Glomerular filtration rate in conscious unrestrained starlings under dehydration

Glomerular filtration rate (GFR) was measured in conscious unrestrained starlings, Sturnis vulgaris. Alzet osmotic minipumps were inserted into the peritoneal cavity under lidocaine local anesthesia and served to infuse the GFR marker [¹⁴C]sodium ferrocyanide. GFR was measured in hydrated birds and again after a 24-h period of water deprivation. GFR in dehydrated birds was 1.15 ± 0.09 ml · min^-1 · kg^-1, significantly lower (P < 0.001) than the 2.71 ± 0.31 ml · min^-1 · kg^-1 in fully hydrated animals. In addition, the GFR on the hydrated birds was significantly lower (P < 0.01) than values of 4.43 ± 0.24 ml · min^-1 · kg^-1 measured in hydrated anesthetized birds during micropuncture studies. A 24-h period of water deprivation caused significant increases in plasma osmolality and concentration of sodium, chloride, and phosphate. The osmolality and concentrations of calcium, magnesium, and phosphate were significantly elevated in both cloacal urine supernatant and in ureteral urine of dehydrated birds. In both hydrated and dehydrated states, the concentration of magnesium, phosphate, potassium, and total osmolality were significantly higher in cloacal urine than in ureteral urine.