Developmental changes of action potential configuration and I(to) in canine epicardium

Developmental changes of the transient outward current (I(to)) and action potential configuration were determined in canine epicardium ranging in age from fetal to 60 wk. The contributions of I(to) to rapid initial repolarization and to terminal repolarization were estimated by measuring the amplitude of phase 1 of the action potential and action potential duration, respectively. Phase 1 amplitude decreased progressively from fetal to 40 wk and remained constant thereafter. Action potential duration decreased from fetal to 2 wk, increased to 20 wk, and tended to decrease thereafter. Peak I(to) at +40 mV increased progressively from 2 to 60 wk. However, I(to) density was less at 2-10 wk than at 20-60 wk. The time constant of decay of I(to) increased with age from 2 to 60 wk, whereas the steady-state voltage dependence of inactivation did not vary with age. The time constant for the initial rapid phase of recovery from inactivation decreased from 2 to 10 wk and remained constant thereafter. The time constant for the more slowly evolving phase did not vary with age. The observation that the age-dependent reduction in phase 1 amplitude did not necessarily coincide with significant increases in I(to) density suggests that maturation of other ionic currents or transport mechanisms may contribute to developmental alterations of phase 1 repolarization.