Myocardial action potential prolongation by calcium channel activation under calcium free-EGTA condition in rats: developmental and regional variations.

1. Prolongation of action potentials upon the addition of isoproterenol, forskolin, isobutylmethyl-xanthine (IBMX) and dibutyril cAMP (dbcAMP) under Ca-free EGTA condition was examined in isolated myocardial preparations from neonatal and adult rats, whose action potential configuration greatly differ. 2. The prolongation of the action potential was previously suggested to be produced by persistent sodium influx through calcium channel due to the lack of calcium-mediated inactivation of calcium channels under such experimental condition. 3. Preparations used were papillary muscles and free walls of the right and left ventricles from neonatal and adult rats. 4. In adult preparations, the prolongation produced by isoproterenol, forskolin and IBMX in the right free wall was smaller than those in the other three regions, while no regional difference was observed with dbcAMP. 5. The degree of prolongation by all of the four drugs were smaller in the neonate than in the adult. No regional difference was observed with any of the drugs in the neonate. 6. Our present results suggest that contribution of calcium-mediated inactivation of calcium channels to the repolarization of rat myocardium may increase postnatally to produce the developmental shortening of its action potential. Also, regional difference in the cAMP related mechanisms may appear postnatally.