An 'engine' phenomenon displayed by monolayers of a pulmonary surfactant cycled to steady state

Monolayers of dipalmitoyl phosphatidylcholine--the predominant lung surfactant--have been studied at the surface of buffered Ringer's solution at 37 degrees C using the Langmuir trough with surface tension measured by both the Wilhelmy and du Noüy methods. When the surface was loaded with surfactant in excess of that needed to give a condensed monolayer on compression, the films displayed the conventional loops of surface tension versus area, demonstrating large hysteresis. However, when this system was cycled several hundred times over a physiological (25%) area change, it reached a steady state in which the surface tension for compression now exceeded that for expansion at all areas. This inversion was also recorded after attaining steady state by two other approaches--low initial concentration and 'aging' for two hours--while the phenomenon was further displayed over a larger non-physiological (75%) area excursion after 90 cycles. Inversion of surface tension hysteresis under physiological conditions implies the conversion of some other form of energy into mechanical work which could aid respiration, i.e. an 'engine'. Calculations are included to show how it might make a contribution of the order of 23% to the energy needed to satisfy the work of breathing under resting conditions.