Exogenous nitric oxide triggers classic ischemic preconditioning by preventing intracellular Ca²⁺ overload in cardiomyocytes

The involvement of nitric oxide (NO) in the late phase of ischemic preconditioning is well established. However, the role of NO as a trigger or mediator of "classic preconditioning" remains to be determined. The present study was designed to investigate the effects of NO on calcium homeostasis in cultured newborn rat cardiomyocytes in normoxia and hypoxia. We found that treatment with the NO donor, sodium nitroprusside (SNP) induced a sustained elevation of intracellular calcium level ([Ca²⁺]_i) followed by a decrease to control levels. Elevation of extracellular calcium, which generally occurs during ischemia, caused an immediate increase in [Ca²⁺]_i and arrhythmia in cultures of newborn cardiomyocytes. Treatment with SNP decreased [Ca²⁺]_i to control levels and re-established synchronized beating of cardiomyocytes. A decrease in extracellular [Na⁺], which inhibits the Na⁺/Ca²⁺ exchanger, did not prevent [Ca²⁺]_i reduction by SNP. In contrast, application of thapsigargin, an inhibitor of sarcoplasmic reticulum Ca²⁺-ATPase (SERCA2a), increased [Ca²⁺]_i, and in its presence, SNP did not reduce [Ca²⁺]_i, indicating that Ca²⁺ reduction is achieved via activation of SERCA2a. The results obtained suggest that activation of SERCA2a by SNP increases Ca²⁺ uptake into the sarcoplasmic reticulum (SR) and prevents cytosolic Ca²⁺ overload, which might explain the protective effect of SNP from hypoxic damage.