The biophysical function of pulmonary surfactant

Abstract

Pulmonary surfactant lowers surface tension in the lungs. Physiological studies indicate two key aspects of this function: that the surfactant film forms rapidly; and that when compressed by the shrinking alveolar area during exhalation, the film reduces surface tension to very low values. These observations suggest that surfactant vesicles adsorb quickly, and that during compression, the adsorbed film resists the tendency to collapse from the interface to form a 3D bulk phase. Available evidence suggests that adsorption occurs by way of a rate-limiting structure that bridges the gap between the vesicle and the interface, and that the adsorbed film avoids collapse by undergoing a process of solidification. Current models, although incomplete, suggest mechanisms that would partially explain both rapid adsorption and resistance to collapse as well as how different constituents of pulmonary surfactant might affect its behavior.

Figure 1. Structure of the hypothetical kinetic intermediate that limits the rate of adsorption

R₁ and R₂ indicate the two principal radii of curvature for the stalk that connects the adsorbing vesicle with the nascent interfacial monolayer. These radii in turn define the principal curvatures (c), according to c ≡ 1/R. Because the standard frame of reference originates at the phosphate group and projects along the acyl tail, R₁ and c₁ have negative values. Modified with permission from (Walters et al., 2000; Schram and Hall, 2001).

Figure 2. Phase behavior and metastability of phospholipid monolayers

After spreading of films containing dipalmitoyl phosphatidylcholine (DPPC) or 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC) at an air/water interface, area was changed at slow or fast rates (20 or 300 Ų/(molecule·min), respectively) at 26°C in the direction indicated by the single-headed arrows. Labels indicate the presence of liquid-expanded (LE), tilted-condensed (TC), and collapsed (bulk) phases detected microscopically in separate experiments. Horizontal double-headed arrows indicate regions of coexisting phases. POPC, which reaches the γ_e of 24 mN/m in the LE phase, has minimal effect on γ during further slow compression because of collapse (dark grey line). DPPC films, which at this temperature form the TC phase at γ ≈ 65 mN/m, sustain γ < γ_e without evidence of collapse above γ = 3 mN/m (black line). If compressed fast enough to reach low γ, supercompressed LE POPC transforms to a metastable film that also resists collapse over the full range of γ between γ_e and at least 5 mN/m (light grey lines). Modified with permission from (Yan et al., 2007).

Figure 3. Mechanism of liquid-crystalline collapse

Microscopic images show that constituents move as a continuous sheet from the monolayer to a stacked smectic liquid-crystal through a locus of restricted area (Schief et al., 2003). Adapted with permission from (Rugonyi et al., 2005).

Figure 4. Temperature-dependence of the γ at which phase separation begins in monomolecular films related to pulmonary surfactant

Monolayers containing extracted calf surfactant (calf lung surfactant extract, CLSE), the complete set of purified phospholipids (PPL) obtained chromatographically from CLSE, or DPPC were spread at an air/water interface and monitored microscopically during slow compression at specific temperatures. Symbols indicate the point at which microscopy first detected distinct domains in the initially homogeneous film. Because the γ of water, γ_o, varies with temperature, results are expressed in terms of the extent to which the film changes γ, given by Δγ = γ − γ_o. Adapted with permission from (Discher et al., 1999a).

Figure 5. Transformation of surfactant monolayers at low γ

Monomolecular films of extracted calf surfactant (calf lung surfactant extract, CLSE) at 37°C were compressed at >100% A_o/sec from an initial area (A_o) established at γ = 25 mN/m. After reaching γ < 5 mN/m, area remained essentially constant, indicating the absence of collapse, during incubation at the lowest γ, and after expansion to γ of 10 and 20 mN/m. Subsequent recompression and reexpansion without hysteresis similarly indicates the absence of collapse as long as γ remains below 25 mN/m. Expansion to 30 mN/m, at which γ remains roughly constant while area increases during respreading of the extracted surfactant, restores the ability of the film to collapse during slow compression. Modified with permission from (Crane and Hall, 2001).

Figure 6. Rates of collapse at different γ < γ_e

Monomolecular films of CLSE were compressed rapidly (32 min⁻¹) at 37°C to specific γ, which were then held constant. Within a few seconds after reaching the constant γ, area decreased along a simple exponential function of time, the time-constant of which provided the rate of collapse. Symbols indicate mean ± S.D. Modified with permission from (Lhert et al., 2007)

Figure 7. Variation of area during the melting of TC and supercompressed fluid films

Monomolecular films contained: TC DPPC; LE POPC not exposed to low γ; or LE POPC after supercompression to γ < 5 mN/m and return to γ = 30 mN/m. Because the γ of the clean air/water interface, γ_o, varies as a function of temperature, the films were held during heating at constant Δγ = γ − γ_o rather than at constant γ. Between 26 and 70°C, TC DPPC and supercompressed POPC both melted from a structure that sustained low γ without collapse (decrease in area with accompanying decrease in Δγ at 26°) to a structure that collapsed at Δγ = −45 mN/m (despite extensive decrease in area at 70°, Δγ reached only −45 mN/m), as expected for a fluid film. Melting of DPPC from the TC to the LE phase at 39–41°C abruptly expanded the film because of the larger molecular area for the LE phase. In contrast, during melting of the supercompressed POPC film, area expanded only to the same extent as POPC that had never reached low γ, indicating that the molecular areas for the films with and without supercompression are similar. Symbols indicate mean ± S.D. Modified with permission from (Smith et al., 2003).