cAMP-signaling pathway acts in selective synergism with glucose or tolbutamide to increase cytosolic Ca2+ in rat pancreatic beta-cells

cAMP and the insulinotropic peptides that raise cAMP glucose-dependently increase the cytosolic free Ca2+ concentration ([Ca2+](i)) in pancreatic beta-cells, which is tightly linked to the potentiation of glucose-induced insulin release. me examined whether cAMP increases [Ca2+](i) in specific cooperation only with glucose or also with other insulin secretagogues that act through different mechanisms. [Ca2+](i) in single rat pancreatic beta-cells was measured by dual-wavelength fura-2 microfluorometry. In the presence of a stimulatory concentration of glucose (8.3 mmol/l) and the moderate elevation in [Ca2+](i) induced by it, forskolin, an activator of adenylyl cyclase, or dibutyryl cAMP produced a marked additional increase in [Ca2(+)](i) but was ineffective at the basal 2.8 mmol/l glucose. These cAMP-elevating agents also potentiated the effect of tolbutamide on [Ca2+](i). The cAMP-induced increase in [Ca2+](i) was completely and selectively inhibited by a blocker of cAMP-dependent protein kinase A (PKA), and by nitrendipine, a blocker of the L-type Ca2+ channel. However, in the presence of high KCl and the [Ca2+](i) elevation induced by it, a rise in cAMP failed to further increase [Ca2+](i), whereas BAY K8644, an agonist of L-type Ca2+ channels, evoked an additional increase in [Ca2+](i). Under low Na+ conditions, the [Ca2+](i) response to cAMP was observed in the majority of the cells. In the cells in which glucose at 4.5-5 mmol/l was inadequate to increase [Ca2+](i), the glucose together with a rise in cAMP often increased [Ca2+](i). Likewise, tolbutamide and a rise in cAMP acted in concert to increase [Ca2+](i). Thus, cAMP left-shifted the concentration-[Ca2+](i) response relationship for glucose and tolbutamide. In conclusion, the cAMP-PKA pathway acts in selective synergism with glucose and tolbutamide to initiate [Ca2+](i) signals in pancreatic beta-cells. cAMP appears to regulate beta-cell sensitivity to glucose and tolbutamide. In contrast, cAMP fails to cooperate with high KCl to increase [Ca2+](i). It is suggested that cAMP acts mainly on a site that is more proximal but functionally linked to the L-type Ca2+ channel, thereby finally increasing Ca2+ flux through this channel.