Modulation of β-Cell Ouabain-Sensitive (86)Rb(+) Influx (Na(+)/K(+) Pump) by D-Glucose, Glibenclamide or Diazoxide

The activity of the β-cell Na(+)/K(+) pump was studied by using ouabain-sensitive (lmM ouabain) (86)Rb(+) influx in β-cell-rich islets of Umeå-ob/ob mice as an indicator of the pump function. The present results show that the stimulatory effect of glucose on ouabain-sensitive (86)Rb(+) influx reached its approximate maximum at 5mM glucose. Pre-treatment of the islets with 20mM glucose for 60 min strongly reduced the glucose-induced stimulation of the Na(+)/K(+) pump. Pre-treatment (60 or 180 min) of islets at 0mM glucose, on the other hand, did not affect the magnitude of the glucose-induced stimulation of (86)Rb(+) influx dunng the subsequent 5-min incubation. Glibenclamide stimulated the ouabain-sensitive (86)Rb(+) uptake in the same manner as glucose. The stimulatory effect, showed its apparent maximum at 0.5μM. Pre-treatment (60 min) of islets with 1μM glibenclamide did not reduce the subsequent stimulation of the ouabain-sensitive (86)Rb(+) influx. The stimulatory effect of glibenclamide and D-glucose were not .additive, suggesting that they may have the same mechanism of action. No direct effect of glibenclamide (0.01-1μM) was observed on the Na(+)/K(+) ATPase activity in homogenates of islets. Diazoxide (0.4mM) inhibited the Na(+)/K(+) pump. This effect was sustained even after 60 min of pre-treatment of islets with 0.4mM diazoxide. The stimulatory effect of glibenclamide and D-glucose were abolished by diazoxide. It is concluded that nutrient as well as non-nutrient insulin secretagogues activate the Na(+)/K(+) pump, probably as part of the membrane repolarisation process.