Antagonistic effect between histamine and norepinephrine on isolated guinea-pig right atrium

Biphasic effect of histamine on spontaneously beating rat atria

1. Effect of histamine (HA) (10(-4)-10(-2) M) on spontaneously beating rat atrium was investigated. HA produced a biphasic effect which is composed of an initial negative chronotropy followed by a positive chronotropic phase, at each single dose. 2. At a submaximal dose of HA (3 x 10(-3) M) we have investigated the effects of some antagonists on the biphasic effect profile. 3. Propranolol (10(-7) M) depressed the secondary phase by abolishing the biphasic pattern of the effect of HA leading it to a monophasic one (negative chronotropy). 4. On the other hand, combination with atropine (10(-7) M) was shown to reduce the negative inotropic effect (in the secondary phase) which was produced by HA in the presence of propranolol. 5. The initial negative chronotropic phase was significantly enhanced by propranolol and reduced by pheniramine (10(-6) M). 6. Pheniramine also potentiated the positive chronotropy at the secondary phase of the response, but cimetidine (10(-4) M) has no significant effect on both phases. 7. Theophylline (10(-4) M) abolished the initial negative chronotropic phase, but did not influence the secondary phase. Dipyridamole (10(-5) M) did not affect the secondary phase of the response of HA, but increased the initial negative chronotropic effect.

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.