Developmental changes in action potential properties of the guinea-pig myocardium

Developmental changes in action potential properties were examined in electrically driven (1 Hz) left atria and right ventricles from foetal, neonatal and adult guinea-pig hearts, using standard micro-electrode recording techniques. In both left atria and right ventricles, the overshoot, resting potential and maximum upstroke velocity of the action potential increased progressively with age until birth, and then remained almost unchanged. Action potential duration (APD) changed markedly with age during foetal and neonatal periods. In left atria, APD at 50% repolarization initially decreased until foetal day 50, and then increased until the adult period. In right ventricles, APD initially increased until approximately foetal day 45, then decreased for 5 days following birth, thereafter it increased again. In addition, after-hyperpolarization was observed only in left atria of younger foetuses. Thus we have demonstrated that in the guinea-pig myocardium developmental changes in action potential properties occur more extensively during the foetal period than during the postnatal period.