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 microM GTP-gamma S did not significantly affect prepulse facilitation or whole-cell Ba2+ current (IBa) density. In contrast, inactivation of G proteins by intracellular GDP-beta S or pertussis toxin (PTX) pretreatment completely abolished or markedly attenuated facilitation of IBa, respectively. GDP-beta S dialysis resulted in nearly a threefold increase in peak IBa density, whereas PTX pretreatment resulted in a 50% increase. Our results indicate that under control recording conditions (200 microM 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 (approximately 50%) in IBa 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.
Doupnik
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CA
Department of Physiology and Biophysics, University of Cincinnati College of Medicine, Ohio 45267-0576.
Pun
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D000255:Adenosine Triphosphate / Q000378:metabolism; D000313:Adrenal Medulla / Q000187:drug effects / Q000378:metabolism*; D000818:Animals; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism*; D002395:Catecholamines / Q000378:metabolism; D002417:Cattle; D002478:Cells, Cultured; D004745:Enkephalins / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000378:metabolism*; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology; D006153:Guanosine Diphosphate / Q000031:analogs & derivatives / Q000494:pharmacology; D008564:Membrane Potentials; D013873:Thionucleotides / Q000494:pharmacology
D015220:Calcium Channels; D002395:Catecholamines; D004745:Enkephalins; D013873:Thionucleotides; D006153:Guanosine Diphosphate; D016244:Guanosine 5'-O-(3-Thiotriphosphate); C023427:guanosine 5'-O-(2-thiodiphosphate); D000255:Adenosine Triphosphate; D019204:GTP-Binding Proteins