Glucose-induced increase in cytoplasmic pH in pancreatic β-cells is mediated by Na⁺/H⁺ exchange, an effect not dependent on protein kinase C

Glucose-induced changes in cytoplasmic pH (pH_i) were investigated using pancreatic β-cells isolated from obese hyperglycemic mice. Glucose, at concentrations above 3-5 mM, depolarized the β-cell and increased pH_i, cytoplasmic free Ca²⁺ ([Ca²⁺]_i), and insulin release. This increase in pH_i was dependent on the presence of extracellular Na⁺ and was inhibited by 5-(N-ethyl-N-isopropyl) amiloride, a blocker of Na⁺/H⁺ exchange. Stimulation of protein kinase C with phorbol ester also induced an alkalinization. However, when protein kinase C activity was down-regulated, glucose stimulation still induced alkalinization. At 20 mM glucose, 10 mM NH₄Cl induced a marked rise in pH_i, paralleled by repolarization, inhibition of electrical activity, and decreases in both [Ca²⁺]_i and insulin release. Reduction in [Ca²⁺]_i was prevented by 200 μM tolbutamide, but not by 10 mM tetraethylammonium. At 4 mM glucose, NH₄Cl induced a transient increase in insulin release, without changing [Ca²⁺]_i. Exposure of β-cells to 10 mM sodium acetate caused a persistent decrease in pH_i, an effect paralleled by a small transient increase in [Ca²⁺]_i. Acidification per se did not change the β-cell sensitivity to glucose, not excluding that the activity of the ATP-regulated K⁺ channels may be modulated by changes in pH_i.