Is vasopressin a ''stress hormone'' in the horse?

The aim of this study was to compare the effects of stress caused by exercise and dehydration and stress induced by restraint and use of a nasogastric tube on the plasma arginine vasopressin (AVP) concentration in the horse. Two experiments were performed. In the first experiment, four horses were studied during three different fluid status (normohydrated, dehydrated and hyperhydrated) when performing an incremental exercise test on a treadmill. In contrast to our expectations, the exercise-induced increase in AVP after hyperhydration was much greater than when the horses were exercised during normo- and dehydration. We hypothesised that the high level of AVP in the hyperhydrated horses was a ''stress reaction'' caused by the naso-gastric administration of fluid. Therefore, in the second experiment, the effects of the use of a naso-gastric tube, combined with different methods of restraint and fluid administration, on plasma AVP concentrations were studied in four horses. Dependant on the method of restraint, AVP increased to different levels when the naso-gastric tube was used. AVP decreased immediately when the tube was withdrawn. In our study the combined use of a naso-gastric tube and twitch induced a much greater AVP response than exercise even following dehydration. Our results suggest a role of AVP in mediating stress responses in the horse. The significance of AVP during exercise and the possible effects of high levels on the haemodynamics in the exercising horse needs further investigations.