Estimating rat renal medullary interstitial oncotic pressures and the driving force for fluid uptake into ascending vasa recta

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Abstract
1 We have investigated the potential contribution of medullary interstitial oncotic pressure to the net balance of forces influencing fluid movement through the walls of the ascending vasa recta (AVR) in the exposed papillae of 2-week-old Sprague-Dawley rats. 2 Using a capillary zone electrophoresis (CZE) assay, hyaluronan (HA) concentrations were measured in fresh tissue slices from the renal papilla. HA content per wet weight of tissue decreased from tip to base of the papilla, but as a function of cell-free interstitial space (IS) HA concentration was relatively constant at 1.6 μg HA per mg IS up to 1600–1800 μm from the tip. Thereafter the concentration fell rapidly to near zero levels. 3 The volume of the IS was determined using a transmission electron micrographic study of the papilla in age-matched rats. Total interstitial volume (i.e. IS + IC, the interstitial cell volume), as a function of total tissue volume, decreased only slightly between 0 and 1800 μm (i.e. from about 40 to 35 %). IS and IC were found to be reciprocally related with IS decreasing from 21.8 to 10.2 %, while IC increased from 18.3 to 25.2 % over the 1800 μm. 4 Total interstitial oncotic pressures were estimated as the sum of the oncotic pressure due to HA alone plus the oncotic pressure of albumin (A) in an HA matrix. Taking into account reflection coefficients to HA and A, there was an effective oncotic pressure (Eπ) of between 3.46 and 6.0 cmH2O on the interstitial side of the AVR. Under free flow conditions an Eπ in this range is sufficient to result in a net inward driving force of between 5.4 and 9.3 cmH2O, sufficient to account for current estimates of water reabsorption by the AVR. 5 HA concentrations in the papilla increased over the first 3 weeks of life from 0.313 ±0.09 μg (mg wet weight of slice)−1 (mean ± s.e.m.) at 1 week to 0.563 ±0.06 at 3 weeks of age (P > 0.01), in parallel with an age-dependent increase in mean urine osmolarity. It is suggested that the increasing interstitial concentrations of HA may contribute to the development of urinary concentrating ability, which occurs concurrently.