Cote R H, Nicol G D, Burke S A, Bownds M D
Laboratory of Molecular Biology, University of Wisconsin, Madison 53706.
J Biol Chem. 1989 Sep 15;264(26):15384-91.
We have used a preparation of isolated, intact rod photoreceptors to correlate the effects of flash illumination on the intracellular cyclic GMP content and the membrane current. We find that the recovery of cyclic GMP levels after brief flash illumination requires approximately twice as much time as the recovery of the membrane current. In contrast, the subsecond kinetics of the cyclic GMP response to light are faster than the kinetics of membrane current suppression. Both cyclic GMP and the membrane current show graded responses to a wide range of flash intensities; however, in a low Ca2+-Ringer's solution, dim flashes can trigger a decrease in cyclic GMP concentration with no corresponding decrease in membrane current. These results suggest that either other factors can regulate the membrane current, or that measurements of total cellular cyclic GMP do not accurately reflect dynamic changes in cyclic GMP concentration in the vicinity of the light-regulated channel. Changes in cyclic GMP concentration in the presence of background illumination exhibit adaptational behavior similar to that observed in a light-adapted photoresponse: acceleration in the response kinetics and a decrease in response amplitude. That this result is observed in rods depleted of internal Ca2+ suggests a Ca2+-independent mechanism by which background illumination can accelerate the cyclic GMP response.
我们使用了分离的完整视杆光感受器制剂，以关联闪光照射对细胞内环鸟苷酸（cGMP）含量和膜电流的影响。我们发现，短暂闪光照射后cGMP水平的恢复所需时间约为膜电流恢复时间的两倍。相比之下，cGMP对光反应的亚秒级动力学比膜电流抑制的动力学更快。cGMP和膜电流对广泛的闪光强度均表现出分级反应；然而，在低钙林格氏溶液中，微弱闪光可引发cGMP浓度降低，而膜电流却无相应降低。这些结果表明，要么其他因素可调节膜电流，要么细胞总cGMP的测量不能准确反映光调节通道附近cGMP浓度的动态变化。在背景光照下cGMP浓度的变化表现出与光适应光反应中观察到的类似的适应性行为：反应动力学加速和反应幅度减小。在耗尽内部钙离子的视杆细胞中观察到这一结果，提示存在一种不依赖钙离子的机制，通过该机制背景光照可加速cGMP反应。
