CA2+ CURRENT ENHANCEMENT BY ALPHA-2/DELTA AND BETA-SUBUNITS IN XENOPUS OOCYTES - CONTRIBUTION OF CHANGES IN CHANNEL GATING AND ALPHA-1 PROTEIN LEVEL

1. A combined biochemical and electrophysiological approach was used to determine the mechanism by which the auxiliary subunits of Ca2+ channel enhance the macroscopic Ca2+ currents. Xenopus oocytes were injected with RNA of the main pore-forming subunit (cardiac: alpha 1C), and various combinations of RNAs of the auxiliary subunits (alpha 2/delta and beta 2A). 2. The single channel open probability (P-o; measured at 0 mV) was increased similar to 3-, similar to 8- and similar to 35-fold by alpha 2/delta, beta 2A and alpha 2/delta + beta 2A, respectively. The whole-cell Ca2+ channel current was increased similar to 8- to 10-fold by either alpha 2/delta or beta 2A, and synergistically > 100-fold by alpha 2/delta + beta 2A. The amount of S-35-labelled alpha 1 protein in the plasma membrane was not changed by coexpression of beta 2A, but was tripled by coexpression of alpha 2/delta (either with or without beta). 3. We conclude that the increase in macroscopic current by alpha 2/delta is equally due to changes in amount of dl in the plasma membrane and an increase in P-0, whereas all of the effect of beta 2A is due to an increase in P-0. The synergy between alpha 2/delta and beta in increasing the macroscopic current is due mainly to synergistic changes in channel gating.