The sodium current underlying the responses of toad rods to light.

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Abstract
Intracellular responses were recorded from single rods in the retina of the toads Bufo bufo and B. marinus during exposure to solutions in which Na was replaced by equimolar amounts of choline. Upon moderate reduction (80 and 50 mM) of the external Na the size of responses to bright flashes decreased as a consequence of both an increase in the resting potential and a decrease of the membrane potential at the peak, while the level of the plateau remained fairly constant. Upon reduction of the external Na to .ltoreq. 22 mM, rods hyperpolarized to about the plateau level and failed to respond to illumination. Under these circumstances, membrane depolarization induced by an increased external K did not restore the cell responsiveness. Addition of 2-5 mM Cs hyperpolarized the membrane and partially restored the photoresponse. Complete replacement of external Na with K depolarized the rod by 40 .+-. 10 mV, and no voltage responses to light could be detected. In the presence of Cs, a nearly complete blockage of the photoresponses was obtained when the external Na was .ltoreq. 5 mM. Further reductions of the external Na did not invert the photoresponses. Application of Cs when the external Na was nominally zero induced a transient hyperpolarization followed by a slow decay. During exposure to steady illumination, the dependence of the plateau level on the external Na slowly increased. The ionic current which is directly modulated by the light seems to depend primarily on external Na. The current associated with the voltage- and time-dependent process responsible for the sag from peak to plateau of the response to a bright flash of light may have multiple components.