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 alpha(1c beta 2a) or alpha(1c)/beta(2a)/alpha(2 delta) 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 (Brum, 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 similar or equal to 0 mV) gates noninactivated channels, while charge 2 (V-1/2 similar or equal to -90 mV) is generated in inactivated channels. In alpha(1c)/beta(2a) cells, the available charge 1 decreased upon inactivating depolarization with a time constant tau similar or equal to 8, while the available charge 2 decreased upon recovery from inactivation (at -200 mV) with tau similar or equal to 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 alpha(2 delta), implies that charges 1 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 alpha(2 delta) subunit did not change the V-1/2 beta of charge 2, but sped up the reduction of charge 1 upon inactivation at 40 mV (to tau = 2 s), while slowing the reduction of charge 2 upon recovery (tau similar or equal to = 2 s). The observations were well simulated with a model that describes ac activation as continuous electrodiffusion (Levitt, D. 1989. Biophys. J. 55:489-498) and inactivation as discrete modal change. The effects of alpha(2 delta) are reproduced assuming that the subunit lowers the free energy of the inactivated mode.