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 (Vmax) 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 (hinfinity) of Vmax, the temperature-dependent shift of hinfinity, nor the time constant for recovery (tauRec) of Vmax changed significantly when Vmax had doubled. Inhibition of Vmax by tetrodotoxin (TTX) was the same in hearts from the 4th, 6th, and 18th incubation days. The relation between Vmax 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 Vmax, a valid measure of the early inward Na+ current (iNa) during the rising phase of the cardiac action potential, can be attributed to an elevated maximal Na+ conductance (-gNa) during ontogenesis. These results indicate that the physicochemical properties of the gNa 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 -gNa increased significantly.