Effects of hydrophobic surfactant proteins on collapse of pulmonary surfactant monolayers

Abstract

To determine if hydrophobic surfactant proteins affect the stability of pulmonary surfactant monolayers at an air/water interface, the studies reported here compared the kinetics of collapse for the complete set of lipids in calf surfactant with and without the proteins. Monomolecular films spread at the surface of captive bubbles were compressed at 37 degrees C to surface pressures above 46 mN/m, at which collapse first occurred. The rate of area-compression required to maintain a constant surface pressure was measured to directly determine the rate of collapse. For films with and without the proteins, higher surface pressures initially produced faster collapse, but the rates then reached a maximum and decreased to values <0.04 min(-1) above 53 mN/m. The maximum rate for the lipids with the proteins (1.22 +/- 0.28 min(-1)) was almost twice the value for the lipids alone (0.71 +/- 0.15 min(-1)). Because small increments in surface pressure produced large shifts in the rate close to the fastest collapse, compressions at a series of constant speeds also established the threshold rate required to achieve high surface pressure as an indirect indication of the fastest collapse. Both samples produced a sharply defined threshold that occurred at slightly faster compression with the proteins present, supporting the conclusion of the direct measurements that the proteins produce a faster maximum rate of collapse. Our results indicate that at 47-53 mN/m, the hydrophobic surfactant proteins destabilize the compressed monolayers and tend to limit access to the higher surface pressures at which the lipid films become metastable.

FIGURE 1

Variation of π and area during isothermal fast and slow compressions for N&PL. Films were spread on the surface of a captive bubble, heated to 37°C, and compressed either continuously at 0.02 min⁻¹ (experiment 1, black/gray lines), or during a brief pulse at 32 min⁻¹, after which area was expanded and recompressed at 0.04 min⁻¹ (experiment 2, red/blue lines). Rates of compression are expressed as A⁻¹·dA/dt. (A) π (left axis) and area (right axis) versus time after beginning of compression. The subscripts indicate data from the same experiment. Surface area is expressed relative to the initial value (A_o) at the beginning of compression from different initial π values. (B) The same data expressed as π-area isotherms. Surface area is expressed relative to the value at 44 mN/m (A₄₄) during the initial compression to facilitate comparison of different experiments. Arrows indicate the temporal progression of the measurements.

FIGURE 2

Kinetics of isobaric collapse for spread films of CLSE. Data indicate the temporal variation of π (shaded symbols, right axis) and area (solid symbols, left axis) during an initial rapid change in area (initial portion of split time axis) followed by an isobaric compression (later time). Each symbol represents the mean of measurements at selected times for at least three experiments, ± standard deviation for area during the isobaric compression. Data at additional times and other π-values were omitted for purposes of clarity. Solid lines at times beyond 2 s give the best fit for data averaged at all times to the equation [A(t) − A_∞]/[A_o − A_∞] = exp (−kt), where A(t), A_o, and A_∞ represent the surface area at times t, t_o = time at onset of constant π, and t = ∞ by extrapolation, respectively. Area is expressed relative to the value at 44 mN/m (A₄₄) before the pulsed compression.

FIGURE 3

Kinetics of isobaric collapse for spread films of N&PL. Conditions are the same as in Fig. 2.

FIGURE 4

Effect of hydrophobic surfactant proteins on the kinetics of collapse. Symbols represent the rate of collapse (mean ± SD) obtained from the time constant k, defined in the legend for Fig. 2, for the experimental curves fit to the area during isobaric compression at different π-values. n ≥ 3 for each set of measurements.

FIGURE 5

Variation of π during compression of N&PL monolayers at different approximately constant rates. Buffer infused at different constant rates produced roughly linear changes in area with time. Area is expressed relative to the initial value (A₄₄) at 44 mN/m.

FIGURE 6

Effect of surfactant proteins on the final π achieved during compression at different constant rates for CLSE and N&PL. Experiments were conducted as illustrated in Fig. 5. Dashed lines connect the points for the highest rate with a final π < 52 mN/m and the lowest rate with π > 65 mN/m, for purposes of illustration only.