Influence of pyruvic acid methyl ester on rat pancreatic islets. Effects on insulin secretion, phosphoinositide hydrolysis, and sensitization of the beta cell.

The methyl ester of pyruvic acid (methyl pyruvate) stimulated a dose-dependent increase in insulin secretion from isolated perifused rat islets. The threshold level for release was about 10 mM, and at 20 mM the addition of MP to perifused islets resulted in a large first phase of secretion followed by an insulin-secretory response that was sustained for at least 40 min. When compared to the effects of 20 mM glucose, peak first-phase release rates in response to 20 mM methyl pyruvate were comparable, but the second phase of release was only about 10-15% of that observed with an equimolar level of the hexose. The stimulatory effects of 20 mM methyl pyruvate on secretion were abolished by the K1+-ATP channel blocker diazoxide (200 microM) and by the calcium channel antagonist nitrendipine (500 nM). The glucokinase inhibitor mannoheptulose (20 mM) had no adverse effect on the secretory response to 20 mM methyl pyruvate, whereas 10 microM forskolin amplified the insulinotropic action of MP. Sodium pyruvate alone or in combination with 10 microM forskolin had no insulinotropic effect. In additional experiments islet phosphoinositide pools were labeled with myo-2-[3H]inositol, and the subsequent accumulation of labeled inositol phosphates was used to monitor the activation of phospholipase C. Methyl pyruvate stimulated a dose-dependent increase in inositol phosphate levels when measured after a 30-min incubation period with a maximal increase of about 300% at 20 mM methyl pyruvate. The increase in phosphoinositide hydrolysis caused by methyl pyruvate (20 mM) was, like insulin secretion, reduced by both diazoxide and nitrendipine but was immune to inhibition by mannoheptulose. Pyruvate (20 mM) had no effect on inositol phosphate accumulation. Prior short-term exposure to methyl pyruvate sensitized islets to subsequent stimulation with 15 mM glucose. Sodium pyruvate did not sensitize islets. These findings support the concept that the mitochondrial metabolism of nutrient molecules is an event sufficient to acutely augment insulin release from the beta cell, to increase phospholipase C-mediated phosphoinositide hydrolysis, and to induce time-dependent potentiation of insulin secretion.