Effect of saline intake on water flux and osmotic homeostasis in Pekin ducks (Anas platyrhynchos).

The physiological regulation of body water volume and concentration was evaluated in Pekin ducks, Anas platyrhynchos, slowly acclimated to increasingly saline drinking water (six equal 75 mM NaCl increments). Body mass, total body water (TBW), water flux, plasma osmolality (Osm(pl)), and ionic and osmoregulatory hormone concentrations were measured at the end of each increment. The salinity at which each variable deviates from its homeostatic set point was calculated by continuous two-phase linear regression. We hypothesized that, as drinking water salinity increases: (1) body water increases in concentration before it decreases in volume and (2) that regulating variables that help determine homeostatically set values (plasma hormone concentrations and water flux) deviate from values of freshwater ducks at lower drinking water salinities than the variables they regulate (Osm(pl), hematocrit, TBW). Osm(pl) was the first variable for which we could calculate a deviation from its homeostatically controlled value. It increases at much lower drinking water salinity than that at which TBW decreases, supporting our first hypothesis, but not our second hypothesis. We further hypothesized that, because the concentration of Pekin duck salt gland secretion is only slightly higher than that of their drinking water, they increase water flux (drinking) as salinity of drinking water increases, until the latter exceeds the secretion concentration and then they drink less. There was no change in water flux until it decreases when TBW decreases, 329 mM NaCl and 335 mM NaCl, respectively. The results do not support our hypothesis that Pekin ducks increase drinking as the salinity of their drinking water increases, but do indicate that, at tolerable salinities, Pekin ducks maintain body water volume while allowing body water osmolality to increase. At higher salinities, ducks decrease drinking and use body water to get rid of the excess salt.