Transport of sodium chloride and water in rat ascending vasa recta

Experiments were undertaken to test the hypothesis that transcapillary small solute (NaCl and urea) gradients drive water across ascending vasa recta (AVR). Axial gradients of NaCl and urea were eliminated with furosemide. AVR were perfused with buffer containing fluorescein isothiocyanate-labeled dextran and 22Na. Perfusion of AVR with isotonic buffer at 10 and 20 nl/min yielded collectate-to-perfusate 22Na ratios of 0.17 +/- 0.05 and 0.34 +/- 0.03, respectively, in AVR of 601 +/- 56 and 583 +/- 46 microns mean length, respectively. A 22Na permeability of 113.2 +/- 12.8 x 10(-5) cm/s was determined. AVR were perfused at 20 nl/min with buffer NaCl of 0 (hypotonic to papilla), 161 (isotonic), or 500 mM (hypertonic). Transcapillary volume flux was not significantly different in these groups (3.8 +/- 1.5, 4.6 +/- 1.5, and 2.1 +/- 1.4 nl.min-1.mm-1, respectively). AVR were perfused in the hydropenic kidney at 5 nl/min antegrade from tip to base and retrograde from base to tip, which was a maneuver designed to impose physiological transcapillary NaCl and urea gradients of opposite direction. Volume fluxes were -1.4 +/- 0.05 and -1.3 +/- 0.04 nl.min-1.mm-1 in these groups, respectively. These data demonstrate that the AVR are highly permeable to NaCl and that physiological small solute gradients do not influence water movement across the AVR wall.