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The effect of exercise dehydration and hypertonic saline infusion on osmolality and vasopressin in donkeys

Elizabeth R. Vint, BS; Nora S. Matthews, DVM; Tex S. Taylor, DVM; John Stallone, PhD; Kerry S. Barling, DVM, PhD

Objective - To determine the effects of work-induced dehydration/rehydration and acute infusion of hypertonic saline on plasma osmolality (Osm) and vasopressin (VP) levels in the donkey and compare these data with that of the horse.

Animals- Five standard donkeys (2 jennets and 3 geldings) weighing 215 to 314 kg, 10 to 14 years old.

Procedure- In part one, donkeys are harnessed to a cart and driven at a walk and trot for 5 hours to induce dehydration.  Blood samples were taken hourly during dehydration and at 30 minutes, 1, and 2 hours during rehydration.  Total protein (TP), hematocrit (Hct), saline, at 50 μmol/kg/min, followed by hypertonic 9% NaCl saline, at 50μmol/kg/min, were infused for 15 minutes each into the coartoid artery and blood samples were taken at 0, 1, 2, 5, 10, and 15 minutes from both jugular veins.   In both parts, plasma VP measurements were made using radioimmunoassay (RIA).

Results- TP, Hct, Osm, and [Na+] increased by 14, 21, 2.5, and 5.3 percent respectively during dehydration while [K+] decreased by 24%.  With hypertonic infusion, carotid side jugular POsm increased 10% while contralateral jugular POsmonly increased 1-2%.  VP results are unavailable at this time.

Conclusions- This data indicates that plasma TP, Hct, POsm, [Na+], and [K+]change significantly upon exercise induced dehydration.  During rehydration POsm fell below baseline before returning to baseline at 2 hours while TP and Hct decreased but failed to reach baseline in 2 hours.