Role of nitric oxide and endothelin-1 in monocrotaline-induced pulmonary hypertension in rats

Objective: Nitric oxide (NO) and endothelin-1 (ET-1) have both been implicated in the pathogenesis of pulmonary hypertension (PH). Therefore, we examined NO-related relaxation and ET-1 levels in rat hilar pulmonary arteries (PA) during the progression of monocrotaline (MCT)-induced PH. Methods: Rats were studied 1 and 2 weeks after a single subcutaneous injection of MCT (80 mg/kg). Pulmonary artery pressure (PAP), right ventricular hypertrophy (RVH), NO-related relaxation and tissue ET-1 levels in PA were evaluated and compared with control (C). Results: One week post-MCT, endothelium (E)-dependent relaxation to 10−5 M adenosine diphosphate (ADP), 10−5 M A23187 and 10−5 M acetylcholine (ACh) and tissue ET-1 levels in PA were normal. Rats in this group did not develop PH or RVH. Two weeks post-MCT, E-dependent relaxation was impaired (ADP, 7 ± 3% vs. C, 62 ± 5%; A23187, 2 ± 7% vs. C, 58 ± 2%; ACh, 33 ± 7% vs. C, 86 ± 2%; P < 0.05) and ET-1 levels were elevated (1925 ± 244 pg/g wwt vs. C, 469 ± 59 pg/g wwt, P < 0.05). In addition, significant PH and RVH were present (PAP 33 ± 4 mmHg vs. C 18 ± 0.8 mmHg, P < 0.05; RVH index 0.40 ± 0.006 vs. C, 0.25 ± 0.01, P < 0.05). Incubation with 10 μM indomethacin, 150 U/ml superoxide dismutase or 300 μM l-arginine failed to restore impaired relaxation to ACh. In E-intact rings, relaxation to 10−6 M glyceryl trinitrate (GTN) was inhibited at 1 week post-MCT (72 ± 2% vs. C, 87 ± 3%, P < 0.05) with further inhibition at 2 weeks (39 ± 4%). Response to GTN in E-denuded rings was normal in MCT groups. Conclusions: These results indicate that MCT injection in rats results in delayed but progressive endothelial injury and PH. Despite mild endothelial dysfunction 1 week post-MCT, NO-related relaxation and ET-1 levels are normal. At 2 weeks post-MCT, inhibition of E-dependent NO-related relaxation and elevation of ET-1 levels are associated with PH and RVH. Thus, inhibition of NO production associated with elevated ET-1 levels may play an important role in the pathophysiology of MCT-induced PH.