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.