Post-Infectious ARDS: Mechanisms of Lung Injury and Repair

Pulmonary fibroblast function in an acute lung injury model

The role of pulmonary fibroblasts (PFBs) in early adult respiratory distress syndrome is poorly understood. To investigate PFB cellular function in acute lung injury, New Zealand rabbits (2 to 3 kg) were given either three daily doses of phorbol myristate acetate (PMA; 65 micrograms/kg, IV), a potent stimulator of oxygen radical formation, or saline (control). On day 4, the lungs were harvested, subjected to enzymatic digestion, and PFBs isolated via serial subculture. Proliferation was assessed via 6-hour pulsed [3H]thymidine incorporation and by creating 5-day growth curves. Confluent PFB cultures were assessed for collagen production and total protein production, as well as interleukin (IL)-1 alpha secretion. Qualitative comparisons using transmission electron micrography were also made. There were no differences between PFBs harvested from control versus PMA-treated animals in terms of growth rates, total protein, and IL-1 alpha production. However, there was a significant difference in collagen production, with the PMA-treated animals' PFBs producing 35% more collagen than controls. Transmission electron micrography revealed PMA fibroblasts to be smaller (two to three times), have more dark staining granules, and have hypertrophied smooth endoplasmic reticulum--all consistent with increased metabolic activity. This suggests that pulmonary fibrosis, a late development in adult respiratory distress syndrome, may be triggered during the acute phase of lung injury. The increase in collagen synthesis is not related to PFB proliferation or the secretion of IL-1 alpha.

Tyrosine kinase activation during lung injury, fibrosis, and compensatory lung growth

Endobronchial instillation of paraquat into left lungs of rats resulted in ipsilateral fibrosis (evaluated by light and electron microscopy) and compensatory growth of the contralateral lung. Tyrosine kinase (Tyr-K) activity rose early in the fibrotic lung, peaked to a value 4 times the controls on day 3, and remained elevated on day 14. In the right lung, slight increase in activity occurred only on day 5. Cellular proliferative activity was inhibited in both lungs early after injury. In the right lung, however, DNA synthesis increased to levels 10 times the controls by day 7. In the left lung, the inhibition decreased gradually after day 7, indicating increased cellular proliferation. We conclude that activation of some cellular Tyr-K may play a role in regulating early cellular proliferation during lung injury, fibrosis, and compensatory growth.