Decay of prepulse facilitation of N type calcium channels during G protein inhibition is consistent with binding of a single G(βγ) subunit

We have examined the modulation of cloned and stably expressed rat brain N type calcium channels (α(1B) + β(1b) + α(2)δ subunits) by exogenously applied purified G protein βγ subunits. In the absence of G(βγ), barium currents through N type channels are unaffected by application of strong depolarizing prepulses. In contrast, inclusion of purified G(βγ) in the patch pipette results in N type currents that initially facilitated upon application of positive prepulses followed by rapid reinhibition. Examination of the kinetics of G(βγ)-dependent reinhibition showed that as the duration between the test pulse and the prepulse was increased, the degree of facilitation was attenuated in a monoexponential fashion. The time constant τ for the recovery from facilitation was sensitive to exogenous G(βγ), so that the inverse of τ linearly depended on the G(βγ) concentration. Overall, the data are consistent with a model whereby a single G(βγ) molecule dissociates from the channel during the prepulse, and that reassociation of G(βγ) with the channel after the prepulse occurs as a bimolecular reaction.