Sands J M, Naruse M, Jacobs J D, Wilcox J N, Klein J D
Department of Medicine, Emory University School of Medicine, Atlanta, Georgia 30322, USA.
J Clin Invest. 1996 Jun 15;97(12):2807-14. doi: 10.1172/JCI118736.
Low-protein diets cause a urinary concentrating defect in rats and humans. Previously, we showed that feeding rats a low (8%) protein diet induces a change in urea transport in initial inner medullary collecting ducts (IMCDs) which could contribute to the concentrating defect. Now, we test whether decreased osmotic water permeability (Pf) contributes to the concentrating defect by measuring Pf in perfused initial and terminal IMCDs from rats fed 18 or 8% protein for 2 wk. In terminal IMCDs, arginine vasopressin (AVP)-stimulated osmotic water permeability was significantly reduced in rats fed 8% protein compared to rats fed 18% protein. In initial IMCDs, AVP-stimulated osmotic water permeability was unaffected by dietary protein. Thus, AVP-stimulated osmotic water permeability is significantly reduced in terminal IMCDs but not in initial IMCDs. Next, we determined if the amount of immunoreactive aquaporin-2 (AQP2, the AVP-regulated water channel) or AQP3 protein was altered. Protein was isolated from base or tip regions of rat inner medulla and Western analysis performed using polyclonal antibodies to rat AQP2 or AQP3 (courtesy of Dr. M.A. Knepper, National Institutes of Health, Bethesda, MD). In rats fed 8% protein (compared to rats fed 18% protein): (a) AQP2 decreases significantly in both membrane and vesicle fractions from the tip; (b) AQP2 is unchanged in the base; and (c) AQP3 is unchanged. Together, the results suggest that the decrease in AVP-stimulated osmotic water permeability results, at least in part, in the decrease in AQP2 protein. We conclude that water reabsorption, like urea reabsorption, responds to dietary protein restriction in a manner that would limit urine concentrating capacity.
低蛋白饮食会导致大鼠和人类出现尿浓缩功能缺陷。此前，我们发现给大鼠喂食低蛋白（8%）饮食会导致初始髓质内层集合管（IMCD）中尿素转运发生变化，这可能是导致浓缩功能缺陷的原因。现在，我们通过测量喂食18%或8%蛋白质饮食2周的大鼠灌注初始和终末IMCD的渗透水通透性（Pf），来测试渗透水通透性降低是否导致了浓缩功能缺陷。在终末IMCD中，与喂食18%蛋白质的大鼠相比，喂食8%蛋白质的大鼠中精氨酸加压素（AVP）刺激的渗透水通透性显著降低。在初始IMCD中，AVP刺激的渗透水通透性不受饮食蛋白质的影响。因此，AVP刺激的渗透水通透性在终末IMCD中显著降低，但在初始IMCD中未受影响。接下来，我们确定免疫反应性水通道蛋白2（AQP2，AVP调节的水通道）或AQP3蛋白的量是否发生改变。从大鼠髓质内层的基部或尖端区域分离蛋白质，并使用针对大鼠AQP2或AQP3的多克隆抗体进行蛋白质印迹分析（由美国国立卫生研究院的M.A. Knepper博士提供，马里兰州贝塞斯达）。在喂食8%蛋白质的大鼠中（与喂食18%蛋白质的大鼠相比）：（a）尖端的膜和囊泡部分中的AQP2均显著减少；（b）基部的AQP2未发生变化；（c）AQP3未发生变化。这些结果共同表明，AVP刺激的渗透水通透性降低至少部分是由于AQP2蛋白减少所致。我们得出结论，水重吸收与尿素重吸收一样，对饮食蛋白质限制的反应方式会限制尿液浓缩能力。
