Glucose-stimulated increase in cytoplasmic pH precedes increase in free Ca2+ in pancreatic beta-cells. A possible role for pyruvate.

The temporal relationship of glucose-induced increases in cytoplasmic pH (pHi) and cytoplasmic free Ca2+ was studied in single mouse pancreatic beta-cells and suspensions of clonal beta-cells (HIT). In both preparations of cells the increase in pHi preceded the cytoplasmic free Ca2+ increase. Therefore the alkalinization cannot be a consequence of the Ca2+ influx. A potential metabolic mechanism for the increase in pHi, involving stimulation of pyruvate transport and oxidation, was demonstrated in a model system of liver mitochondria incubated with pyruvate, ATP, and hexokinase to which glucose was then added to initiate ATP use. The involvement of this mechanism in beta-cells is suggested by the observation that the alkalinization was prevented in most cells by incubation with 3-hydroxycyanocinnamate, a mitochondrial pyruvate transport inhibitor. On the other hand, the inhibited cells exhibited normal Ca2+ responses to glucose stimulation. This indicates that neither pyruvate metabolism nor the alkalinization is of critical importance for the Ca2+ signal, though pyruvate oxidation or its metabolites may be important in downstream regulation of secretion.