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

有相当多的人推测，细胞质中钙离子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+)对光转导级联的稳态作用，我们研究了降低外界Ca~(2+)浓度、磷酸二酯酶抑制剂(IBMX)和几种鸟苷环化酶激动剂对蛙棒闪光反应的影响。膜电流是从一个孤立的、黑暗适应的杆子上记录下来的，方法是将其内段吸入含有林格溶液的玻璃吸管。细胞外段暴露于可改变的浴液中，在长时间(&gt；2min)暴露于1-10muMCa^&lt；2+&gt；溶液中，闪光反应潜伏期明显延长，恢复过程加快。对较暗的闪光的敏感度降低，但闪光刺激的饱和强度似乎与正常林格溶液中相同。长时间暴露(&gt；2分钟)至5-50微米的IBMX溶液增加了闪光反应的潜伏期，减缓了上升和恢复阶段。细胞内应用硝普钠和不饱和脂肪酸(花生四烯酸和亚油酸)只加速了恢复阶段。对闪光反应的早期阶段和最大(饱和)波幅无明显影响。这些结果支持了胞浆内钙离子浓度降低可能降低PDE活性，加速鸟苷环化酶的观点。我们测试的所有物质似乎对闪光反应的最大幅度没有或几乎没有影响。这可能意味着存在一种维持游离cGMP动态平衡的机制。

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.