Basolateral uptake and tubular metabolism of L-citrulline in the isolated-perfused non-filtering kidney of the African clawed toad (Xenopus laevis).

The kidney forms arginine (Arg) by using citrulline (Cit) as precursor, and is the main source of Arg for systemic protein synthesis. Even if the filtered and reabsorbed load (in rats) is sufficient for normal Arg synthesis, the following questions remain. (a) Can Cit be taken up across the contraluminal membrane of the tubule cells also? If so, (b) by what kind of mechanism? And (c) is this Cit, entering the cell from the peritubular side, metabolized to Arg and ornithine (Orn)? Although these questions are raised mainly in connection with mammals, we used the amphibian kidney, which is especially suitable because of its double blood supply, for an initial approach to the problem. After the toad was decapitated, the portal vein, the caval vein and the ureters were catheterized, and the kidneys were perfused through the portal vein (Ringer solution + L- or D-Cit + inulin + p-aminohippurate + L-aspartate). Exclusive peritubular perfusion was assured by showing that inulin perfused into the portal vein did not appear in the urine. During perfusion of the portal vein with L-Cit in a physiological concentration (65 mumol/l), an initial peritubular net uptake of L-Cit of 170 +/- 27 (n = 10) nmol.h-1.g kidney-1 (wet weight) was observed, whereas the value for D-Cit (65 mumol/l) was only 18 +/- 7 (n = 6) nmol.h-1.g-1. After perfusion for 50 min, the uptake of L-Cit reached a steady state with an uptake rate of 108 +/- 5 nmol.h-1.g-1.(ABSTRACT TRUNCATED AT 250 WORDS)