Kidney Secretion in Reptiles

Abstract

In birds there is now rather conclusive evidence that tubular secretion plays a major role in the excretion of uric acid by the kidney. 1 , 2 Uric acid is the main nitrogenous urinary constituent of the arid-living reptiles (snakes and lizards) but no data are available as to its mode of excretion. The experiments reported here indicate that in the lizard, it is chiefly excreted by tubular secretion.

The urine/plasma ratio of uric acid has been compared with that of glucose after administration of phlorizin. Glucose is not secreted by the tubule 3 and phlorizin paralyzes the reabsorptive power of the tubule for glucose. Hence, the urine/plasma ratio for glucose should be a measure of the amount of glomerular filtrate if no glucose is reabsorbed under phlorizin. This error, if present, is probably small and would not affect appreciably the conclusions drawn from the present experiments.

The iguana (Iguana iguana Shaw) has been used as the experimental animal. Phlorizin was injected subcutaneously in dosage of 250 mg. per kilo 2 hours before the experiment or in 2 doses of 200 mg. per kilo the afternoon before and early on the morning of the experiment. The iguanas were anesthetized with urethane (5 cc. of a 25% solution per kilo), the cloaca opened with a small incision, cannulae tied into the ureteral papillae, and urine collected for a period of 10 to 30 minutes. The abdominal cavity was then quickly opened and blood drawn from the aorta. The urine which was milky in appearance was diluted with hot water to an appropriate volume to obtain solution of all the uric acid. The plasma was precipitated with tungstic acid. Uric acid was determined by the method of Benedict 4 and glucose by the Hagedorn-Jensen method. 5