Renal response of the starling (Sturnus ;ulgaris) to an intravenous salt load

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Abstract
Total kidney GFR's, urine flow, inorganic ion excretion, and single nephron glomerular filtration rates (SNGFRs) were evaluated in anesthetized starlings under control conditions (2.5% mannitol infusion) and in starlings subjected to an osmotic stress induced by the intravenous infusion of 5.8% sodium chloride. Under control conditions the GFR was 2.92 ml/kg per min, urine flow 0.20 ml/kg per min, 63% of filtered sodium was reabsorbed, mean mammalian type (MT) SNGFR was 15.6 nl/min, and mean reptilian type (RT) SNGFR was 7.0 nl/min. During osmotic stress the GFR did not change, urine flow increased to 3 times the control level, the percentage of filtered sodium reabsorbed did not change, both the mean MT SNGFR (24.3 nl/min) and the mean RT SNGFR (10.3 nl/min) were significantly higher than the control levels. During the osmotic stress more MT and fewer RT nephrons were filtering than during control conditions. Under control conditions 98.5% of the sodium excreted by the kidney was associated with uric acid. This percentage decreased as the osmotic stress increased. The starlings tended to excrete the osmotic load imposed by the infusion of NaCl to prevent the plasma osmolality from increasing.