Haynes L W, Stotz S C
Department of Physiology and Biophysics, Faculty of Medicine, University of Calgary, Alberta, Canada.
Vis Neurosci. 1997 Mar-Apr;14(2):233-9. doi: 10.1017/s0952523800011378.
Inside-out patches containing cGMP-gated channels were excised from catfish rod or cone outer segments and held under voltage clamp. The net cGMP-dependent currents elicited by saturating and subsaturating concentrations of cGMP at +/-30 mV were measured and the dependence of current upon cGMP concentration was determined. The apparent affinity of the channel for its ligand was estimated by fitting these data with the Hill equation. The concentration of cGMP required to give half the maximum current (K1/2) in rod and cone channels at +30 mV was approximately 28 microM and approximately 37 microM, respectively, and was weakly voltage dependent. Thus, cone channels have an intrinsically higher K1/2 than rod channels. For both types of channel, the Hill coefficient was approximately 2.3. In the presence of calcium-calmodulin, the apparent affinity of the rod channel for cGMP decreased by about twofold, but the apparent affinity of the cone channels was unaffected. These results indicate that the open probability of the cone channel for its ligand cannot be modulated by calmodulin. This represents the first significant departure between rod and cone photoreceptors in mechanisms used by phototransduction and suggests that the beta subunit of the cone channel must be different from that of the rod channel.
从鲶鱼视杆或视锥细胞外段切下包含环鸟苷酸门控通道的内向外膜片，并置于电压钳制下。测量了在+/-30 mV时由饱和和亚饱和浓度的环鸟苷酸引发的净环鸟苷酸依赖性电流，并确定了电流对环鸟苷酸浓度的依赖性。通过用希尔方程拟合这些数据来估计通道对其配体的表观亲和力。在+30 mV时，视杆和视锥通道产生最大电流一半（K1/2）所需的环鸟苷酸浓度分别约为28 microM和约37 microM，且对电压的依赖性较弱。因此，视锥通道的固有K1/2高于视杆通道。对于这两种类型的通道，希尔系数约为2.3。在存在钙调蛋白的情况下，视杆通道对环鸟苷酸的表观亲和力降低了约两倍，但视锥通道的表观亲和力未受影响。这些结果表明，钙调蛋白不能调节视锥通道对其配体的开放概率。这代表了光转导所使用机制中视杆和视锥光感受器之间的第一个重大差异，并表明视锥通道的β亚基必须与视杆通道的不同。
