The Role Metabolism, Cytoplasmic Ca2+, and Ph-regulating Exchangers in Glucose-induced Rise of Cytoplasmic Ph in Normal Mouse Pancreatic-islets

Intact mouse islets were loaded with 2',7'-bis(2-car boxyethyl)-5(6)-carboxyfluorescein to study the effects of glucose on cytoplasmic pH (pH(i)) in pancreatic B-cells. In HCO3- buffer, glucose produced a steady-state increase in pH(i) that required metabolism of the sugar and was concentration-dependent between 0 and 10 mM (K-m similar to 5 mM) before plateauing at a maximum value of similar to 0.2 pH units. In HEPES buffer, glucose concentrations above 7 mM caused an initial rise followed by a secondary decrease and an eventual return to about initial values. Inhibition of Ca2+ influx had little effect on the pH(i) changes produced by glucose in HCO3- medium, but unmasked an alkalinizing effect in HEPES buffer. Raising cytoplasmic Ca2+ by 30 mM potassium caused a larger acidification in HEPES than in HCO3- buffer, but a subsequent rise in glucose now increased pH(i) in both types of buffer. In the presence of 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS; inhibitor of HCO3-/ Cl- exchange), the effect of glucose on pH(i) in HCO3- buffer became similar to that in HEPES buffer. After inhibition of the Na+/H+ exchanger by dimethylamiloride, glucose produced a marked and sustained fall in pH(i) in HEPES buffer. A similar fall was seen in HCO3- buffer only when DIDS and dimethylamiloride were present together. However, if Ca2+ influx was prevented when both exchangers were blocked, glucose increased pH(i). In conclusion, the metabolism of glucose tends to increase pH(i) in B-cells, whereas the concomitant rise in [Ca2+](i) exerts an acidifying action. In HEPES buffer, this acidifying effect of Ca2+ is offset by the operation of the Na+/H+ exchanger. In physiological HCO3- buffer, the activity of the HCO3-/Cl- exchanger overcompensates and leads to an increase in pH(i).