Biphasic insulin response to high glucose and a role of protons and calcium

In order to determine the role of protons and Ca++ in the biphasic insulin response to glucose, we studied the effect of monensin, a carboxylic ionophore, on the first phase and second phase of glucose-induced insulin release and Ca++ efflux from perifused rat pancreatic islets. The agent, 1-100 nM, dose dependently inhibited the insulin release from the islets incubated for 60 min in Krebs-Ringer bicarbonate buffer containing 16.7 mM glucose. Islet 14CO2 production rates from D-(U-14C)glucose were not affected by 10 or 100 nM monensin. Perifusion of the islets prelabeled with 45Ca++ demonstrated that 100 nM monensin had only a slight inhibitory effect on the first phase insulin response to 16.7 mM glucose and no effect on 45Ca++ efflux. This agent inhibited the second phase insulin release and depressed 45Ca++ efflux. When monensin was added at the start of the second phase release, the release was inhibited. When exposed to the agent before the stimulation by glucose, the first phase insulin release was observed, albeit significantly decreased, and the start of the insulin release and 45Ca++ efflux was delayed. The agent, added 30 min after the change to high glucose, immediately inhibited the insulin release. Thus, the first phase insulin release is mediated mainly through a mechanism which is not related to protons generated from glucose metabolism. Protons may be a crucial coupling factor in the second phase insulin release.