Sands J M, Martial S, Isozaki T
Department of Medicine, Emory University School of Medicine, Atlanta, Georgia, USA.
Kidney Int. 1996 Jun;49(6):1611-4. doi: 10.1038/ki.1996.234.
Active transport of urea has been proposed to exist in the 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 1 mM 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 prepared from inner medullae of rats fed 8% protein for three weeks and injected it into Xenopus laevis oocytes. RNA from a 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.
三十多年来，一直有人提出低蛋白饮食的哺乳动物的髓质内集合管（IMCD）中存在尿素的主动转运。我们对喂食标准（18%）或低（8%）蛋白饮食的大鼠的IMCD亚段进行灌注，并测试是否存在主动尿素转运。无论饮食如何，我们在终末IMCD中均未发现主动尿素转运。在喂食18%蛋白或喂食8%蛋白一至两周的大鼠的初始IMCD中，我们同样未发现主动尿素转运。然而，在喂食8%蛋白三至四周的大鼠中，我们发现有显著的净尿素重吸收。当通过添加1 mM哇巴因或去除浴液中的钾来抑制Na +，K（+）-ATP酶活性时，这种主动尿素重吸收受到抑制，提示这是一个继发性主动转运过程。从灌注液中去除钠完全抑制了净尿素重吸收，表明这种主动尿素转运依赖于肾小管管腔中钠的存在。与易化尿素转运体不同，主动尿素转运体不受根皮素抑制，也不受血管加压素刺激，提示它是一种独特的转运蛋白。为了验证这一假设，我们对从喂食8%蛋白三周的大鼠髓质中制备的聚腺苷酸（+）-RNA进行大小分离，并将其注射到非洲爪蟾卵母细胞中。与注射水的卵母细胞或注射其他大小片段的RNA相比，4.4至8.4 kb大小片段的RNA使尿素通透性增加了四倍。我们得出结论，给大鼠喂食三周低蛋白饮食会诱导一种独特的、继发性主动的、钠依赖性尿素转运体的表达，其cDNA大小在4.4至8.4 kb之间。此外，我们的结果表明，通过在非洲爪蟾卵母细胞中表达来克隆这种钠-尿素共转运体的cDNA是可能的。
