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Computer analysis of the significance of the effective osmolality for urea across the inner medullary collecting duct in the operation of a single effect for the counter-current multiplication system

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Abstract

Background

Although urea and water are transported across separate pathways in the apical membrane of the inner medullary collecting duct (IMCD), the existence of a cellular diffusion barrier as an unstirred layer makes it possible to use coefficients of effective osmotic force (σ*) as equivalent to reflection coefficients. The difference in effective osmolality between urea and NaCl across the IMCD becomes a driving force for water if the compositions of solutes are different between tubular lumen and interstitium. Since reported values for σ*urea are discrepant, we compared the efficiency of a single effect in the counter-current system between an ascending thin limb (ATL) and the IMCD, with the interposition of capillary networks (CNW), between two models with σurea* = 0.7 (model 1) and σurea* = 1.0 (model 2).

Methods

The time courses (within 3 s) of solute and the water transport profiles among ATL, CNW, and IMCD were simulated with a computer in the absence of flow in each compartment.

Results

In spite of small differences in the profiles of urea and NaCl concentrations between the two models, model 1 displayed a larger volume flux in the IMCD than model 2, resulting in an increase of osmolality in the IMCD and a decrease of osmolality in the ATL. These findings are vital for the operation of the counter-current multiplication system.

Conclusions

The concept of coefficients for effective osmotic force can be applied to the counter-current model between the IMCD and the ATL with the interposition of CNW. The model of σurea* = 0.7 is more efficient than that of σurea* = 1.0.

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Authors and Affiliations

  1. Division of Molecular Pharmacology, Department of Pharmacology, Jichi Medical University, 3311-1 Yakushiji, Shimotsuke, Tochigi, 329-0498, Japan

    Junichi Taniguchi & Masashi Imai

Authors
  1. Junichi Taniguchi
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  2. Masashi Imai
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Correspondence to Junichi Taniguchi.

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Taniguchi, J., Imai, M. Computer analysis of the significance of the effective osmolality for urea across the inner medullary collecting duct in the operation of a single effect for the counter-current multiplication system. Clin Exp Nephrol 10, 236–243 (2006). https://doi.org/10.1007/s10157-006-0436-5

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  • DOI: https://doi.org/10.1007/s10157-006-0436-5

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