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Diphenylhydantoin Suppresses Glucose-Induced Insulin Release by Decreasing Cytoplasmic H⁺ Concentration in Pancreatic Islets

Department of Diabetes and Clinical Nutrition, Graduate School of Medicine, Kyoto University (K.N., S.F., M.S., R.K., Y.N., S.F., E.M., Y.Y., N.I.), Kyoto 606-8507, Japan; and Kansai-Denryoku Hospital (Y.S.), Osaka 553-0003, Japan

Address all correspondence and requests for reprints to: Dr. Shimpei Fujimoto, 54 Shogoin Kawahara-cho, Sakyo-ku, Kyoto 606-8507, Japan. E-mail: fujimoto{at}metab.kuhp.kyoto-u.ac.jp.

Diphenylhydantoin (DPH), which is clinically used in the treatment of epilepsy, inhibits glucose-induced insulin release from pancreatic islets by a mechanism that remains unknown. In the present study, DPH is shown to suppress glucose-induced insulin release concentration-dependently. In dynamic experiments, 20 µM DPH suppressed 16.7 mM glucose-induced biphasic insulin release. DPH also suppressed insulin release in the presence of 16.7 mM glucose, 200 µM diazoxide, and 30 mM K⁺ without affecting the intracellular Ca²⁺ concentration. DPH suppressed ATP content and mitochondrial membrane hyperpolarization in the presence of 16.7 mM glucose without affecting glucose utilization, glucose oxidation, and reduced nicotinamide adenine dinucleotide phosphate fluorescence. DPH increased cytoplasmic pH in the presence of high glucose, but the increase was abolished under Na⁺-deprived conditions and HCO₃^–-deprived conditions, suggesting that Na⁺ and HCO₃^– transport across the plasma membrane are involved in the increase in cytoplasmic pH by DPH. Alkalization by adding NH₄⁺ to the extracellular medium also suppressed insulin release, ATP content, and mitochondrial membrane hyperpolarization. Because ATP production from the mitochondrial fraction in the presence of substrates was decreased by increased pH in the medium, DPH suppresses mitochondrial ATP production by reducing the H⁺ gradient across mitochondrial membrane. Using permeabilized islets, the increase in pH was shown to decrease Ca²⁺ efficacy at a clamped concentration of ATP in the exocytotic system. Taken together, DPH inhibits glucose-induced insulin secretion not only by inhibiting mitochondrial ATP production, but also by reducing Ca²⁺ efficacy in the exocytotic system through its alkalizing effect on cytoplasm.

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				P. Stiernet, M. Nenquin, P. Moulin, J.-C. Jonas, and J.-C. Henquin Glucose-induced Cytosolic pH Changes in beta-Cells and Insulin Secretion Are Not Causally Related: STUDIES IN ISLETS LACKING THE NA+/H+ EXCHANGER NHE1 J. Biol. Chem., August 24, 2007; 282(34): 24538 - 24546. [Abstract] [Full Text] [PDF]