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

Abstract

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.)

Transport of sodium and urea in outer medullary descending vasa recta.

“…4. The 22Na permeabilities (PNa) of papillary DVR and AVR are similar to previously determined values ( 10,11 ). For the first time, urea permeability (Pu), is also provided.…”

“…4, Table II) based on vessel diameters of 15.6 and 20 ,tm, respectively (33). PNa does not differ significantly from that previously measured ( 10,11). With remarkable consistency, values for P. were nearly identical to those for PNa in both vessel types.…”

“…10) upon exposure to 10 ,gM calcium ionophore A23 187 (n = 6) or 1 nM angiotensin II (n = 15). Contraction is reversible upon removal of A23 187 or angiotensin II.…”

“…A tracheostomy was performed, the left femoral artery cannulated for blood pressure monitoring, and a bladder catheter inserted to measure urine flow from the right kidney. The papilla ofthe left kidney was prepared for micropuncture as previously described (7,10,11,16,17). After surgery, a 30-min equilibration period was permitted to elapse before microperfusion was performed.…”

Physiology of the renal medullary microcirculation

“…That transport proceeds in a direction opposite to inwardly directed Starling forces (hydraulic and oncotic pressure), thus implicating transmural small-solute osmotic gradients as the responsible driving force (98,118,137,138). For NaCl and urea gradients generated by the lag in equilibration between DVR plasma and medullary interstitium to induce water efflux, the water must traverse a pathway of sufficiently small pore size for these solutes to be osmotically active.…”