Excretion in fresh-water turtle (Pseudemys scripta) and desert tortoise (Gopherus agassi)

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Abstract
Renal clearance studies during dehydration, salt loading, and water loading were performed on two species of chelonians. Mild dehydration in fresh-water turtles decreases glomerular filtration rate (GFR) and increases tubular reabsorption of water. Urine osmolality approaches that of the blood. A similar response occurs after a salt load that raises the plasma osmolality less than 20 milliosmols. More severe dehydration or greater salt loads cause anuria. In desert tortoises anuria does not occur with salt loading until the blood osmolality is raised more than 100 milliosmols. With smaller increases in plasma osmolality, GFR and tubular reabsorption of water are unchanged and the urine always remains hypoosmotic to the blood. In both species a water load increases GFR markedly but has little or no effect on tubular function. Changes in GFR reflect changes in the number of functioning glomeruli. Uric acid is actively secreted by the tubules of both species. The fresh-water turtle is predominantly aminoureotelic, the desert tortoise predominantly uricotelic. The bladders of desert tortoises are permeable to water, ions, and some larger molecules. Urine entering the bladder becomes isoosmotic to the blood while urates precipitate. In fresh-water turtles, bladder urine can remain hypoosmotic and urates do not precipitate. The differences in renal and bladder function are related to the habitats of the animals and to the end products of their nitrogen metabolism.

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