Abstract: | The role of theNa+/Ca2+exchanger in intracellular Ca2+regulation was investigated in freshly dissociated catfish retinalhorizontal cells (HC).Ca2+-permeable glutamate receptorsand L-type Ca2+ channels as wellas inositol 1,4,5-trisphosphate-sensitive and caffeine-sensitiveintracellular Ca2+ stores regulateintracellular Ca2+ in these cells.We used the Ca2+-sensitive dyefluo 3 to measure changes in intracellularCa2+ concentration(Ca2+]i)under conditions in whichNa+/Ca2+exchange was altered. In addition, the role of theNa+/Ca2+exchanger in the refilling of the caffeine-sensitiveCa2+ store followingcaffeine-stimulated Ca2+ releasewas assessed. Brief applications of caffeine (1-10 s) producedrapid and transient changes inCa2+]i.Repeated applications of caffeine produced smallerCa2+ transients until no furtherCa2+ was released. Store refillingoccurred within 1-2 min and required extracellularCa2+. Ouabain-induced increases inintracellular Na+ concentration(Na+]i)increased both basal freeCa2+]iand caffeine-stimulated Ca2+release. Reduction of external Na+concentration(Na+]o)further and reversibly increasedCa2+]iin ouabain-treated HC. This effect was not abolished by the Ca2+ channel blocker nifedipine,suggesting that increases inNa+]ipromote net extracellular Ca2+influx through aNa+/Ca2+exchanger. Moreover, whenNa+]owas replaced by Li+, caffeine didnot stimulate release of Ca2+ fromthe caffeine-sensitive store afterCa2+ depletion. TheNa+/Ca2+exchanger inhibitor 2',4'-dimethylbenzamil significantlyreduced the caffeine-evoked Ca2+response 1 and 2 min after store depletion. |