The effects of sodium replacement on the responses of toad rods

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Abstract
The effects of Na+ substitution on the membrane potential and light responses of rods in the superfused retina of the toad, B. marinus, was investigated. When all of the Na+ in the Ringer was replaced with Li+, the effects on the rods depended on the external free Ca2+ concentration ([Ca2+]o). At [Ca2+]o .gtoreq. 10-6 M, the membrane potential (Em) hyperpolarized and light responses were greatly diminished or abolished. At [Ca2+]o .ltoreq. 10-7 M, Li+ replacement had little effect on Em or response amplitude. An Na+-Ca+ counter-transport may exist in rods. At high [Ca2+]o, replacing Na+ with Li+ would have produced an increase in the rod cytosol free Ca2+ concentraiton ([Ca2+]i) and the blockage of the light-dependent conductance, leading directly to the suppression of light responses. At [Ca2+]o .ltoreq. 10-7 M, this presumably would not have occurred. Since at these low Ca2+ concentrations light responses of nearly normal amplitude in Li+ were observed, the light-dependent conductance may be permeable to Li+. Substitution of Na+ with K+ in low Ca2+ produced a complete suppression of the responses. It was still possible to measure large-induced changes in rod input resistance. Substitution of Na+ with tetramethylammonium [TMA], tetraethylammonium [TEA], Tris [tris-(hydroxymethyl) aminomethane] or choline in low Ca2+ produced a large hyperpolarization of the membrane potential and a diminution of response amplitude. A complete suppression of the responses for these cations was not observed. Substitution of Na+ with tetrapropylammonium [TPA] or with an uncharged substance (glucose or urea) in low Ca2+ produced a large hyperpolarization of membrane potential and a considerable decrease in the light responses. In .apprx. 1/2 of the attempts, the responses were observed to decline reversibly to < 20% of their peak amplitude in Na+. Results with TPA were indistinguishable from those of glucose or urea, indicating that the light-dependent conductance probably is not permeable to TPA. The resistance changes measured with K+ substitution and the responses observed in the presence of the organic ions TMA, TEA, Tris and choline suggest that these species may be permeable; alternative explanations could not be discounted.