title: Control of cytosolic calcium activity during low sodium exposure in cultured chick heart cells.
abstract: We investigated the roles of sodium-calcium exchange, sarcoplasmic reticulum, and mitochondria in Cai homeostasis in cultured chick ventricular cells. Specifically, the influence of low sodium medium on contractile state, calcium fluxes, and cytosolic free [Ca] [( Ca]i) was examined. [Ca]i was measured using fura-2. Mean [Ca]i in control medium was 126 +/- 14 nM. Exposure of cells to sodium-free or sodium- and calcium-free medium (choline-substituted) resulted in contracture development, which returned toward the baseline level over 2-3 minutes. The Nao-free contracture was associated with a tenfold increase in [Ca]i (1,280 +/- 110 nM) followed by a gradual decrease to a level fourfold above control [Ca]i (460 +/- 58 nM). Nao- and Cao-free contracture was associated with a fivefold increase in [Ca]i (540 +/- 52 nM) followed by a rapid decrease to below 80 nM. Sodium-free medium failed to produce an increase in [Ca]i or contracture in cells preexposed to calcium-free medium, although caffeine, when subsequently added to sodium- and calcium-free medium, was able to elicit a transient increase in [Ca]i and contracture. Brief, 5-second preperfusion of cells with La3+ (1 mM) or EGTA (1 mM) abolished the Nao-free contracture and the increase in [Ca]i. In the presence of 20 mM caffeine, removal of Nao resulted in minimal changes in the resting position of the cell although 45Ca uptake and [Ca]i were increased in response to sodium-free medium; the subsequent decrease in [Ca]i was greatly slowed. Addition of caffeine during the relaxation phase of the sodium-free contracture produced an additional transient contracture and transient increase in [Ca]i. Ryanodine (1 microM) abolished this effect of caffeine. Caffeine or ryanodine abolished Nao- and Ca-free contracture. CCCP (2 microM), a potent oxidative phosphorylation inhibitor, did not significantly affect calcium efflux rate. In the presence of 2 microM CCCP, removal of sodium resulted in an augmented contracture signal and a rise in [Ca]i, followed by a slow decrease. We conclude that removal of extracellular sodium enhances transsarcolemmal entry of calcium via sodium-calcium exchange, but this effect alone does not lead to the development of sodium-free contracture. Calcium displaceable by lanthanum or EGTA appears to contribute to Nao-free or Nao- and Cao-free contracture. Studies using caffeine and ryanodine suggest that removal of Nao leads to release of calcium from the sarcoplasmic reticulum (presumably via calcium-induced calcium release).(ABSTRACT TRUNCATED AT 400 WORDS)
mesh_terms: D000584:Amiloride / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone / Q000494:pharmacology; D002478:Cells, Cultured; D002642:Chick Embryo; D003600:Cytosol / Q000378:metabolism*; D006321:Heart / Q000187:drug effects; D007811:Lanthanum / Q000494:pharmacology; D008929:Mitochondria, Heart / Q000378:metabolism; D009200:Myocardial Contraction / Q000187:drug effects; D009206:Myocardium / Q000378:metabolism*; D012433:Ryanodine / Q000494:pharmacology; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D012964:Sodium / Q000378:metabolism*
chemical_list: C041470:3',4'-dichlorobenzamil; D012433:Ryanodine; D002110:Caffeine; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone; D007811:Lanthanum; D000584:Amiloride; D012964:Sodium; D002118:Calcium