Schnetkamp P P, Szerencsei R T
University of Calgary, Department of Medical Biochemistry, Alberta, Canada.
J Membr Biol. 1989 Apr;108(1):91-102. doi: 10.1007/BF01870428.
Micromolar concentrations of silver ion activate large Ca2+ fluxes across the plasma membrane of intact rod outer segments isolated from bovine retinas (intact ROS). The rate of Ag+-induced Ca2+ efflux from intact ROS depended on the Ag+ concentration in a sigmoidal manner suggesting a cooperative mechanism with a Hill coefficient between 2 and 3. At a concentration of 50 microM Ag+ the rate of Ca2+ efflux was 7 x 10(6) Ca2+/outer segment/sec; this represents a change in total intracellular Ca2+ by 0.7 mM/outer segment/sec. Addition of the nonselective ionophore gramicidin in the absence of external alkali cations greatly reduced the Ag+-induced Ca2+ efflux from intact ROS, apparently by enabling internal alkali cations to leak out. Adding back alkali cations to the external medium restored Ag+-induced Ca2+ efflux when gramicidin was present. In the presence of gramicidin, Ag+-induced Ca2+ efflux from intact ROS was blocked by 50 microM tetracaine or L-cis diltiazem, whereas without gramicidin both blockers were ineffective. Both L-cis diltiazem and tetracaine are blockers of one kinetic component of cGMP-induced Ca2+ flux across ROS disk membranes. The ion selectivity of the Ag+-induced pathway proved to be broad with little discrimination between the alkali cations Li+, Na+, K+, and Cs+ or between Ca2+ and Mg2+. The properties of the Ag+-induced pathway(s) suggest that it may reflect the cGMP-dependent conductance opened in the absence of cGMP by silver ions.
微摩尔浓度的银离子可激活从牛视网膜分离出的完整视杆细胞外段（完整ROS）质膜上的大量钙离子通量。银离子诱导的完整ROS中钙离子外流速率呈S形依赖于银离子浓度，这表明存在一种协同机制，希尔系数在2到3之间。在50微摩尔银离子浓度下，钙离子外流速率为7×10⁶个钙离子/外段/秒；这相当于细胞内总钙离子浓度以0.7毫摩尔/外段/秒的速度变化。在没有外部碱金属阳离子的情况下添加非选择性离子载体短杆菌肽，可显著降低银离子诱导的完整ROS中钙离子外流，显然是通过使内部碱金属阳离子泄漏出去实现的。当存在短杆菌肽时，向外部介质中重新添加碱金属阳离子可恢复银离子诱导的钙离子外流。在存在短杆菌肽的情况下，50微摩尔的丁卡因或L-顺式地尔硫卓可阻断银离子诱导的完整ROS中钙离子外流，而没有短杆菌肽时，这两种阻断剂均无效。L-顺式地尔硫卓和丁卡因都是环鸟苷酸诱导的钙离子穿过ROS盘膜通量的一个动力学成分的阻断剂。银离子诱导途径的离子选择性很广，对碱金属阳离子Li⁺、Na⁺、K⁺和Cs⁺之间或Ca²⁺和Mg²⁺之间几乎没有区分。银离子诱导途径的特性表明，它可能反映了在没有环鸟苷酸的情况下由银离子打开的环鸟苷酸依赖性电导。
