Intrasynaptic [glutamate] must be kept low in order to maximize the signal-to-noise ratio after the release of transmitter glutamate. This is accomplished by rapid uptake of glutamate into astrocytes, which convert glutamate into glutamine. The latter then is released to neurons, which, via mitochondrial glutaminase, form the glutamate that is used for neurotransmission. This pattern of metabolic compartmentation is the "glutamate-glutamine cycle." This model is subject to the following two important qualifications: 1) brain avidly oxidizes glutamate via aspartate aminotransferase; and 2) because almost no glutamate crosses from blood to brain, it must be synthesized in the central nervous system (CNS). The primary source of glutamate carbon is glucose, and a major source of glutamate nitrogen is the branched-chain amino acids, which are transported rapidly into the CNS. This arrangement accomplishes the following: 1) maintenance of low external [glutamate], thereby maximizing signal-to-noise ratio upon depolarization; 2) the replenishing of the neuronal glutamate pool; 3) the "trafficking" of glutamate through the extracellular fluid in a nonneuroactive form (glutamine); 4) the importation of amino groups from blood, thus maintaining brain nitrogen homeostasis; and 5) the oxidation of glutamate/glutamine, a process that confers an additional level of control in terms of the regulation of brain glutamate, aspartate and gamma-aminobutyric acid.

Glutamate dehydrogenase (GDH) is regulated by both positive (leucine and ADP) and negative (GTP and ATP) allosteric factors. We hypothesized that the phosphate potential of ␤-cells regulates the sensitivity of leucine stimulation. These predictions were tested by measuring leucine-stimulated insulin secretion in perifused rat islets following glucose depletion and by tracing the nitrogen flux of [2-15 N]glutamine using stable isotope techniques. The sensitivity of leucine stimulation was enhanced by long time (120-min) energy depletion and inhibited by glucose pretreatment. After limited 50-min glucose depletion, leucine, not ␣-ketoisocaproate, failed to stimulate insulin release. ␤-Cells sensitivity to leucine is therefore proposed to be a function of GDH activation. Leucine increased the flux through GDH 3-fold compared with controls while causing insulin release. High glucose inhibited flux through both glutaminase and GDH, and leucine was unable to override this inhibition. These results clearly show that leucine induced the secretion of insulin by augmenting glutaminolysis through activating glutaminase and GDH. Glucose regulates ␤-cell sensitivity to leucine by elevating the ratio of ATP and GTP to ADP and P i and thereby decreasing the flux through GDH and glutaminase. These mechanisms provide an explanation for hypoglycemia caused by mutations of GDH in children.In addition to glucose, amino acids and other metabolic fuels are important stimulants of insulin secretion from pancreatic ␤-cells. Leucine, which has been studied intensively, may stimulate insulin release through two different mechanisms. The first involves transamination of leucine to ␣-ketoisocaproate (KIC) 1 and subsequent mitochondrial oxidation. The second promotes insulin release via allosteric activation of glutamate dehydrogenase (GDH) causing oxidation of glutamate to the Krebs cycle intermediate, ␣-ketoglutarate, plus ammonia. The importance of the latter mechanism has been highlighted recently by the discovery of a dominant form of congenital hyperinsulinism associated with mutations of GDH leading to a gain of enzyme activity, because sensitivity to inhibition by GTP and ATP is impaired (1-3). Affected children have increased ␤-cell responsiveness to leucine and are susceptible to acute hypoglycemia following a high protein meal (4). The involvement of GDH may explain the observation that, in contrast to other amino acids, leucine-stimulated insulin secretion (LSIS) is suppressed by high glucose. For example, Gao et al. (5) reported that glucose inhibits leucine stimulation of glutaminolysis and insulin secretion in isolated mouse islets, presumably by increasing intracellular ATP and GTP while decreasing ADP and thus inhibiting GDH activity.GDH has also been proposed by Maechler and Wollheim (6) to play an essential role in glucose-mediated insulin secretion by acting in the reverse direction to catalyze production of glutamate, which is hypothesized to work as a cofactor in the process leading to exocytosis of insulin granules. T...

Children with hypoglycemia due to recessive loss of function mutations of the ␤-cell ATP-sensitive potassium (K ATP ) channel can develop hypoglycemia in response to protein feeding. We hypothesized that amino acids might stimulate insulin secretion by unknown mechanisms, because the K ATP channel-dependent pathway of insulin secretion is defective. We therefore investigated the effects of amino acids on insulin secretion and intracellular calcium in islets from normal and sulfonylurea receptor 1 knockout (SUR1؊/؊) mice. Even though SUR1؊/؊ mice are euglycemic, their islets are considered a suitable model for studies of the human genetic defect. SUR1؊/؊ islets, but not normal islets, released insulin in response to an amino acid mixture ramp. This response to amino acids was decreased by 60% when glutamine was omitted. Insulin release by SUR1؊/؊ islets was also stimulated by a ramp of glutamine alone. Glutamine was more potent than leucine or dimethyl glutamate. Basal intracellular calcium was elevated in SUR1؊/؊ islets and was increased further by glutamine. In normal islets, methionine sulfoximine, a glutamine synthetase inhibitor, suppressed insulin release in response to a glucose ramp. This inhibition was reversed by glutamine or by 6-diazo-5-oxo-L-norleucine, a non-metabolizable glutamine analogue. High glucose doubled glutamine levels of islets. Methionine sulfoximine inhibition of glucose stimulated insulin secretion was associated with accumulation of glutamate and aspartate. We hypothesize that glutamine plays a critical role as a signaling molecule in amino acid-and glucosestimulated insulin secretion, and that ␤-cell depolarization and subsequent intracellular calcium elevation are required for this glutamine effect to occur.