Dehydration elevates osmotic threshold for salt gland secretion in the duck.

Acute salt and water balance measurements were made in two conscious salt water-acclimated Pekin ducks at and above their osmotic threshold for salt gland secretion. Intravneous infusion of 1,000 mosmol/kg H2O NaCl at 0.350 ml/min increased plasma tonicity less than 0.5% and increased secretion from nearly zero to a rate matching the infusion. Continuous secretion at a similar submaximal rate was driven by 5,600 mosmol/kg H2O NaCl infused at 0.070 ml/min. Osmolality of secreted fluid was constant for any secretion rate, so that net water loss occurred when the concentration of infusate exceeded that of secreted fluid. Threshold plasma osmolality increased by 9 mosmol/kg H2O after the loss of 77 g water (3% body wt). Solutes were always secreted at the infusion rate, even when body fluid osmolality increased while body water decreased. We conclude that the salt gland controller is sensitive to more than just extracellular fluid (ECF) tonicity, and we suggest that elevation of the osmotic threshold may occur in response to decreased ECF volume.