The K+-ATP channel-independent pathway of regulation of insulin secretion by glucose: in search of the underlying mechanism.

By closing ATP-sensitive K+ (K+-ATP) channels, glucose promotes depolarization-dependent Ca2+ entry and cytoplasmic free Ca2+ concentration ([Ca2+]i) rise in beta-cells. Ca2+-dependent exocytosis of insulin granules is then potentiated by a K+-ATP channel-independent action of glucose. The underlying mechanisms of this second pathway are still unclear. They were studied by incubating normal mouse islets in the presence of diazoxide to open K+-ATP channels and 30 mmol/l K+ to restore Ca2+ entry. The effect of glucose did not require priming of beta-cells by preincubation in the presence of high glucose and could not be attributed to interaction of the sugar with a "glucoreceptor." There is no evidence that protein kinases A and C are involved in the K+-ATP channel-independent pathway, because inhibitors of the kinases did not alter the effect of glucose. In 3 mmol/l glucose, fatty acids did not influence K+-induced insulin secretion, even in the presence of bromopalmitate, an inhibitor of fatty acid oxidation. Bromopalmitate alone had no effect, but it decreased the potentiation that the fatty acids produce in 20 mmol/l glucose. It is thus unlikely that long-chain acyl CoAs mediate the effect of glucose. The action of glucose was not associated with an increase in arachidonic acid release from the islets and was not mimicked by exogenous arachidonic acid. Phospholipase A2 inhibitors antagonized the effect of glucose, but their action was not reversed by arachidonic acid or palmitate and was associated with a fall in islet ATP. No evidence could be found for the intervention of NO, cGMP, Mg, phosphate, phosphatidylinositol 3-kinase, or pertussis toxin-sensitive G-proteins. Formycin A, an adenosine analog that is converted to formycin A-triphosphate in islets, increased insulin secretion in the absence and presence of glucose. In conclusion, the present and our previous results strongly suggest that among all known potential second messengers, adenine nucleotides are the best candidates as regulators of insulin secretion through the K+-ATP channel-independent pathway.