Effects of administration of water versus an isotonic oral rehydration solution (ORS) at rest and changes during exercise and recovery

The administration of 41 of an isotonic, plasma-like oral rehydration solution (ORS) with an osmotic skeleton and 41 of water (water; no osmotic skeleton), were evaluated in five thoroughbred horses. Solutions were administered by nasogastric tube 4 h after feeding. Uptake of deuterium, concentrations of plasma sodium, potassium, chloride, glucose, total protein and packed cell volume, pH, PCO2, HCO3-, total CO2, actual base excess, standard base excess, plasma volume and weight loss were assessed both at rest, and during and after exercise on a treadmill. Each horse underwent four experimental sessions (water-resting; ORS-resting; water-exercise; ORS-exercise). There was an indication of uptake of both water and ORS by 10 min post-administration. Based on the appearance of deuterium in plasma, there was no significant difference in the rate of uptake of water or ORS at rest, although there was a trend for the uptake of ORS to be slower than water during the exercise session. The mean decrease in total protein (TP, 3.0 g l(-1)) and the increase in plasma volume (PV, 4.6 ml kg(-1)) after administration of ORS at rest was greater (P < 0.05) than that of water (TP, 1.3 g l(-1) and PV, -1.2 ml kg(-1)). There was no significant difference in TP or PV following administration of water or ORS during the exercise treatment. Exercise had little effect on plasma sodium concentration. The results confirmed that administration of 41 of isotonic, plasma-like ORS provided a much more distinct and durable contribution to the maintenance of plasma volume and circulation than administration of an equal amount of water at rest. Subsequent exercise may, however, mask some of the effects observed at rest.