IS VASOPRESSIN A STRESS HORMONE IN THE HORSE

The aim of this study was to compare the effects of stress caused by e xercise and dehydration and stress induced by restraint and use of a n asogastric 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 (normoh ydrated, dehydrated and hyperhydrated) when performing an incremental exercise test on a treadmill. In contrast to our expectations, the exe rcise-induced increase in AVP after hyperhydration was much greater th an when the horses were exercised during normo- and dehydration. We hy pothesised that the high level of AVP in the hyperhydrated horses was a ''stress reaction'' caused by the naso-gastric administration of flu id. Therefore, in the second experiment, the effects of the use of a n aso-gastric tube, combined with different methods of restraint and flu id administration, on plasma AVP concentrations were studied in four h orses. Dependant on the method of restraint, AVP increased to differen t levels when the naso-gastric tube was used. AVP decreased immediatel y when the tube was withdrawn.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 exer cise even following dehydration. Our results suggest a role of AVP in mediating stress responses in the horse. The significance of AVP durin g exercise and the possible effects of high levels on the haemodynamic s in the exercising horse needs further investigations.