Pulmonary Endothelium-Derived Relaxing Factor is Impaired in Hypoxia

1. The vasoconstrictor and vasodilator activity of cultured bovine pulmonary artery endothelial cells was measured to determine how exposure to different partial pressures of O2 [Po2 142, 42 and 15 mmHg (18.9, 5.6 and 2 kPa)] affects the production of endothelial-derived relaxing and constrictor factors 2. A de-endothelialized rat aortic ring [maintained at a Po2 of 142 mmHg (18.9 kPa)] was used to bioassay the effluent from a perfused column of bovine endothelial cells grown on microcarrier beads. The endothelial cells were stimulated by 10−7 mol/l bradykinin given for 1 min at 12 min intervals 3. At the start, middle and end of the experiment the bovine endothelial cells were exposed to a Po2 of 142 mmHg (18.9 kPa) and when stimulated by bradykinin the perfusate caused respectively a 70 ± 4%, 63 ± 6% and 63 ± 6% (mean ± sem) relaxation of an aortic ring which had been pre-contracted by 10−6 mol/l phenylephrine. At a Po2 of 42 mmHg (5.6 kPa) the relaxation induced by the cells was not significantly altered, but this tailed to zero after 26–38 min exposure of the cells to a Po2 of 15 mmHg(2 kPa) 4. These responses were unaltered by the presence of 10−5 mol/l indomethacin, suggesting that prostacyclin is not a significant vasodilator in this system 5. Reduced production of endothelium-derived relaxant factor rather than production of a constrictor factor, or a direct effect on the smooth muscle, may be involved in pulmonary hypoxic vasoconstriction.