The accelerated late adsorption of pulmonary surfactant

Adsorption of pulmonary surfactant to an air-water interface lowers surface tension (γ) at rates that initially decrease progressively, but which then accelerate close to the equilibrium γ. The studies here tested a series of hypotheses concerning mechanisms that might cause the late accelerated drop in γ. Experiments used captive bubbles and a Wilhelmy plate to measure γduring adsorption of vesicles containing constituents from extracted calf surfactant. The faster fall in γreflects faster adsorption rather than any feature of the equation of state that relates γto surface concentration (τ). Adsorption accelerates when γreaches a critical value rather than after an interval required to reach that γ. The hydrophobic surfactant proteins (SPs) represent key constituents, both for reaching the γat which the acceleration occurs and for producing the acceleration itself. The γat which rates of adsorption increase, however, is unaffected by the τ of protein in the films. In the absence of the proteins, a phosphatidylethanolamine, which, like the SPs, induces fusion of the vesicles with the interfacial film, also causes adsorption to accelerate. Our results suggest that the late acceleration is characteristic of adsorption by fusion of vesicles with the nascent film, which proceeds more favorably when the τ of the lipids exceeds a critical value.