Na+/Ca2+exchange in catfish retina horizontal cells: regulation of intracellular Ca2+ store function

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Abstract
The role of the Na+/Ca2+exchanger in intracellular Ca2+regulation was investigated in freshly dissociated catfish retinal horizontal cells (HC). Ca2+-permeable glutamate receptors and L-type Ca2+ channels as well as inositol 1,4,5-trisphosphate-sensitive and caffeine-sensitive intracellular Ca2+ stores regulate intracellular Ca2+ in these cells. We used the Ca2+-sensitive dye fluo 3 to measure changes in intracellular Ca2+ concentration ([Ca2+]i) under conditions in which Na+/Ca2+exchange was altered. In addition, the role of the Na+/Ca2+exchanger in the refilling of the caffeine-sensitive Ca2+ store following caffeine-stimulated Ca2+ release was assessed. Brief applications of caffeine (1–10 s) produced rapid and transient changes in [Ca2+]i. Repeated applications of caffeine produced smaller Ca2+ transients until no further Ca2+ was released. Store refilling occurred within 1–2 min and required extracellular Ca2+. Ouabain-induced increases in intracellular Na+ concentration ([Na+]i) increased both basal free [Ca2+]iand caffeine-stimulated Ca2+release. Reduction of external Na+concentration ([Na+]o) further and reversibly increased [Ca2+]iin ouabain-treated HC. This effect was not abolished by the Ca2+ channel blocker nifedipine, suggesting that increases in [Na+]ipromote net extracellular Ca2+influx through a Na+/Ca2+exchanger. Moreover, when [Na+]owas replaced by Li+, caffeine did not stimulate release of Ca2+ from the caffeine-sensitive store after Ca2+ depletion. The Na+/Ca2+exchanger inhibitor 2′,4′-dimethylbenzamil significantly reduced the caffeine-evoked Ca2+response 1 and 2 min after store depletion.