Calcium and the control of discrete wave latency in the ventral photoreceptor of Limulus.

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Abstract
1. Discrete, transient depolarization (discrete waves) of the ventral photoreceptor of the horseshoe crab, Limulus, occur spontaneously in the dark adapted photoreceptor and are also evoked by light. They form the basic events which comprise the receptor potential. A brief, low energy flash of light evokes variable numbers of discrete waves which have variable latencies. Evidence suggesting that discrete wave latency reflects the kinetics of the chemical reactions of phototransduction is reviewed. 2. The concentration of extracellular Ca influences both the average discrete wave latency and its variability. Lowering extracellular Ca prolongs the latency and increases its variability. Increasing extracellular Ca has the opposite effect. 3. Changes in discrete wave latency caused by changes in extracellular Ca require 10--15 min to become fully manifest, whereas when the concentration of extracellular K is increased the photoreceptor achieves a steady-state depolarization in 10-15 sec. 4. Iontophoresis of the Ca-chelating agent EGTA into the photoreceptor increases both the average discrete wave latency and its variability. Iontophoresis of Ca-EGTA mixtures may either increase or decrease discrete wave latency and its variability depending upon the proportion of Ca mixed with EGTA. 5. It is suggested that the concentration of intracellular rather than extracellular ionized Ca is the prime factor indicating discrete wave latency. The effects of changing extracellular Ca can be explained if the photoreceptor is permeable to Ca in the dark and if it maintains a low intracellular Ca concentration by virtue of active metabolic processes (a pump-leak system). 6. Lowering the temperature of the photoreceptor also has the dual effect of increasing discrete wave latency and its variability. However, effects of lowering temperature and Ca simultaneously are greater than the sum of the two effects in individually. This suggests that Ca may be a reactant in the chemical process of phototransduction. 7. Changing the concentration of extracellular Ca does not change the quantum efficiency of discrete wave production. A previous study showed that quantum efficiency is not changed by temperature. Thus, once initiated by the absorption of light, the reactions that subserve phototransduction may be forced to completion. Ca probably exerts its influence by changing one or more rate constants in the reaction sequence.