Control of plasma angiotensin II in a bird with salt glands (Anas platyrhynchos)

A radioimmunoassay was developed for the measurement of avian angiotensin II (1Asp-5Val-AII) in the blood plasma. This assay was used to investigate the relationship between body fluid parameters and plasma levels of AII in the Pekin duck, an avian species employing salt glands, besides the kidneys, for NaCl excretion in case of chronic salt stress. A plasma AII concentration of 35.3 +/- 3.9 pg X ml-1 (mean +/- SE) was found in normally hydrated, freshwater-adapted (FW) ducks with a plasma osmolality of 294.9 +/- 0.9 milliosmoles (mOsm) X kg-1. In saltwater-adapted (SW) ducks with a plasma osmolality of 312 +/- 1.5 mOsm X kg-1 and functioning salt glands, a plasma AII level of 102.6 +/- 9.3 pg X ml-1 was found and plasma AII was positively correlated with plasma osmolality (r = 0.601; n = 47; 2P less than 0.001). Intravenous infusion of AII at different rates resulted in a strictly linear relationship between the log values of the AII plasma concentrations and the infusion rates, indicating a plasma clearance rate for AII of 119 ml X min-1 X kg-1 when taking into consideration endogenous AII formation. Experiments designed to produce physiological changes in body fluid and plasma volume and tonicity in FW and SW ducks by appropriate salt and/or water loading indicate that although both parameters may contribute to the control of plasma AII, volume is the dominant stimulus.