Dissociation between apparent volumes of distribution for inulin and sodium by salt adaptation in ducks

In domestic ducks acclimated to 2% saltwater (SW ducks), chronic elevations of plasma osmolality and sodium concentration, and of the circulating levels of antidiuretic hormone and angiotensin II indicate a trend towards dehydration when compared to ducks maintained on freshwater (FW ducks). Under steady state conditions of salt and water balance, with prevailing antidiuresis and no salt gland secretion, the apparent volumes of distribution of 22Na and of radiolabeled inulin were calculated after single injections of the tracers. The sodium spaces as percent body mass of FW ducks (33.3 +/- 0.9) and SW ducks (31.5 +/- 0.5) were not different, but apparent volumes for inulin distribution differed significantly between FW ducks (31.2 +/- 0.7) and SW ducks (24.8 +/- 0.7). Plasma volumes and rates of inulin disappearance from the plasma during both the non-uniform and uniform phases of tracer distribution were closely similar in FW and SW ducks and support the hypothesis that the difference between apparent volumes of inulin distribution reflects a difference in extracellular fluid volume which was estimated as 20.9% in FW ducks and 17.2% in SW ducks. For SW ducks the difference between inulin and sodium spaces was significantly greater than for FW ducks. In relation to the extracellular fluid compartment SW ducks have a larger pool of non-extracellular exchangeable sodium.