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

Abstract

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 (P,,). The relation between Jv/A and P, was nonlinear suggesting hydraulic permeabilities (Lp) of 50-100 x 10-7 cm s-1 cmH2T-when P, was between 0-10 cmH2O and 150-200 x 10-7 cm s' cmH20-when P, 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-when P, 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 -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.The reabsorption of fluid from the renal interstitium into the ascending vasa recta (AVR) is the final stage in the conservation of water by the formation of a concentrated urine. The details of this process are unclear. In most microvascular beds, net fluid uptake from tissues to blood occurs when the oncotic pressure of the plasma exceeds the sum of the oncotic pressure of the pericapillary fluid and the hydrostatic pressure difference across the microvascular walls (Starling, 1896). In these tissues there are lymphatics, which act both as an overflow and as a route for clearing plasma protein from the interstitial fluid. It has been argued that in tissues, such as the intestinal mucosae and the renal cortex, the steady uptake of fluid into the blood depends on a parallel but much smaller flow of lymph which, by clearing interstitial proteins, maintains the oncotic pressure differences across the capillary walls (Michel, 1984;Levick, 1991). In the inner zone of the renal medulla, however, lymphatics are difficult to demonstrate and it is concluded that they are either sparse (Cuttino, Jennette, Clark &...

Countercurrent exchange in the renal medulla

“…With respect to transport of macromolecules, the reflection coefficient of the AVR to albumin has been measured by different methods. Values of 0.59-0.72 and an average of 0.78 were obtained by MacPhee and Michel (64) and Pallone (81), respectively (Table 3). Those nonzero values predict significant sieving of albumin by the AVR wall.…”

“…Transmural oncotic pressure gradients favor water uptake across the AVR wall and AVR hydraulic conductivity is very high (Table 3) (64,82,98,113,114). In vivo, transmural gradients in the AVR generated by the osmotic lag between blood and interstitium are directed to favor water uptake (luminal concentration Ͼ interstitial concentration).…”

“…Those nonzero values predict significant sieving of albumin by the AVR wall. Theoretical analysis of macromolecular trafficking has been considered in several studies (64,86,141,153).…”

Physiology of the renal medullary microcirculation

“…The renal medulla is devoid of lymphatics, so albumin must be removed from the interstitium into AVR (10,20). The reflection coefficient of AVR to albumin is relatively low (0.58-0.78) (66)(67)(68)101). Based on this and the ability of AVR to withstand a hydraulic pressure gradient without collapsing, it has been postulated that albumin is cleared from the interstitium by convective solvent drag across the AVR wall (67-69, 100, 160-162).…”

“…Thus, for mass balance, removal is delegated to AVR. The latter are highly fenestrated (110,138) and have very high hydraulic conductivity and solute permeability that exceeds that of DVR (68,69,102,103,116). AVR are less well characterized than DVR because they cannot be isolated for in vitro perfusion.…”