Mitochondrial activation directly triggers the exocytosis of insulin in permeabilized pancreatic beta-cells

Abstract

In the pancreatic beta-cell, insulin secretion is stimulated by glucose metabolism resulting in membrane potential-dependent elevation of cytosolic Ca2+ ([Ca2+]c). This cascade involves the mitochondrial membrane potential (delta psi[m]) hyperpolarization and elevation of mitochondrial Ca2+ ([Ca2+]m) which activates the Ca(2+)-sensitive NADH-generating dehydrogenases. Metabolism-secretion coupling requires unidentified signals, other than [Ca2+]c, possibly generated by the mitochondria through the rise in [Ca2+]m. To test this paradigm, we have established an alpha-toxin permeabilized cell preparation permitting the simultaneous monitoring of [Ca2+] with mitochondrially targeted aequorin and insulin secretion under conditions of saturating [ATP] (10 mM) and of clamped [Ca2+]c at substimulatory levels (500 nM). The tricarboxylic acid (TCA) cycle intermediate succinate hyperpolarized delta psi(m), raised [Ca2+]m up to 1.5 microM and stimulated insulin secretion 20-fold, without changing [Ca2+]c. Blockade of the uniporter-mediated Ca2+ influx into the mitochondria abolished the secretory response. Moreover, glycerophosphate, which raises [Ca2+]m by hyperpolarizing delta psi(m) without supplying carbons to the TCA cycle, failed to stimulate exocytosis. Activation of the TCA cycle with citrate evoked secretion only when combined with glycerophosphate. Thus, mitochondrially driven insulin secretion at permissive [Ca2+]c requires both a substrate for the TCA cycle and a rise in [Ca2+]m. Therefore, mitochondrial metabolism generates factors distinct from Ca2+ and ATP capable of inducing insulin exocytosis.