| Abstract: | Stimulation of the nicotinic receptor of bovine chromaffin cells results in a rise in intracellular free calcium ( Ca2+]i) and subsequent release of catecholamine. This response is totally dependent on the presence of external Ca2+. Monitoring Ca2+]i using quin-2 demonstrated a rise in Ca2+]i in response to muscarinic agonists which was approximately 4-times less than that obtained in response to nicotinic stimulation. This atropine-sensitive Ca2+]i rise occurred after a 10-s lag and was found to be independent of the external Ca2+, implying the existence of an intracellular Ca2+ source. Despite producing this Ca2+]i rise low concentrations of the muscarinic agonist, methacholine (under 1 X 10(-3) M), failed to trigger secretion itself and did not effect the secretory response elicited by nicotine. Challenging the cells with higher methacholine concentrations (over 1 X 10(-3) M) resulted in the same Ca2+]i rise, no secretion, but an inhibition of secretion due to nicotine. This latter response, however, was found to be atropine-insensitive and therefore non-muscarinic. The Ca2+]i rise and secretion due to depolarization by 55 mM K+ were largely unaffected by prior addition 1 X 10(-2) M methacholine, inferring that high concentrations of methacholine inhibit nicotine-induced secretion by interacting with the nicotinic receptor. These results provide evidence consistent with the existence of an intracellular Ca2+ store mobilized by muscarinic receptor activation in bovine chromaffin cells and show that, despite causing a rise in Ca2+]i, bovine chromaffin cell muscarinic stimulation does not effect secretion itself or secretion induced by either nicotine or high K+. |
