Responses of reindeer to water loading, water restriction and ADH

Two female reindeer were hydrated by administration of (10% of b.wt.) water into the rumen. The diuretic response was very fast and strong but the urea and electrolyte excretion were little affected. Dehydration was carried out by not giving the reindeer water for 48 h. This water deprivation caused a loss of up to 20% of their body weight. The urine osmolality did not exceed 840 mosm/kg H2O, although the plasma osmolality rose from 300 to 346 and 368 mosm/kg H2O respectively. The plasma and urine urea concentrations were elevated during dehydration, while the urine urea excretion did not increase. Urine sodium concentration did not increase. When the urine flow rate, after two days of water deprivation, decreased to half of the original, the urine Na+ concentrations, instead of increasing, went down to half of the original. So did the potassium excretion. When ADH was injected intravenously into hydrated animals a dose of 30 mU of ADH was needed to induce antidiuresis or increased excretion of potassium. The resistance to ADH and the low relative thickness of the medulla confirm the limited capacity of reindeer kidney to concentrate urine or to excrete a solute load. On the other hand, reindeer is able rapidly to excrete surplus water without affecting the electrolyte or nitrogen balance.