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 increasing agents under Ca2+ free-EGTA condition, which was previously suggested to be produced by persistent Na+ influx through Ca2+ channel due to lack of Ca(2+)-mediated inactivation, was examined in isolated myocardial 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 preparations from various regions of the heart. Both in the neonate and adult, the prolongation in the right ventricular free wall and atrial preparations was smaller than that in preparations from other parts of the ventricle. In all regions of the heart except the right ventricle, the prolongation by isoproterenol was greater in the adult than in the neonate, 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 developmental difference in contribution of Ca(2+)-mediated inactivation of Ca2+ channels to the repolarization of guinea-pig myocardium. The developmental changes were similar to but much smaller than, those observed in the rat myocardium.