Collagen synthesis and degradation in acutely damaged mouse lung tissue following treatment with prednisolone

Effect of continuous beta-blockade on collagen synthesis in interstitial fibroblasts isolated from adult rat lung. An in vitro model of progressive pulmonary fibrogenesis

Continuous administration of propranolol to adult rats resulted in pulmonary changes indicative of fibrogenesis in the absence of an extensive inflammatory infiltrate. Interstitial fibroblasts, isolated from the lungs of rats continuously exposed to 0.5 mg propranolol-HCl/hr via subcutaneous osmotic pumps for 1 or 3 wk synthesized more acid-insoluble collagenase-sensitive protein than matched controls. This effect could be decreased, but not prevented, by administering zinc salts to the animals in drinking-water during drug treatment. A larger percentage of the overall protein synthesis of the propranolol-treated cells was devoted to collagenous proteins as well. Isolated fibroblasts placed in tissue culture and treated with propranolol secreted more total lactate dehydrogenase than controls; this change was due to increased secretion of lactate dehydrogenase isoenzyme 5. Three weeks of propranolol treatment caused a 31% decrease in the number of beta-adrenergic lung receptors, with no change in their affinity. These results suggest that this in vitro model may be useful in elucidating mechanisms of progressive lung disease following long-term exposure to injurious agents.

Pulmonary hydroxyproline content and production following treatment of mice with O,S,S-trimethyl phosphorodithioate

The systemic administration of O,S,S-trimethyl phosphorodithioate (OSS), a contaminant of various organophosphorus insecticides, induces delayed damage to rat and mouse lung tissue. The lesion, particularly in the rat, closely resembles that produced by butylated hydroxytoluene (BHT) in mice. Although the time course of cell damage and repair has been studied in both species, it is not clear whether excess collagen, indicative of fibrosis, is deposited. Changes in pulmonary hydroxyproline content and synthesis, indices of collagen metabolism, were analysed in mice treated with 45 mg/kg OSS. A significant increase in total lung hydroxyproline was evident 21 days after treatment compared to both pair-fed and ad libitum controls. This increase was not augmented by subsequent treatment with 35 mg/kg 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) or exposure to 70% oxygen for 7 days. The rate at which lung tissue synthesized hydroxyproline was increased 7-14 days after treatment with OSS. These data demonstrate that treatment of mice with OSS results in changes indicative of pulmonary fibrosis. However, in contrast to some other lung-toxic chemicals, this lesion was not enhanced by subsequent treatment with BCNU or hyperoxia.

Comparison of in vitro and in vivo rates of collagen synthesis in normal and damaged lung tissue

The rate of collagen synthesis was measured in vivo and in vitro in both normal and damaged mouse lung tissue. Acute lung damage was induced by the administration of butylated hydroxytoluene (BHT). The production of labeled hydroxyproline, following the administration of labeled proline, was used as an index of collagen production. Total and labeled hydroxyproline in normal and damaged lung tissue were solubilized equally following digestion with purified collagenase. Assuming that the extent of hydroxylation was not altered, this indicated that hydroxyproline was an accurate index of collagen content and production in damaged as well as normal lung tissue. The quantities of hydroxyproline formed at various times both in vivo and in vitro were calculated from the specific activity of free proline in lung tissue. The specific activity of free proline in normal and damaged lung tissue remained constant in vivo for at least 90 minutes after the intravenous injection of labeled proline. Hydroxyproline production was a linear function of time for up to 90 minutes in vivo and three hours in vitro. The in vivo rate of hydroxyproline production was significantly greater than the in vitro rate in lung tissue from similarly treated mice. The difference ranged from five-fold in normal lung tissue to eight-fold in lung tissue damaged by the administration of BHT. Comparable differences were seen between the in vivo and in vitro rates of non-collagen protein synthesis. Despite these differences in rates, the percentage of total protein synthesis committed to collagen in vivo was the same as in vitro in normal lung, and identical increases were seen in damaged lung. These data show that in vivo rates of both collagen and non-collagen protein synthesis are significantly higher than those measured in mouse lung tissue in vitro. Although the relative increases in collagen synthesis that occur in response to lung damage are larger in vivo, measurements of collagen synthesis in vitro do accurately reflect the general changes that accompany acute lung damage.

Increased pulmonary collagen synthesis in mice treated with cyclophosphamide

Cyclophosphamide is a widely used anticancer drug which damages lung tissue and elicits a progressive fibrotic lesion in mice. The early time course of collagen accumulation and the changes in collagen synthesis which may accompany this lung damage and fibrosis have not been reported. The total lung concentration of hydroxyproline, an index of collagen content and the extent of pulmonary fibrosis, was not significantly increased in mice until 12 days after treatment with 200 mg/kg cyclophosphamide. The change in total lung hydroxyproline content was preceded, on Day 6, by an increase in the rate of acid-insoluble hydroxyproline synthesis. This rate remained elevated to Day 21, but was no longer significantly increased by Day 28. The percentage of total protein synthesis devoted to the production of collagen was significantly increased 9 and 12 days after cyclophosphamide, compared to saline-treated controls. The rate of pulmonary non-collagen protein synthesis was significantly decreased 3 days after cyclophosphamide, and increased 6 and 15 days after this treatment. These data indicate that cyclophosphamide-induced increases in pulmonary collagen synthesis precede the accumulation of this protein. Increased collagen synthesis may occur in the absence of changes in overall protein synthesis although cyclophosphamide also alters non-collagen protein synthesis.