Aw. Smits et al., SURFACTANT COMPOSITION AND FUNCTION IN LUNGS OF AIR-BREATHING FISHES, The American journal of physiology, 266(4), 1994, pp. 180001309-180001313
Examination of lung washings from primitive air-breathing fishes (rope
fish, bichirs, and gar) revealed a lipid-based surfactant with an aver
age disaturated phospholipid-to-total phospholipid ratio five times lo
wer than in mammals. The lung lavage of fishes was exceptionally rich
in cholesterol, resulting in average cholesterol-to-phospholipid ratio
s three times higher, and cholesterol-to-disaturated phospholipid rati
os nearly 15 times higher, than those of mammals. Removal of lung surf
actant doubled the pressures necessary to initially open the anterior
regions of collapsed lungs in all three fish species but had little or
no effect on pressures required to fill the lung (i.e., compliance) a
fter the initial opening. The elevated cholesterol content found in pu
lmonary surfactant of these fishes is consistent with such findings in
other ectotherms, suggesting that the proportional elevation of chole
sterol may serve to stabilize the fluidity of the lung surfactant over
broader temperature ranges. The influence of surfactant on lung openi
ng pressures rather than on compliance contrasts with that seen in mam
mals and supports an ''antiglue'' role of pulmonary surfactant in the
simpler open-design lungs of lower vertebrates.