MODULATION OF HYPOXIC PULMONARY VASOCONSTRICTION BY ANTIOXIDANT ENZYMES IN RED-BLOOD-CELLS

To determine whether antioxidant mechanisms within red blood cells (RB Cs) significantly contribute to preserving hypoxic pulmonary vasoconst riction (HPV) in both the absence and the presence of oxidative stress , we investigated HPV changes in isolated rabbit lungs perfused with s olutions containing RBCs treated with various inhibitors of superoxide dismutase (SOD), anion channels, catalase (CAT), or glutathione perox idase (GSH-Px). Perfusion was maintained at a constant flow rate of 70 ml/min, and lung temperature at 37 to 38 degrees C. Hematocrit was ad justed to 7%. In the absence of overt oxidative stress, HPV was signif icantly enhanced in the perfusate containing control RBCs (untreated R BCs) as compared with that in Krebs-Henseleit buffer. Inhibition of SO D, CAT, and GSH-Px within RBCs, as well as anion channels located on t he RBC membrane, had little influence on HPV. Neither exogenous SOD no r CAT altered HPV. In the presence of high levels of reactive oxygen s pecies (ROS), generated by addition of xanthine (100 mu M) and xanthin e oxidase (10 mU/ml) to the reservoir, HPV was considerably suppressed in the perfusate containing only buffer but was restored in the perfu sate with control RBCs. Inhibition of CAT or GSH-Px in RBCs preserved the HPV, whereas inhibition of SOD or anion channels failed to preserv e HPV obtained during exposure to high ROS levels. Addition of exogeno us SOD, but not CAT, to the perfusate containing RBCs in which endogen ous SOD had been inhibited restored HPV under high ROS conditions. in conclusion, (1) although RBCs augment HPV in the absence of ROS, this finding is not attributable to the antioxidants in RBCs. (2) RBCs rest ore HPV upon exposure to high ROS. This finding may well be explained by antioxidant mechanisms operating with in RBCs, especially those of endogenous SOD.