Differential effects of glucose and glyburide on energetics and [Na.sup.+] levels of βHC9 cells: nuclear magnetic resonance spectroscopy and respirometry studies. (Islet Studies)

Inhibition of ATP-sensitive [K.sup.+] channels ([K.sub.ATP]) by an increase in cytosolic [ATP.sup.4-] or a decrease in Mg[ADP.sup.-] and the resultant depolarization are essentially involved in the physiological mechanisms that lead [...]
In the present study, noninvasive [sup.31]P and [sup.23][Na.sup.+]-nuclear magnetic resonance (NMR) technology and respirometry were used to compare the effect of high glucose (30 mmol/l) with the effect of the antidiabetic sulfonylurea (SU) compound glyburide (GLY) on energy metabolism, [Na.sup.+] flux, insulin, and cAMP release of continuously superfused β-HC9 cells encapsulated in microscopic agarose heads. Both high glucose and GLY increased oxygen consumption in β-HC9 cells (15-30%) with a maximal effect at 8 mmol/l for glucose and at 250 nmol/l for GLY. At the same time, insulin release from β-cells increased by 15- and 25-fold with high glucose or GLY, respectively. The P-creatine (PCr) level was greatly increased and inorganic phosphate ([P.sub.i]) was decreased with 30 mmol/l glucose in contrast to the decreased level of PCr and increased [P.sub.i] with GLY. ATP levels remained unchanged during both interventions. Studies on isolated mitochondria of β-HC9 cells showed that GLY added to mitochondria oxidizing glutamine or glutamate abolished the stimulation of respiration by ADP (state 3) meanwhile leaving state 3 respiration unchanged during oxidation of other substrates. Exposure of β-HC9 cells to 5 mmol/l glucose decreased intracellular [Na.sup.+] levels monitored by [sup.23][Na.sup.+]-NMR spectroscopy and 30 mmol/l glucose resulted in a further decrease in cytosolic [Na.sup.+]. In contrast, [Na.sup.+] increased when 1 µmol/l GLY was added to the perfusate containing 5 mmol/l glucose. These data support the hypothesis that glucose activates the β-cell through a 'push mechanism' due to substrate pressure enhancing fuel flux, energy production, and extrusion of [Na.sup.+] from the cells in contrast to SU receptor (SUR)-1 inhibitors, which may modify intermediary and energy metabolism secondarily through a 'pull mechanism' due to higher energy demand resulting from increased ion fluxes and the exocytotic work load.