DIFFERENCES IN OUTWARD CURRENTS BETWEEN NEONATAL AND ADULT-RABBIT VENTRICULAR CELLS

In adult rabbit ventricular preparations, action potential duration is significantly increased when stimulation frequency is increased from 0.1 to 1.0 Hz. In neonatal preparations, a similar change in stimulati on frequency produced no significant increase in action potential dura tion. To identify the ionic basis for this difference, we studied diff erent outward currents in single myocytes from papillary muscle and fr om epicardial tissue of adult and neonatal rabbits. The densities of t he outward currents in neonatal cells were about one-half of the curre nt density in adult cells. The density of the voltage-activated transi ent outward current (It,l) was smaller in cells from papillary muscle than in cells from epicardium in adult and newborn rabbits. We found m ajor differences in the kinetic behavior of I-to1 between adult and ne onatal cells: 1) the rate of apparent inactivation was faster in neona tal cells, and 2) the recovery from inactivation was significantly fas ter in neonatal cells, with a time constant of 113 vs. 1,356 ms. We pr opose that this marked difference in the recovery from inactivation of I-to1 is the basis for the difference in frequency dependence of acti on potential duration.