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+.
Siegmund
B
B
Physiologisches Institut I, Heinrich-Heine-Universität, Dusseldorf, Germany.
Ladilov
Y V
YV
Piper
H M
HM
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D000860:Hypoxia / Q000378:metabolism; D008297:Male; D009200:Myocardial Contraction; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D010042:Ouabain / Q000494:pharmacology; D010100:Oxygen / Q000494:pharmacology*; D051381:Rats; D017208:Rats, Wistar; D012964:Sodium / Q000378:metabolism / Q000502:physiology*; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors
D010042:Ouabain; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D010100:Oxygen; D002118:Calcium