VOLTAGE-DEPENDENT MODULATION OF CALCIUM CURRENT BY GTP-GAMMA-S AND DOPAMINE IN CULTURED FROG PITUITARY MELANOTROPHS

Dopamine (1-mu-M) reversibly scaled down barium current through high-voltage activated (HVA) calcium channels but had little effect on the time course of current activation in cultured frog melanotrophs. Intracellular Perfusion with guanosine-5'-O-(3-thiotriphosphate) (GTP-gamma-S; 100-mu-M) sustained the effect of dopamine. Moreover, GTP-gamma-S drastically slowed down the current activation kinetics. The latter effect was in part reversed by dopamine. A conditioning prepulse to +70 mV facilitated the current in GTP-gamma-S-dialyzed cells but not in cells exposed to dopamine. These results suggest the existence of a dual G protein-mediated mechanism for reducing HVA calcium current.