Effect of Rise in cAMP Levels on Ca²⁺ Influx Through Voltage-Dependent Ca²⁺ Channels in HIT Cells: Second-Messenger Synarchy in β-Cells

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.