On leaking into the lumen, amino acids cross the tubule cells. Secretion of L-citrulline in the isolated-perfused non-filtering kidney of the African clawed toad (Xenopus laevis).

In our previous studies we were able to show that L-citrulline is taken up across the contraluminal membrane into the tubule cells of the isolated perfused non-filtering kidney of the African clawed toad (Xenopus laevis). We have also described the tubular metabolism of the L-citrulline taken up. The remaining question was whether peritubular L-citrulline gains access to the tubular lumen and, if so, by what kind of mechanism. The method was the same as that described in our previous study: the perfusion of the kidneys was performed through both portal veins (2 ml/h into each) with a solution containing inulin. In order to stimulate the urine formation in the non-filtering kidney, we added p-aminohippurate to the perfusion solution. In the urine collected from the ureters no inulin could be detected. Thus, the glomeruli were not reached by the perfusate, and no glomerular filtration took place. Upon adding 65 mumol/l L-citrulline to the perfusion solution (corresponding to the physiological plasma concentration), a urinary excretion rate of this amino acid of 4.5 +/- 0.4 nmol.h-1.g-1 (n = 6) was determined. During perfusion under the same conditions with D-citrulline (65 mumol/l) the excretion rate was only 0.36 +/- 0.1 nmol.h-1.g-1 (n = 5). A similarly low excretion rate was also found with L-citrulline when L-phenylalanine (20 mmol/l) was added to the perfusate or when NaCl was substituted by mannitol. These data show that L-citrulline taken up from the contraluminal side of the renal tubular epithelium enters the tubular lumen to a much greater extent than its D-isomer.(ABSTRACT TRUNCATED AT 250 WORDS)