Responses of gull kidneys and salt glands to NaCl loading

Glomerular filtration rate was measured in Glaucous-winged Gulls, Larus glaucescens, acclimated to freshwater or saline and was also measured (along with salt gland secretion) following intravenous and intragastric infusion of 700 mM sodium chloride (NaCl). Freshwater- and saline-acclimated gulls had the same mean glomerular filtration rate, 5.6 +/- 0.8 and 6.1 +/- 1.0 mL.min-1.kg-1; urine flow rate, 44 +/- 20 and 46 +/- 19 microL.min-1.kg-1; urine/plasma osmolality ratio, 1.2 +/- 0.2 and 1.7 +/- 0.1; urine/plasma [14C]polyethylene glycol ratio, 210 +/- 69 and 211 +/- 64; and Na+ reabsorption, 99.6 +/- 0.2 and 99.6 +/- 0.3%, respectively. Renal function during 143 mM NaCl infusion (glomerular filtration rate, 3.4 +/- 0.3 mL.min-1.kg-1; urine flow rate, 19.7 +/- 2.8 microL.min-1.kg-1; urine/plasma osmolality ratio, 1.7 +/- 0.1; urine/plasma [14C]polyethylene glycol ratio, 223 +/- 26, and Na+ reabsorption 99.9 +/- 0.0%) was not affected by subsequent infusion of 700 mM NaCl, although plasma osmolality was significantly increased. The urine/plasma ratio for [14C]polyethylene glycol, but not osmolality, was higher in gulls than in other avian species. Intravenously infused NaCl elicited less concentrated salt gland secretion at a significantly lower threshold plasma osmolality than intragastric NaCl infusion. Salt gland secretion and glomerular filtration rates were inversely related.