Surface Dilatational Behavior of Pulmonary Surfactant Components Spread on the Surface of a Captive Bubble. 3. Dipalmitoyl Phosphatidylcholine, Surfactant Protein C, and Surfactant Protein B

Abstract

Spread monolayers containing dipalmitoyl phosphatidylcholine (DPPC), surfactant protein C (SP-C), and surfactant protein B (SP-B) were characterized by dynamic and equilibrium surface pressure/area isotherms, surface dilatational rheological measurements, and transient stress relaxation. The maximum surface pressure which can be realized by monolayer compression depends significantly on the compression rate. The surface rheological behavior is strongly influenced by the presence of SP-B, which reduces the characteristic time of stress relaxation. Therefore the surface pressure of a layer containing SP-B (DPPC + SP-B or DPPC + SP-B + SP-C) changes within some seconds after a transient surface pressure jump to a constant value which corresponds to an equilibrium surface pressure plateau at about 51 mN/m, whereas DPPC + SP-C relaxes within tens of seconds to a steady-state value of surface pressure. In the frequency range of human breathing, the surface dilatational viscosity is weak and nearly independent of surface pressure and frequency for all systems investigated. The surface rheological behavior of the phospholipid/surfactant layers in this range is mainly elastic, with maximum elasticity at a surface pressure slightly below the equilibrium surface pressure plateau, whereas the compressibility of the pulmonary film in the surface pressure range of breathing crosses a stationary compressibility maximum. This study suggests that the presence of SP-B is essential for expiration and the presence of both proteins, SP-B and SP-C, is essential for the inspiration process of the breathing cycle.