L-Citrulline acts as potential hypothermic agent to afford thermotolerance in chicks

Recently we demonstrated that L-citrulline (L-Cit) causes hypothermia in chicks. However, the question of how L-Cit mediates hypothermia remained elusive. Thus, the objective of this study was to examine some possible factors in the process of L-Cit-mediated hypothermia and to confirm whether L-Cit can also afford thermotolerance in young chicks. Chicks were subjected to oral administration of L-Cit along with intraperitoneal injection of a nitric oxide synthase (NOS) inhibitor, N^G-nitro-L-arginine methyl ester HCl (L-NAME), to examine the involvement of NO in the process of hypothermia. Food intake and plasma metabolites were also analyzed after oral administration of L-Cit in chicks. To examine thermotolerance, chicks were orally administered with a single dose of L-Cit (15 mmol/10 ml/kg body weight) or the same dose twice within a short interval of 1 h (dual oral administration) before the exposure to high ambient temperature (35 ± 1 °C) for 180 min. Although the rectal temperature was reduced following administration of L-Cit, L-NAME caused a greater reduction. L-NAME reduced total NO₂ and NO₃ (NOx) in plasma, which confirmed its inhibitory effect on NO. A single oral administration of L-Cit mediated a persistent state of hypothermia for the 300 min of the study without affecting food intake. It was further found that plasma glucose was significantly lower in L-Cit-treated chicks. Dual oral administration of L-Cit, but not a single oral administration, afforded thermotolerance without a significant change in plasma NOx in chicks. In conclusion, our results suggest that L-Cit-mediated hypothermia and thermotolerance may not be involved in NO production. L-Cit-mediated thermotolerance further suggests that L-Cit may serve as an important nutritional supplement that could help in coping with summer heat.