Influence of Pyruvic Acid Methyl Ester on Rat Pancreatic Islets

Abstract

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 firstphase 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 K 1؉ -ATP channel blocker diazoxide (200 M) 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 M forskolin amplified the insulinotropic action of MP. Sodium pyruvate alone or in combination with 10 M forskolin had no insulinotropic effect. In additional experiments islet phosphoinositide pools were labeled with myo-2-[ 3 H]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.The regulation of fuel-induced insulin secretion from pancreatic beta cells depends upon the intermediary metabolism of these compounds via several established metabolic pathways (1). In addition to glucose, mannose, glyceraldehyde, and dihydroxyacetone, which are metabolized initially by cytosolic glycolytic enzymes (2-5), a variety of insulinotropic nutrient molecules are metabolized solely within the mitochondria. These include among others leucine, monomethylsuccinate, and ␣-ketoisocaproate (6 -10). Because these latter stimulants for insulin secretion share in common many of the same stimulatory features of glucose on the beta cell, the concept that mitochondrial-derived signals mediate, at least in part, the stimulatory actions of the hexose has been proposed (11, 12). For example, both glucose and ␣-ketoisocaproate stimulate similar changes in i...

Overexpression of monocarboxylate transporter and lactate dehydrogenase alters insulin secretory responses to pyruvate and lactate in β cells

“…We hypothesize that this difference is due to low transport capacity for pyruvate in native β cells. The fact that a membrane-permeable analogue of pyruvate, methylpyruvate, is a potent secretagogue in isolated islets supports this idea (12,13,26). To test this hypothesis, we infected isolated rat islets with AdCAMCT-1 to increase the monocarboxylate transport activity in the β cell.…”

“…Pyruvate and lactate have been reported to be metabolized, but do not cause insulin secretion in pancreatic islets (11)(12)(13)(14)(15). Hence, the oxidation rate of 30 mM [1-14 C]pyruvate was found to be as high as that of 16.7 mM [U-14 C]glucose without any increase in insulin secretion (11).…”

“…In studies using whole rat or mouse islets, it has been observed that pyruvate cannot mimic the effect of glucose on insulin secretion, despite active metabolism (11)(12)(13). This has been referred to as the "pyruvate paradox."…”

Mitochondrial Metabolism Sets the Maximal Limit of Fuel-stimulated Insulin Secretion in a Model Pancreatic Beta Cell

“…The so-called "pyruvate paradox" is one such example because the mitochondrial fuel pyruvate is readily oxidized in islet preparations but does not elicit insulin secretion (20,21). In contrast, glyceraldehyde, which functions both as a cytosolic and mitochondrial fuel, is a potent secretagogue (22)(23)(24).…”

Cells depleted of mitochondrial DNA (p0) yield insight into physiological mechanisms

“…A fundamental question underlying glucose-induced insulin secretion relates to the source of the increased [ATP] in response to glucose uptake. Some investigators have argued that glycolytic, but not Krebs cycle metabolism of glucose is critically involved in this signaling process [24,25]. Studies have demonstrated that inhibitors of glycolysis suppress glucose-stimulated insulin secretion, whereas methyl pyruvate can generate a normal secretory response.…”