© 2004 American Thoracic Society
Analyzing Surfactant Metabolism in HumansAn Important First StepUniversity of Western Ontario London, Ontario, Canada Pulmonary surfactant is a complex mixture of lipids and specific surfactant-associated proteins lining the epithelial surface of lungs. Within the alveoli, its main function is to reduce surface tension at the air–liquid interface and ensure alveolar stability during respiratory motion (1). The clinical importance of surfactant in maintaining lung homeostasis is evident from the significant impact that exogenous surfactant administration has had on preterm infant mortality, as well as the documented contribution of surfactant alterations to the respiratory failure associated with the acute respiratory distress syndrome (ARDS) (2, 3). In addition, pulmonary surfactant also functions as part of the innate host defense system and contributes to the stability and patency of the conducting airways (4, 5). These latter two functions suggest that alterations of surfactant may contribute to the pathophysiology of other diseases involving the mature lung including bacterial pneumonia, bronchial asthma, and cystic fibrosis. Indeed, changes in surfactant composition have been observed in bronchoalveolar lavage samples obtained from these types of patients (6). Studies on animal models of lung injury, including studies using radiolabeled surfactant precursors, have shown that alterations in endogenous surfactant metabolism directly contribute to these observed changes. Unfortunately, even though these studies have enhanced our knowledge about surfactant metabolism in health and disease, most animal models, particularly for diseases such as asthma and cystic fibrosis are not an accurate reflection of the human condition, thus the specific surfactant alterations observed in such models have to be interpreted with caution. In addition, bronchoalveolar lavage samples obtained from humans under different conditions only provide a snapshot of the status of the surfactant system at one time, so that accurate metabolic information cannot be elucidated. Therefore, a more extensive evaluation of surfactant metabolism in patients with these various respiratory conditions would represent an important advance. The study reported by Bernhard and colleagues in this issue of the Journal (pp. 54–58) describes a novel methodological approach to address this issue and is an important first step in this direction (7). Previous reports evaluating surfactant metabolism in humans using stable isotope-labeled precursors involved preterm and/or critically ill infants that were mechanically ventilated (7–10). The current study is the first to evaluate surfactant metabolism in spontaneously breathing adult subjects using the more specific, deuterium-labeled precursor, choline, rather than labeled glucose or fatty acid infusion. In addition to providing a more direct assessment of de novo synthesis of the most abundant surfactant lipid, phosphatidylcholine, the approach used by Bernhard and colleagues involved a significantly shorter infusion time of precursor (3 hours versus 24 hours for infusion of glucose or fatty acids), a relatively small amount of the precursor (3.6 mg/kg of choline versus 245 mg/kg of glucose), and an earlier detection of labeled phosphatidylcholine species in the collected sample (6 hours in sputum versus approximately 9–13 hours in tracheal aspirates of ventilated infants) (7–10). In addition, the specific technique used to analyze the samples obtained from these human volunteers involved electrospray ionization tandem mass spectrometry (ESI-MS/MS). This technique required very small quantities of material for analysis without the extensive preparatory steps necessary with either combustion interface isotope ratio mass spectrometry (CI-IRMS) or high-pressure liquid chromatography (HPLC)-based approaches. These advantages, together with the consistency and specificity of identifying the individual phosphatidylcholine molecular species in these readily available samples suggest that this approach is not only feasible, but will provide reliable and useful information regarding the status of surfactant metabolism in the human lung. Future modification and refinement of this technology may allow not only for the assessment of the major lipid species of surfactant, phosphatidylcholine, but also for the assessment of the metabolism of other surfactant lipids such as phosphatidylglycerol and cholesterol, as well as changes in the surfactant-associated proteins within the lung. Another technical aspect in this study of Bernhard and colleagues was the noninvasive method of collecting induced sputum for the surfactant analysis (7, 11, 12). Although a direct comparison of the sputum samples with material collected via bronchoalveolar lavages will ultimately be required, this non-invasive method of collecting samples has significant advantages. Multiple samples can be collected from individuals at different time points, and the proven feasibility for normal subjects in the present study leads to the opportunity of examining surfactant phosphatidylcholine metabolism in a number of other circumstances. For example, the effects of nutrition, exercise, and some pharmacological agents on surfactant metabolism can easily be investigated using this technique, all of which represents novel and potentially exciting information in the normal lung. Clearly, a major future goal would be to utilize this technique to obtain a more detailed assessment of surfactant metabolism in the setting of lung disease. This particular approach poses minimal risk to spontaneously breathing patients and could reveal new information on the status of the surfactant system in subjects predisposed to developing a certain type of lung disease or at earlier stages of a specific disease. Such data are not currently available and would certainly provide important insight into clinical diseases. Unfortunately, despite its promise in healthy and perhaps mildly impaired lungs, the usefulness of this methodology in more severe lung injuries is still questionable. Lung disorders such as ARDS, asthma, and cystic fibrosis all involve complex pathophysiologies, each of which may change over time depending on the severity of illness. The accurate assessment of phosphatidylcholine species in sputum samples will be affected by many factors, including radiolabeled choline availability to the lung, uptake of the precursor into lung tissue, endogenous choline pools, type II cell activity, and even ciliary function. The consistency of sample collection at different times and severities of the disease will also influence the measurements, as will the relationship between the potentially heterogeneous alveolar environments and the observed outcomes in the collected sputum sample (13). Thus, with regards to the usefulness of assessing surfactant metabolism in patients with severe lung injury, it is not the actual technique itself but rather the interpretation of the differences in outcomes measured between groups of subjects that could limit the usefulness of such an approach. Despite these potential limitations, the technique described by Bernhard and colleagues in this issue (7) represents an exciting contribution to our understanding of the complex mechanisms involved in pulmonary surfactant metabolism in humans. Future studies utilizing this technique may shine new light on the effects of numerous factors influencing surfactant metabolism, not only in normal subjects but those with a variety of clinical disorders. FOOTNOTES Conflict of Interest Statement: J.F.L. has served as a consultant to Altana Pharma Inc. for the past 2 years and received a total of $10,000; R.A.W.V. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. REFERENCES
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
