THE SURFACE-ASSOCIATED SURFACTANT RESERVOIR IN THE ALVEOLAR LINING

A small atmospheric bubble was introduced into a surfactant suspension in a captive bubble surfactometer. After film formation to the equili brium surface tension at the bubble air-liquid interface, the bulk pha se surfactant was depleted by replacing the chamber contents several t imes with a saline-CaCl2 solution. The remaining film adsorbed at the bubble surface was then compressed stepwise in quasi-static fashion to near zero minimum surface tension. This was followed by a series of q uasi-static expansion steps to surface tensions slightly above equilib rium. The surface tension of films from lipid extract surfactants and phospholipid mixtures did not increase in a manner consistent with the presence of a single surface monolayer. After the initial, rapid rise in surface tension at each expansion step, a decrease in surface tens ion to a new value was observed. This decrease in surface tension is l ikely due to the adsorption of 'surplus' material from a 'surface-asso ciated reservoir' into the surface active film. The presence of surplu s non-monolayer surfactant material in situ at the alveolar surface wa s also demonstrated by electron microscopy. SP-A acted as a potent pro moter for the movement of excess material (equivalent to 2-3 monolayer s) at the interface into the surface active film. In contrast, inhibit ory serum proteins prevented the formation of a surface-associated res ervoir or the adsorption of excess material into a surface active film .