Differential actions of albumin and plasma on capillary solute permeability

We tested the hypothesis that albumin regulates the solute permeability coefficients (P(s)) of individually perfused exchange microvessels of the frog mesentery by mechanisms similar to those described for hydraulic conductivity. Using unbound Evans blue dye (T-1824, mol wt 960, 1.3 nm radius) as a test solute, we demonstrated a hysteresis of P(s)(T-1824) on luminal albumin content as perfusate albumin concentration was first reduced in three steps from 1 mg/ml to zero and then returned to 1 mg/ml. P(s)(T-1824) did not return to initial control values. The same result was found with tetramethylrhodamine isothiocyanate-α-lactalbumin (mol wt 14,176; 2.0 nm radius) even when the perfusate albumin concentration was increased by 10-fold to 10 mg/ml. In contrast, frog plasma at the same total protein concentration restored P(s)(α-lactalbumin) to values below those measured with control albumin perfusates before Ringer perfusion. Our results conform to the hypothesis that a plasma factor (possibly orosomucoid) that modifies the charge selectivity of the microvessel wall is removed from the microvessel membrane during protein-free Ringer perfusion and is not restored by reperfusion with albumin alone.