Excretion of Urea

Kidney International (1996) 49, 1611–1614; doi:10.1038/ki.1996.234

Active urea transport in the rat inner medullary collecting duct: Functional characterization and initial expression cloning

Jeff M Sands1, Sonia Martial1 and Taisuke Isozaki1

1Renal Division, Department of Medicine, Emory University School of Medicine, Atlanta, Georgia, USA

Correspondence: Dr Jeff M Sands, Emory University School of Medicine, Renal Division, 1364 Clifton Road, NE, Atlanta, Georgia 30322, USA. E-mail: jsands@emory.edu

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Abstract

Active urea transport in the rat inner medullary collecting duct: Functional characterization and initial expression cloning. Active transport of urea has been proposed to exist inthe inner medullary collecting duct (IMCD) of low-protein fed mammals for over 30 years. We perfused IMCD subsegments from rats fed a standard (18%) or a low (8%) protein diet and tested for the presence of active urea transport. We found no active urea transport in terminal IMCDs, regardless of diet. In initial IMCDs from rats fed 18% protein or fed 8% protein for one to two weeks, we again found no active urea transport. However, in rats fed 8% protein for three to four weeks, we found significant net urea reabsorption. This active urea reabsorption was inhibited when Na+,K+-ATPase activity was inhibited by adding 1mM ouabain or removing bath potassium, suggesting a secondary active transport process. Removing sodium from the perfusate completely inhibited net urea reabsorption, demonstrating that this active urea transport is dependent upon the presence of sodium in the tubule lumen. Unlike the facilitated urea transporter, the active urea transporter was not inhibited by phloretin nor stimulated by vasopressin, suggesting that it is a distinct transport protein. To test this hypothesis, we size-separated poly(A)+-RNA preparedfrom inner medullae of rats fed 8% protein for three weeks and injected it into Xenopus laevis oocytes. RNA froma 4.4 to 8.4 kb size fraction increased urea permeability fourfold compared to water-injected oocytes or injecting RNA from other size-fractions. We conclude that feeding rats a low-protein diet for three weeks induces the expression of an unique, secondary active, sodium-dependent urea transporter whose cDNA is between 4.4 and 8.4 kb in size. In addition, our results suggest that it will be possible to clone the cDNA for this sodium-urea cotransporter by expression in Xenopus laevis oocytes.

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