ONTOGENETIC INCREASE OF THE MAXIMAL RATE OF RISE OF THE CHICK EMBRYONIC HEART ACTION POTENTIAL - RELATIONSHIP TO VOLTAGE, TIME, AND TETRODOTOXIN

The maximal rate of rise (V̇(max)) of the embryonic chick ventricular action potential increased about 2-fold from 112 ± 6 V/sec and 113 ± 5 V/sec on the 4th and 6th incubation days, respectively, to 217 ± 7 V/sec on the 12th incubation day. Neither the steady state inactivation (h(∞)) of V̇(max), the temperature-dependent shift of h(∞), nor the time constant for recovery (T(Rec) of V̇(max) changed significantly when V̇(max) had doubled. Inhibition of V̇(max) by tetrodotoxin (TTX) was the same in hearts from the 4th, 6th, and 18th incubation days. The relation between V̇(max) and TTX concentration could be described by a one-to-one binding curve with an apparent dissociation constant of 2 x 10-8 M. The increase of V̇(max), a valid measure of the early inward Na+ current (i(Na)) during the rising phase of the cardiac action potential, can be attributed to an elevated maximal Na+ conductance (ḡ(Na)) during ontogenesis. These results indicate that the physicochemical properties of the g(Na) unit responsible for the rapid rising phase of the action potential and of the TTX binding site associated with it may remain constant during a period of embryonic development when ḡ(Na) increased significantly.