Initially, surfactant was thought to be a key player only in the biophysical behavior of the lung. It is known that during the cycle of inspiration and expiration, fast and repeated alteration of alveolar surface size and, correspondingly, the area of surfactant cover occur. The surface tension of water which covers glicocalex of alveolar cells is 72mN/m. Surfactant adsorption on alveolar surface decreases the surface tension to 23mN/m, which facilitates the work of breath and provides respiratory mechanics (42).
Experimental data in vitro (42) and in vivo (43-45) shows that the surface tension at compression (expiration) falls to about 0mN/m at the water-air interface (42). However, both we and other investigators have been confused by the lack of physical sense in this finding (46). We think that the following statements can explain surface phenomenon in inspiration/ expiration cycle more profoundly. The quantity of surface-active molecules in water phase of alveoli is much more than necessary for monolayer formation on the air-water interface. Therefore, the molecule adsorption on the surface is maximum, and the surface tension coincides with one on the PL-air interface and is about 25mN/m (42,47,48). Furthermore, many experimental data show that the surfactant film on the air-water interface may consist (probably partly) of not one but three layers (42,49,50).
The high concentration of surfactant molecules on the interface means that when the surface area decreases, they come tightly to each other; and on reaching the tightest packing, repulsive force will result in exertion in the film, which will compensate the force compressing the surface. In rheology, it is named concatenation of viscosity and elasticity. The force that compresses the surface is surface tension on air-water interface in alveolar (25 mN/m after adsorption). At pressure reduction (expiration), this force tries to reduce the surface. Finally, elastic stress will balance the surface tension force, and the resulting "force" will be equal to zero. This is the resulting surface force, which is measured as surface tension. Surfactant surface tension cannot be less than 25mN/m, (PL surface tension on air-water interface), while the resulting surface "force" can fall to zero. Because surfactant film is not solid, its molecules are squeezed out of the surface of the water phase. Surfactant bilayer located under the monolayer may prevent molecule squeezing out and increase the stability of the film.
When the surface area is the least at expiration, and surfactant film is in the condition of its maximum compression, the force of elastic tension is practically completely balanced by surface tension force and resulting "force" is equal to zero. Therefore, there are no reasons for the following reduction of alveolar surface and its collapse. The available data on surface forces in surfactant films on air-water interface can be explained by this concept.
Although stabilizing the lungs is undoubtedly the major physiological function of surfactant, there is evidence that surfactant system may also serve other functions: it affects the permeability of the alveolar-capillary barrier to soluble compounds (51) and contributes to innate and adaptive immunity of the lung. Surfactant proteins act as a first-line defense against invading microorganisms and viruses (51-53). Moreover, they possess binding capacity for aeroallergens, highlighting the possible role of the pulmonary surfactant system in allergic diseases such as asthma (54,55).
Every component of surfactant complex plays its own role in polyfunc-tional surfactant activities. The key element in all pulmonary surfactants, DPPC, is considered to be the most important component with respect to its biophysical function (56). Anionic PL, especially PG, are responsible for modulating the properties of surfactant interfacial films, improving their stability during compression, and facilitating the adsorption and refining of PL on the air-lipid interface. PG can stimulate uptake of liposomal PC by type II cells (57). PA interacts with DPPC and/or SP-B to increase the movement of surfactant from the subphase and to stabilize the surfactant complex at the air-water interface (58-61). Cholesterol may play an important role in the lateral phase organization of surfactant structures (62).
Of particular interest are the specific surfactant-associated proteins that control the normal lifecycle of endogenous surfactant. SP-B and SP-C are mainly important for the biophysical properties of surfactant. SP-A and SP-D contribute essentially to host defense, which is realized in two ways: interaction with potentially injurious agents and alteration of the behavior of immune cells (63). SP-A and SP-D bind various microorganisms (64,65), lipids, and other exogenous substances. They stimulate alveolar macrophages (AM) (5,65-68) and influence the behavior of mast cells, dend-ric cells, and lymphocytes (69). SP-A inhibits the maturation of dendric cells, whereas SP-D enhances the ability of the cells to take up and present antigen, thereby enhancing adaptive immunity. SP-D may reduce the number of apo-ptotic cells (70,71). Transgenic models (SP-A null mice and SP-D null mice) demonstrates the importance of these proteins in the setting of bacterial and virus pneumonia (72). SP-A and SP-D have differential roles in modulating the inflammatory response to noninfectious lung injury (73). The overall effect of SP-D might be anti-inflammatory, whereas SP-A can contribute to both pro- and anti-inflammatory activity.
SP-B and SP-C play an important role in lung mechanics. Genetic deactivation of the SP-B gene induces irreversible and lethal respiratory failure both at birth (74,75) and in adults (76) due to incapability to maintain an opened respiratory surface. However, the controversial role of SP-B in monolayer refining and formation of a DPPC enriched layer is being discussed. It is thought now that SP-B brings lateral stability to the DPPC-rich monolayer of PL by both electrostatic and hydrophobic interactions (77). The analysis of the structure of lipid films at the nanoscopic level suggests that SP-B and SP-C alter the structure of surfactant films to optimize film rheological behavior under the dynamic conditions imposed by the lungs (78,79). Besides SP-A, SP-B is necessary for the formation of tubular mielin from secreted LB material. SP-B plays a role in host defense of the lung together with SP-A (80-82). SP-C, the smallest pulmonary surfactant-associated polypeptide, can have several functions: it contributes to the formation and dynamics of surfactant films at the air-liquid interface (83,84), prevention of surfactant inactivation by serum proteins, modulation of surfactant PL turnover, and binding to bacterial lipopolysaccharides (LPS).
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