Importance of sodium for recovery of calcium control in reoxygenated cardiomyocytes.

The role of Na+ in the recovery from severe anoxic Ca2+ overload was investigated in isolated quiescent ventricular cardiomyocytes from adult rat. Changes of cytosolic Ca2+ and Na+ concentrations were followed by the fura 2 and Na(+)-binding benzofuran isophthalate techniques, respectively. When the fura 2 ratio (340/380 nm) reached saturation in anoxic cells, indicating a severe cytosolic Ca2+ overload, the cells were reoxygenated. This caused a rapid initial drop of cytosolic Ca2+ to a lower but still elevated level (phase I), followed by oscillatory Ca2+ transients at this level (phase II) and, within 10 min, the reestablishment of a stable cytosolic Ca2+ concentration at the normal resting level (phase III). As previously shown [B. Siegmund, R. Zude, and H. M. Piper. Am. J. Physiol. 263 (Heart Circ. Physiol. 32): H1262-H1269, 1992], Ca2+ shifts in phase I and II are mainly due to uptake and release of Ca2+ by the sarcoplasmic reticulum. Phase I was unchanged, and phase II was much prolonged (> 60 min) in cells reoxygenated under Na+ pump inhibition (0.2 mM ouabain) or Na+ depletion. Phase III could only be reestablished (< 10 min) when ouabain was eluted or external Na+ replenished, respectively. The results show that full recovery of cytosolic Ca2+ control (phase III) requires an active sarcolemmal Na+ pump and the availability of external Na+. This indicates that phase III is determined by the transsarcolemmal extrusion of Ca2+ by a tandem mechanism consisting of 1) the Na+ pump, generating an extracellular-to-intracellular Na+ gradient, and 2) the sarcolemmal Na+/Ca2+ exchange, driven by that gradient to extrude Ca2+.