Glucose-induced Cytosolic pH Changes in β-Cells and Insulin Secretion Are Not Causally Related.

The contribution of Na+/H+ exchange (achieved by NHE proteins) to the regulation of β-cell cytosolic pHc, and the role of pHc changes in glucose-induced insulin secretion are disputed and were examined here. Using real-time PCR, we identified plasmalemmal NHE1 and intracellular NHE7 as the two most abundant NHE isoforms in mouse islets. We, therefore, compared insulin secretion, cytosolic free Ca2+ ([Ca2+]c) and pHc in islets from normal mice and mice bearing an inactivating mutation of NHE1 (Slc9A1-swe/swe). The experiments were performed in HCO-3/CO2 or HEPES/NaOH buffers. PCR and functional approaches showed that NHE1 mutant islets do not express compensatory pH-regulating mechanisms. NHE1 played a greater role than HCO-3-dependent mechanisms in the correction of an acidification imposed by a pulse of NH4Cl. In contrast, basal pHc (in low glucose) and the alkalinization produced by high glucose were independent of NHE1. Dimethylamiloride, a classic blocker of Na+/H+ exchange, did not affect pHc but increased insulin secretion in NHE1 mutant islets, indicating unspecific effects. In control islets, glucose similarly increased [Ca2+]c and insulin secretion in HCO-3 and HEPES buffer, although pHc changed in opposite directions. The amplification of insulin secretion that glucose produces when [Ca2+]c is clamped at an elevated level by KCl was also unrelated to pHc and pHc changes. All effects of glucose on [Ca2+]c and insulin secretion proved independent of NHE1. In conclusion, NHE1 protects -cells against strong acidification, but has no role in stimulus-secretion coupling. The changes in pHc produced by glucose involve HCO-3-dependent mechanisms. Variations in β-cell pHc are not causally related to changes in insulin secretion. [ABSTRACT FROM AUTHOR]