In the present study, vitamin C was used as a positive control. Its beneficial effects have already been confirmed in chicken growth performance, pulmonary hypertensive response, and intestinal morphology by our previous study [27]. Then, the comparison of vitamin C effects with marjoram could be helpful.
The results based on the RV: TV ratio showed that marjoram could reduce pulmonary hypertensive response in the broilers. Chen and Chen [28] reported that phenolic acids and tanshinones improve atherosclerosis, thrombosis, and myocardial injuries. Bina and Rahimi [5] also determined that marjoram inhibited lipid peroxidation and nitric oxide synthesis in the heart tissue to improve cardiovascular dysfunction.
Although many reports have shown that marjoram improves body weight gain, food consumption, or feed conversion ratio [7, 8, 29], there are also conflicting reports [6, 30, 31]. In these reports, different products of marjoram (oil, essential oil, powder, and extract) with different concentrations have been used. Indeed, the compounds and characteristics of marjoram would be different in these products, and then considerably change the effects of marjoram in the chickens. In the present study, marjoram extract was used in developmental pulmonary hypertension, only marjoram 0.2% extract led to improve FCR in 22–35 days.
In the studies of Solis de Los Santos et al. [32] and Zamani Moghaddam et al. (2009), it was reported that digestive system damages occur in the PHS. Many studies suggested different additives and supplements (e.g., vitamin C, L-arginine, garlic) in the diets of pulmonary hypertensive chickens for improving the PHS and intestinal damage [20, 21, 27, 33]. In our study, we determined that oral supplementation of marjoram extracts changes the intestinal morphology (duodenum) and increases the thickness of lamina propria in pulmonary hypertensive broilers. The effect of marjoram on lamina propria thickness could be evidence of its improvement in intestinal secretion. The beneficial effect of marjoram on the intestine may be associated with its antioxidant and anti-apoptotic effects [2, 29, 34]. It was indicated that dietary antioxidants such as vitamin C protect enterocytes against oxidative stress, promote epithelial proliferation, and improve villus morphology [27, 35]. Therefore, the antioxidant effect of marjoram could be an essential factor in protecting the intestine against oxidative damage in PHS. This useful effect of marjoram may be also observed in the heart, lung, and blood vessels that are crucial organs in PHS. Different studies have determined that compounds such as Terpinen-4-ol, Spathulenol, c-Terpinene, and α-Terpineol have strong anti-oxidative, anti-inflammatory, antifungal and antibacterial effects [36–39]. These compounds were considered the major constituents of marjoram leaf extract in our study. Thus, this strong effect of marjoram constituents in the oxidant and antioxidant status could justify its improvement in the developmental PHS and its other following advantages in the intestine.
In our study, the decrease of lipid peroxidation and increase of CAT activity could partially confirm the antioxidant property of marjoram in PHS. [15][15][15][15][15][15][15][15]Abdel-Wahab [29] reported that high doses of marjoram (1-2.5% of diet) increased GPX activity in healthy chickens. Kamel [40] found that marjoram increased CAT, SOD, and GPX activities in diabetic rats. However, the antioxidant effect of marjoram may be related to its dose and cell condition. There is a complex antioxidant protection system in the cells and organs. This system consists of enzymatic and non-enzymatic antioxidants with several members. The members work together and regulate each other to neutralize and scavenge free radicals. Enzymatic antioxidants (such as SOD, CAT, and GPX) are the essential members of this protective system that play a crucial role in cellular defense [41]. In this study, marjoram did not significantly affect the enzymatic antioxidants (except for CAT). Marjoram even down-regulated the gene expression of those enzymes in the lung and heart. Therefore, marjoram may also influence the other members of the total antioxidant system to reduce lipid peroxidation.