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Heat stress adversely affects the growth performance, muscle development, and protein metabolism in poultry. L-Citrulline (L-Cit), is a non-essential amino acid that is known to stimulate muscle protein synthesis under stress conditions. This study investigated whether L-Cit could influence the growth performance, amino acid profile, and protein metabolism in broilers exposed to high ambient temperature. In a 2 × 2 factorial arrangement, Arbor acre broilers (288 chickens) were fed with basal diet (CON) or 1% L-Cit supplemented diet and later subjected to either thermoneutral (TNZ: 24°C, 24 h/d) or heat stress (HS: 35°C for 8 h/d) environment for 21 days. The results showed that L-Cit diet promoted the body weight and body weight gain of broilers higher than the CON diet, and it further alleviated HS suppression of body weight and feed intake at certain periods (p < 0.05). Plasma urea, uric acid, glucose, and total cholesterol were elevated during HS, whereas, the triglyceride content was decreased (p < 0.05). Serum amino acids including citrulline, alanine, aspartate, and taurine were decreased by HS. L-Cit supplementation restored the citrulline level and alleviated HS induction of 3-methylhistidine (p < 0.05). L-Cit supplementation increased the plasma growth hormone (GH) and insulinlike growth factor-1 (IGF-1) concentration, as well as the GH concentration in the breast muscle (p < 0.05). The mRNA expression showed that HS elicited tissue-specific responses by upregulating some growth factors in the breast muscle, but downregulated the GH receptor, GH binding protein, and IGF-1 expression in the hypothalamus. L-Cit supplementation upregulated the GHRH and IGFBP2 expression in the hypothalamus. L-Cit also upregulated the expression of IGF-1R and IGFBP2 in the breast muscle of HS broilers. The total mTOR protein level in the breast muscle of HS broilers was also increased by L-Cit diet (p < 0.05). Therefore, this study demonstrated that HS negatively affected the growth performance of broilers and dysregulated the expression of growth factors related to protein metabolism. Contrarily, L-Cit promoted the growth responses of broilers via its stimulation of circulating GH/IGF-1 concentration. To certain extents, L-Cit supplementation elicited protective effects on the growth performance of HS broilers by diminishing protein catabolism. [ABSTRACT FROM AUTHOR]

L-Citrulline is a non-protein amino acid that promotes arginine recycling and muscle protein synthesis. This study investigated whether L-citrulline can exert these functions in heat-stressed chickens. Arbor acre broilers were fed either basal diets (Control) or basal diets supplemented with 1% L-citrulline (L-Cit). At 28 d old, broilers were subjected to two environmental temperatures, 35 °C for 8 h/d (HS) or 24 °C for 24 h/d (TNZ) for 2 weeks. The experiment was designed as a 2 by 2 factorial arrangement. Results showed that HS increased the core body temperature (CBT) and rectal temperature of broilers compared to the TNZ condition. The CBT and mean CBT decreased by ∼0.5oC in TNZ + L-Cit broilers compared to the TNZ + Control group (P < 0.05). L-Cit supplementation at TNZ significantly (P < 0.05) lowered the plasma malondialdehyde content but this was increased during HS. Exposure to HS significantly (P < 0.05) elevated plasma malondialdehyde compared to TNZ condition. Plasma immunoglobulin A, G, and M were increased (P < 0.05) by TNZ + L-Cit compared to the TNZ + Control group. Plasma nitric oxide and endothelial nitric oxide synthase (NOS) activity was increased during HS, whereas the total and inducible NOS enzymes were decreased. In addition, L-Cit supplementation increased both the inducible and endothelial NOS isoforms (P < 0.05). The mRNA expression of peroxisome proliferator-activated receptor γ coactivator 1-α, avian uncoupling protein, cytochrome C oxidase subunit 3, and ATP synthase F1 subunit beta (ATP5β) in the breast muscle were downregulated (P < 0.05) during HS. However, L-Cit supplementation upregulated the mitochondrial transcription factor A and during HS, L-Cit increased ATP5β expression similar to TNZ housed broilers. Therefore, this study demonstrates that dietary L-Cit can lower the body temperature, decrease lipid peroxidation and promote the immune status of broilers under thermoneutrality. Also, L-Cit would act to promote muscle ATP generation during heat stress in broilers. • Heat stress adversely impacts the growth performance and immune status of broilers. • L-Citrulline is involved in body temperature regulation and nitric oxide synthesis. • L-Citrulline reduced the core body temperature and improved plasma immunoglobulin. • L-Citrulline induced TFAM expression in breast muscle tissues. • L-Citrulline promoted muscle ATP generation during heat stress. [ABSTRACT FROM AUTHOR]