DEVELOPMENTAL-CHANGE IN FAST NA CHANNEL PROPERTIES IN EMBRYONIC CHICKVENTRICULAR HEART-CELLS

To assess developmental changes in kinetic properties of the cardiac s odium current, whole-cell voltage-clamp experiments were conducted usi ng 3-, 10-, and 17-day-old embryonic chick ventricular heart cells. Ex perimental data were quantified according to the Hodgkin-Huxley model. While the Na current density, as examined by the maximal conductance, drastically increased (six- to seven-fold) with development, other cu rrent-voltage parameters remained unchanged. Whereas the activation ti me constant and the steady-state activation characteristics were compa rable among the three age groups, the voltage dependence of the inacti vation time constant and the steady-state inactivation underwent a shi ft in the voltage dependence toward negative potentials during embryon ic development. Consequently, the steady-state (window current) conduc tance, which was sufficient to induce automatic activity in the young embryos, was progressively reduced with age.