Phosphatidylcholine metabolism of rat trachea in relation to lung parenchyma and surfactant

Pulmonary surfactant prevents alveolar collapse and contributes to airway patency by reducing surface tension. Although alveolar surfactant, consisting mainly of phospholipids (PL) together with neutral lipids and surfactant-specific proteins, originates from type II pneumocytes, the contribution of airway epithelia to the PL fraction of conductive airway surfactant is still debated. We, therefore, analyzed the composition, synthesis, and release of phosphatidylcholine (PC) molecular species as the main surfactant PL of the rat trachea compared with the lung. Analyses of individual PC molecular species with HPLC and electrospray ionization mass spectrometry revealed that the rat trachea contained and synthesized much more palmitoyloleoyl-PC, palmitoyllinoleoyl-PC, and palmitoylarachidonoyl-PC, together with increased amounts of alkylacyl-PC, and less surfactant-specific species such as dipalmitoyl-PC than the lung. Organ cultures with [methyl-3H]choline as precursor of PC revealed that, in the trachea, synthesized PC was retained in the tissue, rather than secreted. [Methyl-3H]choline-labeled dipalmitoyl-PC was a negligible component in the trachea, and, in contrast to the lungs, palmitoyloleoyl-PC was enriched in tracheal secretions. We conclude that the surfactant fraction in the airways does not originate from the airways but is produced in the alveolar space and transported upward.