G-PROTEIN ACTIVATION MEDIATES PREPULSE FACILITATION OF CA2+ CHANNEL CURRENTS IN BOVINE CHROMAFFIN CELLS

The effects of G-protein activation were investigated on tonic, large depolarization-induced Ca2+ channel facilitation in cultured bovine adrenal chromaffin cells. Under whole-cell voltage clamp, activation of G proteins by intracellular dialysis with 200 mu M GTP-gamma S did not significantly affect prepulse facilitation or whole-cell Ba2+ current (I-Ba) density. In contrast, inactivation of G proteins by intracellular GDP-beta S or pertussis toxin (PTX) pretreatment completely abolished or markedly attenuated facilitation of I-Ba, respectively. GDP-beta S dialysis resulted in nearly a threefold increase in peak I-Ba density, whereas PTX pretreatment resulted in a 50% increase. Our results indicate that under control recording conditions (200 mu M intracellular GTP), G proteins are tonically activated and suppress high-voltage-activated (HVA) Ca2+ channels in a voltage-dependent and voltage-independent manner. Local superfusion of chromaffin cells with normal bath solution produced a rapid and reversible increase (similar to 50%) in I-Ba amplitudes that also abolished prepulse facilitation. Together, these results demonstrate that tonic facilitation of HVA Ca2+ channels in bovine chromaffin cells involves the voltage-dependent relief of a G-protein-mediated suppression, imposed by chromaffin cell secretory products that feedback and activate G-protein-coupled autoreceptors.