To elucidate the postnatal development of electrogenesis in the rat ventricular myocardium, we investigated specimens of the right ventricles of rats of different ages (newborn, 2 days, 5 days, 10 days and adult) by means of glass microelectrodes and a programmable stimulator. The resting membrane potential rose from -82 mV (newborn) to -89.6 mV (adult). During the first 10 postnatal days the plateau phase of the action potential (AP) shortened from 46 ms to 16 ms; in adulthood its duration was 9.6 ms. An increase in the stimulation frequency from 2 Hz to 10 Hz led to pronounced shortening of the myocardial AP of newborn and 2-day-old rats; with advancing age the influence of a mounting frequency diminished and the myocardium of adult rats was frequency-insensitive. The first AP evoked after a rest interval in the myocardium of newborn animals was prolonged proportionally to the logarithm of the duration of the interval; the AP of adult rats was unaffected by the length of the interval. During the rest interval diastolic depolarization appeared on the myocardium of newborn rats; with advancing age it slowed down and the decrease in membrane voltage was smaller. In the adult rats myocardium there was no diastolic depolarization.

Effect of verapamil on contractile function of the isolated perfused rat heart during postnatal ontogeny