Direct measurements of the free intracellular calcium concentration in the outer segment of the isolated vertebrate rod have shown that free [Ca]j declines after a flash of light. Free [Ca]. is found to be determined by the balance between influx through light-sensitive channels and efflux pumped by a 3Na + :lCa 2+ exchange, with 90-95% of the exchangeable Ca being sequestered inside the cell by a rapid and reversible binding mechanism. A rise in free [Ca] ; has effects on the dim flash time course which are similar but opposite in sign to the effects of light adaptation. It seems probable that the decline in free [Ca] s occurring in the presence of light mediates the changes in response time course characteristic of the light-adapted state.
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McNaughton et al. (1986) studied this question.
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