A new hypothesis on the maturation of urea-dependent urine concentrating mechanism

虽然新生儿肾脏在浓缩尿液方面还不强大，但它们已经和成人肾脏一样有效地稀释尿液了。为了阐明这种不成熟的尿浓缩能力的基础，我们研究了新生儿大鼠肾脏内髓质的亨勒袢和收集管（IMCDs）的功能。通过测量膜转运蛋白mRNA表达、经上皮电压和同位素水离子通量，对新生大鼠和成年大鼠肾内髓单个肾小管进行了分析。用多克隆抗体对新生儿和成人肾片的rCCC2和rCLC-K1进行免疫荧光鉴定。第1天，薄升肢（tALs）和imcd的上皮传导电压（VTs）分别为14.6 + 1.1 mV （n=27）和？42.7 + 6.1 mV （n=14）。第1天，薄降肢VTs为0。此外，管腔布美他胺或基底外侧钠离子均能强烈抑制TAL内的VTs，表明其存在与厚升肢（TAL）相似的NaCl重吸收机制。经上皮NaCl梯度引起的tAL扩散电压（VD）对氯离子通道阻滞剂5-硝基-2-（3-苯基丙胺）苯甲酸酯（NPPB）几乎不敏感。在imcd中，腔内酰胺强烈抑制VTs。在第1天，tDL和tAL都不透水，表明整个环路的不透水性。imcd的扩散透水性（Pdw）和尿素透水性（Purea）在第1天显示对后、尿素的基本不透性。抗利尿激素刺激（1 nmol/L）显示，到第14天，只有Pdw对抗利尿激素敏感。在新生儿IMCD中，用NaCl代替部分等摩尔尿尿引起的水通量可以忽略不计，这表明新生儿IMCD中不存在尿素依赖的体积通量。每个新生儿小管的这些运输特征与鹌鹑肾脏相似。rAQP-1、rCCC2、rCLC-K1、rENaCb亚基、rAQP-2和rUT-A1等特异性转运体的mrna鉴定和免疫荧光研究支持了这些发现。我们有理由认为，新生大鼠的肾髓小管组织与鸟类的肾髓有很大的相似之处，而肾髓小管组织的质变是哺乳动物新生儿未成熟尿液集中能力的主要原因。少

While neonatal kidneys are not powerful in concentrating urine, they already dilute urine as efficiently as adult kidneys. To elucidate the basis for this paradoxical immaturity in urine-concentrating ability, we investigated the function of Henle' s loop and collecting ducts (IMCDs) in the inner medulla of neonatal rat kidneys.Analyses of individual renal tubules in the inner medulla of neonatal and adult rat kidneys were performed by measurering mRNA expression of membrane transporters, transepithelial voltages, and isotopic water and ion fluxes. Immunofluorescent identification of the rCCC2 and rCLC-K1 using polyclonal antibodies was also performed in neonatal and adult kidney slices.On day 1, the transepithelial voltages (VTs) in the thin ascending limbs (tALs) and IMCDs were 14.6 + 1.1 mV (n=27) and? 42.7 + 6.1 mV (n=14), respectively. The VTs in the thin descending limbs (tDLs) were zero on day 1. The VTs in the tALs were strongly inhibited by luminal bumetanide or basolateral ou … More abain, suggesting the presence of a NaCl reabsorptionmechanism similar to that in the thick ascending limb (TAL). The diffusional voltage (VD) of the tAL due to transepithelial NaCl gradient was almost insensitive to a chloride channel blocker 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB). The VTs in the IMCDs were strongly inhibited by luminal amiloride.On day 1, both the tDL and tAL were impermeable to water, indicating the water impermeability of the entire loop. Diffusional water permeability (Pdw) and urea permeabilities (Purea) in the IMCDs indicated virtual impermeability to after and urea on day 1. Stimulation by vasopressin (1 nmol/L) revealed that only Pdw was sensitive to vasopressin by day 14. A partial isoosmolar replacement of luminal urea by NaCl evoked negligible water flux across the neonatal IMCDs, indicating the absence of urea-dependent volume flux in the neonatal IMCD.These transport characteristics in each neonatal tubule are similar to those in quail kidneys.Identification of mRNAs and immunofluorescent studies for specific transporters including rAQP-1, rCCC2, rCLC-K1, rENaCb subunit, rAQP-2 and rUT-A1 supported these findings.Conclusion. We propose with reasonable assurance that the renal medullary tubule organization of neonatal rats shares a tremendous similarity with avian renal medulla, and that the qualitative changes in the organization of medullary tubules are primarily responsible for the immature urine-concentrating ability in mammalian neonates. Less