Glutamate Is Not a Messenger in Insulin Secretion

Abstract

Experiments do not support a recent claim that glutamate formed from the amination of citric acid cyclederived ␣-ketoglutarate is a messenger in glucose-induced insulin secretion (Maechler, P., and Wollheim, C. (1999) Nature 402, 685-689). Glucose, leucine, succinic acid methyl ester, and ␣-ketoisocaproic acid all markedly stimulate insulin release but do not increase glutamate levels in pancreatic islets. Increasing the intracellular glutamate levels to 10-fold higher than basal levels by adding glutamine to islets does not stimulate insulin release. When leucine, in addition to glutamine, is applied to islets, insulin release is almost as high as with glucose alone. This is consistent with the known ability of leucine to allosterically activate glutamate deamination by glutamate dehydrogenase, which can supply ␣-ketoglutarate to the citric acid cycle. Experiments with mitochondria from pancreatic islets suggest that flux through the glutamate dehydrogenase reaction is quiescent during glucose-induced insulin secretion. These experiments support the traditional idea that when insulin release is associated with flux through glutamate dehydrogenase, the flux is in the direction of ␣-ketoglutarate.Recently Maechler and Wollheim (1), on the basis of intricate and broad-based experiments, proposed that glutamate generated from citric acid cycle-derived ␣-ketoglutarate is a messenger in glucose-induced insulin secretion. The glutamate effect was not robust, and its demonstration seemed to require rather narrowly defined conditions. For example, dimethylglutamate, a glutamate precursor that is permeable to the plasma membrane, caused a leftward shift in the concentration dependence of glucose-stimulated insulin release in INS-1 insulinoma cells. Dimethylglutamate did not stimulate insulin release at a basal concentration of glucose (2.5 mM) or augment insulin release at concentrations of glucose (16.7 to 25 mM) optimal for insulin release. Insulin release by dimethylglutamate was potentiated only at intermediate glucose concentrations. It was reported that when rat insulinoma INS-1 cells were incubated in the presence of a concentration of glucose (12.8 mM) that stimulates insulin release, cellular glutamate levels increased 4.8-fold to a stimulated level of 0.22 mM within 30 min. It was also observed that glucose (16.7 mM) augmented glutamate levels in human pancreatic islets from a basal level of 0.78 to a stimulated level of 3.93 nmol of glutamate per mg of islet protein. However, even these stimulated levels of glutamate are quite low. Our calculations indicate that the basal (0.04 to 0.08 mM) and stimulated glutamate levels (0.2 to 0.4 mM) reported by Maechler and Wollheim (1) are far lower than the basal concentration of glutamate of 1 to 7 mM found in many tissues (2) including the pancreatic islets used in our current study (see below) and in islets studied by others (3, 4).1 In addition, others have observed previously that glucose does not increase glutamate in islets (3). Thus, even though sophisticated app...

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The Mitochondrial Citrate/Isocitrate Carrier Plays a Regulatory Role in Glucose-stimulated Insulin Secretion

“…However, studies in several laboratories have failed to identify increases in glutamate levels during glucose stimulation of ␤-cells (8,9,40). Moreover, glutamate probably serves as an insulin secretagogue via glutaminolysis or oxidation of glutamate in the TCA cycle (41).…”

Mitochondrial regulation of insulin production in rat pancreatic islets

“…However, the kinetics of insulin release and of proinsulin biosynthesis, and the metabolic signals mediating these processes, are not identical [3,8]. In fact, the role of various glycolytic and TCA cycle intermediates in glucose-stimulated insulin secretion is controversial [7,[9][10][11][12][13][14][15][16], and even less is known about the metabolic signals for glucose-stimulated insulin production. A previous report by Alarcon et al suggested that succinate is the key signal for glucose stimulation of preproinsulin mRNA translation [3].…”