AQUAPORINS IN COMPLEX TISSUES .2. SUBCELLULAR-DISTRIBUTION IN RESPIRATORY AND GLANDULAR TISSUES OF RAT

The molecular pathways for fluid transport in pulmonary, oral, and nas al tissues are still unresolved. Here we use immunocytochemistry and i mmunoelectron microscopy to define the sites of expression of four aqu aporins in the respiratory tract and glandular epithelia, where they r eside in distinct, nonoverlapping sites. Aquaporin-1 (AQP1) is present in apical and basolateral membranes of bronchial, tracheal, and nasop haryngeal vascular endothelium and fibroblasts. AQP5 is localized to t he apical plasma membrane of type I pneumocytes and the apical plasma membranes of secretory epithelium in upper airway and salivary glands. In contrast, AQP3 is present in basal cells of tracheal and nasophary ngeal epithelium and is abundant in basolateral membranes of surface e pithelial cells of nasal conchus. AQP4 resides in basolateral membrane s of columnar cells of bronchial, tracheal, and nasopharyngeal epithel ium; in nasal conchus AQP4 is restricted to basolateral membranes of a subset of intra- and subepithelial glands. These sites of expression suggest that transalveolar water movement, modulation of airway surfac e liquid, air humidification, and generation of nasopharyngeal secreti ons involve a coordinated network of aquaporin water channels.