Effects of dietary L-citrulline supplementation on nitric oxide synthesis, immune responses and mitochondrial energetics of broilers during heat stress

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.