SUMMARY 1. Glucocorticoids are an effective treatment in the amelioration of chronic lung disease in neonates. However, systemic administration of glucocorticoids to neonates is associated with significant side‐effects that preclude them as an early intervention to prevent onset of the condition. Conversely, local intratracheal administration of glucocorticoids may prevent inflammatory insult to the lungs without the development of systemic side‐effects. We therefore investigated whether local intratracheal delivery of corticosteroids could be facilitated using surfactant as a vehicle. 2. Addition of dexamethasone to either diluted or commercial artificial surfactant, Survanta (Abbott Industries, Sydney, NSW, Australia), did not alter the surface properties of the surfactant. 3. After intratracheal instillation to rats, radiolabelled dexamethasone in Survanta was well distributed throughout all four lobes of the lungs. A concentration gradient of the steroid was observed between the root and the peripheral sections of all lobes. 4. Our results suggest that surfactant is an effective vehicle for intratracheal delivery of glucocorticoids. Moreover, we propose that prophylactic intratracheal administration of glucocorticoids administered shortly after birth may prevent inflammatory insult to the lungs and thereby reduce the likelihood of chronic lung disease developing.

Alveolar proteinosis (AP) is an idiopathic condition characterized by excess alveolar surfactant. Although the surfactant proteins (SP) are known to be aberrant, little is known of their variation between patients or their abundance relative to the lipids. We have examined surfactant composition in lavage fluid from 16 normal subjects and 13 patients with AP, one of whom was lavaged on 11 occasions over approximately 13 mo. In this patient we have examined composition on each occasion and in each sequential lavage aliquot. Composition was constant between right and left lung, but it differed markedly between patients. The cholesterol/disaturated phospholipid ratios (CHOL/DSP) were invariably elevated, on average by approximately 7-fold, whereas the SP-A/DSP and SP-B/DSP ratios were generally elevated, in some cases by as much as approximately 40- and approximately 100-fold, respectively. Although AP lavage generally contained more non-thiol-dependent SP-A aggregates and low Mr isoforms, the two-dimensional immunochemical staining patterns varied between patients and right and left lung. In the patient lavaged on multiple occasions, the SP-A/DSP and SP-B/DSP ratios progressively decreased as the patient's condition resolved. Because the SP-B/SP-A ratio was normal in all cases, we suggest that structural changes to the proteins occurred secondarily and that caution must be used in comparing functional data derived using SP-A obtained from patients with AP.

Although acute lung injury (ALI) is associated with inflammation and surfactant dysfunction, the precise sequence of these changes remains poorly described. We used oleic acid to study the pathogenesis of ALI in spontaneously breathing anesthetized rats. We found that lung pathology can occur far more rapidly than previously appreciated. Lung neutrophils were increased approximately threefold within 5 min, and surfactant composition was dramatically altered within 15 min. Alveolar cholesterol increased by approximately 200%, and even though disaturated phospholipids increased by approximately 30% over 4 h, the disaturated phospholipid-to-total phospholipid ratio fell. Although the alveolocapillary barrier was profoundly disrupted after just 15 min, with marked elevations in lung fluid ((99m)Tc-labeled diethylenetriamine pentaacetic acid) and (125)I-labeled albumin flux, the lung rapidly began to regain its sieving properties. Despite the restoration in lung permeability, the animals remained hypoxic even though minute ventilation was increased approximately twofold and static compliance progressively deteriorated. This study highlights that ALI can set in motion a sequence of events continuing the respiratory failure irrespective of the alveolar surfactant pool size and the status of the alveolocapillary barrier.

Although endotoxin-induced acute lung injury is associated with inflammation, alveolocapillary injury, surfactant dysfunction, and altered lung mechanics, the precise sequence of these changes is polemic. We have studied the early pathogenesis of acute lung injury in spontaneously breathing anesthetized rats after intravenous infusion of Salmonella abortus equi endotoxin. The animals became hypoxic, and airway resistance, tissue resistance, lung elastance, and static compliance all deteriorated well before any change in alveolar neutrophils, macrophages, lung fluid (99mTc-labeled diethylenetriamine pentaacetic acid), or 125I-albumin flux, which were only appreciably increased at 8.5 hours. Lung elastance deteriorated before airway resistance, indicating that the compliance change was specific rather than caused by reduced lung volume. The subcellular and alveolar content of surfactant proteins A and B, cholesterol, disaturated phospholipids, and phospholipid classes remained normal in the face of a dramatic increase in the synthesis and turnover of 3H-disaturated phosphatidylcholine. Our findings indicate that the increase in surfactant disaturated phospholipid turnover reflects, at least in part, an approximately five-fold increase in "sigh frequency." We suggest that endotoxin has direct effects on tissue resistance and lung elastance independent of surfactant composition and that the initial respiratory failure results primarily from endotoxin-induced ventilation/perfusion mismatch independent of edema or alveolocapillary injury per se.

Treatment of rats with 10 mg.kg body wt-1 day-1 4-aminopyrazolo[3,4-d]pyrimidine (APP) for 2 days markedly reduced serum cholesterol and phospholipids. This was associated with large decreases in the principal component of alveolar surfactant, the disaturated phospholipids (DSP), in the lamellar body and in the tubular myelin-rich and -poor alveolar fractions, but with no concomitant change in cholesterol or surfactant protein A. These decreases in DSP were associated with a decrease in the synthesis of surfactant phospholipids. Despite these large changes in composition of alveolar surfactant, we could detect no change in either static or dynamic lung compliance. However, the treatment markedly increased both the minimum and maximum surface tension of the lipid extract of the tubular myelin-rich fraction, as measured by bubble surfactometry. Whereas these changes appeared unimportant in the isolated perfused lung at resting tidal volume, they were associated with edema after an increase in tidal volume. The ability of APP to inhibit phospholipid synthesis selectively makes it a useful tool in investigating the surfactant system.

Alveolar disaturated phospholipids (DSPA) increase in vivo in rats with hyperpnea and in isolated perfused lungs (IPL) in response to either salbutamol or increasing tidal volume (VT). Because surfactant protein-A (SP-A) may play a role in surfactant homeostasis, we have examined the relationship between SP-A and DSP in the alveolus lamellar bodies (LB-A), and in a vesicular (LB-B) lung subfraction. Whereas 2 h swimming increased total DSPA (approximately 48%), it had no effect on alveolar SP-A (SP-AA). In the IPL, salbutamol increased total DSPA (approximately 30%) and SP-AA (approximately 41%); increasing VT (2.5-fold) only increased DSPA (approximately 22%). SP-A and DSP also varied differentially in the tubular myelin-rich and -poor subfractions. In both the IPL and in vivo, we found inverse relationships between DSPA and SP-AA/DSPA, indicating that although SP-AA and DSPA are related, they vary independently. Whereas total SP-AA/DSPA varied between 0.046 and 0.074, it remained constant in LB-A (approximately 0.015) and LB-B (approximately 0.010), suggesting that DSP and SP-A are secreted differentially and that only a small portion of SP-AA is derived from lamellar bodies.

The application of impedance pneumography for monitoring respiration in small animals has been limited by problems with calibration. With improved instrumentation, we describe the calibration of tidal volume in anesthetized rats. The detection of changes in voltage, reflecting the electrical impedance variations associated with respiration, was optimized by using disposable adhesive silver-silver chloride electrodes, advanced circuitry, and analog-to-digital recording instrumentation. We found a linear relationship between change in impedance and tidal volume in individual rats (R2 >/= 98%), which was strongly influenced by rat weight. Consequently, a calibration equation incorporating change in impedance and rat weight was derived to predict tidal volume. Comparison of the predicted and true tidal volumes revealed a mean R2 >/= 98%, slopes of approximately 1, intercepts of approximately 0, and bias of approximately 0.07 ml. The predicted volumes were not significantly affected by either frequency of respiration or pulmonary edema. We conclude that impedance pneumography provides a valuable tool for the noninvasive measurement of tidal volume in anesthetized rats.

We conclude that training markedly alters the composition of alveolar surfactant both at rest and with exercise; the physiological significance of these changes remains to be determined.