Baylor D A, Nunn B J
J Physiol. 1986 Feb;371:115-45. doi: 10.1113/jphysiol.1986.sp015964.
The light-sensitive conductance of isolated rods from the retina of the tiger salamander was studied using a voltage-clamp method. The membrane current of the outer segment was collected with a suction electrode while the internal voltage was measured and controlled with a pair of intracellular electrodes. Saturating light blocked the outer segment current at all potentials, the residual conductance usually becoming less than 20 pS. This suggests that light-sensitive channels comprise the main ionic conductance in the surface membrane of the outer segment. Current-voltage relations determined 10-40 ms after changing the voltage showed outward-going rectification, the outward current increasing e-fold for a depolarization of 11-14 mV. The reversal potential of the light-sensitive current was estimated as 5 +/- 4 mV. This is consistent with other evidence indicating that the channel is not exclusively permeable to Na. Applying steady light, lowering external Ca, or changing the intracellular voltage to a new steady level scaled the light-sensitive current without altering the reversal potential or the form of the rectification. This suggests that all three manipulations change the number of channels in the conducting state without changing the ionic concentration gradients or the mechanism of permeation through an 'open' channel. Hyperpolarizing voltage steps slowly increased the light-sensitive current and depolarizing steps reduced it. A gating variable Y expressing the fractional activation of the light-sensitive conductance in the steady state was derived from the ratio of the instantaneous and steady-state currents. Y declined at voltages positive to -100 mV and usually reached a minimum near 0 mV, with a secondary rise positive to 0 mV. Around the dark voltage Y changed e-fold in roughly 25 mV. The voltage-dependent gating in (6). appeared to involve two delays similar in magnitude to those of the four principal delays in the rod's response to a dim flash. Steady background light shortened the time-scale of gating and flash responses to a similar degree. Clamping the voltage at the dark level had little effect on the photocurrent evoked by a flash. The small, delayed effect actually observed is explained by the slow voltage-dependent gating of the light-sensitive conductance. Hyperpolarization had little effect on the kinetics of the response to a flash, but depolarization slowed the response, causing it to reach a larger, later peak. Depolarization also prolonged the blockage of the light-sensitive current after a saturating flash.(ABSTRACT TRUNCATED AT 400 WORDS)
采用电压钳法研究了虎螈视网膜分离视杆细胞的光敏电导。用吸力电极收集外段的膜电流，同时用一对细胞内电极测量和控制内部电压。饱和光在所有电位下均阻断外段电流，残余电导通常小于20 pS。这表明光敏通道构成了外段表面膜中的主要离子电导。在改变电压后10 - 40毫秒测定的电流 - 电压关系显示出外向整流，去极化11 - 14 mV时外向电流增加e倍。光敏电流的反转电位估计为5±4 mV。这与其他证据一致，表明该通道并非仅对Na通透。施加稳定光、降低细胞外Ca或改变细胞内电压至新的稳定水平会使光敏电流按比例缩放，而不改变反转电位或整流形式。这表明所有这三种操作改变了处于导通状态的通道数量，而不改变离子浓度梯度或通过“开放”通道的通透机制。超极化电压阶跃缓慢增加光敏电流，去极化阶跃则降低它。从瞬时电流与稳态电流的比值得出一个表示稳态下光敏电导分数激活的门控变量Y。Y在高于 - 100 mV的电压下下降，通常在0 mV附近达到最小值，在高于0 mV时二次上升。在暗电压附近，Y在约25 mV内变化e倍。（6）中的电压依赖性门控似乎涉及两个延迟，其大小与视杆细胞对弱闪光反应中的四个主要延迟相似。稳定的背景光在相似程度上缩短了门控和闪光反应的时间尺度。将电压钳制在暗水平对闪光诱发的光电流影响很小。实际观察到的小的延迟效应可由光敏电导的缓慢电压依赖性门控来解释。超极化对闪光反应的动力学影响很小，但去极化会减慢反应，使其达到更大、更晚的峰值。去极化还会延长饱和闪光后光敏电流的阻断时间。（摘要截于400字）
