DECAY OF PREPULSE FACILITATION OF N-TYPE CALCIUM CHANNELS DURING G-PROTEIN INHIBITION IS CONSISTENT WITH BINDING OF A SINGLE G(BETA-GAMMA) SUBUNIT

We have examined the modulation of cloned and stably expressed rat bra in N type calcium channels (alpha(1B) + beta(1b) + alpha(2) delta subu nits) by exogenously applied purified G protein beta gamma subunits, I n the absence of G(beta gamma), barium currents through N type channel s are unaffected by application elf strong depolarizing prepulses, In contrast, inclusion of purified G(beta gamma) In the patch pipette res ults in N type currents that initially facilitated upon application of positive prepulses followed by rapid reinhibition, Examination of the kinetics of G(beta gamma)-dependent reinhibition showed that as the d uration between the test pulse and the prepulse was increased, the deg ree of facilitation was attenuated In a monoexponential fashion, The t ime constant tau for the recovery from facilitation was sensitive to e xogenous G(beta gamma), so that the inverse of tau linearly depended o n the G(beta gamma) concentration. Overall, the data are consistent wi th a model whereby a single G(beta gamma) molecule dissociates from th e channel during the prepulse, and that reassociation of G(beta gamma) with the channel after the prepulse occurs as a bimolecular reaction.