Influence of L-type Ca channel α₂/δ-subunit on ionic and gating current in transiently transfected HEK 293 cells

We have measured ionic and gating currents in human embryonic kidney (HEK 293) cells transiently transfected with cDNAs encoding subunits of the cardiac voltage-gated L-type Ca²⁺ channel. Robust recombinant ionic current and associated nonlinear charge movement could be measured over a broad voltage range without contamination by endogenous channel activity. Coexpression of the α₂/δ-subunit along with α₁- and β₂-subunits speeded activation and deactivation kinetics and significantly increased the maximal conductance of ionic current. Charge movement was measured at voltages negative to the threshold for activation of ionic current, and gating charge could be immobilized at positive holding potentials that did not inactivate ionic current. The ratio of maximal ionic conductance to maximal charge moved remained the same in the absence or presence of the α₂/δ-subunit. However, the maximal amount of charge moved was increased about twofold in the presence of the α₂/δ-subunit. These results suggest that coexpression of the α₂/δ-subunit enhances the expression of functional L-type channels and, in addition, provide evidence that most of the L-type channel-associated nonlinear charge movement is caused by transitions between nonconducting states of the channel protein that precede the open and inactivated states.