Critical Dependence of the Biophysical Activity of Pulmonary Surfactant Films on Temperature

Abstract

Pulmonary surfactant is a lipid-protein complex that stabilizes the respiratory surface of lungs. Once secreted into the alveolar spaces, surfactant adsorbs rapidly at the air-liquid interface reducing surface tension upon compression to near 0 mN/m. Within a given animal species, surfactant composition is influenced by development, disease, respiratory rate, and/or body temperature. In principle, surfactant collected from animals functions optimally at the body temperature 3of the animal at the time of sample collection.Changes in temperature can alter the physical state and the molecular packing of surfactant membranes and films, potentially altering their biophysical performance. We have analyzed the effect of temperature on the structure of native surfactant, by differential scanning calorimetry (DSC) and fluorescence spectroscopy with the fluorescent probes DPH (Diphenylhexatriene) and Laurdan (6-Lauroyl-2-(N,N-dimethylamino)naphthalene). The spectral properties of these probes have been used to assess lipid packing and fluidity in surfactant as a function of temperature and compression state. The effect of temperature on the interfacial performance of surfactant has been evaluated by analyzing spreading capabilities in a surface balance and compression-expansion dynamics in a Captive Bubble Surfactometer. Native surfactant from porcine lungs showed optimal adsorption at temperatures around 37 oC, reaching minimal surface tensions below 2 mN/m upon quasi-static or dynamic compression-expansion cycling. Critical structural transitions at temperatures above 39oC led to reduced interfacial adsorption and impossibility of compressed films to reduce surface tensions below 20 mN/m, suggesting that surfactant composition has been optimized to work at a narrow interval of temperatures and that regulatory factors may be involved in adaptation of surfactant structures to changes in body temperature.