Endothelium-derived relaxing factor in pulmonary and renal circulations during hypoxia

The pulmonary and renal vasculatures, in contrast to the systemic vasculature, constrict during hypoxia. The endothelium has been implicated in mediating these vascular responses to acute hypoxia via the production of endothelium-derived vasoactive factors. The present study, performed in anesthetized dogs, was designed to investigate the role of endothelium- derived relaxing factor (EDRF) to attenuate the vasoconstrictor response of the pulmonary and renal circulations during acute hypoxia. In response to hypoxia, pulmonary (2.2 ± 0.3 to 4.5 ± 0.6 mmHg · l^-1 · min) and renal (0.60 ± 0.07 to 0.90 ± 0.14 mmHg · ml^-1 · min) vascular resistances increased. Inhibition of endogenous EDRF with N(G)-monomethyl-L-arginine resulted in similar increases in pulmonary (3.0 ± 0.1 to 4.8 ± 0.4 mmHg · l^-1 · min) and renal (0.67 ± 0.07 to 0.90 ± 0.09 mmHg · ml^-1 · min) vascular resistances as in hypoxia. However, in the presence of both hypoxia and EDRF inhibition, an exaggerated pulmonary vascular response was observed (2.2 ± 0.2 to 7.4 ± 0.9 mmHg · l^-1 · min), in contrast to the renal vascular response to EDRF inhibition during hypoxia (0.61 ± 0.05 to 0.95 ± 0.10 mmHg · ml^-1 · min), which was not different from hypoxia or EDRF inhibition individually. The endothelium-derived contracting factor endothelin, which modestly increased during hypoxia (11.7 ± 1.9 to 15.6 ± 2.4 pg/ml), may also contribute to this vasoconstrictive response to hypoxia. This study suggests in the intact animal that EDRF serves to oppose the pulmonary vasoconstrictor response to hypoxia and further characterizes the role of endothelium-derived factors in the regulation of vascular function during hypoxia.