Effects of aging on surfactant forms in rats

Surfactant present in the alveolar space exists in two major forms: functional large aggregate forms (LA) and nonfunctional small aggregate forms (SA), but there is no information about the changes of surfactant forms and the rate of conversion of LA to SA in the aged lungs. The purpose of the present study was to investigate the developmental aspects of surfactant forms in newborn, young, middle-aged and aged rats, LA and SA were recovered from alveolar lavages of rats. The rate of conversion from LA to SA was then analysed using a surface-area cycling technique. Age-related changes of saturated phosphatidylcholine (Sat-PC) and surfactant protein A (SP-A) pool sizes were also evaluated in alveolar lavages. The alveolar lavages recovered from aged rats contained a significantly higher proportion of LA than did those obtained from young or newborn rats. There was also an age-related decrease in the rate of conversion from LA to SA in vitro. The Sat-PC pool sizes in the alveolar lavages decreased with age, but the SP-A contents were similar between young and aged rats. These results suggested that decreased form conversion may contribute to maintaining functional surfactant pool sizes in the lungs of aged rats.