Endothelial ROS in lung microvessels in situ. (A,B) Color-coded endothelial DCF fluorescence in venules (V) and septal capillaries (S) under normoxic (left) and hypoxic (right) conditions. Vessel margins are depicted (white lines). The alveolar septum is indicated (arrows). Venules were given RBC-containing perfusion at indicated hematocrits. (C) Tracings of DCF fluorescence intensity from an identical endothelial cell at baseline and hypoxic conditions. Perfusate was RBC-free or RBC-containing (hematocrit, 20%) with or without catalase (50 U/mL) replicated 5 times. (D) Hematocrit (Hct) dependency of endothelial DCF fluorescence. Data obtained after 30 minutes of hypoxia are mean plus or minus SE (n = 4 each point; *P < .05 vs hematocrit of 20%). (E) Group data are for hypoxia-induced responses in venules given RBC-containing perfusions (hematocrit, 20%). The unfilled bar corresponds to pO₂ of 40 mmHg attained after 17 minutes of hypoxia. The bar immediately to the right corresponds to pO₂ of 21 mmHg at 30 minutes of hypoxia exposure. For the other bars, RBCs were perfused together with catalase (50 U/mL) or RBCs were pretreated with the indicated agents (L-NAME, 250 μM; azide, 1 mM; nitrite, 5 mM). Data are mean plus or minus SE; n = 5 for each bar (*P < .05 vs untreated RBC perfusion at pO₂ of 21 mmHg, hematocrit of 20%). (F) Data are for venules after 30 minutes of hypoxia. RBC indicates RBCs from wild-type (WT) and BERK-trait (BT) mice were perfused (hematocrit, 20%) in WT and BT lungs as indicated. Catalase (50 U/mL) was included in the perfusion as indicated. Data are mean plus or minus SE; n = 5 for each group (*P < .05 vs first bar; §P < .05 vs 3rd bar).