Induction of nitric oxide synthase activity in pulmonary arteries from normoxic and chronically hypoxic rats

Chronic hypoxia has recently been shown to upregulate inducible nitric oxide synthase (iNOS) gene expression in rat lung. In the present study, we questioned whether induction of NO synthesis could alter the reactivity of pulmonary arteries (PA) from chronically hypoxic (CH) rats. Dose-response curves to phenylephrine (PE) 10(-9) to 5 x 10(-6) M) were examined in PA rings as well as response to L-arginine analogues in isolated lungs from CH or normoxic (N) rats after various incubation times. Although maximal contraction to PE did not differ in PA from CH rats compared to N rats at time 0 (361 +/- 53 vs 506 +/- 52 mg, respectively), it was markedly decreased after prolonged incubation (149 +/- 28 vs 386 +/- 47 mg, respectively, at 4 h; p < 0.001). This phenomenon persisted after endothelial-denudation, but was reversed by NG-monomethyl-L-arginine (L-NMMA) (5 x 10(-4) M) and prevented by actinomycin D (2 x 10(-6) M). In contrast, maximal contraction to PE in aorta from CH rats was similar at time 0 and 4 h. After a short incubation, PA contraction to L-NMMA was greater in CH than in N rats (96 +/- 17 vs 33 +/- 9 mg at 90 min; p < 0.05), was abolished after endothelial denudation, but persisted in CH rats in the presence of calmidazolium (5 x 10(-4) M). At 4 h, contraction to L-NMMA was abolished in endothelium-denuded PA from N rats but only attenuated in those from CH rats. In salt solution perfused lungs, L-NMMA added 30 or 90 min after isolation did not alter baseline pressure in N rats but caused its increase in CH rats. Whereas iNOS messenger ribonucleic acid (mRNA) was detectable by reverse-transcriptase polymerase chain reaction in the PA wall of N or CH rats after 4 h of incubation, it was absent in both at the time of isolation. In contrast, there was evidence of iNOS mRNA in lungs from CH rats at the time of isolation but no signal in those from N rats. In conclusion, there is induction of nitric oxide synthase activity in pulmonary arteries from normoxic and chronically hypoxic rats after prolonged incubation, but this effect is more pronounced in pulmonary arteries from chronically hypoxic rats.