MODIFICATION OF SODIUM-CHANNEL INACTIVATION BY ALPHA-CHYMOTRYPSIN IN CANINE CARDIAC PURKINJE-CELLS

Introduction: Studies of tetrodotoxin-sensitive sodium current (I(Na)) after modification of inactivation by intracellular enzymes in mammal ian cells have demonstrated a marked increase in peak I(Na) at test po tentials near current threshold causing a large, negative shift of the peak I(Na) conductance-voltage relationship by approximately -20 mV. These findings support a kinetic model in which the unmodified Na chan nel has rapid and voltage-independent inactivation from the open state . However, the kinetics of cardiac Na channels differ from those of ma mmalian neuronal Na channels. In particular, inactivation of cardiac N a channels has been proposed to be more voltage dependent than that of tetrodotoxin-sensitive Na channels. To help understand the role of in activation in cardiac Na channel kinetic behavior, we studied Na curre nts before and after modification of inactivation by the proteolytic e nzyme, alpha-chymotrypsin. Methods and Results: Whole cell I(Na) was m easured in single canine cardiac Purkinje cells that were voltage clam ped and internally perfused with a large-bore suction pipette. The dec ay of I(Na) in response to step depolarizations was dramatically slowe d after perfusion with intracellular alpha-chymotrypsin consistent wit h modification of inactivation. In contrast to mammalian tetrodotoxin- sensitive Na current, Boltzmann distribution fits to peak I(Na) conduc tance-voltage (G(Na)-V) relationships after alpha-chymotrypsin showed no change in either the potential at half maximum conductance (V1/2), after correction for the spontaneous background shift of I(Na) kinetic s, or in the voltage-dependence of conductance (i.e., slope factor of G(Na)-V relationships). Maximal peak I(Na) conductance by 18%. I(Na) t ail-current relaxations at potentials less-than-or-equal-to -110 mV, a fter correction for spontaneous shifts in Na channel kinetics, were al so similar before and after modification by alpha-chymotrypsin. Conclu sion: alpha-chymotrypsin modified inactivation of cardiac I(Na) with l ittle or no change in activation, and cardiac Na channel inactivation was slow near threshold and played little role in determining V1/2 for peak I(Na) conductance-voltage relationships.