Inactivation of gating currents of L-type calcium channels. Specific role of the alpha 2 delta 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. Ríos. 1987. J. Physiol. (Camb.). 387:489-517) and cardiac Ca channels (Shirokov, R., R. Levis, N. Shirokova, and E. Ríos. 1992. J. Gen. Physiol. 99:863-895). Charge 1 (voltage of half-maximal transfer, V1/2 approximately 0 mV) gates noninactivated channels, while charge 2 (V1/2 approximately -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 approximately 8, while the available charge 2 decreased upon recovery from inactivation (at -200 mV) with tau approximately 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 V1/2 of charge 2, but sped up the reduction of charge 1 upon inactivation at 40 mV (to tau approximately 2 s), while slowing the reduction of charge 2 upon recovery (tau approximately 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 alpha 2 delta are reproduced assuming that the subunit lowers the free energy of the inactivated mode.