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

Abstract

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 (sigma*) 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 sigma*(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 sigma(urea)* = 0.7 (model 1) and sigma(urea)* = 1.0 (model 2). 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. 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. 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 sigma(urea)* = 0.7 is more efficient than that of sigma(urea)* = 1.0.