Action potentials of the right ventricular working myocardium of newborn and adult rabbits under paired stimulation conditions.

The effect of paired stimulation on the action potentials of the working ventricular myocardium of newborn and adult rabbits was studied with the aim of elucidating the postnatal development of electrogenesis in cardiac cells. After a 2 minutes' rest pause we applied two identical rectangular electric pulses (duration 0.1-1 ms, voltage double the stimulation threshold) with an exactly defined interval, T (100 ms to 20 s). In the recordings we evaluated the duration of the plateau phase of the first (D1) and the second (D2) action potential for given stimulation pairs. The area formed by the course of the first (A1) and second (A2) action potential above the resting membrane potential value was determined planimetrically. Graphs of the correlations of D2/D1 to T and of A2/A1 to T were constructed for the evaluation. The D2/D1 curve for adult animals rose from minimum T values, attained the maximum at T=260 ms and then gradually fell. In newborn rabbits, the corresponding curve, in the majority of cases, displayed a value of T=1. The correlations of A2/A1 to T in adult animals were similar to the course of the D2/D1 curve, and the same applied to newborn animals. The results show that the working right ventricular myocardium of the adult rabbit is capable of reacting to altered stimulation intervals by a change in the duration of the plateau phase, while the myocardium of newborn animals is frequency-insensitive. The rest pause did not affect the duration of action potentials in newborn rabbits, but in adult animals it caused marked shortening of the action potential. The differences found between the neonatal and adult myocardium are probably related to rhe characteristics of the channel systems of the slow Ca-dependent inward current.