PROTEIN MOVEMENT ACROSS CULTURED GUINEA-PIG TRACHEA - SPECIFICITY ANDEFFECT OF TRANSCYTOSIS INHIBITORS

Airway surface liquid (ASL) is a complex fluid with solutes including electrolytes, lipids, mucins, and proteins. The proximal airways are a bsorptive for most solutes, including proteins. We investigated the pr ocess of protein movement across confluent primary cultures of guinea pig trachea grown on filters using fluorescent-labeled bovine serum al bumin (BSA), ovalbumin (OA), and 70-kDa dextran (Dex). We found marked asymmetry of BSA and OA transepithelial flux, with apical-to-basolate ral flux (J(A-->B)) 10 times greater than the opposite direction (J(B- ->A)) for both proteins. The apparent permeability for Dex was the sam e as that for proteins in the basolateral-to-apical direction and show ed no asymmetry. Increasing concentrations of unlabeled BSA, OA, or tr ansferrin inhibited J(A-->B) for both BSA and OA without affecting Dex movement. Cooling reduced J(A-->B) for BSA without affecting J(B-->A) . Monensin and nocodazole each reduced JA-B for BSA and OA without aff ecting J(B-->A) Monensin eliminated all asymmetry for BSA movement. Br efeldin A did not affect J(A-->B) for either protein but did increase J(B-->A) for BSA. Treatment with the protease inhibitors increased J(A -->B) for BSA. Western immunoblotting demonstrated immunologically int act protein in the downstream compartment. We conclude that there is t ranscytosis of proteins across cultured trachea epithelium in the apic al-to-basolateral direction, which is monensin sensitive, involves mic rotubules, is not dependent on proteolysis, and is not protein species specific. This process may be important for maintenance of the ASL, a nd defects in this process may contribute to the abnormally thickened airway secretion seen in airway diseases.