Molecular sieving of small solutes by outer medullary descending vasa recta.

Molecular sieving of small solutes by outer medullary descending vasa recta (OMDVR). Descending vasa recta (DVR) plasma equilibrates with the medullary interstitium by volume efflux (Jv), as well as by influx of solutes. Jv is driven by transmural osmotic pressure gradients due to small hydrophilic solutes (delta pi s), NaCl and urea. DVR endothelium probably contains a "water-only" pathway most likely mediated by the aquaporin-1 (AQP1) water channel. We measured the ability of microperfused OMDVR to concentrate lumenal 22Na and [3H]raffinose when Jv was driven by transmural NaCl gradients. Collectate-to-perfusate ratios of 2 x 10(6) M(r) fluorescein isothiocyanate-labeled dextran volume marker (RDx), 22Na (RNa), and [3H]raffinose (Rraf) were measured in the absence and presence of Jv. During volume efflux (Jv > 0), RDx was 1.37 +/- 0.31. RNa increased from 0.64 +/- 0.03 when Jv = 0 to 0.82 +/- 0.05 when Jv > 0 and Rraf increased from 0.83 +/- 0.03 to 1.13 +/- 0.05: Mathematical simulations predict RNa and Rraf most accurately when the OMDVR reflection coefficient to the tracers is assigned a value near unity. This indicates that the OMDVR wall contains a pathway for osmotic volume flux that excludes small hydrophilic solutes, a behavior consistent with that of aquaporins.