Hodgkin A L, McNaughton P A, Nunn B J, Yau K W
J Physiol. 1984 May;350:649-80. doi: 10.1113/jphysiol.1984.sp015223.
The effect of ions on the light-sensitive current of isolated retinal rods from the toad Bufo marinus was studied by sucking the inner segment into a tightly fitting pipette. The outer segment projected into flowing solution whose composition could be changed rapidly. Reducing the external Na concentration, [Na]o, round the outer segment caused rapid and reversible reductions in the light-sensitive current. With the outer segment in the pipette, reductions of [Na]o round the inner segment had little effect on the light-sensitive current. The current about 15 s after a change in [Na]o was approximately proportional to [Na]2o. The current decreased in elevated external Ca concentration, [Ca]o, and increased in reduced [Ca]o. Between 10 and 0.5 mM-external Ca the current 15 s after a change was approximately inversely proportional to [Ca]o. Reducing [Ca]o from 1 mM to 1 microM or less transiently increased the current by about 15-fold. After a change in [Na]o or [Ca]o the current did not approach its final value monotonically but with a characteristic overshoot or underswing, followed by a slow relaxation of current which may reflect the time course of change in internal Na. Reducing [Na]o from 110 to 70 mM or less prolonged the response to a flash; very long responses were observed in solutions containing Li rather than Na and also in rods that had been returned to Ringer solution after exposure to low Ca. All these effects might be explained if Ca extrusion in exchange for Na determines the reactivation of current after a flash. The rod current was not changed if the ratio [Na]No/[Ca]o was held constant, N being about 2.5. Between 5 mM and 10 microM-Ca the change in peak current produced by absorption of a single quantum was roughly proportional to the dark current. Responses in the absence of external Na were not normally seen if the solution contained 0.1 mM-Ca or more. Responses of normal polarity were regularly observed in 0 Na, 0 Ca EGTA solutions containing 1.6 mM-Mg. Removal of Mg from such solutions gave inverted responses. Other conditions which promote responses of normal and inverted polarity in Na-free solutions are described briefly. We conclude that Li, Ca, Mg and perhaps K can pass through the light-sensitive channel. The above results suggest that external Na has two distinct effects: (1) it provides ions to carry inward current, and (2) it keeps the light-sensitive conductance open by maintaining the internal Ca concentration, [Ca]i, at a low level.
通过将蟾蜍（海蟾蜍）离体视网膜视杆细胞的内段吸入紧密适配的移液管，研究了离子对其光敏电流的影响。外段伸入流动的溶液中，溶液成分可迅速改变。降低外段周围的外部钠离子浓度（[Na]o）会导致光敏电流迅速且可逆地降低。当外段置于移液管中时，降低内段周围的[Na]o对光敏电流影响很小。[Na]o改变后约15秒时的电流大致与[Na]o²成正比。在外部钙离子浓度（[Ca]o）升高时电流降低，在[Ca]o降低时电流增加。在10到0.5 mM的外部钙离子浓度之间，改变后15秒时的电流大致与[Ca]o成反比。将[Ca]o从1 mM降低到1 microM或更低会使电流瞬时增加约15倍。在[Na]o或[Ca]o改变后，电流并非单调地接近其最终值，而是有特征性的过冲或下冲，随后电流缓慢松弛，这可能反映了内部钠离子变化的时间进程。将[Na]o从110 mM降低到70 mM或更低会延长对闪光的响应；在含有锂而非钠的溶液中以及在暴露于低钙后再回到林格氏液的视杆细胞中观察到非常长的响应。如果钙离子与钠离子的交换性外流决定了闪光后电流的重新激活，那么所有这些效应都可以得到解释。如果[Na]No/[Ca]o的比值保持恒定（N约为2.5），视杆细胞电流不会改变。在5 mM到10 microM的钙离子浓度之间，吸收单个量子产生的峰值电流变化大致与暗电流成正比。如果溶液中含有0.1 mM或更多的钙离子，通常不会观察到无外部钠离子时的响应。在含有1.6 mM镁离子的0 Na、0 Ca EGTA溶液中经常观察到正常极性的响应。从这种溶液中去除镁离子会产生反向响应。简要描述了在无钠溶液中促进正常和反向极性响应的其他条件。我们得出结论，锂、钙、镁以及可能的钾可以通过光敏通道。上述结果表明外部钠离子有两种不同的作用：（1）它提供离子来携带内向电流，（2）它通过将内部钙离子浓度（[Ca]i）维持在低水平来保持光敏电导开放。
