Abstract 15229: Critical Protective Role of Endogenous Nitric Oxide in Established Severe Pulmonary Arterial Hypertension in Rats

Abstract

Background: It is widely believed that impared bioavailability of endothelium-derived nitric oxide (NO) contributes to the development of pulmonary arterial hypertension (PAH). However, there are reports that show high pulmonary endothelial NO synthase (eNOS) expression in PAH patients, and the functional roles of endogenous NO in established PAH remain unclear. Hypothesis: We tested the hypothesis that pulmonary eNOS expression is increased, and that endogenous NO plays an important functional role in a rat model of PAH. Method and Results: Adult male Sprague Dawley rats (BW: 150 - 200g) were injected subcutaneously with Sugen5416 (20 mg/kg; VEGF receptor blocker), and exposed to hypoxia (10% O2) for 3 weeks followed by a return to normoxia for 5 weeks. Hemodynamic examination was performed at the 1-, 3-, 5- and 8-week (wk) time points after Sugen5416 injection. RV systolic pressure increased time-dependently (RVSP: 47 ± 6, 83 ± 14, 101 ± 22, and 122 ± 16 mmHg at 1-, 3-, 5- and 8-wk time points, n=4-6) compared with normal rats (31 ± 2 mmHg, n=4, p<0.01, respectively). Blockade of NOS with Nω-nitro-L- arginine methyl ester (L-NAME, 30 mg/kg, iv) further increased RVSP acutely (ΔRVSP from each baseline: 7 ± 11, 25 ± 11, 52± 21, and 43 ± 13 at 1-, 3-, 5- and 8-wk time points) compared with normal rats (0.8 ± 1.1 mmHg). At the 8-wk time point, inhibition of NOS caused a marked RVSP elevation (165 ± 11 mmHg), leading to RV failure and death in all rats tested. In vivo pulmonary angiogram using the synchrotron radiation microangiogram system showed severe small pulmonary arterial narrowing in response to L-NAME. Western blot analysis showed increased eNOS protein levels in PAH rat lungs (n=4 each time point; p<0.01 vs. normal) and immunohistochemisty revealed high expression of eNOS in remodeled pulmonary arteries at the 5- and 8-time points. Conclusion: Based on our findings that L-NAME caused marked pulmonary vasoconstriction and further elevation in RVSP and that there was high lung eNOS protein levels in advanced PAH rats, we conclude, contrary to the general assumption, that endogenous NO presumably via eNOS may play a critical protective role by counteracting severe pulmonary vasoconstriction in established PAH.