INACTIVATION OF GATING CURRENTS OF L-TYPE CALCIUM CHANNELS - SPECIFICROLE 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 comb inations in tsA201 cells, we found that long-lasting depolarization sh ifted the distribution of mobile charge to very negative potentials. T he phenomenon has been termed charge interconversion in native skeleta l muscle (Brum, G., and E. Rios. 1987. J. Physiol. (Camb.). 387:489-51 7) 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 noninactivate d channels, while charge 2 (V-1/2 similar or equal to -90 mV) is gener ated in inactivated channels. In alpha(1c)/beta(2a) cells, the availab le charge 1 decreased upon inactivating depolarization with a time con stant tau similar or equal to 8, while the available charge 2 decrease d upon recovery from inactivation (at -200 mV) with tau similar or equ al to 0.3 s. These processes therefore are much slower than charge mov ement, which takes <50 ms. This separation between the time scale of m easurable charge movement and that of changes in their availability, w hich.was even wider in the presence of alpha(2 delta), implies that ch arges 1 and 2 originate from separate channel modes. Because clear mod al separation characterizes slow (C-type) inactivation of Na and K cha nnels, this observation establishes the nature of voltage-dependent in activation of L-type Ca channels as slow or C-type. The presence of th e alpha(2 delta) subunit did not change the V-1/2 beta of charge 2, bu t 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 si milar or equal to = 2 s). The observations were well simulated with a model that describes ac activation as continuous electrodiffusion (Lev itt, D. 1989. Biophys. J. 55:489-498) and inactivation as discrete mod al change. The effects of alpha(2 delta) are reproduced assuming that the subunit lowers the free energy of the inactivated mode.