Importance of bicarbonate transport for protection of cardiomyocytes against reoxygenation injury

Isolated cardiomyocytes from adult rats were incubated in anoxic bicarbonate-buffered media at extracellular pH (pH_o) 6.4 until a cytosolic Ca²⁺ overload and intracellular pH (pH_i) of 6.4 were reached. On reoxygenation, the pH of the medium was changed to 7.4 to activate the Na⁺/H⁺exchanger (NHE) and the Na⁺-${HCO}_3^{-}$ symporter (NBS). The reoxygenation was performed in the absence or presence of the NHE inhibitor HOE-642 (3 μmol/l) and/or the NBS inhibitor DIDS (0.5 mmol/l), as in bicarbonate-free media. In reoxygenated control cells pH_i rapidly recovered to the preanoxic level, and a burst of spontaneous oscillations of cytosolic Ca²⁺ occurred, accompanied by the development of hypercontracture. When NBS and NHE were simultaneously inhibited during reoxygenation, pH_i recovery was prevented, Ca²⁺oscillations were attenuated, and hypercontracture was abolished. Sole inhibition of NBS or NHE showed no protection against hypercontracture. In the absence of cytosolic acidosis, HOE-642 or DIDS did not prevent hypercontracture induced by Ca²⁺ overload. The results demonstrate that simultaneous inhibition of NHE and NBS is needed to protect myocardial cells against reoxygenation-induced hypercontracture.