UREA PRODUCTION BY KIDNEY COLLECTING DUCTS IN-VITRO - EFFECT OF AMINO-ACID ADDITION

Urea production by cortical (CCD) and medullary (OMCD) collecting duct s of the rat kidney was measured in vitro by incubating single microdi ssected pieces of tubule in the presence of L-[guanido-C-14]arginine ( 0.2 mM). The [C-14]urea released from the cells was hydrolysed in pres ence of urease added to the incubation medium and the (CO2)-C-14 forme d was trapped in KOH and counted. The effect of various amino acids (A A) on urea production was investigated by adding unlabelled AA (either in combination or singly) at concentrations close to those present in blood plasma. A mixture of 17 AA decreased urea production from [C-14 ]arginine by 46% in CCD and by 58% in OMCD. When lysine and proline we re omitted from the mixture, the inhibition was less marked (19% in CC D and 43% in OMCD, respectively). When AA were tested singly, lysine i nduced the larger inhibition (40% in CCD and 45% in OMCD), than ornith ine and glutamine (about 15% each, in CCD and OMCD), whereas proline i nhibition (7% in CCD, 10% in OMCD) was not statistically significant. Branched-chain amino acids (BCAA) in combination (leucine, isoleucine and valine) also markedly reduced urea production by CCD and OMCD. The ir effect was dose dependent. Solubilization of CCD and OMCD cell memb ranes with Triton X-100 resulted in a twofold increase in urea product ion by control samples; the relative inhibition (per cent) induced by BCAA was enhanced, whereas that induced by lysine was decreased. The d ata suggest that, in living tubules, the inhibition obtained with lysi ne resulted, for a large part, from competition between lysine and arg inine fro cell uptake via a common membrane carrier, whereas the inhib ition induced by BCAA corresponded to an effect on arginase activity i tself.