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

2000 Fiscal Year Final Research Report Summary

• Project/Area Number: 11670734
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Single-year Grants
• Section: 一般
• Research Field: Pediatrics
• Research Institution: Tohoku University
• Principal Investigator: KONDO Yoshiaki Tohoku University, School of Medicine, Associate Professo, 大学院・医学系研究科, 助教授 (00221250)
• Project Period (FY): 1999 – 2000
• Keywords: urine-concentrating mechanism / kidney medulla / ontogeny / phylogeny / urea / newborn

Research Abstract

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 abain, suggesting the presence of a NaCl reabsorption
mechanism 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.