cAMP-Signaling Pathway Acts in Selective Synergism With Glucose or Tolbutamide to Increase Cytosolic Ca²⁺ in Rat Pancreatic β-Cells

cAMP and the insulinotropic peptides that raise cAMP glucose-dependently increase the cytosolic free Ca²⁺ concentration ([Ca²⁺]_i) in pancreatic β-cells, which is tightly linked to the potentiation of glucose-induced insulin release. We 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 β-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 β-cells. cAMP appears to regulate β-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⁺ influx through this channel.