title: Diphenylhydantoin suppresses glucose-induced insulin release by decreasing cytoplasmic H+ concentration in pancreatic islets.
abstract: 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 microm DPH suppressed 16.7 mm glucose-induced biphasic insulin release. DPH also suppressed insulin release in the presence of 16.7 mm glucose, 200 microm 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.
authors.affiliation: Department of Diabetes and Clinical Nutrition, Graduate School of Medicine, Kyoto University, Kyoto 606-8507, Japan.
authors.forename: Koichiro
authors.initials: K
authors.lastname: Nabe
authors.forename: Shimpei
authors.initials: S
authors.lastname: Fujimoto
authors.forename: Makiko
authors.initials: M
authors.lastname: Shimodahira
authors.forename: Rieko
authors.initials: R
authors.lastname: Kominato
authors.forename: Yuichi
authors.initials: Y
authors.lastname: Nishi
authors.forename: Shogo
authors.initials: S
authors.lastname: Funakoshi
authors.forename: Eri
authors.initials: E
authors.lastname: Mukai
authors.forename: Yuichiro
authors.initials: Y
authors.lastname: Yamada
authors.forename: Yutaka
authors.initials: Y
authors.lastname: Seino
authors.forename: Nobuya
authors.initials: N
authors.lastname: Inagaki
mesh_terms: D000255:Adenosine Triphosphate / Q000032:analysis / Q000096:biosynthesis; D000818:Animals; D000927:Anticonvulsants / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism; D003981:Diazoxide / Q000494:pharmacology; D005947:Glucose / Q000494:pharmacology*; D006863:Hydrogen-Ion Concentration; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000187:drug effects* / Q000378:metabolism; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D009249:NADP / Q000032:analysis; D010672:Phenytoin / Q000494:pharmacology*; D011188:Potassium / Q000494:pharmacology; D000644:Quaternary Ammonium Compounds / Q000032:analysis / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar
chemical_list: D000927:Anticonvulsants; D007328:Insulin; D000644:Quaternary Ammonium Compounds; D009249:NADP; D010672:Phenytoin; D000255:Adenosine Triphosphate; D005947:Glucose; D003981:Diazoxide; D011188:Potassium; D002118:Calcium