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)

The role of sarcolemmal Ca2+‐ATPase in the regulation of resting calcium concentration in rat ventricular myocytes

“…5, extracellular Na + was removed first under control conditions and then after loading with carboxyeosin. In agreement with previous work (Allen et al 1984; Kim et al 1987), the Na + removal under control conditions displays an initial increase in [Ca 2+ ] i which then decays to a lower level which is still greater than the resting level. The initial increase in [Ca 2+ ] i is due to Ca 2+ entry on ‘reverse’ Na + ‐Ca 2+ exchange.…”

The sarcolemmal mechanisms involved in the control of diastolic intracellular calcium in isolated rat cardiac trabeculae

“…7A, B) and then declined to a steady level that was generally slightly higher than that in the sodiumcontaining solution. Previous work [2,9] has shown that, in the absence of extracellular sodium, [Ca 2+] ; could be maintained at a relatively low level for prolonged periods, suggesting that there is another mechanism removing calcium from the cytoplasm -presumably the sarcolemmal ATPase [5]. Overall, the steady-state [Ca2+] ; was significantly higher (P<0.05) in the absence of extracellular sodium than under control conditions.…”

Inhibition by KB-R7943 of the Reverse Mode of the Na+/Ca2+ Exchanger Reduces Ca2+ Overload in Ischemic-Reperfused Rat Hearts.

“…19 To induce Ca 2+ influx via the reverse mode of NCX, 10,[20][21][22] hearts were then exposed to the low-Na + solution (110 mmol/L Tris aminomethane, 30 mmol/L NaCl, 6 mmol/L KCl, 1 mmol/L MgCl2, 2 mmol/L CaCl2, 10 mmol/L HEPES, 10 mmol/L glucose, 10 mmol/L caffeine, gassed with 100% O2, pH 7.4, 37°C) for 30 s. This procedure was followed by 2 min of perfusion with the Tyrode solution containing 10 mmol/L caffeine to extrude Ca 2+ from the myocytes. That sequence of perfusions was designed to specifically detect NCX-derived changes in [Ca 2+ ]i; according to a previous study using cardiac myocytes, 20,22 low-Na + exposure alone induced not only Ca 2+ influx via the reverse mode of the NCX but also induced intracellular Ca 2+ release from the SR. 20,22,23 Verapamil-sensitive Ca 2+ channels were not involved in the increase in [Ca 2+ ]i resulting from low-Na + exposure, 20 so we used caffeine to exclude SR Ca 2+ release during lowNa + exposure. The effect of caffeine was reversible.…”