Increased endothelium-derived NO in hypertensive pulmonary circulation of chronically hypoxic rats

The hypothesis that the endothelium-derived relaxing factor/nitric oxide (EDNO) activity is elevated in chronic hypoxic pulmonary hypertension (CH- PHT) was tested using isolated Krebs-albumin-perfused rat lungs. Concentration of the EDNO decomposition products (NO(x)) in the lungs' effluent was measured by a modified chemiluminescence assay. The functional significance of basal EDNO production was studied by measuring the vasoconstrictor response to an EDNO synthesis inhibitor, N(ω)-nitro-L- arginine methyl ester (L-NAME). Reactivity to the endothelium-dependent vasodilator substance P and to exogenous NO was also studied. More NO(x) was found in effluent from CH-PHT (22.3 ± 9.8 nM) than control (0.4 ± 3.9 nM) lungs. The L-NAME-induced vasoconstriction was greater in CH-PHT than in control rats. The sensitivity, but not the maximal vasodilation, to exogenous NO was elevated in CH-PHT. The substance P-induced vasodilation was potentiated in CH-PHT compared with control rats and blocked by L-NAME in both groups. We conclude that basal and agonist-stimulated pulmonary EDNO activity is enhanced in this model of CH-PHT. The EDNO synthesis may play a counterregulatory role in CH-PHT.