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Unmasking of a Periodic Na⁺ Entry into Glucose-Stimulated Pancreatic ß-Cells after Partial Inhibition of the Na/K Pump¹

Department of Medical Cell Biology, Uppsala University, S-751 23 Uppsala, Sweden

Address all correspondence and requests for reprints to: Dr. E. Grapengiesser, Department of Medical Cell Biology, Biomedicum, Box 571, S-751 23 Uppsala, Sweden. E-mail: eva.grapengiesser{at}medcellbiol.uu.se

The cytoplasmic concentration of Na⁺ ([Na⁺]_i) was measured in individual mouse ß-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 µM ouabain resulted in oscillations of [Na⁺]_i in 65% of the cells (frequency, 0.13 ± 0.01 min^-1; amplitude, 4.4 ± 0.3 mM). The oscillations were unaffected by the presence of 3 µM tetrodotoxin, but disappeared when the medium was depleted of Ca²⁺ or supplemented with 10 µM 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 ß-cells exposed to 11 mM glucose. Ouabain (50 µM) 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.

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