Metabolic and K-ATP channel-independent actions of keto acid initiators ofinsulin secretion

Insulin-releasing effects of 2-ketobutyric acid (KB), 2-ketoisocaproic acid (KIC), 2-keto-3-methylvaleric acid (KMV), and 3-phenylpyruvic acid (PP) we re examined by using clonal beta cells. Whereas KIC, KMV, and PP dose-depen dently initiated insulin secretion and potentiated the effects of 4.2-16.7 mM glucose, equimolar KB was without effect. Transport inhibition by using 10 mM valine, isoleucine, 2-cyano-3 hydroxycinnamate or 2-cyano-4 hydroxyci nnamate, or metabolic inhibition by 15 mM mannoheptulose, 5 mM sodium azide , 5 mM sodium cyanide, or removal of HCO3 reduced the secretory effects of KIC, KMV, and PP. Whereas K+ depletion reduced keto acid-induced insulin ou tput, depolarizing concentrations of L-leucine and L-arginine potentiated t he keto acid-induced effects. Under depolarizing conditions (25 mM KCl and 16.7 mM glucose), 10 mM KIC, KMV, or PP induced insulin secretion, suggesti ng K-ATP channel-independent actions. Furthermore, the K-ATP channel opener diazoxide reduced, but did not abolish, the keto acid-induced effects. How ever, voltage-dependent Ca2+ channel blockade with verapamil or removal of extracellular Ca2+ abolished keto acid-induced insulin release. Collectivel y, these results indicate that KIC, KMV, and PP initiate insulin secretion at least partially independently of K-ATP channel activity, through both mi tochondrial metabolism and regulation of Ca2+ influx.