Abstract
Dopamine (1 μM) 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) (GTPγS; 100 μM) sustained the effect of dopamine. Moreover, GTPγS 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 GTPγS-dialyzed cells but not in cells exposed to dopamine. These reults suggest the existence of a dual G protein-mediated mechanism for reducing HVA calcium current.
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