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Alveolar surfactant exists in several structural forms that are believed to be secular products of the secreted form, lamellar bodies. The conversion of tubular myelin to the small vesicular form appears to require the action of a novel serine protease, surfactant convertase. As the in vitro activity of this enzyme is quite sensitive to inhibition by the serine protease inhibitor alpha 1-antitrypsin, a normal constituent of the alveolar fluid lining layer, we explored the possibility that the alveolar level of alpha 1-antitrypsin might affect the rate of subtype conversion in vivo. When the alveolar alpha 1-antitrypsin level was augmented by tracheal instillation of exogenous alpha 1-antitrypsin, the newly synthesized surfactant phospholipids of spontaneously breathing mice accumulated in the heavy subtype, and virtually none was found in the light subtype form up to 72 h later. An in vivo turnover study suggested that when the alveolar alpha 1-antitrypsin level was raised by the same means, the flux of surfactant through the light (small vesicular) compartment was virtually shut off, despite the availability of abundant amounts of its precursor, heavy subtype (tubular myelin). Finally, mouse lungs that were lavaged to remove resident surfactant and mechanically ventilated for 1-5 h released surfactant that progressed through heavy and light subtype compartments as in vivo. But in mice whose lung alpha 1-antitrypsin might affect the metabolism of alveolar surfactant in addition to its well-known role in lung defense.

The accompanying paper [Am. J. Physiol. 260 (Lung Cell. Mol. Physiol. 4): L302-L310, 1991] showed that in the radiation pneumonitis model of adult respiratory distress syndrome (ARDS) there was an excess of the proximate, higher buoyant density subtypes of alveolar surfactant, and a decrease in the light buoyant density form. Because the surfactant subtypes normally evolve from the former to the latter a delay in the alveolar metabolism of surfactant could explain this disproportion. Three possible mechanisms of a delay in surfactant metabolism in radiation pneumonitis were explored using an in vitro model of surfactant subtype metabolism called "cycling". The first was that the surfactant of mice with radiation pneumonitis was intrinsically less capable of conversion to the light subtype. It was found, however, that the proximate forms of surfactant of mice with radiation pneumonitis were as capable of generating light subtype as those of control mice. The second was that there was a deficit in the serine protease activity, called "convertase", that mediates the conversion. But it was found that lungs of mice with radiation pneumonitis released convertase activity to the same extent as control lungs. The third was that an inhibitor of convertase activity was present in the alveoli. It was found that the alveolar lavage fluid of mice with radiation pneumonitis inhibited the conversion of exogenous surfactant by exogenous convertase. Moreover, it contained an 18-fold excess of antiprotease activity. The present data are interpreted as suggesting that an inhibitor in the alveolar space is responsible for the delay in surfactant subtype metabolism in radiation pneumonitis, resulting in the disproportion of surfactant subtypes in radiation pneumonitis.(ABSTRACT TRUNCATED AT 250 WORDS)

Radiation pneumonitis, a chronic form of adult respiratory distress syndrome (ARDS), is known to be associated with physiological and biophysical abnormalities of the surface element of the lungs suggesting an impairment of the surfactant system. The alveolar surfactant of mice with radiation pneumonitis was fractionated into subtypes on continuous sucrose density gradients to examine their relative amounts, composition, ultrastructure, surface activity, and turnover kinetics. The total phospholipid and protein contents of the alveolar lavage were increased. The proportions of high buoyant density subtypes (normally surface active) were increased about twofold and that of the low buoyant density subtype (not surface active) was decreased or absent. The buoyant densities, ultrastructure, and phospholipid compositions of the major surfactant subtypes were not significantly altered. The surface activity of the normally surface-active subtypes, when purified free of extraneous material, was close to those of normal controls. Turnover studies of the kinetics of surfactant subtype phospholipids suggested increased secretion of surfactant but a delay in the conversion of the heavier subtypes into their low-density product. Most of the heavier material appeared not to enter the lighter pool, in contrast to findings in control mice. It is concluded that in this form of ARDS the surfactant subtypes are qualitatively normal but that their surface activity is impaired, presumably by extraneous material in the alveoli, and that proportions of surfactant subtypes are radically altered by a combination of increased synthesis and decreased metabolism of the heavier subtypes.

We have previously shown that the fall in lung compliance in radiation pneumonitis is mainly due to abnormality of the alveolar surface lining layer and speculated that this, in turn, is due to an increased amount of protein in the alveolar lining fluid layer. In the present study, this extra protein has been partially characterized, and the kinetics of permeation from the blood to the lung and the AF have been investigated by using intravenously injected of 125I-labeled albumin. Mice which had received 3000 rads to the thorax were used. A fourfold to fivefold increase in the total protein in AF obtained by pulmonary lavage was again found in mice irradiated 4 months previously, as compared to control littermates. With immunologic techniques and polyacrylamide disc-gel electrophoresis the excess protein in the alveolar lavage fluid was shown to be derived from the circulation. Following injection of 125I-labeled albumin into the tail vein, APR in the perfused, lavaged lung relative to radioactivity in simultaneously obtained blood rose to a plateau at 6 hr, which was approximately twice as high in irradiated mice as in controls. APR in the AF relative to that in simultaneously obtained blood rose rapidly and continued to rise throughout the period of study, reaching a level which was six times that in control mice at 24 hr. A major abnormality in radiation pneumonitis appears to be a large increase in permeability of the capillary and alveolar membranes to both small and large protein molecules. These abnormalities are not acute terminal events but probably persist for some weeks before death. It is speculated that leakage of plasma proteins onto the alveolar surface is responsible for the fall in compliance of the AF lining layer in radiation pneumonitis.

Mice received 2400 rads in two fractions to the thorax; 4 months later at a time when deaths began, mechanical and biochemical properties of the lungs were studied and compared to those of littermate controls. The principal mechanical change was a large increase in the elastance of the alveolar surface element and abnormal surface tension properties of the AF. Alveolar fluid phospholipid content and saturation were only minimally change; however the alveolar fluid contained a large excess of protein, possibly of circulatory origin. The latter may explain the fall in compliance of the alveolar surface and hence of the lung in radiation pneumonitis. A small increase in elastance of the lung tissue element was also found; this correlated with a decrease in lung volume and a small increase in lung hydroxyproline content. These changes may signify the early development of radiation fibrosis. Other features of this model are the frequent occurrence of pleural effusions and the presence of increased numbers of alveolar macrophages in the alveolar lavage.

Because previous studies have suggested that some of the toxic effects of oxygen may result from impaired surfactant phospholipid metabolism, we examined the effects of methylprednisolone on the surfactant system in adult mice exposed to greater than 98% O2. Mice pretreated for 7 days with methylprednisolone, 10 mg/kg/day, had a significantly greater mortality in hyperoxia than saline-pretreated, oxygen-exposed control mice. Their bronchoalveolar lavage fluid contained significantly less desaturated phospholipid, and static pressure-volume curves with air and saline showed decreased lung compliance caused by stiffness of the surface layer. The synthesis of desaturated phospholipids by lung slices in vitro was markedly impaired by the first day of hyperoxia, although in the absence of hyperoxia, it was mildly increased. The increased inhibition of phospholipid synthesis in steroid-treated mice was present even when mice were first given steroids on the day of entry into hyperoxia. These results suggest that methylprednisolone has an unfavorable effect on the surfactant system in oxygen toxicity by accentuating the inhibition of phospholipid synthesis. The possibility that these results could have been due to infection promoted by steroid treatment was considered. No cultural, histologic, or serologic evidence in support of this possibility was found, although infection cannot be conclusively excluded.

Corticosteroid administration during radiation pneumonitis in mice markedly improves the physiologic abnormalities and decreases mortality, an effect that has been attributed to the stimulation of surfactant synthesis and secretion by type 2 alveolar epithelial cells. In the present experiments we explored the effects of corticosteroids on the replicative activity of type 2 cells of lethally irradiated lungs at the height of the radiation reaction. The labeling index of type 2 cells of irradiated mice was increased threefold above that of sham-irradiated controls. Corticosteroids given continuously from 10 weeks after thoracic irradiation further increased the type 2 cell labeling index another threefold above that of irradiated untreated mice. The enhanced reproductive activity of type 2 cells following thoracic irradiation is seen as a protective response that is augmented by corticosteroids, whose effect may be both to improve the physiology of the alveolar surface and to maintain the population of alveolar epithelial cells. The bearing of this result on the controversial role of the type 2 cell as a target in radiation pneumonitis is discussed.