Calcium Sodium Binding Competition in the Gating of Light-Activated Membrane Conductance Studied by Voltage Clamp Technique in Limulus Ventral Nerve Photoreceptor

The membrane current vs. voltage dependence was measured in Limulus ventral nerve photo­receptors at various external Ca2+ and Na+ concentrations, using the voltage clamp technique. Lowering the external concentration of the divalent cations Ca2+ and Mg+ to < 1 μmol/l by adding EDTA causes 1)the light-induced transient conductance increase to disappear and 2)the reversal potential of the membrane current in the dark to shift to a positive value between + 10 and + 20 mV. This value is about the same as the (VrevJD), reversal potential of the total light current under normal ionic conditions. If the external Na+ is lowered to 50 mmol/1 (i.e. 10% of the normal concentration) simultaneously with the lowering of the divalent cation concentration described above, the light response is not abolished and VrevJD is shifted less. The extent of this antagonism depends on the sodium sub­stitute; it is stronger if choline is used instead of lithium. Lowering of sodium alone to 50 mmol/1, in a saline containing normal Ca2+ and Mg2+ concen­trations, does not change the membrane dark current vs. voltage curve and so VrevJD is not altered; Vrev⊿JL, the reversal potential of the light-induced current, however, is reduced by 10 mV (from +20 to +10 mV). This reduction in Vrev⊿JL can be accounted for by the reduction of the sodium gradient across the cell membrane. Raising the external Ca2+ concentration to 40 or 100 mmol/l has no conspicuous effect on the membrane current vs. voltage dependence and the gating of the light-induced conductance in­crease. The results are consistent with our working hypothesis that the gating of the light-activated ion channels in Limulus photoreceptor is controlled by negative binding sites for which calcium- and sodium ions compete with antagonistic actions.