IONIC MECHANISMS INVOLVED IN THE REGULATION OF INSULIN-SECRETION BY MUSCARINIC AGONISTS

The effects of the muscarinic agonist oxotremorine-m (oxo-m) on insuli n secretion, K+-permeability and electrical activity from isolated mou se pancreatic islets were studied. Oxo-m potentiated glucose-induced i nsulin secretion in a dose-dependent manner, saturating at ca. 10 mu M . At 11.2 mM glucose, oxo-m (0.1 and 10 mu M) had two distinct effects on beta-cell electrical activity. Both concentrations increased the s teadystate burst frequency, however, at 10 mu M an initial and transie nt polarization was measured, and the subsequent activity was accompan ied by a slight depolarization. The polarizing effect of oxo-m was alm ost completely suppressed by charybdotoxin (ChTX), a blocker of the la rge conductance (maxi) [Ca2+](i)-activated potassium channel (K-(Ca)). In the presence of 11.2 mM glucose, oxo-m (50 mu M) provoked a signif icant and transient increase in the Rb-86 efflux from perifused islets . This effect was inhibited by ChTX. ChTX also potentiated oxo-m stimu lated insulin secretion in the presence of glucose. Finally, the balan ce between the polarizing and depolarizing effects of oxo-m was variab le in different islets and depended on glucose concentration. Insulin secretion stimulated by oxo-m in the presence of glucose was more clos ely correlated to the agonist induced increase in burst frequency than to an increase in plateau fraction. We conclude that muscarinic stimu lation has at least two effects on beta-cell electrical activity, an i nitial hyperpolarization, owing to activation of K-(Ca) channels, foll owed by depolarization and high-frequency bursts, proposed to reflect the activation of a current sensitive to the depletion of intracellula r Ca2+ stores (CRAC).