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

Abstract

Effects of 5x10-5 of dibenamine, 10-5 of chlorpromazine and yohimbine were studied on the non-pacemaker potentials of the isolated atria of the rabbit pretreated with reserpine. Competitive effects of epinephrine, norepinephrine and acetylcholine against these adrenolytics were compared between reserpinized and untreated atria. The action potentials in the isolated atrium of the reserpinized rabbit showed the considerable decrease in rate and the prolongation of the 90% repolarization time, especially the phase 3 of the repolarization. The initial rate-increasing effect of dibenamine, and the rate-decreasing effect of dibenamine, chlorpromazine and yohimbine were manifested more markedly in the reserpinized atrium than in the untreated one. The lag time needed for the standstill of the atrium was shortened by the reserpinization. The application of the adrenolytics produced less prolongation of the depolarization time and less frequent appearance of the notch on the depolarization phase in the reserpinized atrium than in the untreated one. More marked prolongation of repolarization phase, especially the phase 3, was observed in the reserpinized preparation. The reserpinized atrium in which the spontaneous action potentials had been abolished by the adrenolytics failed to restart by washing-out, but was easily restarted by epinephrine or norepinephrine as well as the untreated one. However, the repetitions of the addition either amine and washing-out recovered the depressed transmembrane potential more effectively in the reserpinized atrium than in the untreated one. Especially, the prolonged repolarization time of the reserpinized atrium was fully recovered to the normal level. However, the recovery by epinephrine or norepinephrine was also incomplete and transient in the reserpinized atria. The action potentials in the reserpinized atria disappeared again at the variable time-length after the addition as well as in the untreated one. The addition of acetylcholine restarted the action potential which had been abolished by the adrenolytics in the reserpinized atria. The decreased resting potential, the depressed amplitude and prolonged depolarization time of the action potential were completely recovered to initial level, but the prolonged repolarization time and decreased rate of the action potential failed to recover. The recovering effect of acetylcholine was longer lasting than that of epinephrine or norepinephrine. The prolongation of the repolarization phase was effectively restored to the normal level by the combined addition of acetylcholine and norepinephrine. Occasionally, the addition of norepinephrine produced a hyperpolarization at the phase following the repolarization. From the results described above, it is assumed as a possibility that acetylcholine regulates and maintains the depolorization phase of the action potential in the rabbit''s atrium, while norepinephrine does the repolarization one.