To determine the effect of D-glucose on the β-cell
Na+/k+ pump, R86b+ 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 R86b+
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.
