The struggle to maintain hydration and osmoregulation in animals experiencing severe dehydration and rapid rehydration: the story of ruminants. 
Ruminants in tropical and desert areas routinely experience cycles of severe dehydration and rapid rehydration.  These animals can withstand severe dehydration (18-40% of initial body weight), which exceeds considerably the capacity of most monogastric mammals.  This capacity is related to their ability to use, during the course of dehydration, their large water reservoir in the rumen, which contributes 50-70% of the total water loss.  As rumen fluid is in approximate isotonicity with systemic fluid, the utilization of gut water during the course of dehydration involves a considerable load of sodium and potassium.  Consequently, the effectiveness of utilization of rumen fluid during dehydration depends on the capacity of the kidney to 'desalt' the water absorbed from the gut and on maintenance of salivary flow to the rumen.  Following rehydration, ruminants can imbibe their entire water deficit in one drinking and the entire amount ingested is first retained in the rumen.  The rumen volume at this stage may exceed the extracellular fluid volume and the sudden drop in rumen osmolality creates a huge osmotic gradient (200-300 mosmol kg-1) between the rumen and systemic fluid.  Ruminant animals are confronted at this stage by two opposing tasks, each of them of vital importance: (i) the need to prevent the osmotic hazard leading to water intoxication; and (ii) the need to retain the ingested water, so that it is not missing for the next dehydration cycle.  The most prevalent view until recently was a theory which attributes an osmotic protective mechanism to the rumen wall.  However, such a capacity has not yet been demonstrated and is challenged by contradictory observations that large amounts of water are absorbed from the rumen following rehydration.  The view that is most consistent with available information is that gustatory-alimentary and hepatoportal signals regarding the presence of large amounts of water in the rumen and the absorption of water from the gut activate a range of homeostatic responses involved in fluid and sodium restitution.  The efferent elements, presumably activated by the CNS, include: a dramatic increase in secretion of hypotonic saliva and, reciprocally, a dramatic drop in urine flow.  The enhanced saliva secretion recycles a considerable portion of the water absorbed from the gut back to the rumen, which allows effective retention of water while avoiding the danger of osmotic threat to the red blood cells.  The enhanced saliva secretion also drains large amounts of sodium and bicarbonate from the blood.  Accompanying responses are marked retention of sodium and carbonic acid in the kidney.(ABSTRACT TRUNCATED AT 400 WORDS)