Metabolic and K(ATP) channel-independent actions of keto acid initiators of insulin secretion

Insulin-releasing effects of 2-ketobutyric acid (KB), 2-ketoisocaproic acid (KIC), 2-keto-3-methylvaleric acid (KMV), and 3-phenylpyruvic acid (PP) were examined by using clonal beta cells. Whereas KIC, KMV, and PP dose- dependently 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 hydroxycinnamate, 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 output, depolarizing concentrations of L-leucine and L-arginine potentiated the keto acid-induced effects. Under depolarizing conditions (25 mM KCl and 16.7 mM glucose), 10 mM KIC, KMV, or PP induced insulin secretion, suggesting K(ATP) channel-independent actions. Furthermore, the K(ATP) channel opener diazoxide reduced, but did not abolish, the keto acid-induced effects. However, voltage-dependent Ca²⁺ channel blockade with verapamil or removal of extracellular Ca²⁺ abolished keto acid-induced insulin release. Collectively, these results indicate that KIC, KMV, and PP initiate insulin secretion at least partially independently of K(ATP) channel activity, through both mitochondrial metabolism and regulation of Ca²⁺ influx.