin infusion in the conscious, unrestrained rat. Infusion of E. coli endotoxin (41.7 ng kg-1 min-1) for 4 h produced a fall in mean arterial pressure, an increase in heart rate, a transient hyperglycaemia (at 1 h) followed by hypoglycaemia (evident at 6 h), an elevation in plasma lactate and a profound thrombocytopenia. The above changes were accompanied by a marked elevation in plasma thromboxane B2 concentrations (e.g. endotoxin-treated 935 +/- 150 pg ml-1 at 1 h compared with pre-endotoxin values of 125 +/- 30 pg ml-1). The administration of either dazoxiben (30 mg kg-1 i.v., given 30 min before starting the endotoxin infusion) or UK 38485 (15 mg kg-1 given 30 min before, and again 4 h after, starting the endotoxin infusion) prevented the rise in plasma thromboxane B2 concentrations. Neither dazoxiben nor UK 38485 prevented the metabolic, cardiovascular or thrombocytopenic effects of endotoxin and did not modify mortality. These results suggest that, although large amounts of thromboxane are generated in response to endotoxin, they do not play an important role in the major pathophysiological consequences of acute endotoxaemia.

The effects of timolol on arrhythmias and prostanoid release during canine myocardial ischaemia and reperfusion

Timolol (50 micrograms kg-1), administered intravenously to chloralose-anaesthetized open-chest greyhounds 30 min prior to occlusion of the left anterior descending coronary artery, reduced heart rate and mean arterial blood pressure. This dose caused a 20 fold increase in the dose of isoprenaline required to increase heart rate by 25 beats min-1. During the first 30 min of myocardial ischaemia the number of extrasystoles in the timolol-treated dogs (327 +/- 179) was less than in the control group (888 +/- 168) and none of the dogs that received timolol fibrillated. The haemodynamic changes induced by coronary artery occlusion (decreased cardiac output and stroke volume, increased peripheral vascular resistance) were similar in both control and timolol-treated dogs as were the increases in PCO2 and decreases in PO2 and pH in blood draining from the ischaemic myocardium. Timolol did not alter the release during myocardial ischaemia, of either thromboxane B2 or prostacyclin (measured as 6-keto PGF1 alpha). Reperfusion-induced ventricular fibrillation occurred in 7 out of 8 control dogs and in 5 out of 10 timolol-treated dogs. The overall survival following occlusion and reperfusion was improved by 10% to 50% by timolol.

The effects of nafazatrom on arrhythmias and prostanoid release during coronary artery occlusion and reperfusion in anaesthetized greyhounds

The effects of the antithrombotic drug nafazatrom (BAY g 6575) were investigated in chloralose-anaesthetized greyhounds subject to coronary artery occlusion and reperfusion. Pretreatment with nafazatrom 10 mg/kg p.o. did not significantly reduce the number of extrasystoles or the incidence of ventricular fibrillation (VF) during the first 30 min occlusion of the left anterior descending coronary artery. However, the incidence of VF resulting from release of a 40-min coronary artery occlusion was markedly reduced (from 88% in the controls to 14% in the nafazatrom group). Both thromboxane B2 (TxB2) and 6-keto PGF1 alpha (breakdown products of TxA2 and prostacyclin respectively) were released from the acutely ischaemic myocardium in control dogs. Nafazatrom did not alter the release of TxB2 but the concentrations of 6-keto PGF1 alpha were elevated in blood draining from both the ischaemic and normal regions of the myocardium. The pronounced anti-fibrillatory effect of nafazatrom during reperfusion of the ischaemic myocardium may be related to the ability of this drug to elevate prostacyclin concentrations in the coronary circulation.

The effect of a selective 5-HT2 antagonist, ketanserin, on the pulmonary responses to Escherichia coli endotoxin

5-Hydroxytryptamine (5-HT, 5-160 microgram kg-1) injected intravenously in pentobarbitone-anaesthetized, open-chest cats caused dose-dependent increases in pulmonary arterial and intratracheal pressures. There was also a marked systemic hypotension and bradycardia. The pulmonary effects were completely prevented by ketanserin (0.2 mg kg-1), a selective 5-HT2 blocking drug. Ketanserin (0.2 mg kg-1) itself lowered arterial pressure (by 30-40 mmHg) but this systemic hypotension was relatively transient. Endotoxin (E. coli) administration resulted in pulmonary hypertension, increases in intratracheal pressure and airways resistance and reductions in lung compliance and in arterial PO2. Only the airways resistance response was modified by ketanserin (0.2 mg kg-1), suggesting a relatively unimportant role for 5-HT in mediating the acute, pulmonary effects of endotoxin in this species. The reductions in arterial (mixed venous) pH and in PO2 that resulted from endotoxin administration were not affected by pretreatment with ketanserin.

The effect of exaprolol (MG 8823) on epicardial ST-segment changes in a feline model of acute myocardial ischaemia

A model is described (anaesthetized, open-chest cats subjected to acute coronary artery occlusion) which allows the effects of drug interventions to be determined on one major electrocardiographic index of myocardial ischaemia. Epicardial ST-segment changes were continuously recorded from five individual sites on the surface of the left ventricle. Coronary artery occlusion (left anterior descending branch) resulted in marked and consistent elevations of the ST-segment in all sites in nearly all experiments. These changes started within 1 min of the onset of ischaemia and reached a peak at between 30 and 60 min; thereafter there was a gradual reduction over the next 4 h. The one significant haemodynamic effect of coronary artery occlusion was an increase in left ventricular (LV) end-diastolic pressure (LVEDP). Ventricular ectopic activity was not pronounced in this model (about 50 ectopic beats over the initial 30 min post-occlusion period). Exaprolol (1.0 mg kg-1, intravenously) a potent beta-adrenoceptor blocking agent with 'membrane stabilising activity', when given 1 h after the onset of ischaemia, reduced heart rate and LV dP/dtmax and increased LVEDP. These effects were prolonged (i.e. little recovery in heart rate 3 h after administration). Exaprolol decreased total ST-segment elevation immediately after administration; this was significantly different from the effect of intravenous saline and lasted for at least 3 h. The effects appeared to be greater at sites of less pronounced ischaemia. Intramyocardial temperature records were taken to indicate a reduction in blood flow to the ischaemic region; however the alleviation of epicardial ST-segment elevation suggests an improved myocardial oxygen demand:supply ratio. Reperfusion was unsuccessfully attempted after a 4 h occlusion period; reperfusion after a shorter period (30 min) resulted in ventricular ectopic activity but no fibrillation.

The effect of beta-adrenoceptor blocking agents, with differing ancillary properties, on the arrhythmias resulting from acute coronary artery ligation in anaesthetized rats

The effects of several beta-adrenoceptor blocking agents, [+), (-) and (+/-)-oxprenolol, p-oxprenolol, practolol, propranolol and timolol) were investigated on the ventricular arrhythmias occurring within the first 30 min of acutely ligating the main left coronary artery in anaesthetized rats. The degree of cardiac and vascular beta-adrenoceptor blockade was also assessed. All the compounds exhibited antiarrhythmic activity under these conditions. The degree of cardiac beta-adrenoceptor blockade required for this protection was less for the cardioselective agents, p-oxprenolol and practolol, than for the non-selective beta-adrenoceptor blocking agents. A comparison of the two isomers of oxprenolol demonstrated that the (-)-isomer markedly suppressed ischaemic arrhythmias (ventricular ectopic beats, incidence and duration of ventricular tachycardia and duration of ventricular fibrillation) more effectively than the (+)-isomer. Compounds possessing intrinsic sympathomimetic activity (ISA) caused less marked haemodynamic changes (in equivalent beta-blocking doses) than those that did not possess this ancillary property. The membrane stabilizing activity of oxprenolol and p-oxprenolol did not appear to contribute to the antiarrhythmic activity of these agents; however, the membrane stabilizing activity of propranolol may contribute to its effectiveness. In all the drugs studied, the main pharmacological property required to suppress early postischaemic arrhythmias is blockade of cardiac beta-adrenoceptors.

Prostacyclin--antiarrhythmic or arrhythmogenic? Comparison of the effects of intravenous and intracoronary prostacyclin and ZK36374 during coronary artery occlusion and reperfusion in anaesthetised greyhounds

The effects of prostacyclin and ZK36374, a more stable analogue of prostacyclin, were examined in greyhounds anaesthetised with chloralose. Intravenous drug administration (100 ng kg-1 min-1) exacergbated the arrhythmias induced by occlusion of the left anterior descending coronary artery. The incidence of ventricular fibrillation (VF) was 50% in both drug groups compared with 10% in controls. An antiarrhythmic effect was observed when a lower dose (5 ng kg-1 min-1) was infused directly into the coronary circulation. The number of extrasystoles occurring during the first 30 min of occlusion was reduced from 720 +/- 136 in the controls to 327 +/- 167 in the prostacyclin group and 309 +/- 110 (p less than 0.05) in the dogs receiving ZK36374. The only haemodynamic change observed in the dogs receiving intracoronary drug infusions was a small decrease in systemic arterial blood pressure. In the intravenous groups systemic hypotension was accompanied by tachycardia, suggesting that there may have been a reflex increase in sympathetic drive. Increased catecholamine release could account for the higher incidence of VF in the dogs treated with intravenous prostacyclin or ZK36374. The intracoronary administration of both drugs markedly reduced the incidence of VF induced by the release of a 40-min coronary artery occlusion. This latter result suggests that the release of endogenous prostacyclin may have protective effects during reperfusion of the ischaemic myocardium.

Pharmacological agents to aid the clinician in combating myocardial ischaemia

This paper summarizes the effects of various drugs (sodium channel inhibitors, alpha and beta-adrenoceptor blocking agents, calcium antagonists and substances that interfere in various ways with the metabolism of arachidonic acid (e.g. aspirin, dazoxiben, nafazatrom) on the early consequences of experimental acute myocardial ischaemia. Particular attention is paid to effects on the serious early arrhythmias that arise within a few minutes of the onset of ischaemia, on those ventricular arrhythmias that result from the subsequent reperfusion of the ischaemic myocardium and on the severity and extent of myocardial ischaemic damage. From these experimental studies conclusions are drawn regarding possible approaches to early intervention with drugs in clinical myocardial infarction. Among those agents where, based on results from studies in animals, early administration should prove beneficial are beta-adrenoceptor blocking drugs, calcium antagonists, drugs that inhibit thromboxane synthesis or its effects at receptor level, agents that 'promote' prostacyclin, nitroglycerin and analgesics with marked antiarrhythmic properties (e.g. meptazinol). However, the answer to the question as to which (if any) of these drugs should be given early in clinical infarction is still dependent upon the results of considerably more experimental and clinical studies.

Cardioprotection with calcium antagonists by suppression of early ischaemia and reperfusion-induced arrhythmias

This paper discusses the relevance of certain animal models for the production of ischaemia and reperfusion-induced ventricular arrhythmias to clinical problems of ischaemia, for example, sudden cardiac death. It then describes experiments in anaesthetised rats subjected to acute coronary artery occlusion in which nifedipine (administered both intravenously and orally) prevented the occurrence of ventricular fibrillation and markedly increased survival. In anaesthetised dogs, a continuous intravenous infusion of nifedipine reduced the severity and incidence of ventricular ectopic activity and also dramatically improved survival on reperfusion fro 12.5% (in the controls) to 88%. The mechanisms of this protection are discussed.