Interaction of osmotic and volemic components in initiating salt-gland secretion in Pekin Ducks

Pekin Ducks (Anas platyrhynchos) were infused with 1,000 mM NaCl. Changes in plasma osmolality (Osm(pl)); relative plasma volume (RPV); and plasma concentrations of electrolytes, arginine vasotocin, and angiotensin II were determined at initiation of salt-gland secretion and at 5 and 11 h post-infusion. Changes in these parameters and in volume and concentration of secretion produced were not different between the sexes. Although Osm(pl) and RPV were not related prior to infusion, they were linearly related at the onset of secretion (r(2) = 0.51, P = 0.013). Osm(pl) and RPV were equally effective stimuli for secretion. Initial RPV was a strong determinant of secretion volume (P < 0.01) and total sodium (P < 0.001) and potassium (P < 0.001) secreted, but initial Osm(pl) was not. Plasma arginine vasotocin concentration was increased by NaCl infusion but not by subsequent dehydration. Plasma angiotensin II concentration was not significantly affected by NaCl infusion or by dehydration. These experiments are the first to directly relate the secretory response of the salt glands to the physiological state of ducks prior to salt loading and to describe the interactive simultaneous changes in Osm(pl) and RPV associated with initiation of secretion.