Effects of SK&F 93479 on experimentally induced ventricular arrhythmias in dogs, rats and mice

The effects of exogenous histamine in isolated rat hearts

The role of histamine in cardiac physiology and pathophysiology is not clarified, but is dependent on species. The effects of exogenous histamine in Langendorff-perfused rat hearts were investigated. 1 mM, 100, 10, 1 and 0.1 microM of histamine (n = 7 each) as 15 min infusions were employed in a dose-response study, and compared to control perfused hearts (n = 7). In another experimental series, 100 microM histamine (n = 15) was added during reperfusion after 25 min global ischemia, and compared to control ischemia-reperfusion (n = 15). The maximal response to histamine in the dose-response study (100 microM) was an increase of left ventricular developed pressure to 126 +/- 8% of initial value (mean +/- SEM, p < 0.04), and increase of coronary flow to 152+6% (p < 0.02) after 5 min infusion. 100 microM histamine did not significantly influence heart rate or rhythm. The lowest concentration (0.1 microM) did not have effects cardiac performance. Reperfusion with histamine for 2 min after ischemia reduced left ventricular developed pressure to 68 +/- 10% of initial value versus 116+17% in ischemic controls (p < 0.05), and increased left ventricular end-diastolic pressure to 24 +/- 8 mmHg compared to 6 +/- 2 mmHg in controls (p < 0.04). Left ventricular pressures were similar in hearts reperfused with histamine and in ischemic controls for the rest of the observation. Coronary flow increased during reperfusion in hearts given histamine. Histamine had a dose-dependent positive inotropic and vasodilatory effect in isolated rat hearts. Exogenous histamine had only minor effects on post-ischemic cardiac function.

Histamine release and its effects in ischaemia-reperfusion injury of the isolated rat heart

Histamine is released from the heart during ischaemia-reperfusion injury. As histamine has cardiac effects, we investigated the role of histamine in ischaemia-reperfusion injury of isolated rat hearts. A Langendorff-model with 30 min global (37 degrees C) ischaemia followed by 60 min reperfusion was employed. The effects of ischaemia alone (n = 10, group 1.1 + n = 10, group 2.1, 2 different series), and ischaemia with H1- and H2-receptor blockade with cimetidine (10 microM, n = 10), chlorpheniramine (10 microM, n = 8), terfenadine (10 microM, n = 8), and promethazin (10 microM, n = 9), or both cimetidine and chlorpheniramine (n = 8), were studied. Histamine was measured in the coronary effluent and cardiac tissue of group 1.1. Release of histamine increased from 6.5 +/- 1 pmol min-1 before ischaemia to 19 +/- 3 pmol min-1 at the start of reperfusion. Ischaemia decreased left ventricular developed pressure to 18 +/- 11% (1.1) and 50 +/- 11% (2.1) of initial value (mean +/- SEM) at the start of reperfusion. Left ventricular end-diastolic pressure increased from 0 to 79 +/- 8 mmHg (1.1) and 39 +/- 9 (2.1) mmHg, while left ventricular systolic pressure was unchanged (101 +/- 12% in 1.1 and 101 +/- 10% in 2.1). Severe arrhythmias were induced in 90 (1.1) and 30 (2.1)% of the hearts, while coronary flow decreased during reperfusion. H2-blockade did not modify the changes in left ventricular pressures, coronary flow, or heart rate induced by ischaemia. Three different H1-blockers increased left ventricular systolic pressure, inhibited the decrease of developed pressure, attenuated the increase of end-diastolic pressure, and totally inhibited reperfusion arrhythmias. The effect of both blockers together was similar to that of H1-blockers alone. Coronary flow was increased during reperfusion in two of the groups with H1-blocker compared with ischaemic controls. Increased release of histamine from ischaemic-reperfused rat hearts concurred with depression of left ventricular function and arrhythmias during early reperfusion. Cardiac dysfunction during reperfusion was attenuated by three different H1-receptor blockers.

Arrhythmia models in the rat

The rat has been used extensively for arrhythmia studies, and a large number of models have been developed, which include chemical, electrical, and pathological procedures for including arrhythmias. Such models have been used to investigate mechanisms for inducing arrhythmias as well as the actions of antiarrhythmic drugs. Arrhythmia models in rats are of particular importance in examination of the antiarrhythmic action of various drugs.

[Histamine effects on the heart with special reference to cardiac side effects of H2 receptor antagonists]

The existence of cardiac h1- and h2-receptors is evidenced by pharmacologic studies. Despite of the relatively high content of cardiac histamine it is not clarified whether histamine actually plays a physiologic role - apart from pharmacologic effects - in the regulation of myocardial function and coronary blood flow. Under pathophysiologic conditions (during anaphylaxis, surgical procedures, accidents, stress etc.), however, when a local or systemic histamine release occurs both hemodynamic and arrhythmogenic effects are evident. Numerous studies in animal models conclusively demonstrated a role of cardiac histamine as a major mediator of serious arrhythmias. Consequently, a combination of h1- and h2-receptor antagonists (f.e. Dimetinden/Cimetidin) was recommended as a prophylactic treatment against severe anaphylaxis including life-threatening arrhythmias for cardiac patients at risk. There is pharmacologic evidence of both a positive inotropic and chronotropic effect in the human heart via h2-receptor and stimulation of adenylate cyclase. Histamine-induced coronary effects such as vasoconstriction via h1-receptor and coronary dilatation via h2-receptor are not yet sufficiently validated. Studies on the human heart in vitro using coronary strips from explanted hearts and in vivo investigations on the intact coronary system yielded conflicting results. H2-receptor blocking agents cimetidine, ranitidine and famotidine have qualitatively a different pharmocodynamic spectrum of side effects due to differences in chemical structure. Data on cardiac arrhythmias are mostly associated to cimetidine. Symptomatic bradycardia were reported for both ranitidine and cimetidine. A possible negative inotropic effect of famotidine, although presently not validated, requires further studies. Causative and adverse side effects of cimetidine on the cardiovascular system, however, are to be expected extremely seldom due to easily reversible competetive h2-receptor binding. For prophylaxis rapid intravenous injections of h2-blockers, particularly in elder patients with cardiac diseases, should be avoided. Compared to cimetidine, a tendency of explainable difference seems to become apparent for ranitidine and famotidine due to higher receptor affinity.

Ventricular arrhythmias parallel cardiac histamine efflux after coronary artery occlusion in the dog

Release of cardiac histamine by immunologic and pharmacologic stimuli is known to provoke ventricular arrhythmias. Augmented histamine efflux from ischemic myocardium has been proposed but remains controversial. The purpose of this study was to determine whether cardiac histamine efflux is precipitated by coronary artery occlusion and if so, whether histamine efflux is associated with the development of early ischemic ventricular arrhythmias. The left anterior descending coronary artery was occluded while recording a continuous electrocardiogram and coronary sinus blood was sampled frequently during the first 30 min of coronary artery occlusion in pentobarbital-anesthetized, open-chest dogs. Coronary sinus histamine concentration rose from a mean baseline of 0.06 +/- 0.10 ng/ml (+/- SD) before coronary artery occlusion to a mean peak of 0.61 +/- 0.40 ng/ml after coronary artery occlusion (p less than 0.0001; n = 14). The median peak coronary sinus histamine concentration was significantly greater in dogs that suffered ventricular fibrillation after coronary artery occlusion (n = 4) than in those that did not (n = 10) (0.86 ng/ml vs. 0.37 ng/ml; p = 0.05). The area under the coronary sinus histamine concentration-vs.-time curve ("total cardiac histamine efflux") correlated directly with the total number of ventricular premature contractions during the first 30 min after coronary artery occlusion (r = 0.81; p less than 0.005; n = 10), and with infarct size (r = 0.91; p less than 0.01; n = 6). Thus, during acute myocardial ischemia, the coronary sinus histamine concentration increases simultaneously with the development of early ischemic ventricular arrhythmias and in proportion to their severity.

Ventricular histamine concentrations in naive and morphine-treated rats during acute myocardial ischaemia

The ventricular histamine concentrations of naive and morphine-treated rats subjected to acute left coronary artery ligation were examined. In naive animals, there was a significant increase in the right ventricular histamine level at 5 min following ligation, but not at 3 or 10 min. Left ventricular histamine concentrations tended to decrease, but the changes were not statistically significant. In shamoperated rats, neither acute nor chronic morphine treatment significantly altered either right or left ventricular histamine levels. Acute morphine treatment also did not significantly affect the ventricular histamine content at 5 min following coronary artery ligation. However, both right and left ventricular histamine concentrations were found to be significantly lower in chronic morphine-treated rats than in the naive animals when they were subjected to acute myocardial ischaemia. If the hypothesis that histamine release may contribute to the genesis of early ventricular arrhythmias resulting from acute myocardial ischaemia is accepted, the present findings suggest that the previously reported decreased incidence and delayed onset of early ventricular arrhythmias induced by acute left coronary artery ligation in chronic morphine-treated rats may be attributed to the reduced ventricular histamine concentrations.

Ventricular histamine concentrations and arrhythmias during acute myocardial ischaemia in rats

The relation between ventricular histamine concentrations and the occurrence of early ventricular arrhythmias during acute myocardial ischaemia was investigated in pentobarbitone-anaesthetized rats. There was significant decrease in the left, but not the right, ventricular histamine level at 5 min following acute left coronary artery ligation. Pretreatment with rhodanine caused remarkable reduction in ventricular histamine concentrations as well as significantly lower incidence and slower onset of ventricular tachycardia and fibrillation resulting from acute myocardial ischaemia. On the contrary, aminoguanidine pretreatment did not significantly alter ventricular histamine levels nor did it influence the occurrence of early ventricular arrhythmias induced by coronary artery ligation. The responses of blood pressure and heart rate to acute coronary artery ligation were not noticeably affected by rhodanine or aminoguanidine pretreatment. These findings support the hypothesis that histamine release from cardiac tissues may contribute to the genesis of early ventricular arrhythmias, but not to the changes in blood pressure and heart rate, during acute myocardial ischaemia.

Cardiovascular effects of ranitidine and cimetidine during acute myocardial ischaemia in anaesthetized dogs

The effects of ranitidine and cimetidine on ventricular fibrillation threshold and haemodynamics were studied in pentobarbitone-anaesthetized dogs subjected to acute coronary artery ligation. These drugs did not significantly change the ventricular fibrillation threshold nor haemodynamics before coronary artery ligation, except for remarkable haemodynamic depression by ranitidine 1 mg/kg. Ligation of the left anterior descending coronary artery reduced the ventricular fibrillation threshold, decreased systemic and left ventricular pressures and myocardial contractility, and slightly increased heart rate. Pretreatment with ranitidine 0.25 or 1 mg/kg, or with cimetidine 2 mg/kg, significantly abolished the reductions in ventricular fibrillation threshold, but did not noticeably alter the haemodynamic changes. These findings further support the hypothesis that histamine release may contribute to the increased ventricular vulnerability resulting from acute myocardial ischaemia. However, the role of histamine in the haemodynamic responses to coronary artery ligation remains obscure.

Effects of ranitidine and cimetidine on experimentally induced ventricular arrhythmias in anaesthetized rats

The effects of two histamine H2-receptor antagonists, ranitidine and cimetidine, on ventricular arrhythmias induced by acute coronary artery ligation and by aconitine infusion were studied in pentobarbitone-anaesthetized rats. The changes in arterial blood pressure and heart rate were also observed. It was found that both drugs significantly reduced the incidence, and prolonged the time of onset, of ventricular tachycardia and ventricular fibrillation following acute coronary artery ligation; however, they did not significantly alter the incidence or time of onset of ventricular dysrhythmias caused by aconitine infusion. These findings further support the hypothesis that histamine release may contribute to the genesis of early ventricular arrhythmias resulting from acute myocardial ischaemia. Since the decreased blood pressure induced by coronary artery ligation was not significantly prevented by pretreatment with either histamine H2-receptor blocker, this suggests that histamine may not be responsible for the blood pressure changes during acute myocardial ischaemia.