Effects of the dihydropyridine receptor subunits γ and α₂δ on the kinetics of heterologously expressed L-type Ca²⁺ channels

Abstract

Ba²⁺ currents through L-type Ca²⁺ channels were measured in tsA201 cells transiently transfected with expression vectors encoding the dihydropyridine (DHP) receptor subunits α_1C, β_1a-GFP, α₂δ and γ. The subunit effect on channel function was studied by omitting either α₂δ or γ from the transfection mixture and analyzing the voltage dependence and kinetics of activation, inactivation and recovery from inactivation. Activation could be described by a single exponential function while the time course of inactivation of the Ba²⁺ current followed a double exponential function. Progressively longer depolarization led to increasingly slower recovery, indicating the successive occupancy of several inactive states. Activation parameters remained largely unaffected in γ-deficient cells whereas the voltage dependence of inactivation was shifted by 16 mV to more positive potentials and the larger one of the two inactivation time constants was increased by one-third. On the other hand, α₂δ-deficient cells showed decreased current density and slowed activation and inactivation. Recovery from inactivation was significantly slowed by γ coexpression. This and the effect of the γ subunit on steady-state inactivation were independent of the presence of α₂δ. We conclude that γ stabilizes L-type Ca²⁺ channel inactivation in a way similar to certain Ca²⁺-antagonistic drugs. α₂δ is not needed for this effect.