D-glucose Stimulates the Na(+)/K(+) Pump in Mouse Pancreatic Islet Cells

To determine the effect of D-glucose on the β-cell Na(+)/K(+) pump, (86)Rb(+) influx was studied in isolated, -cell-rich islets of Umeå-ob/ob mice in the absence or presence of lmM ouabain. D-glucose (20 mM) stimulated the ouabain-sensitive portion of (86)Rb(+) influx by 65%, whereas the ouabain-resistant portion was inhibited by 48%. The Na(+)/K(+) ATPase activity in homogenates of islets of Umeå-ob/ob mice or normal mice was determined to search for direct effects of D-glucose. Thus, ouabain-sensitive ATP hydrolysis in islet homogenates was measured in the presence of different D-glucose concentrations. No effect of D-glucose (3–20 mM) was observed in either ob/ob or normal islets at the optimal Na(+)/K(+) ratio for the enzyme (135 mM Na(+) and 20 mM K(+)). Neither D-glucose (3–20 mM) nor L-glucose or 3-O-methyl-D-glucose (20 mM) affected the enzyme activity at a high Na(+)/K(+) ratio (175 mM Na(+) and 0.7mM K(+)). Diphenylhydantoin (150 μM) decreased the enzyme activity at optimal Na(+)/K(+) ratio, whereas 50 μM of the drug had no effect. The results suggest that D-glucose induces a net stimulation the Na(+)/K(+) pump of β-cells in intact islets and that D-glucose does not exert any direct effect on the Na(+)/K(+) ATPase activity.