Ionic Dependence of Reversal Voltage of the Light Response in Limulus Ventral Photoreceptors

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