Assessment of pulmonary surfactant function using a captive-bubble surfactometer

Abstract

A captive-bubble surfactometer was built to measure the surface properties of reconstituted pulmonary surfactants ( l-α-dipalmitoyl phosphatidylcholine (DPPC), Exosurf, Alveofact) and whole lung surfactants obtained from normal and injured rat lungs (silica induced Lipoproteinosis). Whole surfactants from normal or injured rats were further characterized by protein and phospholipid measurements. We used soluble test substances (ethanol, water) to ascertain the precision, reliability, and cleanliness of the captive-bubble surfactometer. We performed three types of experiments by controlling the system pressure on the air bubble: adsorption, dynamic, and quasi-static cycling. Lateral views of the bubble were continuously recorded. The bubble height and diameter were determined by image analysis and were used for the calculation of bubble tension and related parameters. Adsorption was slow with DPPC, faster with Exosurf and Alveofact, and fastest with whole lung surfactants. During the dynamic cycling experiments, DPPC immediately exhibited low bubble tension. The bubble tension for Exosurf, Alveofact, and whole lung surfactants showed both decreasing values at minimum area and increasing values at maximum area during dynamic cycling. In initial cycles of the quasi-static cycling experiments, isotherm hystereses were different for the various surfactants, and the bubble tension and compressibility were least with pure DPPC as compared to Exosurf and Alveofact. In contrast to all other surfactants, DPPC showed minimal refinement of film characteristics after repeated cycling. Whole lung surfactants exhibited surface properties comparable to those of reconstituted surfactants, albeit at only half the phospholipid concentration. Whole lung surfactant from injured rat lungs was characterized by increased lipid and protein content, whereas changes in the phospholipid composition and surface activity were only subtle compared to surfactant obtained from normal rat lungs. The characterization of the surface properties of biological substances using the captive-bubble surfactometer is feasible and discriminates between different reconstituted and whole surfactants. This method is useful for studying biological materials because it enables measurements of surface properties under dynamic non-equilibrium conditions without film leakage, which is in contrast to other methods.