Yaekura K, Kakei M, Yada T
Department of Physiology, Kagoshima University School of Medicine, Japan.
Diabetes. 1996 Mar;45(3):295-301. doi: 10.2337/diab.45.3.295.
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. 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 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+ influx through this channel.
环磷酸腺苷（cAMP）以及能以葡萄糖依赖方式提高cAMP水平的促胰岛素肽，可增加胰腺β细胞胞质中的游离钙离子浓度（[Ca2+]i），这与葡萄糖诱导的胰岛素释放增强紧密相关。我们研究了cAMP是仅与葡萄糖特异性协同增加[Ca2+]i，还是也与通过不同机制起作用的其他胰岛素促分泌剂协同增加[Ca2+]i。采用双波长fura-2显微荧光测定法测量大鼠单个胰腺β细胞中的[Ca2+]i。在存在刺激浓度的葡萄糖（8.3 mmol/L）并由其诱导[Ca2+]i适度升高的情况下，腺苷酸环化酶激活剂福斯可林或二丁酰环磷腺苷（dibutyryl cAMP）可使[Ca2+]i显著进一步升高，但在基础葡萄糖浓度2.8 mmol/L时无效。这些升高cAMP的药物还增强了甲苯磺丁脲对[Ca2+]i的作用。cAMP诱导的[Ca2+]i升高被环磷酸腺苷依赖性蛋白激酶A（PKA）阻滞剂和L型钙离子通道阻滞剂尼群地平完全且选择性地抑制。然而，在存在高钾（KCl）并由其诱导[Ca2+]i升高的情况下，cAMP升高未能进一步增加[Ca2+]i，而L型钙离子通道激动剂BAY K8644可使[Ca2+]i额外升高。在低钠条件下，大多数细胞中观察到了对cAMP的[Ca2+]i反应。在葡萄糖浓度为4.5 - 5 mmol/L不足以增加[Ca2+]i的细胞中，葡萄糖与cAMP升高共同作用时往往可增加[Ca2+]i。同样，甲苯磺丁脲与cAMP升高协同作用可增加[Ca2+]i。因此，cAMP使葡萄糖和甲苯磺丁脲的浓度 - [Ca2+]i反应关系左移。总之，cAMP - PKA途径与葡萄糖和甲苯磺丁脲选择性协同作用，在胰腺β细胞中启动[Ca2+]i信号。cAMP似乎调节β细胞对葡萄糖和甲苯磺丁脲的敏感性。相比之下，cAMP与高钾（KCl）不能协同增加[Ca2+]i。提示cAMP主要作用于一个更靠近L型钙离子通道但在功能上与之相连的位点，从而最终增加通过该通道的钙离子内流。
