Glucose inhibits GABA release by pancreatic beta-cells through an increase in GABA shunt activity

GABA is the major inhibitory neurotransmitter in the nervous system. It is also released by the insulin-producing beta-cells, providing them with a potential paracrine regulator. Because glucose was found to inhibit GABA release, we investigated whether extracellular GABA can serve as a marker for glucose-induced mitochondrial activity and thus for the functional state of beta-cells. GABA release by rat and human beta-cells was shown to reflect net GABA production, varying with the functional state of the cells. Net GABA production is the result of GABA formation through glutamate decarboxylase (GAD) and GABA catabolism involving a GABA-transferase (GABA-T)-mediated shunt to the TCA cycle. GABA-T exhibits K(m) values for GABA (1.25 mM) and for alpha-ketoglutarate (alpha-KG; 0.49 mM) that are, respectively, similar to and lower than those in brain. The GABA-T inhibitor gamma-vinyl GABA was used to assess the relative contribution of GABA formation and catabolism to net production and release. The nutrient status of the beta-cells was found to regulate both processes. Glutamine dose-dependently increased GAD-mediated formation of GABA, whereas glucose metabolism shunts part of this GABA to mitochondrial catabolism, involving alpha-KG-induced activation of GABA-T. In absence of extracellular glutamine, glucose also contributed to GABA formation through aminotransferase generation of glutamate from alpha-KG; this stimulatory effect increased GABA release only when GABA-T activity was suppressed. We conclude that GABA release from beta-cells is regulated by glutamine and glucose. Glucose inhibits glutamine-driven GABA formation and release through increasing GABA-T shunt activity by its cellular metabolism. Our data indicate that GABA release by beta-cells can be used to monitor their metabolic responsiveness to glucose irrespective of their insulin-secretory activity.