Sasaki A, Doi S, Mizutani S, Azuma H. Roles of accumulated endogenous nitric oxide synthase inhibitors, enhanced arginase activity, and attenuated nitric oxide synthase activity in endothelial cells for pulmonary hypertension in rats. Am J Physiol Lung Cell Mol Physiol 292: L1480-L1487, 2007. First published February 23, 2007 doi:10.1152/ajplung.00360.2006 has been suggested to play a key role in the pathogenesis of pulmonary hypertension (PH). To determine which mechanism exists to affect NO production, we examined the concentration of endogenous nitric oxide synthase (NOS) inhibitors and their catabolizing enzyme dimethylarginine dimethylaminohydrolase (DDAH) activity and protein expression (DDAH1 and DDAH2) in pulmonary artery endothelial cells (PAECs) of rats given monocrotaline (MCT). We also measured NOS and arginase activities and NOS protein expression. Twenty-four days after MCT administration, PH and right ventricle (RV) hypertrophy were established. Endothelium-dependent, but not endothelium-independent, relaxation and cGMP production were significantly impaired in pulmonary artery specimens of MCT group. The constitutive NOS activity and protein expression in PAECs were significantly reduced in MCT group, whereas the arginase, which shares L-arginine as a common substrate with NOS, activity was significantly enhanced in PAECs of MCT group. The contents of monomethylarginine (MMA) and asymmetric dimethylarginine (ADMA), but not symmetric dimethylarginine (SDMA), were increased in PAECs of MCT group. The DDAH activity and DDAH1, but not DDAH2, protein expression were significantly reduced in PAECs of MCT group. These results suggest that the impairment of cGMP production as a marker of NO production is possibly due to the blunted endothelial NOS activity resulting from the downregulation of endothelial NOS protein, accumulation of endogenous NOS inhibitors, and accelerated arginase activity in PAECs of PH rats. The decreased overall DDAH activity accompanied by the downregulation of DDAH1 would bring about the accumulation of endogenous NOS inhibitors.L-arginine-nitric oxide pathway; dimethylarginine dimethylaminohydrolase PULMONARY HYPERTENSION (PH) is a fatal disease characterized by intimal and medial hyperplasia and adventitial fibrosis in peripheral pulmonary arteries. Impairment of endogenous release of nitric oxide (NO), which causes vasodilation (38) and inhibition of both platelet aggregation (3) and vascular smooth muscle cell proliferation (17), has been proposed to be important in developing the PH (1, 13). Actually, under clinical settings, NO level in lung tissue is lower in patients with PH compared with healthy controls (22), and NO inhalation is a specific pulmonary vasodilator therapy (39). In addition, it has been reported that short-term oral administration of L-arginine improves hemodynamics and exercise capacity in patients with precapillary PH (35).NO production, on one hand, depends on the nitric oxide synthase (NOS) activity and protein expression. There is a report describin...