RECOVERING EFFECT OF ACETYLCHOLINE ON THE DEPRESSED ATRIAL TRANSMEMBRANE POTENTIAL OF THE RESERPINIZED RABBIT BY THE ADRENOLYTICS

Abstract

In the previous paper (1) the effects of the adrenolytics on the transmembrane potential of the extirpated rabbit's heart were reported. The adrenolytics exerted depressive effects on the action potential, i.e. decrease in the rate, prolongation of the de- and re- polarization phases and depression of the amplitude with subsequent complete abolition of the atrial non-pacemaker potential. The resting potential was slightly decreased. However, the pacemaker potential, though it was slightly or moderately depressed, continued to fire rhythmically. In this condition addition of adrenaline or noradrenaline in the concentration which exerted a positive inotropic effect on the intact heart restarted the potential rhythm. However, the recovery of the transmembrane potential was transient and incomplete even by addition of higher concentration of either amine.
Physiological role of noradrenaline in the heart (2-5) on the spontaneous automaticity of the atrium has not been hitherto fully elucidated. Though the effects of the adrenolytics on the transmembrane potential of the heart may relate with the adrenergic mechanism, Matsuo et al. (6) have shown the increase of noradrenaline in the rabbit's heart in response to the intravenous injection of dibenamine (10 mg/kg) but not to the same procedure of yohimbine (5 mg/kg) or chlorpromazine (5 mg/kg).
The biphasic responses of the heart to acetylcholine have been observed by several authors (7-14). The restarting effect of acetylcholine on the atrial preparation in which rhythmical contraction had been abolished by quinidine, eserine and paludrine (12, 13) and by the lowering of environmental temperature (14) has been reported.
The present investigation has been attempted to elucidate the mechanism of the standstill of the atrial transmembrane potential induced by the adrenolytics by studying the restarting and recovering effects of adrenaline, noradrenaline and acetylcholine in the isolated atrium of rabbit which had been pretreated with reserpine. It has been also expected to explain the mode of action of the endogenous catecholamine and acetylcholine on the atrial non-pacemaker potential.