The effects of sodium replacement on the responses of toad rods.

1. We have investigated the effects of Na(+) substitution on the membrane potential and light responses of rods in the superfused retina of the toad, Bufo marinus.2. When all of the Na(+) in the Ringer was replaced with Li(+), the effects on the rods depended upon the external free Ca(2+) concentration ([Ca(2+)](o)). At [Ca(2+)](o) >/= 10(-6) M, the membrane potential (E(m)) hyperpolarized and light responses were greatly diminished or abolished. At [Ca(2+)](o) </= 10(-7) M, Li(+) replacement had little effect on E(m) or response amplitude.3. We interpret these results as revealing a Na(+)-Ca(+) counter-transport in rods. At high [Ca(2+)](o), replacing Na(+) with Li(+) would have produced an increase in the rod cytosol free Ca(2+) concentration ([Ca(2+)](i)) and the blockage of the light-dependent conductance, leading directly to the suppression of light responses. At [Ca(2+)](o) </= 10(-7) M, this presumably would not have occurred.4. Since at these low Ca(2+) concentrations we observed light responses of nearly normal amplitude in Li(+), our results suggest that the light-dependent conductance is permeable to Li(+).5. Substitution of Na(+) with K(+) in low Ca(2+) produced a complete suppression of the responses. However, it was still possible to measure large light-induced changes in rod input resistance.6. Substitution of Na(+) with tetramethylammonium, tetraethylammonium, Tris, or choline in low Ca(2+) produced a large hyperpolarization of the membrane potential and a diminution of response amplitude. However, we were unable to observe a complete suppression of the responses for these cations.7. Substitution of Na(+) with tetrapropylammonium or with an uncharged substance (glucose or urea) in low Ca(2+) produced a large hyperpolarization of membrane potential and a considerable decrease in the light responses. In about half our attempts, the responses were observed to decline reversibly to less than 20% of their peak amplitude in Na(+).8. Results with tetrapropylammonium 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, but we are unable to discount alternative explanations.