Action potentials of the working ventricular myocardium of newborn and adult dogs in the steady state and after a pause.

The aim of the study was to find and measure any differences in the electrophysiological manifestations of the dog working ventricular myocardium during postnatal development. The experiments were performed with the right working ventricular myocardium of eight adult (A) and seven newborn (N) beagles. Using the glass microelectrode technique and programmed stimulation we measured the resting membrane potential (RMP), the duration of action potentials (AP) in the plateau phase (D0) and AP duration at the level of repolarization at -60 mV (D60). Two stimulation programmes were used: 1) stimulation at different frequencies in the steady state (0.1 Hz to 5 Hz) and 2) the effect of a pause (Tp, duration 10 s to 600 s) on D0 and D60 of the first post-rest AP. During postnatal life the RMP value rose from -80 mV (N) to -86.6 mV (A). AP duration in the steady state at 1 Hz stimulation frequency was significantly longer, at both the given levels, in adult animals (D0: N = 56.1 ms, A = 106.1 ms; D60: N = 141.2 ms, A = 212 ms). In the steady state the frequency sensitivity of the neonatal myocardium differed from that of the adult myocardium. In both groups, stimulation frequencies of over 1 Hz shortened the AP, while in A frequencies under 1 Hz markedly prolonged D60 and slightly prolonged D0. Conversely, a decrease in the stimulation frequency shortened both D0 and D60 in N. In the case of A a pause led to marked lengthening of D60 and weak lengthening of D0.(ABSTRACT TRUNCATED AT 250 WORDS)