The light-induced current as measured using a voltage clamp (holding voltage at resting potential) is attenuated when sodium ions in the bathing solution, Nao, are replaced by Tris, choline, or Li or when NaCl is replaced by sucrose. After replacement of NaCl by sucrose, the reversal voltage, Vrev, for the light response becomes more negative. In this case, the slope of the Vrev vs. log Nao near Nao = 425 mM is approximately 55 mV/decade increase of Nao (mean for 13 cells). The slope decreases at lower values of Nao. Choline is not impermeant and partially substitutes for Na; the slope of Vrev vs. log Nao is 20 mV/decade (mean for three cells). Vrev does not change when Na is replaced by Li. Decreases in the bath concentrations of Ca, Mg, Cl, or K do not affect Vrev. When Nao = 212 mM, Vrev becomes more positive when Ko is increased. Thus, light induces a change in membrane permeability to Na and probably also to K.
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