Effect of superfusate albumin on single capillary hydraulic conductivity

It has been proposed that albumin interacts with the endothelial cell surface to form part of the molecular filter at the capillary wall. Previous investigations of the "protein effect" have been limited to sites accessible to albumin from the capillary lumen. We tested for a specific interaction of albumin with the ablumenal surface of the capillary wall. The hydraulic conductivity (Lp) of single capillaries in frog mesentery, perfused initially with albumin-free Ringer, was reduced when albumin (concentration greater than 1 mg/ml) was added to the superfusate (mean fractional reduction 0.56 +/- 0.05 SE, n = 15). A similar reduction was measured when the mesothelial barrier to water and solute flows between the superfusate and the albuminal surface of the capillary was destroyed. When the albumin was extensively dialyzed against Ringer to remove small vasoactive molecules, no change in the fractional reduction of Lp was observed. Lp was reduced to a minimum value in any capillary when the albumin concentration on both sides of the capillary wall was greater than 1 mg/ml. Our data conform to the hypothesis that albumin modifies structures throughout the capillary wall to maintain normal permeability. We predict that extravascular albumin reduces the ability of a Ringer perfusate to increase permeability in many isolated perfused organs.