Modulation ofβ-Cell Ouabain-SensitiveR86b+Influx (Na+/k+Pump) by D-Glucose, Glibenclamide or Diazoxide

Abstract

The activity of theβ-cellNa+/k+pump was studied by using ouabain-sensitive (lmM ouabain)R86b+influx inβ-cell-rich islets of Umeå-ob/obmice as an indicator of the pump function. The present results show that the stimulatory effect of glucose on ouabain-sensitiveR86b+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 theNa+/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 ofR86b+influx dunng the subsequent 5-min incubation. Glibenclamide stimulated the ouabain-sensitiveR86b+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-sensitiveR86b+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 theNa+/k+ATPase activity in homogenates of islets. Diazoxide (0.4mM) inhibited theNa+/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 theNa+/k+pump, probably as part of the membrane repolarisation process.