Oxotremorine-m potentiation of glucose-induced insulin release from rat islets involves M3 muscarinic receptors

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Abstract
CDNAs encoding for M1 and M3 muscarinic acetylcholine (ACh) receptors were detected in rat pancreatic islet cells by polymerase chain reaction (PCR) amplification techniques. A new cholinergic agonist, oxotremorine-m (oxo-m), in the presence of glucose (5.6 mM), produced a dose-dependent potentiation of insulin secretion saturating at approximately 5 microM. This effect was suppressed by the L-type Ca2+ channel blocker nifedipine. Higher doses of oxo-m (50 microM) induced a biphasic insulin response both at low (5.6 mM) or high (16.7 mM) glucose concentrations. In a Ca(2+)-deficient medium containing glucose (5.6 mM), oxo-m evoked only a reduced first phase of insulin secretion. The potentiating effects of oxo-m were inhibited by the muscarinic receptor antagonists 4-diphenylacetoxy-N-methylpiperidine methiodide (M3), hexahydro-sila-difenidol hydrochloride, p-fluoro analogue (M3 > M1 > M2), and pirenzepine (M1) in a dose-dependent manner; half-maximal inhibitory concentration values were approximately 5, 20, and 340 nM, respectively. The PCR results demonstrate the presence of M1 and M3 muscarinic ACh receptors in the islet tissue, and the secretion data strongly suggest that the potentiation of glucose-induced insulin release evoked by oxo-m depends on the activation of a muscarinic M3-subtype receptor present in the beta-cell membrane.