High energy and pelleted diets increase growth rate and improve feed conversion ratio (FCR) in broilers. However, these diets can increase the incidence of metabolic disorders such as ascites. The aims of this study were to investigate effects of pelleting or increasing the level of energy in the diet on ascites incidence, and biochemical and histopathological changes in broilers’ pulmonary artery. 608 male broilers (ROSS 308) were used in a completely randomized design. Treatments (n = 4) included high (3200 kcal/kg) and low (2868 kcal/kg from 7 - 21 day and 2955 kcal/kg from 21 - 42 day) levels of dietary energy each combined with two forms of mash or pellet. Chickens were sacrificed at 3, 4, 5 and 6 weeks. Histopatological analysis (H E staining) was performed on pulmonary artery. Collagen and elastin were measured in pulmonary artery by cyanogen bromide treatment. Gelatin zymography performed for detection of matrix metalloproteinases (MMPs). Data were analyzed by GLM procedure of SAS. A P -value less than 0.05 was considered statistically significant. At 42 days, there was no difference in arterial wall thickness, medial thickness, external diameter and lumen diameter between high energy pelleted diet (HP), low energy pelleted diet (LP) and high energy mashed diet (HM) and low energy mashed diet (control). Right ventricular weight/total ventricular weight was the lowest in control as compared other treatments. (HP = 0.16 ± 0.007, LP = 0.15 ± 0.006, HM = 0.16 ± 0.007 vs. control= 0.12 ± 0.006). Collagen content was higher in all diets than control (HP = 493.78 ± 10.02, LP = 483.02 ± 9.14, HM = 475.54 ± 9.55 vs. control= 439.4 ± 9.14 ?g/cm). These data showed that high energy and pelleted diets increase collagen synthesis, however elastin content did not change. FCR improved in pelleted diets compared to mashed diets. The activation of MMP-9 increased on 5 and 6 weeks. at d 42, high energy pelleted diet resulted in biochemical changes at cellular-molecular level. Although, treatments did not result in incidence of clinical ascites, pelleting the high energy diets can induce significant biochemical changes that may eventually lead to ascites. Key words: energy density, pelleted diet, elastin, collagen, MMP-9, ascites.
