Endothelium-derived Contracting and Relaxing Factors Contribute to Hypoxic Responses of Pulmonary Arteries

The response of porcine pulmonary arteries to hypoxia depended on their location in the vasculature and the degree and duration of the hypoxic challenge. In rings of pulmonary artery suspended for isometric tension recording (37 degrees C, 16% O2 and 5% CO2), moderate hypoxia (10% and 4% O2) caused endothelium-dependent relaxation in distal arteries but transient endothelium-dependent contraction in proximal arteries. In both proximal and distal arteries, the initial response to anoxia (0% O2) was a transient endothelium-dependent contraction. This was followed by a slowly developing, sustained endothelium-dependent contraction in proximal arteries, or by an endothelium-independent relaxation in distal arteries. The endothelium-dependent relaxation to moderate hypoxia in distal arteries was inhibited only by combined inhibition of endothelium-derived relaxing factor (EDRF)-nitric oxide (NO) synthase [N omega-nitro-L-arginine methyl ester (L-NAME)] and cyclooxygenase (indomethacin), suggesting mediation by EDRF-NO and prostacyclin. Transient endothelium-dependent contractions to moderate hypoxia (proximal arteries) or anoxia (all arteries) were abolished by L-NAME, but the late endothelium-dependent anoxic contraction observed in proximal arteries was not reduced by L-NAME and/or indomethacin. Therefore, hypoxia/anoxia may initiate contraction of pulmonary arteries by decreasing the activity of EDRF-NO, but the contractions appear to be maintained by an increased activity of an endothelium-derived contracting factor.