Similarities in the Stimulus-Secretion Coupling Mechanisms of Glucose- and 2-Keto Acid-Induced Insulin Release*

Abstract

The stimulus-secretion coupling of 2-keto acid-induced insulin release was investigated using 2-ketoisocaproate (4-methyl-2-oxopentanoate) as the principal model secretagogue. 2-Ketoisocaproate and 2-ketocaproate (2-oxo-, hexanoate) provoked changes in B cell electrical behavior characterized by an initial depolarization of the membrane potential, followed by rapid spike activity, which appeared either in a bursting pattern or as continuous activity. The onset of spike activity induced by 2-ketoisocaproate (5 mM) was biphasic in nature. The dynamic pattern of 2-ketoisocaproate-induced insulin release was also biphasic. 2-[U-14C]Ketoisocaproate (10 mM) was oxidized in islet tissue at a rate equivalent to that of [U-14C]glucose (17 mM) and a t a higher rate than 2-ketoisovalerate (3-methyl-2-oxobutyrate) and 2-keto-3-methyl-valerate, which were poor secretagogues. Like glucose, 2-ketoisocaproate provoked characteristic changes in 86Rb and 45Ca efflux from prelabeled islets and stimulated 45Ca net uptake. Proinsulin synthesis was stimulated by 2-ketoisocaproate through both a general effect on protein synthesis and a specific effect on hormonal biosynthesis. 2-Ketoisocaproate and 2-ketocaproate reproduced the effect of glucose on the islet content of ATP, ADP, AMP, NAD+, NADH, NADP+, and NADPH. These findings together with a series of observations on the effects upon the above parameters of site-specific inhibitors, e.g. respiratory inhibitors, suloctidil, theophylline, and epinephrine, suggested that the stimulus-secretion-coupling mechanisms for 2-ketoisocaproate- and glucose-induced release are similar. It is postulated that glucose- and 2-keto acid-induced insulin release may be initiated by a common signal.