Site of action of endogenous nitric oxide on pulmonary vasculature in rats

The effect of endogenous nitric oxide (NO) on the pulmonary hypoxic vasoconstriction was studied in isolated and blood perfused rat lungs. By applying the occlusion technique we partitioned the total pulmonary vascular resistance (PVR) into four segments: (1) large arteries (R(a)), (2) small arteries (R(a')), (3) small veins (R(v')), and (4) large veins (R(v)). The resistances were evaluated under baseline (BL) conditions and during; hypoxic vasoconstriction and acetylcholine (Ach) which was injected during hypoxic vasoconstriction. After recovery from hypoxia and Ach, N(ω)-nitro-L-arginine (L-NA) was added to the reservoir and the responses to hypoxia and Ach were reevaluated. Before L-NA, hypoxia caused significant increase in the resistances of all segments (P < 0.05), with the largest being in R(a) and R(a'). Ach-induced relaxation during hypoxia occurred in R(a), R(a') and R(v') (P < 0.05). L-NA did not change the basal tone of the pulmonary vasculature significantly. However, after L-NA, hypoxic vasoconstriction was markedly enhanced in R(a), R(a'), and R(v') (P < 0.01) compared with the hypoxic response before L-NA. Ach-induced relaxation was abolished after L-NA. We conclude that, in rat lungs, inhibition of NO production during hypoxia enhances the response in the small arteries and veins as well as in the large arteries. The results suggest that hypoxic vasoconstriction in the large pulmonary arteries and small vessels is attenuated by NO release.