Influence of the Membrane Potential on the Intracellular Light Induced Ca2+-Concentration Change of the Limulus Ventral Photoreceptor Monitored by Arsenazo III under Voltage Clamp Conditions

The light induced transmission change (Arsenazo signal) of an Arsenazo III injected ventral photoreceptor cell of Limulus polyphemus was studied under voltage clamp. The transmission change which represents a change of free intracellular calcium ion concentration, [Ca2+]j, was investigated for its dependence upon membrane voltage. The peak amplitude of the Arsenazo signal decreases in a linear fashion with the clamp voltage in the examined voltage range (from 80 to + 40 mV). In low Ca2+ saline ([Ca2+]e = 250 |j m ) this decrease in the amplitude of the Arsenazo signal was more pronounced, while in saline with increased Ca2+ ([Ca2+]e = 40, 50 and 100 m M ), there is almost no change of the Arsenazo signal with varied membrane voltage. The recovery of the Arsenazo signal (i.e. recovery of the transmission back to the value before the light flash) is faster during hyperpolarization, this recovery being slowed down when the cell is depolarized. From these experiments it is concluded that a substantial part of the Arsenazo signal is due to a light induced influx of Ca2+ from the extracellular space across the cell membrane into the cytoplasma. Conceivably the Ca2+ could pass through light activated Na+ channels. Subsequently the increased intracellular Ca2+ is lowered to the preillumination level, by a membrane voltage dependent mechanism possibly an Na+-Ca2+ exchange. The data do not exclude the possibility that a part of the Ca2+ responsible for the Arsenazo