Effect of Sodium Chloride Gradients on Water Flux in Rat Descending Vasa Recta

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 attrib-uted 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 (hy-pertonic). Collectate was obtained from the perfused DVR by micropuncture and the collectate-to-perfusate ratios of FITC-Dx and 22Na were measured. A mathematical model was em-ployed to determine DVR permeability (P.) and reflection coef-ficient 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, re-spectively (N. = number ofDVR perfused). The collectate-to-perfusate 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 transcapil-lary NaCl concentrations gradients induce water movement across the wall of the DVR. (J. Clin. Invest. 1991. 87:12-19.) Key words: descending vasa recta * sodium chloride * urinary concentration * rat * micropuncture * microperfusion in viv