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，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+>在光导级联中的稳态作用，我们研究了降低外部Ca^<2+>浓度、一种磷酸二酯酶抑制剂（IBMX）和几种鸟苷酸环化酶刺激剂对蛙杆闪光反应的影响。膜电流记录从一个孤立的，黑暗适应棒吮吸其内部部分到一个玻璃移液管含有林格溶液。细胞的外部部分暴露在可以改变的浴液中。在1-10muM Ca^<2+>溶液中长时间暴露（bbb2min），闪光反应潜伏期明显增加，恢复过程加快。对相对较暗的闪光的敏感性降低，但闪光刺激的饱和强度似乎与正常林格溶液相同。长时间暴露于5-50 μ m的IBMX溶液（bbb2min）增加了闪光反应的延迟，减慢了上升和恢复阶段。钠硝普苷和不饱和脂肪酸（花生四烯酸和亚油酸）与贴片电极在细胞内施用只产生加速恢复阶段。它们似乎对闪光反应的早期阶段和最大（饱和）幅度没有影响。这些结果支持了细胞质Ca^<2+>浓度降低可能降低PDE活性并加速鸟苷酸环化酶的观点。我们测试的所有物质似乎对闪光反应的最大振幅没有或只有很小的影响。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.