Role of the oscillatory afterpotentials in the arrhythmogenic action of histamine

Electrophysiological characterization of histamine receptor subtypes in sheep cardiac Purkinje fibers

The histamine-receptor-subtype-mediated effects on action potentials of electrically driven and spontaneously active isolated sheep cardiac Purkinje fibers were investigated using H1- and H2-selective agonists and antagonists. In electrically stimulated Purkinje fibers, histamine (3 mumol/l) increased the action potential plateau height, decreased the action potential duration measured at a repolarization level of -60 mV and enhanced the pacemaker activity. These effects were abolished by the H2-selective antagonist cimetidine (30 mumol/l), but were not impaired by the H1-selective antagonist dimetindene (0.3 mumol/l). In spontaneously active Purkinje fibers, histamine (10 mumol/l) increased the spontaneous rate by 24%, the slope of diastolic depolarization by 45% and shortened the duration of the diastole by 32% of the respective control measurements. These effects were blocked by 30 mumol/l cimetidine, but remained unchanged in the presence of 0.3 mumol/l dimetindene. Concentration-response curves of histamine were shifted to the right by approximately 2 logarithmic units in the presence of 30 mumol/l cimetidine, but were not influenced in the presence of 0.3 mumol/l dimetindene. The H2-selective agonist impromidine (0.001-0.3 mumol/l) had similar actions as histamine on spontaneously active Purkinje fibers, while the H1-selective agonist 2-(2-pyridyl-)ethylamine was ineffective. It is concluded that the pronounced stimulatory action of histamine on spontaneous activity in sheep cardiac Purkinje fibers is exclusively mediated by H2 receptors.

Histamine and abnormal automaticity in barium- and strophanthidin-treated sheep Purkinje fibers

We used intracellular microelectrodes to study the effects of histamine on both normal and abnormal automaticity in sheep Purkinje fibers. Histamine, dimaprit and 4-methylhistamine caused a similar reduction of the action potential duration in driven Purkinje fibers. Histamine (10-6 M) induced spontaneous activity in p previously quiescent preparations more often than did equimolar concentrations of dimaprit and 4-methylhistamine. The effects of histamine on automaticity were enhanced in the presence of barium In fact histamine, at concentrations which were unable to induce automaticity in normal preparations, induced it in the presence of barium. In Purkinje fibers manifesting barium-induced automatic activity, histamine (10-7--10-6M) significantly increased the average number of spontaneous action potentials and shortened the time of their appearance. In the same range of concentrations, histamine dose-dependently increased the iological manifestation of calcium overload. Histamine (10-6--10-4M) increased the OAP amplitude of strophanthidin -treated Purkinje fibers, eventually inducing triggered extrabeats. All these previously described effects were selectively blocked by cimetidine (10-5 M). It is concluded that histamine may induce cardiac arrhythmias under conditions of calcium overload and that this effect may be due to induction or enhancement of oscillatory afterpotentials.

Histamine enhances hypoxia-induced ventricular arrhythmias in isolated rat hearts

1. The effects of hypoxia, histamine-receptor agonist perfusion, and their combination on cardiac rhythm were studied in isolated rat hearts. 2. While hypoxia induced a high incidence of ventricular tachycardia or fibrillation, only a few preparations developed ventricular arrhythmias in response to perfusion with high concentration of histamine, 2-pyridylethylamine or impromidine. 3. The times of onset of hypoxia-induced ventricular arrhythmias were significantly shortened by perfusion with either histamine, 2-pyridylethylamine or impromidine. The accelerated occurrence of hypoxia-induced ventricular arrhythmias by histamine was significantly abolished by pretreatment with either diphenhydramine or cimetidine. 4. The results indicate that hypoxia and histamine can increase ventricular vulnerability of the rat heart to each other. It is also suggested that the arrhythmogenic actions of histamine in hypoxic rat hearts are mediated by both histamine H1-and H2-receptors.

Histamine attenuates the arrhythmogenic effects of norepinephrine in hearts of spontaneously hypertensive rats

A potential physiological role for cardiac histamine and its interaction with norepinephrine were investigated in isolated left ventricles from spontaneously hypertensive rats (SHR). Prior to drug administration, left ventricle-to-body weight ratios and spontaneous firing rates (beats per min) were significantly increased in SHR ventricles vs. age- and sex-matched controls (WKY). Also, action potential duration was significantly prolonged in SHR at all levels of repolarization. In all hearts, norepinephrine (10(-7)-10(-4) M) increased spontaneous rate and the percent incidence of arrhythmias. The H2-receptor antagonist cimetidine (10(-5) M) potentiated the rate and arrhythmogenic effects of norepinephrine in SHR and, to a lesser extent, in WKY preparations; propranolol (10(-6) M) reduced them. Histamine (10(-7) M) also inhibited the norepinephrine-induced increase in arrhythmias in SHR, but not in WKY. The attenuation of adrenergically induced rhythm disturbances by histamine and their potentiation by cimetidine in hypertensive hearts support the hypothesis that histamine plays a role as a postjunctional modulator of adrenoceptor function in a setting of hypertension and myocardial hypertrophy.

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.