Films of pulmonary surfactant in the lung are metastable at surface pressures well above the equilibrium spreading pressure of 45 mN/m but commonly collapse at that pressure when compressed in vitro. The studies reported here determined the effect of compression rate on the ability of monolayers containing extracted calf surfactant at 37°C to maintain very high surface pressures on the continuous interface of a captive bubble. Increasing the rate from 2 Å2/phospholipid/min (i.e., 3% of (initial area at 40 mN/m)/min) to 23%/s produced only transient increases to 48 mN/m. Above a threshold rate of 32%/s, however, surface pressures reached >68 mN/m. After the rapid compression, static films maintained surface pressures within ±1 mN/m both at these maximum values and at lower pressures following expansion at <5%/min to ≥45 mN/m. Experiments with dimyristoyl phosphatidylcholine at 37°C produced similar results. These findings indicate that compression at rates comparable to values in the lungs can transform at least some phospholipid monolayers from a form that collapses readily at the equilibrium spreading pressure to one that is metastable for prolonged periods at higher pressures. Our results also suggest that transformation of surfactant films can occur without refinement of their composition.
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