Oncotic effects across isolated perfused renal tubular basement membrane

Peritubular protein concentration affects the rate of volume absorption by the proximal tubule, and this effect may be mediated across the tubular basement membrane. To examine the permeability properties of this membrane, segments of mouse proximal straight tubule were isolated and perfused, and cells were removed using deoxycholate. Volume loss from the isolated perfused basement membrane was measured using blue dextran as a volume marker. Each segment was exposed alternately to solutions containing either no colloid or 6% bovine serum albumin. With rates of perfusion averaging 15.9 +/- 1.3 nl/min, rates of volume loss averaged 0.7 +/- 1.7 with no protein in the bath but were significantly higher with albumin, 16.5 +/- 2.1 nl.min-1.mm-1 (n = 14; mean length = 0.4 +/- 0.04 mm). Significant volume flux was also seen when the bath was changed to contain dextran [weight-averaged mol wt (Mw) approximately 70,000 or approximately 500,000]. In separate experiments, the net flux of albumin with albumin in the bath averaged 4.2 +/- 1.2 pmol.min-1.mm-1 (n = 5). The results show that this basement membrane has a high water permeability and significantly restricts the movement of albumin. Permeabilities for osmotic water flow and albumin were estimated from modeling fluxes in individual segments and found to average 70.6 +/- 9.8 cm/s (sigma = 0.1; 11.4 +/- 1.6 for sigma = 1) and 1.6 +/- 0.3 micron/s (assuming sigma = 0.1; 0.8 +/- 0.2 for sigma = 1), respectively. Using these permeabilities, it is demonstrated that interstitial protein may significantly reduce hydrostatic pressure in the intercellular space by oncotic action across the basement membrane.