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Background  High-altitude pulmonary hypertension (HAPH) is a life-threating condition for animals in high altitude, and disturbance of endothelial nitric oxide (NO) synthesis contributes to its pathogenesis. N-carbamylglutamate (NCG), which enhances arginine synthesis, promotes endogenous synthesis of NO. In this study, we determined the effects of NCG on alleviating HAPH in Holstein heifers that ascended to Tibet (Lhasa, 3,658 m).    Methods  Exp. 1, 2,000 Holstein heifers were transported from low elevation (1,027 m) to Lhasa. After being exposed to hypoxia for 1 yr, Holstein heifers were assigned to a healthy group (Control,  n  = 6) with mean pulmonary hypertension (mPAP) < 41 mmHg, and an HAPH affected group (HAPH,  n  = 6) with mPAP > 49 mmHg. Lung tissues were collected to evaluate histopathological changes and the expression of endothelial nitric oxide synthase (eNOS). Exp. 2, ten healthy heifers and 10 HAPH affected heifers were supplemented with NCG (20 g/d per heifer) for 4 wk. Physiological parameters were determined and blood samples were collected on d − 1 and d 28 of the feeding trial.    Results  Expression of eNOS in small pulmonary arteriole intima was higher in the healthy than HAPH group ( P  = 0.006), whereas HAPH group had significantly thicker media and adventitia than healthy group (all  P  < 0.05). The mRNA of  eNOS  and protein level of eNOS were higher in the lungs of heifers in the healthy group than in the HAPH group (both  P  < 0.001), whereas endothelin-1 protein levels were higher in HAPH group than in the healthy group ( P  = 0.025). NCG supplementation decreased mPAP and ammonia (both  P  = 0.001), whereas it increased the expression of eNOS, arginine, and plasma NO (all  P  < 0.05).    Conclusions  The expression of eNOS was decreased in Holstein heifers with HAPH. NCG supplementation decreased mPAP through the restoration of eNOS and endogenous NO synthesis.    Electronic supplementary material  The online version of this article (10.1186/s40104-018-0277-6) contains supplementary material, which is available to authorized users.

journal of animal science and biotechnology/journal of animal science and biotechnology

N-carbamylglutamate restores nitric oxide synthesis and attenuates high altitude-induced pulmonary hypertension in Holstein heifers ascended to high altitude