Calcium signalling mediated through a7 and non-a7 nAChR stimulation is differentially regulated in bovine chromaffin cells to induce catecholamine release

Abstract

Ca2+ signalling and exocytosis mediated by nicotinic receptor (nAChR) subtypes, especially the a7 nAChR, in bovine chromaffin cells are still matters of debate.We have used chromaffin cell cultures loaded with Fluo-4 or transfected with aequorins directed to the cytosol or mitochondria, several nAChR agonists (nicotine, 5-iodo-A-85380, PNU282987 and choline), and the a7 nAChR allosteric modulator PNU120596. Minimal [Ca2+]c transients, induced by low concentrations of selective a7 nAChR agonists and nicotine, were markedly increased by the a7 nAChR allosteric modulator PNU120596. These potentiated responses were completely blocked by the a7 nAChR antagonist a-bungarotoxin (a7-modulated-response). Conversely, high concentrations of the a7 nAChR agonists, nicotine or 5-iodo-A-85380 induced larger [Ca2+]c transients, that were blocked by mecamylamine but were unaffected by a-bungarotoxin (non-a7 response). [Ca2+]c increases mediated by a7 nAChR were related to Ca2+ entry through non-L-type Ca2+ channels, whereas non-a7 nAChR-mediated signals were related to L-type Ca2+ channels; Ca2+-induced Ca2+-release contributed to both responses. Mitochondrial involvement in the control of [Ca2+]c transients, mediated by either receptor, was minimal. Catecholamine release coupled to a7 nAChRs was more efficient in terms of catecholamine released/[Ca2+]c.