Unmasking of a periodic Na+ entry into glucose-stimulated pancreatic beta-cells after partial inhibition of the Na/K pump.

The cytoplasmic concentration of Na+ ([Na+]i) was measured in individual mouse beta-cells using dual wavelength microfluorometry and the indicator sodium-binding benzofuran isophtalate. Under conditions known to induce large amplitude oscillations in cytoplasmic Ca2+ (1.3 mM Ca2+; 11 mM glucose), [Na+]i remained low and stable at 10-14 mM. Partial suppression of the Na/K pump with 50 microM ouabain resulted in oscillations of [Na+]i in 65% of the cells (frequency, 0.13+/-O.O1 min(-l); amplitude, 4.4 +/-0.3 mM). The oscillations were unaffected by the presence of 3 microM tetrodotoxin, but disappeared when the medium was depleted of Ca2+ or supplemented with 10 microM methoxyverapamil. The analysis of the ouabain effect was facilitated by replacing extracellular Ca2+ with 5 mM Sr2+. In the Sr2+-containing medium, oscillations of [Na+]i were seen in more than 70% of the beta-cells exposed to 11 mM glucose. Ouabain (50 microM) modified the [Na+]i oscillations by increasing their amplitudes almost 3-fold and reducing the frequency from once every 3 min to once every 10 min. A relationship between oscillations of cytoplasmic Sr2+ and Na+ was apparent both from observations of similar frequencies and for the modifications obtained with ouabain. It is concluded that the glucose-induced oscillations of cytoplasmic Ca2+ result in a rhythmic entry of Na+, usually balanced by the Na/K pump. A resulting periodic consumption of ATP in the Na/K pump might have implications for the release of insulin by affecting ATP-dependent processes associated with the plasma membrane.