Zawalich W S, Zawalich K C
Yale University School of Nursing, New Haven, Connecticut 06536-0740, USA.
J Biol Chem. 1997 Feb 7;272(6):3527-31. doi: 10.1074/jbc.272.6.3527.
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.
丙酮酸甲酯（甲基丙酮酸）刺激分离的经灌注大鼠胰岛的胰岛素分泌呈剂量依赖性增加。释放的阈值水平约为10 mM，在20 mM时，向经灌注的胰岛中添加MP会导致大量的第一阶段分泌，随后是持续至少40分钟的胰岛素分泌反应。与20 mM葡萄糖的作用相比，对20 mM甲基丙酮酸的第一阶段峰值释放速率相当，但第二阶段的释放仅为等摩尔水平己糖观察到的约10 - 15%。20 mM甲基丙酮酸对分泌的刺激作用被K1 + -ATP通道阻滞剂二氮嗪（200 microM）和钙通道拮抗剂尼群地平（500 nM）消除。葡萄糖激酶抑制剂甘露庚酮糖（20 mM）对20 mM甲基丙酮酸的分泌反应没有不利影响，而10 microM福斯可林增强了MP的促胰岛素作用。单独的丙酮酸钠或与10 microM福斯可林联合使用没有促胰岛素作用。在额外的实验中，胰岛磷酸肌醇池用肌醇-2-[3H]肌醇标记，随后标记的肌醇磷酸的积累用于监测磷脂酶C的激活。在与20 mM甲基丙酮酸孵育30分钟后测量时，甲基丙酮酸刺激肌醇磷酸水平呈剂量依赖性增加，在20 mM甲基丙酮酸时最大增加约300%。由甲基丙酮酸（20 mM）引起的磷酸肌醇水解增加，与胰岛素分泌一样，被二氮嗪和尼群地平降低，但不受甘露庚酮糖抑制。丙酮酸（20 mM）对肌醇磷酸积累没有影响。先前短期暴露于甲基丙酮酸会使胰岛对随后15 mM葡萄糖的刺激敏感。丙酮酸钠不会使胰岛敏感。这些发现支持这样的概念，即营养分子的线粒体代谢足以急性增强β细胞的胰岛素释放，增加磷脂酶C介导的磷酸肌醇水解，并诱导胰岛素分泌的时间依赖性增强。
