Fesenko E E, Kolesnikov S S, Lyubarsky A L
Nature. 1985;313(6000):310-3. doi: 10.1038/313310a0.
Vertebrate rod photoreceptors hyperpolarize when illuminated, due to the closing of cation-selective channels in the plasma membrane. The mechanism controlling the opening and closing of these channels is still unclear, however. Both 3',5'-cyclic GMP and Ca2+ ions have been proposed as intracellular messengers for coupling the light activation of the photopigment rhodopsin to channel activity and thus modulating light-sensitive conductance. We have now studied the effects of possible conductance modulators on excised 'inside-out' patches from the plasma membrane of the rod outer segment (ROS), and have found that cyclic GMP acting from the inner side of the membrane markedly increases the cationic conductance of such patches (EC50 30 microM cyclic GMP) in a reversible manner, while Ca2+ is ineffective. The cyclic GMP-induced conductance increase occurs in the absence of nucleoside triphosphates and, hence, is not mediated by protein phosphorylation, but seems rather to result from a direct action of cyclic GMP on the membrane. The effect of cyclic GMP is highly specific; cyclic AMP and 2',3'-cyclic GMP are completely ineffective when applied in millimolar concentrations. We were unable to recognize discrete current steps that might represent single-channel openings and closings modulated by cyclic GMP. Analysis of membrane current noise shows the elementary event to be 3 fA with 110 mM Na+ on both sides of the membrane at a membrane potential of -30 mV. If the initial event is assumed to be the closure of a single cyclic GMP-sensitive channel, this value corresponds to a single-channel conductance of 100 fS. It seems probable that the cyclic GMP-sensitive conductance is responsible for the generation of the rod photoresponse in vivo.
脊椎动物的视杆光感受器在受到光照时会发生超极化，这是由于质膜中阳离子选择性通道的关闭所致。然而，控制这些通道开闭的机制仍不清楚。3',5'-环鸟苷酸（cGMP）和钙离子都被认为是细胞内信使，用于将视色素视紫红质的光激活与通道活性相偶联，从而调节光敏感电导。我们现在研究了可能的电导调节剂对从视杆外段（ROS）质膜上切除的“内向外”膜片的影响，发现从膜内侧起作用的cGMP能以可逆方式显著增加此类膜片的阳离子电导（cGMP的半数有效浓度为30微摩尔），而钙离子则无效。cGMP诱导的电导增加在没有核苷三磷酸的情况下也会发生，因此不是由蛋白质磷酸化介导的，而是似乎源于cGMP对膜的直接作用。cGMP的作用具有高度特异性；当以毫摩尔浓度应用时，环磷酸腺苷（cAMP）和2',3'-环鸟苷酸完全无效。我们无法识别可能代表由cGMP调节的单通道开闭的离散电流阶跃。膜电流噪声分析表明，在膜电位为-30毫伏、膜两侧均有110毫摩尔钠离子的情况下，基本事件为3飞安。如果假定初始事件是单个cGMP敏感通道的关闭，这个值对应于单通道电导为100飞西门子。cGMP敏感电导很可能是体内视杆光反应产生的原因。
