Albumin absorption by canine bronchial epithelium

Albumin concentrations in airway surface liquid are low compared with plasma. To investigate the mechanisms that generate albumin gradients across airway epithelia, we have investigated whether active albumin absorption is a feature of bronchial epithelia. Freshly excised canine bronchi were mounted in Ussing chambers and short-circuited. Permeability coefficients of 14C-labeled canine albumin (P(alb)) were measured in the mucosal-to-submucosal (M→S) and submucosal-to-mucosal (S→M) directions in conductance-matched tissues. Mean steady-state values for P(alb) in the absorptive (M→S) direction (5.97 ± 1.89 x 10-7 cm/s) were significantly greater than rates in the S→M direction (1.09 ± 0.41 x 10-7 cm/s). Simultaneous measurements detected no asymmetry of transport of the fluid phase marker [3H]inulin. Gel filtration chromatography demonstrated that the majority of the radiolabel released into the submucosal bathing solution represented albumin fragments. Albumin fragments per se were not transported because no asymmetries in permeabilities of albumin fragments isolated from spontaneous degradation of tracer were detected. Decreasing the temperature of the bathing solution from 37 to 4°C completely inhibited net albumin absorption. [14C]albumin transport was saturated by addition of high concentrations of unlabeled albumin (estimated Michaelis constant = 1.6 x 10-3 M). These results demonstrate that albumin is absorbed by a low-affinity process that may contribute to the maintenance of low albumin concentrations in secretions.