With a glucose-responsive (β-cell line (HIT cells), we tested the hypothesis that the cytosolic free Ca2+ level ([Ca2+],) is an intracellular signal through which a rise in cyclic AMP (cAMP) levels is transmitted to potentiate glucose-stimulated insulin secretion. In these cells, glucose stimulates the acute release of insulin without increasing [Ca2+], or altering cAMP content. Either forskolin or 3-isobutylmethylxanthine (IBMX) potentiated glucose-stimulated insulin secretion and increased cAMP levels. At either a submaximal glucose concentration or maximally stimulatory glucose concentration, both IBMX and forskolin triggered a rapid rise in [Ca2+], (1.9- and 1.5-fold increase over basal levels, respectively). Similarly, glucagon stimulated a 1.3-fold increase in [Ca2+]1 over basal levels. The effect on [Ca2+]1 required glucose and was secondary to Ca2+ influx through voltage-dependent Ca2+ channels because it was blocked by either chelation of extracellular Ca2+ with EGTA or by the Ca2+-channel blockers verapamil and nimodipine. Verapamil also inhibited IBMX potentiation of glucose-stimulated insulin secretion and the IBMX-induced rise in [ca2+]1 in a dose-dependent manner with IC50s of 2 × 10−5 and 4 × 10−6 M, respectively. We conclude that in the β-cell, a rise in cAMP levels increases Ca2+ influx through voltage-dependent Ca2+channels and that this represents a mechanism by which cAMP potentiates glucose-stimulated insulin secretion in β-cells.

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