Transcellular thiocyanate transport by human airway epithelia

Human airway mucosa synthesizes and secretes lactoperoxidase (LPO). As H₂O₂ and thiocyanate (SCN^-) are also present, a functional LPO antibacterial defence system exists in the airways. SCN^- concentrations in several epithelial secretions are higher than in serum, although the mechanisms of transepithelial transport and accumulation in these secretions are unknown. To examine SCN^- accumulation in secretions, human airway epithelial cells, re-differentiated at the air-liquid interface, were used in open-circuit conditions. [¹⁴C]SCN^-, in the basolateral medium, was transported across the epithelium and concentrated tenfold at the apical surface. Measurement of the transepithelial potential showed that the basolateral compartment was positive relative to the apical surface (13.7 ± 1.8 mV) and therefore unfavourable for passive movement of SCN^-. Transport was dependent on basolateral [SCN^-] and saturable (K_m,app = 69 ± 25 μM); was inhibited by increased apical [SCN^-]; and was dependent on the presence of basolateral Na⁺. Perchlorate (K_i,app = 0.6 ± 0.05 μM) and iodide (K_i,app = 9 ± 8 μM) in the basolateral medium reversibly inhibited transport, but furosemide did not. Iodide was also transported (K_m,app = 111 ± 69 μM). RT-PCR and immunohistochemistry confirmed expression of Na⁺ - I^- symporter (NIS) in the airways. SCN^- transport was insensitive to apical disulphonic acid Cl^- channel blockers, but sensitive to apical glibenclamide and arylaminobenzoates. Forskolin and dibutyryl cAMP increased transport. These data suggest SCN^- transport may occur through basolateral NIS-mediated SCN^- concentration inside cells, followed by release through an apical channel, perhaps cystic fibrosis transmembrane conductance regulator.