Abstract

In the present study, noninvasive ³¹P and ²³Na⁺–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⁺ flux, insulin, and cAMP release of continuously superfused β-HC9 cells encapsulated in microscopic agarose beads. 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_i) was decreased with 30 mmol/l glucose in contrast to the decreased level of PCr and increased P_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⁺ levels monitored by ²³Na⁺-NMR spectroscopy and 30 mmol/l glucose resulted in a further decrease in cytosolic Na⁺. In contrast, Na⁺ 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⁺ 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.