Energetic Requirement of Carbachol-Induced Ca2+Signaling in Single Mouse β-Cells1

Insulin secretion is under multifactorial control by glucose and neurohumoral factors like acetylcholine (ACH), which activate the Ca2+/phospholipase C signaling pathway. All insulin secretagogues elevate cytosolic free Ca2+ ([Ca2+]i) that is central to the stimulation of insulin secretion. The actions of ACH on [Ca2+]i are glucose dependent but the metabolic steps involved are only partly understood. Here we have characterized the metabolic steps by which glucose exerts its synergistic effects on ACH-linked Ca2+-signals. [Ca2+]i was measured in single fura-2 loaded beta-cells. The ACH analog carbachol (3 microM) caused rise in [Ca2+]i that was strongly dependent on the extracellular glucose concentration ranging from 0-10 mM. Iodoacetate, which blocks glycolysis, thereby preventing the generation of NADH and ATP from glucose metabolism, and rotenone or antimycin, which inhibit complex 1 and 2 of the mitochondrial respiratory chain, respectively, inhibited in glucose (6 mM) the carbachol-induced Ca2+ signal to a similar extent as glucose deprivation. This demonstrates that glucose metabolism and generation of ATP through oxidative phosphorylation of energy rich substrates like NADH and FADH2 are required for carbachol-induced Ca2+ signals. While sodium arsenate, which prevents net glycolytic production of ATP without inhibiting glycolysis, had no significant effect on the carbachol-induced Ca2+-signal, the mitochondrial pyruvate transport inhibitor alpha-cyano-4-hydroxycinnamate and the Krebs cycle inhibitor monofluoroacetate strongly suppressed the rise in [Ca2+]i elicited by carbachol. While pyruvate was ineffective, methyl pyruvate, a membrane-permeant pyruvate analog, and alpha-ketoisocaproate in combination with glutamine, which are both substrates for mitochondrial ATP production, could restore the carbachol-induced Ca2+ signal in glucose-free medium. These data demonstrate for the first time that Krebs cycle metabolism of glucose and ATP formation through oxidative phosphorylation is critical for the glucose dependency of ACH-linked Ca2+-signals in mouse beta-cells, and they suggest that mitochondrial metabolism plays a key role in the interactive regulation of beta-cells by neurohumoral factors activating the Ca2+/phospholipase C signaling pathway.