Kidney response to rapid rehydration in the donkey

Donkeys in the desert are often subjected to severe dehydration and are able to replenish their water deficit rapidly. Underlying this capacity is a large water reservoir that is similar in volume to those in desert-dwelling goats, sheep, and camels, but differs in anatomical location: whereas donkeys store water in their hindgut, posterior to the highly permeable parts of the intestine, ruminants store water in the rumen, a foregut compartment. In the present study, blood parameters and kidney function in donkeys were monitored in animals watered daily (control conditions), following four days of water deprivation, and again up to 9-10 h post-drinking. The results were compared to data for Bedouin goats that share the same desert environment and watering regimen with donkeys. Plasma volume in dehydrated donkeys (body mass 113 ± 15.1 kg) was 24% lower than under control conditions. Their plasma osmotic and sodium concentrations were elevated (osmolality from 264 ± 3.1 to 306 ± 18.2 mosmol/kg H₂O and sodium from 138.6 ± 3.1 to 149.0 ± 3.1 mmol/1 H₂O). Unlike newly rehydrated goats, which regain their baseline status gradually and over a period of time that exceeds the post-drinking measuring session, donkeys achieved this within the first 2 h after drinking and maintained it at a constant level. It is suggested that the difference between these two patterns is related to the anatomical location of the water reservoir. Similar to the goat kidney, the donkey kidney efficiently conserves both water and sodium during dehydration and continues to do so following drinking.