平滑肌细胞膜的钙激活Cl~-通道

在多种平滑肌上都已发现Ｃａ２＋激活Ｃｌ－通道。胞内游离钙升高是钙激活氯通道的必要条件。多种刺激剂诱导胞内钙库释放钙而同时激活钾通道［ＩＫ（Ｃａ）］和氯通道［ＩＣｌ（Ｃａ）］。平滑肌细胞上激活ＩＣｌ（Ｃａ）的［Ｃａ２＋］ｉ阈值因动物种属和组织差异而不同。用荧光指示剂直接测定大鼠门静脉平滑肌细胞上的［Ｃａ２＋］ｉ得出激活ＩＫ（Ｃａ）的最小［Ｃａ２＋］ｉ应大于７０～８０μｍｏｌ·Ｌ－１，比激活ＩＣｌ（Ｃａ）的最低浓度１８０μｍｏｌ·Ｌ－１要小，因此认为ＩＫ（Ｃａ）要比ＩＣｌ（Ｃａ）对［Ｃａ２＋］ｉ更敏感。胞外钙通过电压依赖性钙通道进入胞内，［Ｃａ２＋］ｉ升高也能激活氯通道。Ｇ蛋白与某些受体偶联激活胞内第二信使ＩＰ３而激活氯通道。钙激活氯通道的电导很小，从全细胞电流分析应小于１０ｐＳ。平滑肌细胞上的氯平衡电位（ＥＣｌ）正于静息膜电位，因此Ｃｌ－通道开放Ｃｌ－外流驱动膜电位向ＥＣｌ方向靠近，形成膜的去极化。Ｃａ２＋激活Ｃｌ－通道开放使细胞膜去极化并引起细胞的兴奋，这种通道在由激素或神经递质引起平滑肌细胞的兴奋过程中起重要作用。

Calcium activated chloride channel on smooth muscle cell membrane

Calcium activated chloride channel existed on several kind of smooth muscle cells. The necessary condition for activating calcium activated chlorede channel is intracellular calcium Ca 2+ ] i level rising. Both potassium channel and chloride channel are activated by several kind of activators induced calcium releasing from calcium store. The threshold values for activating I Cl(Ca) are differente from animal categories and tissues. Using flourometric measurement of Ca 2+ ] i of rat portacaval smooth muscle cells get that least Ca 2+ ] i value of activated I K(Ca) should be considered more than 70～80 nmol·L -1 . This value is smaller than the least Ca 2+ ] i of 180 nmol·L -1 of I Cl(Ca) , therefore considered that I K(Ca) is more sensitive than that of I Cl(Ca) for Ca 2+ ] i. I Cl(Ca) is activated by Ca 2+ ] i rising resulting from extracellular calcium pass through the voltage dependent calcium channels. Because of IP 3 is activated by G protein coupling with some receptor, so that I Cl(Ca) is activated. According to analyse whole cell currents, the conductance of I Cl(Ca) should be considered smaller than 10 pS. The chloride equilibrium potential (ECl) of smooth muscle cells is more positive than resting membrane potential. The chloride outflow from chloride channels opening, which promote membrane potential approaching to ECl, therefore membrane depolarizing. When the calcium activated cholride channels are opening the cell membrane depolarized, so that induced cells exciting. This channel plays an important role during smooth muscle cells exciting which induced by hormones and neuro transmiters.