Na1/Ca21 exchange in catfish retina horizontal cells: regulation of intracellular Ca21 store function

Micci, M. A., and Burgess N. Christensen. Na1/Ca21 exchange in catfish retina horizontal cells: regulation of intracellular Ca21 store function. Am. J. Physiol. 274 (Cell Physiol. 43): C1625–C1633, 1998.—The role of the Na1/Ca21 exchanger in intracellular Ca21 regulation was investigated in freshly dissociated catfish retinal horizontal cells (HC). Ca21-permeable glutamate receptors and L-type Ca21 channels as well as inositol 1,4,5-trisphosphate-sensitive and caffeine-sensitive intracellular Ca21 stores regulate intracellular Ca21 in these cells. We used the Ca21-sensitive dye fluo 3 to measure changes in intracellular Ca21 concentration ([Ca]i) under conditions in which Na1/Ca21 exchange was altered. In addition, the role of the Na1/Ca21 exchanger in the refilling of the caffeine-sensitive Ca21 store following caffeinestimulated Ca21 release was assessed. Brief applications of caffeine (1–10 s) produced rapid and transient changes in [Ca]i. Repeated applications of caffeine produced smaller Ca21 transients until no further Ca21 was released. Store refilling occurred within 1–2 min and required extracellular Ca21. Ouabain-induced increases in intracellular Na1 concentration ([Na]i) increased both basal free [Ca]i and caffeinestimulated Ca21 release. Reduction of external Na1 concentration ([Na]o) further and reversibly increased [Ca]i in ouabain-treated HC. This effect was not abolished by the Ca21 channel blocker nifedipine, suggesting that increases in [Na]i promote net extracellular Ca21 influx through a Na1/ Ca21 exchanger. Moreover, when [Na]o was replaced by Li1, caffeine did not stimulate release of Ca21 from the caffeinesensitive store after Ca21 depletion. The Na1/Ca21 exchanger inhibitor 28,48-dimethylbenzamil significantly reduced the caffeine-evoked Ca21 response 1 and 2 min after store depletion.