title: Na+/Ca2+ exchange in catfish retina horizontal cells: regulation of intracellular Ca2+ store function.
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+]i and caffeine-stimulated Ca2+ release. Reduction of external Na+ concentration ([Na+]o) further and reversibly increased [Ca2+]i in ouabain-treated HC. This effect was not abolished by the Ca2+ channel blocker nifedipine, suggesting that increases in [Na+]i promote net extracellular Ca2+ influx through a Na+/Ca2+ exchanger. Moreover, when [Na+]o was 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.
authors.affiliation: Department of Physiology and Biophysics, University of Texas Medical Branch, Galveston, Texas 77555-0641, USA.
authors.forename: M A
authors.initials: MA
authors.lastname: Micci
authors.forename: B N
authors.initials: BN
authors.lastname: Christensen
mesh_terms: D000584:Amiloride / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D004563:Electrochemistry; D006706:Homeostasis; D007059:Ictaluridae / Q000378:metabolism*; D012160:Retina / Q000187:drug effects / Q000378:metabolism*; D012964:Sodium / Q000008:administration & dosage / Q000378:metabolism; D019831:Sodium-Calcium Exchanger / Q000502:physiology*; D014638:Vanadates / Q000494:pharmacology
chemical_list: D019831:Sodium-Calcium Exchanger; C053494:2',4'-dimethylbenzamil; D002110:Caffeine; D014638:Vanadates; D000584:Amiloride; D012964:Sodium; D002118:Calcium