Résumé
Glucose increases insulin secretion by raising cytoplasmic Ca2+([Ca2+]i) in β-cells (triggering pathway) and augmenting the efficacy of Ca2+ on exocytosis (amplifying pathway). It has been suggested that glutamate formed from α-ketoglutarate is a messenger of the amplifying pathway (Maechler, P., and Wollheim, C. B. (1999) Nature 402, 685–689). This hypothesis was tested with mouse islets depolarized with 30 mm KCl (+ diazoxide) or with a saturating concentration of sulfonylurea. Because [Ca2+]i was elevated under these conditions, insulin secretion was stimulated already in 0 mm glucose. The amplification of secretion produced by glucose was accompanied by an increase in islet glutamate. However, glutamine (0.5–2 mm) markedly augmented islet glutamate without affecting insulin secretion, whereas glucose augmented secretion without influencing glutamate levels when these were elevated by glutamine. Allosteric activation of glutamate dehydrogenase by BCH (2-amino 2-norbornane carboxylic acid) lowered islet glutamate but increased insulin secretion. Similar insulin secretion thus occurred at very different cellular glutamate levels. Glutamine did not affect islet [Ca2+]i and pHi, whereas glucose and BCH slightly raised pHi and either slightly decreased (30 mm KCl) or increased (tolbutamide) [Ca2+]i. The general dissociation between changes in islet glutamate and insulin secretion refutes a role of β-cell glutamate in the amplification of insulin secretion by glucose.