DAF fluorescence showing endothelial NO production in lung venular capillaries. (A) Capillaries loaded with DAF-FM. Images were obtained under normoxic conditions (left) and 0 (middle) and 30 (right) minutes after switching to hypoxia. (B) Tracings of DAF-FM fluorescence in a single endothelial cell in a venular capillary that was perfused at either 0% (gray) or 20% (black) hematocrit. (C) Group data show DAF-FM fluorescence as ratio of initial (0 minutes). Data are mean plus or minus SE; n = 4 each bar.