Modification of Na channel inactivation by alpha-chymotrypsin in single cardiac myocytes

The effects of alpha-chymotrypsin and trypsin on the macroscopic Na current in isolated guinea pig ventricular myocytes at 16 degrees C were investigated using the whole-cell voltage-clamp technique. Intracellular application of both enzymes reduced the extent of Na current inactivation during 20- to 50-ms depolarizing pulses. Elimination of fast inactivation by alpha-chymotrypsin was accompanied by a slowing of the rate of Na current decay through changes in both the time constants of current decay and the proportions of current undergoing a fast vs slow rate of decay. Treatment that reduced Na current decay to less than or equal to 10% within 20 ms was accompanied by a hyperpolarizing shift of the Na conductance/voltage relationship and an increase in the time-to-peak current that was most prominent for small depolarizations. Evidence for a significant slow inactivation process was obtained following removal of fast inactivation. The effect of trypsin (0.15-0.3 mg/ml) was less specific than alpha-chymotrypsin in that it also reduced Na conductance and increased leak current.