Surfactant subtypes of mice: metabolic relationships and conversion in vitro

Mouse alveolar surfactant can be separated by equilibrium centrifugation on continuous sucrose gradients into three subtypes which we call "ultraheavy", "heavy", and "light" on the basis of their buoyant densities. We examined their metabolic relationship by in vivo labeling studies and by physical manipulation, cycling the surface area in vitro in an attempt to convert one subtype into another. Labeling studies indicated rapid quantitative progression of surfactant through ultraheavy, heavy, and light subtypes in sequence. To mimic the in vivo conversion of subtypes in vitro we "cycled" the surface area of surfactant in plastic tubes. Newly secreted surfactant obtained from incubated lungs, as well as surfactant obtained by alveolar lavage and lamellar bodies, exhibited conversion of material from heavier to lighter subtypes. The conversion between subtypes was quantal and was dependent on cycling, temperature, and time. We conclude that the three subtypes are discrete forms of alveolar surfactant that evolve from one into another. Cycling may provide a means to study the mechanisms of their interconversion in vitro.