ACTIN DEPOLYMERIZATION IS DEVELOPMENTALLY-REGULATED IN RAT TYPE-II CELLS EXPOSED TO TERBUTALINE

The type II alveolar epithelial tell synthesizes and secretes pulmonar y surfactant. Terbutaline enhances phospholipid release from adult and fetal type Il cells. Our hypothesis is that the actin network of micr ofilaments regulates the secretory activity of the type II cell. To ex amine the developmental regulation of the changes in actin subfraction s associated with secretory activity, cultures of type II cells derive d from adult and 19-d fetal rat lung were incubated with or without IO mu M terbutaline for 1, 30, and 60 min. Dose-response effects of terb utaline were examined in adult type II cells. Effects of phorbol ester were also examined. Globular (G-actin) and filamentous (F-actin) frac tions were extracted from the cells and analyzed separately. Specified cellular equivalent volumes of each subfraction were analyzed by West ern blotting, visualized by a color reaction, and quantified by densit ometry. There was a decrease in the cytoskeletal F-actin pool along wi th an increase in the G-actin fraction within 1 min in adult type II c ells exposed to terbutaline, indicating that depolymerization of F-act in occurs. Values returned to control levels by 60 min. In contrast, t he decrease in F-actin, with a concomitant increase in G-actin, was ma ximal at 60 min in fetal cells exposed to terbutaline. There was a dos e-dependent increase in actin depolymerization with maximal effects at 10 mu M terbutaline. Phorbol ester also caused an increase in actin d epolymerization. Depolymerization of the actin microfilament network m ay regulate transport and exocytosis of lamellar bodies in type II cel ls. We speculate that there is an early secretory mechanism that invol ves depolymerization of actin microfilaments and a late, actin-indepen dent secretory mechanism present in adult type II cells. The timing of the response of the actin-dependent pathway is developmentally regula ted. This may explain the developmental differences in the secretion o f surfactant that we have previously shown.