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

Abstract

The membrane current vs. voltage dependence was measured in Limulus ventral nerve photo­receptors at various external Ca 2+ and Na + concentrations, using the voltage clamp technique. Lowering the external concentration of the divalent cations Ca 2+ 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 (V rev J D ), 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 V rev J D 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 Ca 2+ and Mg 2+ concen­trations, does not change the membrane dark current vs. voltage curve and so V rev J D is not altered; V rev ⊿J L , the reversal potential of the light-induced current, however, is reduced by 10 mV (from +20 to +10 mV). This reduction in V rev ⊿J L can be accounted for by the reduction of the sodium gradient across the cell membrane. Raising the external Ca 2+ 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.