Fast inward current properties of voltage-clamped ventricular cells of embryonic chick heart

To determine ionic properties of the fast inward current of embryonic chick hearts, whole cell voltage-clamp experiments were carried out on single cardiomyocytes. Ventricular myocytes trypsin dispersed (spherical, 10-15 microns in diameter, 10.7 pF in membrane capacitance) from 16- to 18-day-old embryonic chick hearts were used. Cells were bathed in solutions containing 140 mM Na+ at room temperature of 22 degrees C. Action potentials recorded via patch electrodes were approximately 103 mV in amplitude, -73 mV in resting potential, and 400 ms in duration. The maximum amplitude of the fast inward-current peak was -1.34 nA (-0.126 mA/microF) and the voltage at which the current was maximum was -29 mV. The reversal potential (Vrev) for this inward current was +35 mV. The inward current was sensitive to change in extracellular Na+ concentration ([Na+]o) (18-mV change in Vrev for halved [Na+]o) and to tetrodotoxin (TTX; 10(-6)-10(-5) M). The inactivation time constant (tau h) and the time-to-peak current decreased from several milliseconds at -60 mV to 1 ms at positive potentials. The steady-state inactivation (h infinity) characteristics showed an S-shaped dependence on voltages between -110 and -60 mV, giving about -90 mV for the half-inactivation voltage. The steady-state activation characteristics (m infinity) also showed sigmoidal voltage dependence, and the half-activation voltage was -54 mV. Both h infinity and m infinity curves crossed each other in the vicinity of -73 mV, giving a maximum window conductance (m infinity 3.h infinity) of 0.00044 at -50 mV. Compared with inactivation, the reactivation of the fast inward channel proceeded much more slowly. It was concluded that except for the low current density of embryonic chick ventricular cells the voltage- and time-dependent kinetics of the fast inward current channel were quite similar to those reported for the fast Na+ channels in single myocytes of adult hearts of other animal species.