Methyl pyruvate stimulates pancreatic β-cells by a direct effect on KATP channels, and not as a mitochondrial substrate

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Abstract
In pancreatic beta-cells, methyl pyruvate is a potent secretagogue and is widely used to study stimulus-secretion coupling. In contrast with pyruvate, which barely stimulates insulin secretion, methyl pyruvate was suggested to act as an effective mitochondrial substrate. We show that methyl pyruvate elicited electrical activity in the presence of 0.5 mM glucose, in contrast with pyruvate. Accordingly, methyl pyruvate increased the cytosolic free Ca(2+) concentration after an initial decrease, similar to glucose. The initial decrease was inhibited by thapsigargin, suggesting that methyl pyruvate stimulates ATP production. This assumption is supported by the observation that methyl pyruvate hyperpolarized the mitochondrial membrane potential, similar to glucose. However, in contrast with glucose, methyl pyruvate even slightly decreased NAD(P)H autofluorescence and did not influence ATP production or the ATP/ADP ratio. This observation questions the suggestion that methyl pyruvate acts as a powerful mitochondrial substrate. The finding that methyl pyruvate directly inhibited a cation current across the inner membrane of Jurkat T-lymphocyte mitochondria suggests that this metabolite may increase ATP production in beta-cells by activating the respiratory chains without providing reduction equivalents. We conclude that this mechanism may account for a slight and transient increase in ATP production. We further show that methyl pyruvate inhibited the K(ATP) current measured in the standard whole-cell configuration, an effect that was at least partly antagonized by diazoxide. Accordingly, single-channel currents in inside-out patches were blocked by methyl pyruvate. We conclude that inhibition of K(ATP) channels, and not activation of metabolism, mediates the induction of electrical activity in pancreatic beta-cells by methyl pyruvate.