Background: Alterations in surfactant phospholipid compositions are a recognized feature of many acute and chronic lung diseases. Investigation of underlying mechanisms requires assessment of surfactant phospholipid molecular composition and kinetics of synthesis and turnover. Such studies have recently become possible in humans due to the development of stable isotope labelling combined with advances in analytical methods in lipidomics. The objectives of this study are to compare phospholipid molecular species composition and phosphatidylcholine synthesis and turnover in surfactant isolated from various endobronchial compartments in healthy adults.
Methods: Healthy adults (N = 10) were infused with methyl-D9-choline chloride and samples of induced sputum, tracheal wash and small volume bronchoalveolar lavage fluid were obtained subsequently at intervals up to 96 hours. Surfactant phospholipid composition and incorporation of stable isotope into surfactant phosphatidylcholine were determined by electrospray ionisation mass spectrometry.
Results: While molecular species compositions of phospholipids were similar for all three sample types, dipalmitoylphosphatidylcholine content was highest in lavage, intermediate in tracheal wash and lowest in sputum. Methyl-D9-choline incorporation into surfactant phosphatidylcholine was lower for sputum at 24 hours but reached equilibrium with other sample types by 48 hours. Fractional methyl-D9-dipalmitoylphosphatidylcholine incorporation for all sample types was about 0.5% of the endogenous composition. Lysophosphatidylcholine enrichment was twice than that of phosphatidylcholine.
Conclusions: Tracheal secretions may be of value as a surrogate to assess bronchoalveolar lavage fluid surfactant molecular composition and metabolism in healthy people. Despite minor differences, the phospholipid molecular composition of induced sputum also showed similarities to that of bronchoalveolar lavage fluid. Detailed analysis of newly synthesized individual phosphatidylcholine species provided novel insights into mechanisms of surfactant synthesis and acyl remodelling. Lysophosphatidylcholine methyl-D9 incorporation patterns suggest that these species are secreted together with other surfactant phospholipids and are not generated in the air spaces by hydrolysis of secreted surfactant phosphatidylcholine. Application into patient populations may elucidate potential underlying pathophysiological mechanisms that lead to surfactant alterations in disease states.