Molecular sieving of albumin by the ascending vasa recta wall.

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 125I-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 10(6) 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 (Ralb) 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 Ralb and RDX in the same AVR revealed a difference, 1.58 +/- 0.06 vs 1.72 +/- 0.08, respectively (P less than 0.01). AVR perfusions in hydropenic animals showed similar results, Ralb = 1.70 +/- 0.07 and RDX = 2.00 +/- 0.07 (P less than 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.