Methyl pyruvate initiates membrane depolarization and insulin release by metabolic factors other than ATP

The role of mitochondria in stimulus-secretion coupling of pancreatic beta -cells was examined using methyl pyruvate (MP). MP stimulated insulin secre tion in the absence of glucose, with maximal effect at 5 mM. K+ (30 mM) alo ne, or in combination with diazoxide (100 muM), failed to enhance MP-induce d secretion. Diazoxide (100 muM) inhibited MP-induced insulin secretion. MP depolarized the beta -cell in a concentration-dependent manner (5-20 mM). The sustained depolarization induced by 20 mM MP was not influenced by 100 muM diazoxide, but the continuous spiking activity was suppressed by 500 mu M diazoxide. Pyruvate failed to initiate insulin release (5-20 mM) or to de polarize the membrane potential, ATP production in isolated beta -cell mito chondria was detected as accumulation of ATP in the medium during incubatio n in the presence of malate or glutamate in combination with pyruvate or MP . There was no difference in ATP production induced by pyruvate/malate or M P/malate in isolated beta -cell mitochondria. ATP production by MP/glutamat e was higher than that induced by pyruvate/glutamate, but it was much lower than that induced by alpha -ketoisocaproate/glutamate. Pyruvate (5 mM) or MP (5 mM) had no effect on the ATP/ADP ratio in whole islets, whereas gluco se (20 mM) significantly increased the whole islet ATP/ADP ratio. It is con cluded that MP-induced beta -cell membrane depolarization or insulin releas e does not relate directly to mitochondrial ATP production. Instead MP may exert a direct extramitochondrial effect, or it may stimulate beta -cell mi tochondria to produce coupling factors different from ATP to initiate insul in release.