Introduction
Exercising horses do not always rehydrate voluntarily despite extensive fluid losses. This can cause clinical problems as well as being detrimental for performance capacity, as dehydration eventually will reduce plasma volume enough to compromise cardiovascular functions and heat dissipation. Since horse sweat is hyper- or isotonic compared to plasma (McCutcheon et al. 1995), fluid losses during exercise will not necessarily cause an increase in plasma sodium concentration (pNa+) and osmolality (pOSM). If these parameters are unchanged, thirst might not be stimulated. To evaluate this, post-exercise water intake was correlated to plasma osmolality (pOSM), plasma sodium concentration (pNa+), and the proportional increase in total plasma protein concentration (D%TPP).
Methods
The Uppsala Local Ethics Committee had approved the protocol of this study. Four, trained Standardbred horses (5-14 years, 500 ±8 kg) which received their daily maintenance requirement of sodium in the feed and had access to a salt block (pure NaCl) were used. Their voluntary daily water intake was measured during the pre-trial period and two methods of supplying water was used alternatively, either graded buckets (2x20 L) or a self-filling automatic water bowl (3 L/min). The method was changed every fourth day during a total of 32 days in a changeover design. Because these experiments revealed that the horses drank 41% more from the buckets, only this method was used during the exercise trials. The horses performed an incremental exercise test and a 40-min constant velocity exercise test on a treadmill during normohydration, after dehydration for 24 h, and after hyperhydration with 12 L of water. All data are presented as mean values (with SE).
Results
Post-exercise water intake was positively correlated to pOSM (y=0.9x-246.0, R2 = 0.71, p< 0.001) and pNa+ (y=1.6x-221.0, R2 = 0.61, p< 0.001). The correlation between water intake and D%TPP (y=0.9x-0.5, R2 = 0.36, p< 0.001) was less pronounced. However, there was an individual positive correlation between D%TPP and water intake. Two of the horses, A and C, had much lower slopes than B and D and the former two horses also had lower mean daily water intake (17±1 respectively 16±1 L) than horse B (26±1 L) and D (23±1 L) when both water supplying methods were considered.
Conclusions
Our results suggest an individual sensitivity for hypovolemic stimulus of thirst, which might explain why some horses are more prone to voluntary dehydration than others, although further investigations are needed to verify this.
References
McCutcheon, L.J., R.J., Geor, M.J., Hare, J.K. Kingston, and H.R., Staempfli (1995) Equine vet. J., Suppl. 18, 279-284.