The stimulus-secretion coupling of amino acid-induced insulin release. Biosynthetic and secretory responses of rat pancreatic islet to L-leucine and L-glutamine

L-glutamine dramatically augments insulin release evoked by L-leucine. The dynamics, specificity and environmental modifications of the insulin secretory response to the combination of L-glutamine and L-leucine were investigated. L-glutamine alone failed to stimulate proinsulin biosynthesis or insulin release in rat pancreatic islets, but augmented synthesis and secretion evoked by L-leucine. In relation to the secretory response, L-glutamine could be replaced by L-asparagine but not L-glutamate; L-leucine could be replaced by L-norvaline or L-isoleucine but not by L-valine, L-norleucine, glycine, L-serine, L-lysine, L-phenylalanine or L-arginine. The secretory response to L-leucine was rapid and biphasic whereas the enhancing action of L-glutamine upon insulin release was progressive. The release of insulin evoked by the combination of L-leucine and L-glutamine was inhibited by antimycin A, menadione, ammonium ions, verapamil, adrenaline, and by the absence of extracellular Ca2+. It was increased at high carbon dioxide tension, and by glucose, theophylline and cytochalasin B. It is concluded that the enhancing action of L-glutamine upon insulin release evoked by certain amino acids represents a phenomenon dependent on the integrity of oxidative metabolism and involving essentially the same sequence of metabolic, ionic and motile events as that characterizing the process of glucose-induced insulin release.