Site of uptake of nonfiltered amino acid in the rabbit kidney.

During passage from renal artery to vein, nonfiltered amino acids are known to be extracted by the kidney, an observation generally attributed to their basolateral uptake by tubular epithelium. This attribution is here tested in the rabbit, using the nonmetabolizable analogue cycloleucine as test compound. Uptake of cycloleucine is diffusion limited and could be maximized by lengthening its artery-to-vein transit time by short aortic occlusion. The transient anoxia did not abolish active solute transport in the kidney; the technique permits acute loading of the kidney with, for instance, a nephrotoxicant without excessive exposure of the whole animal. The major portion of cycloleucine taken up by nonfiltering kidneys during occlusion returned to renal venous plasma with a mean delay of 45 sec, as if it had accumulated in the same cellular transport pool through which reabsorbed cycloleucine has to pass. A fraction of the amino acid taken up also reached the tubular lumen. These results support the suggested role of tubule cells in the extraction of amino acids from postglomerular blood.