Liu Yi-Jia, Vieira Elaine, Gylfe Erik
Department of Medical Cell Biology, Uppsala University Biomedical Centre, Husargatan 3, Box 571, SE-752 37 Uppsala, Sweden.
Cell Calcium. 2004 Apr;35(4):357-65. doi: 10.1016/j.ceca.2003.10.002.
The glucagon-releasing pancreatic alpha-cells are electrically excitable cells but the signal transduction leading to depolarization and secretion is not well understood. To clarify the mechanisms we studied Ca(2+) and membrane potential in individual mouse pancreatic alpha-cells using fluorescent indicators. The physiological secretagogue l-adrenaline increased Ca(2+) causing a peak, which was often followed by maintained oscillations or sustained elevation. The early effect was due to mobilization of Ca(2+) from the endoplasmic reticulum (ER) and the late one to activation of store-operated influx of the ion resulting in depolarization and Ca(2+) influx through voltage-dependent L-type channels. Consistent with such mechanisms, the effects of adrenaline on Ca(2+) and membrane potential were mimicked by inhibitors of the sarco(endo)plasmic reticulum Ca(2+) ATPase. The alpha-cells express ATP-regulated K(+) (K(ATP)) channels, whose activation by diazoxide leads to hyperpolarization. The resulting inhibition of the voltage-dependent Ca(2+) response to adrenaline was reversed when the K(ATP) channels were inhibited by tolbutamide. However, tolbutamide alone rarely affected Ca(2+), indicating that the K(ATP) channels are normally closed in mouse alpha-cells. Glucose, which is the major physiological inhibitor of glucagon secretion, hyperpolarized the alpha-cells and inhibited the late Ca(2+) response to adrenaline. At concentrations as low as 3mM, glucose had a pronounced stimulatory effect on Ca(2+) sequestration in the ER amplifying the early Ca(2+) response to adrenaline. We propose that adrenaline stimulation and glucose inhibition of the alpha-cell involve modulation of a store-operated current, which controls a depolarizing cascade leading to opening of L-type Ca(2+) channels. Such a control mechanism may be unique among excitable cells.
释放胰高血糖素的胰腺α细胞是电兴奋性细胞，但导致去极化和分泌的信号转导机制尚未完全清楚。为了阐明这些机制，我们使用荧光指示剂研究了单个小鼠胰腺α细胞中的[Ca(2+)]i和膜电位。生理性促分泌剂l-肾上腺素增加了[Ca(2+)]i，引起一个峰值，随后常伴有持续振荡或持续升高。早期效应是由于内质网（ER）中Ca(2+)的动员，后期效应是由于储存-操作性离子内流的激活，导致去极化和Ca(2+)通过电压依赖性L型通道内流。与这些机制一致，肾上腺素对[Ca(2+)]i和膜电位的影响被肌浆网Ca(2+) ATP酶抑制剂模拟。α细胞表达ATP调节的K(+)（K(ATP)）通道，其被二氮嗪激活会导致超极化。当K(ATP)通道被甲苯磺丁脲抑制时，对肾上腺素的电压依赖性[Ca(2+)]i反应的抑制作用被逆转。然而，单独的甲苯磺丁脲很少影响[Ca(2+)]i，表明K(ATP)通道在小鼠α细胞中通常是关闭的。葡萄糖是胰高血糖素分泌的主要生理性抑制剂，它使α细胞超极化并抑制对肾上腺素的后期[Ca(2+)]i反应。在低至3mM的浓度下，葡萄糖对ER中的Ca(2+)螯合有明显的刺激作用，放大了对肾上腺素的早期[Ca(2+)]i反应。我们提出，肾上腺素对α细胞的刺激和葡萄糖的抑制涉及对储存-操作性电流的调节，该电流控制导致L型Ca(2+)通道开放的去极化级联反应。这种控制机制在可兴奋细胞中可能是独特的。
