Glucagon induces Ca2+-dependent increase of reduced pyridine nucleotides in mouse pancreatic beta-cells

Glucagon enhances the electrical activity of pancreatic beta-cells. The mechanism of the glucagon-evoked enhancement of electrical activity was investigated in terms of glucose metabolism. ICR mice aged 6-12 weeks were used for experiments. Intracellular Ca2+ increased in parallel with the enhancement of electrical activity. The stimulating effect of glucagon on Ca2+ oscillation was suppressed by calmodulin-antagonists (Chlorpromazine, W-7, and trifluoperazine). To trace the glucagon-evoked change in glucose metabolism, the reduced pyridine nucleotide (NAD(P)H) fluorescence was monitored using the microfluorometry with the excitation of 360 nm and the emission of 465 nm in islet cell clusters mainly consisting of beta-cells. In the presence of 2.5 mM Ca2+ glucagon (8.6 x 10(-8) M) increased the NAD(P)H fluorescence, while in the absence of Ca2+ the hormone had no effect on the fluorescence. Extracellular Ca2+ removal from the glucagon-containing perifusion solution decreased the fluorescence to the level which had been attained before glucagon was added. Chlorpromazine (10 mu M) reversed the glucagon-induced increase of NAD(P)H fluorescence as well as removing Ca2+. W-7 (15 mu M) and trifluoperazine (30 mu M) also suppressed the glucagon-induced increase of NAD(P)H. These results suggest that Ca2+/calmodulin system is involved in the acceleration of glycogenolysis by glucagon in beta-cells. On the basis of these observations, the mechanism of glucagon-induced enhancement of electrical activity and the relative ineffectiveness of glucagon at low glucose concentrations were discussed.