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Abstract

The regulation of intracellular pH (pH_i) and its role in the insulin-secretory process were evaluated, by using the clonal insulin-secreting cell line RINm5F. Glyceraldehyde, lactate and dihydroxyacetone decreased pH_i, but only the first two released insulin. In the presence of extracellular Na⁺ the cells counteracted the acidification. Blocking the Na⁺/H⁺ exchange in acidic cells resulted in a drastic further lowering of pH_i, an effect not obtained under basal conditions. Whereas glyceraldehyde depolarized the cells, lactate was without effect. Dihydroxyacetone hyperpolarized the cells in the presence of extracellular Na⁺, but this effect disappeared when Na⁺ was excluded from the medium. Stimulation with glyceraldehyde resulted in increased free cytoplasmic Ca²⁺ concentration ([Ca²⁺]_i). Dihydroxyacetone and lactate had no effect on [Ca²⁺]_i in the presence of Na⁺, but lactate induced a decrease in [Ca²⁺]_i in Na⁺-deficient medium. In RINm5F cells the activity of the Na⁺/H⁺ antiport could not be augmented by activation of protein kinase C (PKC). Hence, in insulin-secreting cells a PKC-insensitive Na⁺/H⁺ antiport is the major mechanism restoring a decrease in pH_i. Acidification itself does not affect membrane potential, but may directly interact with the mechanisms regulating exocytosis.