Nifedipine increases oxygen saturation level of myoglobin in the rat heart during hypoxia

Rat hearts were perfused by means of the Langendorff's technique and paced at 150/min. Oxygen saturation level of myocardial myoglobin was measured continuously according to an optical method, by which the intracellular oxygen level in the myocardial cells can be monitored continuously. Krebs-Henseleit solution was bubbled with a gas mixture containing either 95% O2 + 5% CO2 (high oxygen solution) or 95% N2 + 5% CO2 (anoxic solution), or both (low oxygen solution). Hypoxia was produced by changing the perfusion fluid from high to low oxygen solution. After 10 min of hypoxia, drugs were infused into the inflow tube at a rate of 0.1 ml/min for 10 min. Hypoxia increased coronary flow by about 25%, decreased oxygen saturation level of myoglobin by 23.1-35.7%, without a marked change in left ventricular pressure. Nifedipine (1.0 microgram/ml) increased oxygen saturation level, which had been reduced by hypoxia, by 16-27%. Nifedipine at the concentration of 0.1 microgram/ml also increased the oxygen saturation level of myocardial myoglobin slightly, but at the concentration of 10 micrograms/ml it decreased. Nitroglycerin (100 micrograms/kg) increased the oxygen saturation level, which had been decreased by hypoxia, by 6-8%, whereas dipyridamole (100 micrograms/ml) did not. It is concluded that nifedipine as well as nitroglycerin can increase the oxygen saturation level of myocardial myoglobin during hypoxia, suggesting that both drugs may increase the intracellular oxygen tension in the hypoxic heart.