The Effects of Intracellular Iontophoretic Injection of Calcium and Sodium Ions on the Light Response of Limulus Ventral Photoreceptors

During intracellular iontophoretic injection of Ca(++) into Limulus ventral photoreceptor cells, there is a progressive diminution of the light response. Following Ca(++) injection, the size of the light response slowly recovers. Similarly, there is a progressive diminution of the light response during intracellular injection of Na(+) and recovery after the injection is stopped. The rate of diminution during Na(+) injection is greater for higher [Ca(++)](out). In solutions which contain 0.1 mM Ca(++), there is nearly no progressive decrease in the size of the light response during Na(+) injection. Intracellular injections of Li(+) or K(+) do not progressively decrease the size of the light response. We propose that an increase in [Na(+)](in) leads to an increase in [Ca(++)](in) and that an increase in [Ca(++)](in) by any means leads to a reduction in responsiveness to light.