Karpen J W, Zimmerman A L, Stryer L, Baylor D A
Department of Neurobiology, Sherman Fairchild Science Center, Stanford University School of Medicine, CA 94305.
Proc Natl Acad Sci U S A. 1988 Feb;85(4):1287-91. doi: 10.1073/pnas.85.4.1287.
The gating kinetics of the cGMP-activated cation channel of salamander retinal rods have been studied in excised membrane patches. Relaxations in patch current were observed after two kinds of perturbation: (i) fast jumps of cGMP concentration, generated by laser flash photolysis of a cGMP ester ("caged" cGMP), and (ii) membrane voltage jumps, which perturb activation of the channel by cGMP. In both methods the speed of activation increased with the final cGMP concentration. The results are explained by a simple kinetic model in which activation involves three sequential cGMP binding steps with bimolecular rate constants close to the diffusion-controlled limit; fully liganded channels undergo rapid open-closed transitions. Voltage perturbs activation by changing the rate constant for channel closing, which increases with hyperpolarization. Intramolecular transitions of the fully liganded channel limit the kinetics of activation at high cGMP concentrations (greater than 50 microM), whereas at physiological cGMP concentrations (less than 5 microM), the kinetics of activation are limited by the third cGMP binding step. The channel appears to be optimized for rapid responses to changes in cytoplasmic cGMP concentration.
蝾螈视网膜视杆细胞中环鸟苷酸（cGMP）激活的阳离子通道的门控动力学已在膜片钳实验中进行了研究。在两种扰动后观察到膜片电流的松弛现象：（i）cGMP浓度的快速跃升，通过激光闪光光解cGMP酯（“笼化”cGMP）产生；（ii）膜电压跃升，它干扰cGMP对通道的激活。在这两种方法中，激活速度均随最终cGMP浓度的增加而加快。这些结果由一个简单的动力学模型解释，其中激活涉及三个连续的cGMP结合步骤，双分子速率常数接近扩散控制极限；完全结合配体的通道经历快速的开闭转换。电压通过改变通道关闭的速率常数来干扰激活，该速率常数随超极化而增加。在高cGMP浓度（大于50μM）时，完全结合配体通道的分子内转换限制了激活动力学，而在生理cGMP浓度（小于5μM）时，激活动力学受第三个cGMP结合步骤限制。该通道似乎针对细胞质中cGMP浓度的变化进行了快速响应的优化。
