Surface adsorption of pulmonary surfactant in relation to bulk-phase concentration and presence of CaCl2

The adsorption rate of natural surfactant lipids was studied with a surface balance at 37 degrees C, with particular reference to the influence of various bulk-phase concentrations and to the presence of CaCl2. On a hypophase of 0.9% NaCl, natural surfactant had rapid adsorption (1-5 s) in concentrations greater than or equal to 0.5 mg/ml, whereas lipids extracted from the same surfactant showed a comparable adsorption rate only in concentrations greater than or equal to 3.5 mg/ml. This indicates that natural surfactant contains some not yet identified nonlipid component enhancing the adsorption rate. On a hypophase of 0.9% NaCl + 0.05% CaCl2, the adsorption rate of extracted surfactant lipids was similar to that of natural surfactant on 0.9% NaCl. The concentration of surfactant phospholipids in fetal pulmonary fluid at term is about 0.6 mg/ml; our data suggest that at this concentration the adsorption of natural surfactant does not require the presence of bivalent cations to spread effectively in an air-liquid interface.