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,
NG-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 (15mmol/10ml/kg
body weight) or the same dose twice within a short interval of 1h (dual
oral administration) before the exposure to high ambient temperature (35 ± 1°C) for 180min. 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 300min 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.