Slow inhibition of N-type calcium channels with GTP gamma S reflects the basal G protein-GDP turnover rate.

The inhibition of N-type Ca channels via a G protein pathway is a rapid mechanism for modulating Ca influx. It has been noted, however, that when G proteins are activated by guanosine 5'- O-(3-thiotriphosphate) (GTPgammaS), the speed of inhibition is greatly reduced, despite the pathway having fewer molecular steps. We explored this anomaly in chick dorsal root ganglion neurons by comparing Ca current inhibition using GTPgammaS with application of the G protein receptor agonist noradrenaline. Noradrenaline caused rapid Ca channel inhibition (tau~5 s), contrasting greatly with the ~70-fold slower rate observed with GTPgammaS. Additionally, the slow rate with GTPgammaS could be accelerated to near agonist-induced rates by application of noradrenaline, demonstrating that the inhibition with GTPgammaS was not perfusion limited and that the rate-limiting step was upstream from GTPgammaS binding. Our results suggest that in the absence of noradrenaline, G protein activation by GTPgammaS is impeded by the slow resting turnover of GDP/GTP. The rate at which inhibition develops with GTPgammaS (tau~350 s) is thus a direct and sensitive measure of resting GDP turnover.