Role of calcium ions the structure and function of pulmonary surfactant

Pulmonary surfactant isolated by centrifugation in buffers containing ions contains at least three different morphologic structures. The presence of one of these, tubular myelin, is dependent on calcium ions, since chelation of the calcium ions causes disruption of this structure. Addition of EDTA also decreases the ability of the surfactant to absorb rapidly to air-water interfaces and lower surface tension. Titration with calcium ions (2.5 or 5 mM) restores rapid surface adsorption and restores the tubular myelin structural forms. Magnesium ions cannot substitute for calcium ions in these processes. The reversibility of structure and function induced by calcium ions and EDTA is also accompanied by reversible isopycnic density shifts probably related to aggregation and disaggregation of the lipid-protein complex with calcium ions and EDTA, respectively.