A study on the mechanism of light adaptation of vertebrate photoreceptors with the patch clamp method.

有相当多的推测认为，细胞质Ca^<2+>浓度的降低可能在脊椎动物光感受器的光适应中起作用。为了研究Ca^2+对光转导级联反应的稳态作用，我们检测了降低外部Ca^2+浓度、磷酸二酯酶抑制剂（IBMX）和几种鸟苷酸环化酶兴奋剂对蛙视杆闪光反应的影响。膜电流记录从一个孤立的，黑暗适应杆吸其内段到玻璃移液管含有林格氏溶液。将细胞外节暴露于可更换的浴液中，在1- 10 μ M Ca^<2+>溶液中长时间暴露（> 2 min），闪光反应的潜伏期明显延长，恢复过程加快。对相对暗淡的闪光的灵敏度降低，长时间（> 2 min）暴露于5-50 μ M IBMX溶液中，使闪光反应潜伏期延长，上升期和恢复期减慢。（花生四烯酸和亚油酸）的细胞内应用贴片电极仅产生恢复期的加速。这些结果支持了胞浆Ca^2+浓度降低可能导致PDE活性降低和鸟苷酸环化酶活性增强的观点。我们测试的所有物质似乎对闪光反应的最大振幅没有或几乎没有影响。这可能意味着存在一种维持游离cGMP稳态的机制。

There has been considerable speculation that decreased cytoplasmic Ca^<2+> concentration may play a role in light adaptation of vertebrate photoreceptors. In order to study the steady-state action of Ca^<2+> on the phototransduction cascade, we examined the effects of lowered external Ca^<2+> concentration, an phosphodiesterase inhibitor (IBMX), and several guanylate-cyclase excitants on flash responses of frog rods. Membrane current was recorded from an isolated, dark-adapted rod by sucking its inner segment into a glass pipette containing Ringer solution. The outer segment of the cell was exposed to bath solution which could be changed.During prolonged exposure ( >2min ) to 1-10muM Ca^<2+> solutions, the latency of flash responses was markedly increased and the recovery process was accelerated. The sensitivity to relatively dim flashes was reduced, but the saturating intensity of flash stimulation seemed to be same as that in the normal Ringer solution.Prolonged exposure ( >2min ) to 5-50)muM IBMX solutions increased the latency of flash responses and slowed both of the rising and recovery phases.Na-nitroprusside and unsaturated fatty acids (arachidonic acid and linoleic acid) which were intracellularly applied with patch electrodes produced only acceleration of the recovery phase. They appeared to have no effect on the early phase and maximal(saturated) amplitude of the flash responses.These results supports the idea that decreased cytoplasmic Ca^<2+> concentration may decrease PDE activity and accelerate guanylate cyclase. All substances we tested seemed to make no or little influence on the maximal amplitude of flash responses. This may imply the existence of a mechanism that maintains the free cGMP homeostasis.