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Effect of saline intake on osmotic homeostasis in ducks Bennett, Darin Chris

Abstract

Salinity appears to be an important barrier to habitat selection by ducks, yet little is known about the osmoregulatory abilities of ducks that use saline environments. This thesis examines the physiological mechanisms by which ducks maintain osmotic balance when exposed to saline. I first used domesticated Pekin ducks (Anas platyrhynchos) as a model to identify the osmoregulatory traits that confer saline tolerance in ducks. I then compared these in wild species selected to represent ducks that utilize freshwater, estuarine and marine habitats: Mallards (Anasplatyrhynchos), Canvasbacks (Aythya valisineria) and Barrow's Goldeneyes (Bucephala islandica), respectively. My major hypothesis is that among the three species of wild ducks, the ability to maintain osmotic homeostasis during acclimation to saline, habitat affinity, and their ability to secrete excess Na+ are correlated. When Pekin ducks were given increasing concentrations of saline to drink, they maintained water and osmotic balance at salinities up to 300 mM NaCl by producing salt gland secretion slightly higher than 300 mM Na+ . This concentration is roughly half that produced in response to intravenous saline infusion (500-600 mM Na+ ) . When the three species of wild ducks were similarly acclimated to saline, Canvasbacks and Goldeneyes were both more saline tolerant than Pekin ducks and maintained water balance at salinities equivalent to full strength seawater. Mallards did not tolerate salinities greater than 225 mM NaCl . Evaluation o f the relationship between maximum saline tolerance and maximum concentrating ability of the salt glands could not be tested because ducks did not secrete spontaneously when handled. Pekin ducks acclimated to 300 mM NaCl move extracellular water and Na+ into the cells, a mechanism that may be important to the ability to initiate salt gland secretion. I then examined the relative roles of extracellular fluid volume and concentration in the stimulation of salt gland secretion. Increases in extracellular fluid volume and concentration worked interactively to stimulate salt gland secretion. Ducks with a small initial extracellular fluid volume took longer to initiate salt gland secretion and secreted less of the imposed saline load than ducks with a larger extracellular fluid volume. Initial volume of extracellular fluid and its expansion in response to saline loading may be the dominant determinants of the secretory response. The results obtained from the Pekin duck experiments showed that redistribution of extracellular water and Na+ might be important in the control of salt gland secretion and in conferring tolerance to saline in ducks. When wild ducks drank freshwater, total volume of body water did not differ among the three species, but Goldeneyes, the most marine species, had the largest extracellular fluid volume and was the only species that shifted water and Na+ into the cells in response to saline acclimation. The last study examined kidney and salt gland functions by which wild ducks eliminate excess Na+ , but maintain water balance. I found that renal filtration was unaffected by saline intake in any of the three species, but saline tolerance was determined by rates of renal tubular water and Na+ reabsorption and salt gland Na+ secretion. Goldeneyes had the highest rates of all these processes and were the only species that secreted all the infused Na+ via the salt glands. Mallards had lower rates, although saline acclimated Mallards eliminated all the infused Na+ by combined renal and extrarenal excretion. Canvasbacks tolerated higher drinking water salinities than Mallards, even though their renal and extrarenal Na+ excretion rates during acute saline loading indicated they should not be able to do so. This suggests osmoregulation in Canvasbacks may involve Na+ and water regulation at other sites, such as the gut. Postrenal modification o f urine may play an important role in conservation of water in Canvasback.

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