Mechanisms of reoxygenation-induced calcium overload in cardiac myocytes: dependence on pHi.
This study investigated the selective effects of intracellular (pHi) or extracellular change in pH on reoxygenation-induced Ca2+ overload in simulated myocardial hypoxia. Experiments were performed in cultured cardiomyocytes isolated from the ventricle of neonatal ICR mouse. A model of chemical hypoxia with 2 mM sodium cyanide was developed to mimic the ATP depletion of hypoxia. This chemical hypoxia was "reoxygenated" and the dynamics in intracellular Ca2+ concentration ([Ca2+]i) and pHi were monitored using the fluorescent dyes fura-2 and 2', 7'-bis (2-carboxyethyl)-5(6)-carboxyfluorescein, respectively. During a 40-min chemical hypoxia, pHi progressively fell from 7.2 to 6.6. Reoxygenation with control solution caused rapid recovery of pHi and a marked increase in [Ca2+]i (1884 +/- 136 nM). Intracellular acidotic reoxygenation produced by lactate apparently prolonged the time course of pHi recovery and significantly suppressed reoxygenation-induced Ca2+ overload (1170 +/- 118 nM, P = 0.008). Extracellular acidotic reoxygenation with 2-(N-morpholino) ethanesulfonic acid (pK = 5.96) buffer somewhat suppressed the Ca2+ overload; however, the maximal value of [Ca2+]i was not reduced significantly compared with the control (1790 +/- 122 nM, P = 0.130). In addition, inhibition of Na(+)-H+ exchange by amiloride potentiated prolongation of intracellular acidosis during reoxygenation and resulted in a minimal increase in [Ca2+]i (985 +/- 102 nM, P = 0.004). These results suggest that reoxygenation-induced Ca2+ overload is closely correlated with intracellular pH in the initial phase of reoxygenation, and the protective effects of extracellular acidosis is principally mediated by intracellular acidification of reoxygenated cardiomyocytes.
Matsuda
N
N
Department of Surgery, Tottori University School of Medicine, Yonago, Japan.
Mori
T
T
Nakamura
H
H
Shigekawa
M
M
D000143:Acids / Q000494:pharmacology; D000818:Animals; D000831:Animals, Newborn; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D006863:Hydrogen-Ion Concentration; D000860:Hypoxia / Q000378:metabolism; D007773:Lactates / Q000494:pharmacology; D019344:Lactic Acid; D051379:Mice; D008813:Mice, Inbred ICR; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D010100:Oxygen / Q000494:pharmacology*
D000143:Acids; D007773:Lactates; D019344:Lactic Acid; D010100:Oxygen; D002118:Calcium