Fluid uptake from the renal medulla into the ascending vasa recta in anaesthetized rats

1. We have investigated fluid movements between superficial ascending vasa recta (AVR) and the interstitium of exposed papillae of the renal medullae in 15-day-old Sprague-Dawley rats anaesthetized with Hypnorm and Hypnovel. 2. Using a development of the red cell micro-occlusion technique, fluid filtration and reabsorption rates per unit area of vessel wall (Jv/A) were determined in 54 single perfused AVR at known microvascular pressures (Pc). The relation between Jv/A and Pc was non-linear suggesting hydraulic permeabilities (Lp) of 50-100 x 10(-7) cm s-1 cmH2O-1 when Pc was between 0-10 cmH2O and 150-200 x 10(-7) cm s-1 cmH2O-1 when Pc was 10-15 cmH2O. 3. Rates of fluid reabsorption into the AVR estimated by a densitometric technique in a further fourteen vessels were consistent with Lp values of 50-100 x 10(-7) cm s-1 cmH2O-1 when Pc was -2 to 0 cmH2O. 4. The effective oncotic pressures of perfusates containing bovine serum albumin (BSA) were consistent with minimum values for the reflection coefficients of the walls of the AVR to BSA of between 0.59 and 0.72. 5. The concentration of native serum albumin in the papillary interstitial fluid was 9.1 +/- 0.6 mg ml-1 (mean +/- S.E.M., n = 16, from 9 rats), which is approximately 25% of the plasma level. 6. After their microinjection into the medullary interstitium, Patent Blue V and Evans Blue-albumin cleared within 1 min. There was no evidence of preferential movement of either dye towards the base of the exposed renal medulla. 7. Because Lp of the AVR is high, mean pressures of only approximately 3 cmH2O are necessary to account for the total clearance of fluid from the medullary interstitium into the AVR. From published data and from our own observations, it appears that differences in hydrostatic and oncotic pressure across the walls of the AVR are more than sufficient to provide this driving force. The possibility of the clearance of protein from the interstitium into the AVR is discussed.