Calcium interactions in pulmonary surfactant

The surfactant properties of natural bovine pulmonary surfactant, its lipid extracts and acetone precipitates of lipid extracts have been examined with an artificial alveolus model, the pulsating-bubble surfactometer. At bulk concentrations of 0.4% (wt/vol.) phospholipid in saline, all three preparations exhibited surfactant activity i.e., were capable of reducing the surface tension of the pulsating bubble to approx. 27 dynes/cm at maximum bubble radius and to near zero at minimum bubble radius. At a concentration of 0.1% (wt./vol.) in saline, only natural surfactant was effective. Acetone-precipitated surfactant at 0.1% (wt./ vol.) achieved these criteria in the presence of 5 mM calcium, but 15–20 mM calcium was required to restore the surfactant activity of lipid extract surfactant. Chemical analysis revealed that lipid extraction decreases the protein content but does not alter the endogenous calcium levels. A calcium requirement for natural surfactant could only be demonstrated after repeated treatment with chelators for divalent cations. Surfactant activity was restored by low levels of calcium or high levels of magnesium. Paradoxically, a calcium requirement could not be demonstrated by treating acetone-precipitated lipid extract with chelators. The subtle differences noted between natural, lipid extract and acetone-precipitated lipid extract surfactant with the pulsating-bubble assay show that the latter preparations do not represent simplified model systems for the natural product.