Glucose-dependent increase in mitochondrial membrane potential, but not cytoplasmic calcium, correlates with insulin...

We examined the effects of different physiological concentrations of glucose on cytoplasmic Ca(2+) handling and mitochondrial membrane potential (Deltapsi(m)) and insulin secretion in single mouse islet cells. The threshold for both glucose-induced changes in Ca(2+) and Deltapsi(m) ranged from 6 to 8 mM. Glucose step-jumps resulted in sinusoidal oscillations of cytoplasmic Ca(2+), whereas Deltapsi(m) reached sustained plateaus with oscillations interposed on the top of these plateaus. The amplitude of the Ca(2+) rise (height of the peak) did not vary with glucose concentration, suggesting a "digital" rather than "analog" character of this aspect of the oscillatory Ca(2+) response. The average glucose-dependent elevation of cytoplasmic Ca(2+) concentration during glucose stimulation reached saturation at 8 mM stimulatory glucose, whereas Deltapsi(m) showed a linear glucose dose-response relationship over the range of stimulatory glucose concentrations (4-16 mM). Glucose-dependent increases in insulin secretion correlated well with Deltapsi(m), but not with average Ca(2+) concentration. These data show that an ATP-dependent K(+) channel-independent pathway is operative at the single cell level and suggest mitochondrial metabolism may be a determining factor in explaining graded, glucose concentration-dependent increases in insulin secretion.