Pulmonary Hypertension and the Nitric Oxide System

Abstract: Pulmonary hypertension (PH) is a pathophysiological state associated with several medical conditions, leading to progressive rise in pulmonary vascular resistance (PVR) and right ventricular failure. The clinical PH classification encompasses five main World Health Organization (WHO) groups; pulmonary arterial hypertension (PAH), PH due to left heart disease, PH due to lung diseases and/or hypoxia, chronic thromboembolic PH, and PH with unclear multifactorial mechanisms. Nitric oxide (NO) is a potent vasodilator. Impaired NO production via the classical L-arginine-NO synthase (NOS) pathway has been implicated in PH. Phosphodiesterase-5 (PDE5) inhibitors augment NO signalling, and are considered as one of the cornerstone treatments in PAH. The studies in this thesis aim at to explore and expand the understanding of the NO system in patients with PH.In paper I, we found that PAH patients (WHO group 1) have lower bronchial NO flux compared to healthy controls and patients with PH (WHO group 2–4). This implies reduced bronchial NO formation in PAH. Compared to healthy controls, increased alveolar NO levels were found in patients with PH (WHO group 1-4) and patients with PAH. This may reflect NO diffusion disturbances in the alveoli. PAH patients had higher plasma and salivary levels of nitrite than healthy controls, which may reflect a compensatory upregulation of NOS-independent NO generating pathways.In paper II, we observed that a single oral dose of vardenafil (a PDE5 inhibitor) causes rapid changes in cardiopulmonary haemodynamics in PH patients with PH (WHO group 1-4). We found a correlation between plasma vardenafil concentrations and the changes in mean pulmonary arterial pressure as well as PVR.In paper III, we show that a single oral dose of vardenafil to patients with PH (WHO group 1-4) alter the plasma levels of arginine, asymmetric dimethylarginine (ADMA), symmetric dimethylarginine (SDMA), and the arginine/ADMA ratio in a favourable manner. The increase in arginine and the arginine/ADMA ratio were associated with improved cardiac index, and the increase in the arginine/ADMA ratio at 540 min correlated with the exposure to vardenafil. Higher baseline plasma levels of ADMA and SDMA and a low arginine/ADMA ratio was associated with a more severe pulmonary haemodynamic disease state in patients with PH.In paper IV, we found that ingestion of beetroot juice, containing inorganic nitrate, increased plasma and salivary levels of nitrate and nitrite, increased exhaled NO, decreased plasma ornithine levels and increased relative arginine availability in patients with PAH compared to placebo. Higher plasma levels of nitrite after the placebo period, reflecting basal conditions, were associated with a more severe PAH phenotype. Our findings indicate that the nitrate-nitrite-NO pathway is active and upregulated in PAH patients.