Background

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.

Methods

Analyses of individual renal tubules in the inner medulla of neonatal and adult rat kidneys were performed by measuring 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.

Results

On day 1, the transepithelial voltages (V_Ts) 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 V_Ts in the thin descending limbs (tDLs) were zero on day 1. The V_Ts in the tALs were strongly inhibited by luminal bumetanide or basolateral ouabain, suggesting the presence of a NaCl reabsorption mechanism similar to that in the thick ascending limb (TAL). The diffusional voltage (V_D) of the tAL due to transepithelial NaCl gradient was almost insensitive to a chloride channel blocker 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB). The V_Ts 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 (P_dw) and urea permeabilities (P_urea) in the IMCDs indicated virtual impermeability to water and urea on day 1. Stimulation by vasopressin (1 nmol/L) revealed that only P_dw 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, rENaC subunit, rAQP-2, and rUT-A1, supported these findings.

Conclusion

We hypothesize that the renal medullary tubule organization of neonatal rats shares a tremendous similarity with avian renal medulla. The qualitative changes in the organization of medullary tubules may be primarily responsible for the immature urine-concentrating ability in mammalian neonates.