Hydraulic and oncotic pressure measurements in inner medulla of mammalian kidney

Molecular sieving of albumin by ascending vasa recta. Evidence exists to support the presence of an extravascular pool of albumin in the renal medullary interstitium. This study used microperfusion in vivo to measure the transport of '25I-labeled albumin from descending (DVR) and ascending vasa recta (AVR) to the papillary interstitium. Perfusions were performed during furosemide diuresis with a buffer containing FITC-labeled dextran (FITC-Dx) 2 X 106 mol wt and 125I-albumin. Perfusate albumin and collection pressure were adjusted to induce either zero transcapillary volume flux (Jv) or high volume flux. When Jv was zero, the collectate-to-perfusate ratios of FITC-Dx (RDX) and 125I-albumin (Rkb) in the DVR and AVR were identical implying that diffusive efflux of albumin was immeasurably small. In contrast, when Jv was increased, paired comparison of R.Jb and RDX in the same AVR revealed a difference, 1.58±0.06 vs 1.72±0.08, respectively (P < 0.01). AVR perfusions in hydropenic animals showed similar results, R.Jb = 1.70±0.07 and RDX = 2.00±0.07 (P < 0.01).These data suggest that albumin transport across vasa recta in vivo is likely to be governed by solvent drag. The reflection coefficient of the AVR wall to 125I-albumin is estimated to be 0.78. (J. Clin. Invest. 1992. 90:30-34.) Key words:

supporting

confidence: 85%

mentioning

confidence: 99%

Molecular sieving of albumin by the ascending vasa recta wall.

Pallone

1992

“…The rats remained nondiuretic throughout the study as indicated by the mean ratios for urine-to-systemic plasma osmolality and urine-to-systemic plasma inulin. (21) and, in a later study (22), of VR at the papillary tip were slightly lower than 7 g/100 ml. In correcting the present data for plasma water we used the highest mean value.…”

Section: Resultsmentioning

confidence: 61%

Evidence for a concentration gradient favoring outward movement of sodium from the thin loop of Henle.

Johnston¹,

Battilana²,

Lacy³

et al. 1977

A B S T R A C T Recent models of the urinary concentrating mechanism have postulated that urea in the medullary interstitium creates a transtubular concentration gradient for sodium between fluid at the end of the descending limb of Henle's loop and the medullary interstitium, favoring the passive outward movement of sodium from Henle's thin ascending limb. These experiments were designed to determine whether such a gradient normally exists. Young nondiuretic Munich-Wistar rats were prepared for micropuncture of the exposed left renal papilla. Samples of loop of Henle fluid and vasa recta plasma (assumed to reflect the composition of interstitial fluid) were obtained from adjacent sites. Loop fluid values in 21 comparisons from 18 rats (mean+SE)were: sodium, 344+ 12 meq/liter; potassium, 26+2 meq/liter; osmolality, 938+37 mosmol/kg H20. Vasa recta plasma values (in corresponding units ofmeasurement) were: sodium, 284±11; potassium, 34±2; osmolality, 935±34. Mean values of paired differences (loop fluid minus vasa recta plasma) were: A sodium, 60±11.1 (P < 0.001); A potassium, -8.0±2.1 (P < 0.001); A osmolality, 4±16 (NS). Corrected for plasma water, the loop fluid minus vasa recta differences (in milliequivalents per kilogram H20) were:A sodium, 40±11.4 (P < 0.005); A potassium, -9.7 ±1.9 (P < 0.001). We interpret these findings to indicate that in the papilla of nondiuretic rats, a significant difference in sodium concentration exists across the thin loop of Henle favoring outward movement of sodium, which confirms a key requirement of the pasPortions of this work were presented to the American Society for Clinical Investigation.

“…Water is transported from the lumen of DVR to the papillary interstitium despite the fact that plasma oncotic pressure exceeds hydraulic pressure in the hydropenic Munich-Wistar rat. Sanjana et al recognized that a large negative pressure within the papillary interstitium could not explain this observation because water is transported from the papillary interstitium into ascending vasa recta (3,16,17). Based upon this analysis Experimental model.…”

Section: Discussionmentioning

confidence: 99%

Effect of sodium chloride gradients on water flux in rat descending vasa recta.

Pallone

1991

In the hydropenic kidney, volume efflux from descending vasa recta (DVR) occurs despite an intracapillary oncotic pressure that exceeds hydraulic pressure. That finding has been attributed to small solute gradients which may provide an additional osmotic driving force favoring water transport from DVR plasma to the papillary interstitium. To test this hypothesis, axial gradients of NaCl and urea in the papilla were eliminated by administration of furosemide and saline. DVR were then blocked with paraffin and microperfused at 10 nl/min with a buffer containing albumin, fluorescein isothiocyanate labeled dextran (FITC-Dx), 'Na, and NaCl in a concentration of 0 (hypotonic to the interstitium), 161 (isotonic) or 322 mM (hypertonic). Collectate was obtained from the perfused DVR by micropuncture and the collectate-to-perfusate ratios of FITCDx and 22Na were measured. A mathematical model was employed to determine DVR permeability (P.) and reflection coefficient to NaCl (aNca). The rate of transport of water from the DVR lumen to the papillaryuinterstitium was 2.8±0.3 (N, = 22), -0.19±0.4 (N, = 15), and -2.3±0.3 nl/min (N, = 21) (mean±SE) when perfusate NaCl was 0, 161, or 322 mM, respectively (N. = number of DVR perfused). The collectate-toperfusate 22Na concentration ratios were 034±0.04,0.36+0.04 and 0.37±0.03 for those groups, respectively. Based on these data, Ps is calculated to be 60.4 X 10-5±4.0 X 10' cm/s and aN~a < 0.05.-The results of this study confirm that transcapillary NaCl concentrations gradients induce water movement across the wall of the DVR. (J. Clin. Invest. 1991. 87:12-19.)