An increase in intracellular [Na⁺] during Ca²⁺ depletion is not related to Ca²⁺ paradox damage in rat hearts

Abstract

Ca2+paradox damage has been suggested to be determined by Na+ entry during Ca2+ depletion and exchange of Na+ for Ca2+ during Ca2+ repletion. With the use of23Na nuclear magnetic resonance, we previously observed a Ca2+ paradox without a prior Na+ increase. We have now demonstrated a Na+ increase during Ca2+ and Mg2+ depletion without the occurrence of the Ca2+ paradox during Ca2+ repletion. Isolated rat hearts were perfused for 20 min with a Ca2+-free or a Ca2+- and Mg2+-free (Ca2+/Mg2+-free) solution under hypothermic conditions (20 and 25°C). Intracellular Na+ concentration ([Na+]i) increased from 11.9 ± 1.2 to 26.9 ± 5.8 mM ( P < 0.001) during Ca2+/Mg2+-free perfusion at 20°C, whereas no significant change in [Na+]ioccurred during 20 min of Ca2+-free perfusion at 20°C. In addition, we confirmed that [Na+]idid not change significantly during 20 min of normothermic Ca2+-free perfusion. Creatine kinase release during normothermic Ca2+ repletion in the 20°C groups was ∼10% and in the 25°C groups 75% of the release in the normothermia group. Recovery of rate-pressure product was ∼50% in the 20°C groups versus 0% in the normothermia group. In conclusion, hypothermic Ca2+/Mg2+-free perfusion results in a significant increase of [Na+]i, which does not contribute to the extent of the Ca2+ paradox on normothermic Ca2+ repletion.

Na + /Ca 2+ exchange during Ca 2+ repletion is not a prerequisite for the Ca 2+ paradox in isolated rat hearts

“…However, the experimental evidence for this notion has been obtained in the simultaneous absence, or at low levels, of extracellular Mg 2+ or in the presence of chelators, which may lower [Mg 2+ ]. In previous studies using 23 Na-NMR, we have not observed an increase in [Na + ] i during 30 min Ca 2+ -free perfusion [16,24,25], whereas Ca 2+ repletion evoked a full Ca 2+ paradox. [Na + ] i increases only when Mg 2+ as well as Ca 2+ are omitted from the perfusate [25].…”

“…This prevented a large contracture and coronary flow did not collapse but remained constant. The use of verapamil immediately before Ca 2+ -free perfusion had no effect on the extent of the Ca 2+ paradox, because CK release during Ca 2+ repletion in group III was the same as without the use of verapamil [16]. In our experiments, we observed a decrease in [Na + ] i during perfusion with 1 mg/l verapamil, probably due to decreased influx of Na + through the fast Na + channel (at high concentrations, verapamil has an adventitious effect on this channel [15]).…”

“…Verapamil prevents this increase in [Na + ] i indicating that, in the absence of both Ca 2+ and Mg 2+ in the perfusate, Na + enters the cell through the Ltype Ca 2+ channels. Furthermore, a full Ca 2+ paradox can be prevented by performing the Ca 2+ /Mg 2+ -free perfusion at 20°C, although a rise in [Na + ] i still occurs under these conditions [16]. However, 23 Na-NMR (like most other techniques for measuring Na + i ) provides information about the total [Na + ] i and possible inhomogeneity of subcellular distribution cannot be resolved.…”

“…Na + loading was believed to be a necessary prerequisite for the Ca 2+ paradox [14]. Using 23 Na-nuclear magnetic resonance (NMR), however, we have demonstrated recently a rise in intracellular [Na + ] ([Na + ] i ) during hypothermic Ca 2+ /Mg 2+ -free perfusion without the occurrence of Ca 2+ paradox during normothermic Ca 2+ repletion, whereas during normothermic Ca 2+ -free perfusion a full Ca 2+ paradox was observed during Ca 2+ repletion without a rise in [Na + ] i during the previous Ca 2+ -free period [16]. 23 Na-NMR studies of isolated hearts provide information only about the total [Na + ] i ; inhomogeneities in subcellular distribution cannot be resolved.…”

Cardiac basal metabolism: energetic cost of calcium withdrawal in the adult rat heart

“…The increased myothermal response to Ca 2+ o withdrawal may reflect the activation of active transport mechanisms that would counteract the altered ionic homeostasis induced by 0Ca 2+ . It is known that Ca 2+ ‐free perfusion elicits Na + influx into cardiomyocytes (Bhojani & Chapman 1990, Jansen et al. 1998, Bosteels et al.…”