MYOCARDIAL ACTION-POTENTIAL PROLONGATION BY CALCIUM-CHANNEL ACTIVATION UNDER CALCIUM FREE-EGTA CONDITIONS IN GUINEA-PIGS - DEVELOPMENTAL AND REGIONAL VARIATIONS

Prolongation of action potentials upon the addition of various cAMP in creasing agents under Ca2+ free-EGTA condition, which was previously s uggested to be produced by persistent Na+ influx through Ca2+ channel due to lack of Ca2+-mediated inactivation, was examined in isolated my ocardial preparations from neonatal and adult guinea-pigs. Application of isoproterenol under Ca2+ free-EGTA condition produced prolongation of the action potential duration in both neonatal and adult preparati ons from various regions of the heart. Both in the neonate and adult, the prolongation in the right ventricular free wall and atrial prepara tions was smaller than that in preparations from other parts of the ve ntricle. In all regions of the heart except the right ventricle, the p rolongation by isoproterenol was greater in the adult than in the neon ate; no developmental difference was observed in the right free wall. In right ventricular free-wall preparations, the prolongation produced by forskolin and isobutyl-methylxanthine in the adult was larger than in neonatal ones. Our present results suggest regional and developmen tal difference in contribution of Ca2+-mediated inactivation of Ca2+ c hannels to the repolarization of guinea-pig myocardium. The developmen tal changes were similar to, but much smaller than, those observed in the rat myocardium.