Glucose regulation of glutaminolysis and its role in insulin secretion

Leucine or the nonmetabolized leucine analog +/- 2-amino-2-norbornane-carbo xylic acid (BCH) (both at 10 mmol/l) induced biphasic insulin secretion in the presence of 2 mmol/l glutamine (Q2) in cultured mouse islets pretreated for 40 min without glucose but with Q2 present. The beta-cell response con sisted of an initial peak of 20- to 25-fold above basal and a less marked s econdary phase. However, BCH produced only a delayed response, while leucin e was totally ineffective when islets were pretreated with 25 mmol/l glucos e plus Q2. With Q2, 10 mmol/l BCH or leucine caused a nearly threefold incr ease, a twofold increase, or had no effect on cytosolic Ca2+ levels in isle ts pretreated for 40 min with 0, 5, or 15 mmol/l glucose, respectively. Thu s, pretreatment of islets with high glucose inhibited BCH- and leucine-indu ced cytosolic Ca2+ changes and insulin release. Glucose decreased glutamine oxidation in cultured rat islets when BCH was present at 10 mmol/l, but no t in its absence, with a lowest effective level of similar to 0.1 mmol/l, a maximum of 18-30 mmol/l, and an inhibitory concentration, 50%, of similar to 3 mmol/l. The data are consistent with the hypothesis that glucose inhib its glutaminolysis in pancreatic beta-cells in a concentration-dependent ma nner and hence blocks leucine-stimulated insulin secretion. We postulate th at in the basal inter-prandial state, glutaminolysis of beta-cells is partl y turned on because glutamate dehydrogenase (GDH) is activated by a decreas ed beta-potential due to partial fuel depletion and sensitization to endoge nous activators such as leucine. Additionally, it may contribute significan tly to basal insulin release, which is known to be responsible for about ha lf of the insulin released daily. The data explain "leucine-hypersensitivit y" of 3-cells during hypoglycemia and contribute to the elucidation of the GDH-linked syndrome of hyperinsulinism associated with elevated serum ammon ia levels. Thus, understanding the precise regulation and role of beta-cell glutaminolysis is probably central to our concept of normal blood glucose control.