FLUID UPTAKE BY THE RENAL MEDULLARY VASA-RECTA - AN ESTIMATE BASED ONA QUANTITATIVE-ANALYSIS OF THE DISTRIBUTION OF FENESTRAE IN THE VASA-RECTA OF YOUNG SPRAGUE-DAWLEY RATS

Transmission electron microscopic techniques were used to carry out a quantitative analysis of the density of fenestration in the inner medu llary vasa recta of the rat kidney. Measurements were made at 200 mu m intervals from the tip to the base of the papilla (1800 mu m from the tip). Fenestral diaphragms were estimated to be 65.4 +/- 0.78 nm in d iameter (mean +/- S.E.M.), and were arranged in plaques with a mean in terfenestral distance of 1148 +/- 2.6 nm. Near the tip of the papilla there was no correlation between vessel size and degree of fenestratio n; density of fenestration, however, began to decrease about 1400 mu m from the tip. The ratio of fenestrated to non-fenestrated profiles of vasa recta was found to be linear with respect to distance from the t ip (r = 0.991), with values ranging from about 40:1 near the tip to 2: 1 near the base of the papilla. We have estimated the proportion of th e total surface area of a fenestrated vasa recta occupied by fenestral diaphragms to be 0.057 at 1000 mu m from the tip. The total potential conductance (K) of a 200 mu m segment of fenestrated vessel at 1000 m u m from the tip was calculated to be 0.319 mu m(3) s(-1) cmH(2)O(-1), giving a hydraulic conductivity (L-p) of 0.030 mu m s(-1) cmH(2)O(-1) . We have also examined the reverse question of the conductance of a s ingle fenestra if all the fluid flux across the vessel wall occurred t hrough the fenestrae and none via the intercellular clefts or water ch annels; single fenestral conductance was estimated to be 1.94 x 10(-3) mu m(3) s(-1) cmH(2)O(-1).