Woodruff M L, Fain G L, Bastian B L
J Gen Physiol. 1982 Oct;80(4):517-36. doi: 10.1085/jgp.80.4.517.
To measure the influx of Na+ and other ions through the light-dependent permeability of photoreceptors, we superfused the isolated retina of the toad, Bufo marinus, with a low-Ca2+ (10(-8) M), low-Cl- Ringer's solution containing 0.5 mM ouabain. Under these conditions, the membrane potential of the rod is near zero and there is no light-induced potential change either in the rod or in more proximal neurons. The photoreceptors, however, continue to show a light-dependent increase in membrane resistance, which indicates that the light-sensitive channels still close with illumination. Dark-adapted retinas show a larger 22Na+ accumulation than do light-adapted retinas. The extra accumulation of 22Na+ into dark-adapted retinas can be removed if the retinas are washed in darkness with low-Ca2+ Ringer's solutions, or if the ionophore gramicidin D is present in the perfusate. The additional accumulation in dark retinas corresponds to a flux of at least 10(9) Na+ per receptor per second, which is close to the value of the photoreceptor dark current. The light-dependent uptake of 22Na+ can be prevented by exposing the retinas to Ca2+ during the incubation period, but is restored if the phosphodiesterase inhibitor IBMX is added to the perfusate. A significant light-dependent ion accumulation can be observed for the cations K+, Rb+, Cs+, and Tl+, in addition to Na+, but not for methylamine, choline, or tetraethylammonium.
为了测量通过光感受器的光依赖性通透性的Na⁺和其他离子的内流，我们用含有0.5 mM哇巴因的低Ca²⁺（10⁻⁸ M）、低Cl⁻林格氏液对蟾蜍（Bufo marinus）的离体视网膜进行灌流。在这些条件下，视杆细胞的膜电位接近零，视杆细胞或更靠近近端的神经元中也没有光诱导的电位变化。然而，光感受器的膜电阻仍表现出光依赖性增加，这表明光敏感通道在光照下仍然关闭。暗适应视网膜比光适应视网膜显示出更大的²²Na⁺积累。如果在黑暗中用低Ca²⁺林格氏液冲洗视网膜，或者如果灌流液中存在离子载体短杆菌肽D，暗适应视网膜中额外积累的²²Na⁺可以被去除。暗视网膜中的额外积累对应于每个感受器每秒至少10⁹个Na⁺的通量，这接近光感受器暗电流的值。在孵育期间将视网膜暴露于Ca²⁺可阻止²²Na⁺的光依赖性摄取，但如果将磷酸二酯酶抑制剂异丁基甲基黄嘌呤（IBMX）添加到灌流液中，则摄取恢复。除了Na⁺之外，对于阳离子K⁺、Rb⁺、Cs⁺和Tl⁺，可以观察到明显的光依赖性离子积累，但对于甲胺、胆碱或四乙铵则没有。
