Inactivation of gating currents of L-type calcium channels: Specific role of the α₂δ subunit

In studies of gating currents of rabbit cardiac Ca channels expressed as α(1C)/β(2a) or α(1C)/β(2a)/α₂δ subunit combinations in tsA201 cells, we found that long-lasting depolarization shifted the distribution of mobile charge to very negative potentials. The phenomenon has been termed charge interconversion in native skeletal muscle (Bruin, G., and E. Rios. 1987. J. Physiol. (Camb.). 387:489-517) and cardiac Ca channels (Shirokov, R., R. Levis, N. Shirokova, and E. Rios. 1992. J. Gen. Physiol. 99:863-895). Charge 1 (voltage of half-maximal transfer, V(1/2) = 0 mV) gates noninactivated channels, while charge 2 (V(1/2) ≃ -90 mV) is generated in inactivated channels. In α(1C)/β(2a) cells, the available charge 1 decreased upon inactivating depolarization with a time constant τ = 8, while the available charge 2 decreased upon recovery from inactivation (at -200 mV) with τ = 0.3 s. These processes therefore are much slower than charge movement, which takes <50 ms. This separation between the time scale of measurable charge movement and that of changes in their availability, which was even wider in the presence of α₂δ, implies that charges I and 2 originate from separate channel modes. Because clear modal separation characterizes slow (C-type) inactivation of Na and K channels, this observation establishes the nature of voltage-dependent inactivation of L-type Ca channels as slow or C-type. The presence of the α₂δ subunit did not change the V(1/2) of charge 2, but sped up the reduction of charge 1 upon inactivation at 40 mV (to τ = 2 s), while slowing the reduction of charge 2 upon recovery (τ = 2 s). The observations were well simulated with a model that describes activation as continuous electrodiffusion (Levitt, D. 1989. Biophys. J. 55:489-498) and inactivation as discrete modal change. The effects of α₂δ are reproduced assuming that the subunit lowers the free energy of the inactivated mode.