Insulin secretion by isolated islets of Langerhans from 19 human donors (9 women and 10 men) was studied in vitro to test the hypothesis that human islets contain both the K(ATP) channel-dependent and the K(ATP) channel-independent signaling pathways. The results demonstrated the presence of both of these major pathways of glucose signaling. Thus, insulin secretion was stimulated by high glucose concentrations, by the sulfonylurea tolbutamide, and by a depolarizing concentration of potassium chloride. Diazoxide, which activates the K(ATP) channel, completely blocked the stimulation of release by glucose. Stimulation of insulin release by tolbutamide, which inhibits the K(ATP) channel and depolarizes the beta-cell, and inhibition of glucose-stimulated release by diazoxide, which activates the channel and repolarizes the beta-cell, confirm the involvement of the K(ATP) channel-dependent pathway in glucose signaling. The participation of the K(ATP) channel-independent pathway in the stimulation of insulin release by glucose was demonstrated for the first time in human islets. This was done in two ways. The first method, in the presence of diazoxide, blocked the action of glucose on the K(ATP) channel in combination with a depolarizing concentration of KCl to raise [Ca2+]i. Under these conditions, glucose stimulated insulin release. A second method to demonstrate the involvement of the K(ATP) channel-independent pathway was to close the K(ATP) channels with tolbutamide. Again, with no possibility of further action on the K(ATP) channel, glucose stimulated insulin release. In a final series of experiments, glucose-stimulated insulin release was profoundly inhibited by somatostatin, clonidine, and prostaglandin E2, but not by galanin.

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