BIOCHEMICAL HETEROGENEITY OF ARGININE METABOLISM ALONG KIDNEY PROXIMAL TUBULES

By using an in vitro single tubule micromethod of high specificity, in four different species of mammals it has been observed that (a) argin ine synthesis from citrulline (arginine synthase activity, E.C. 6.3.4. 5 and E.C. 4.3.2.1) is restricted to the early portions of proximal co nvoluted tubules, whereas (b) urea production from arginine (arginase activity, E.C. 3.5.3.1.) is present mainly in the cortical (CPST) and even more in the outer medullary (OSPST) portions of straight proximal tubules. The data suggest that (a) in early PCT cells, the citrulline reabsorbed from glomerular filtrate is converted into arginine, which in turn crosses peritubular cell membranes together with reabsorbed a rginine, and (b) the urea formed in CPST and OSPST cells might passive ly diffuse into the luminal fluid entering Henle's loops. Such urea se cretion might contribute to sustain the process of urea recycling in k idney medulla and thereby participate in the mechanism of urine concen tration.