Am. J. Respir. Cell Mol. Biol., Vol 13, No. 2, Aug 1995, 161-166.
The evolutionary significance of pulmonary surfactant in lungfish (Dipnoi)
S Orgeig and CB Daniels
Department of Physiology, University of Adelaide, Australia.
In this study, we characterized surfactant lipids from the lungs of the
lungfish, Protopterus annectens, Lepidosiren paradoxa, and Neoceratodus
fosteri (Sarcopterygiia: Dipnoi). We quantified the types of phospholipids
present, the amounts of total phospholipid, disaturated phospholipid
(DSP)--purported to be the primary surface tension- controlling agent--and
cholesterol (CHOL), an important fluidizer. The surfactant phospholipid
profiles of all three lungfish were very similar to each other and those of
many other animals reported previously. Phosphatidylcholine was the
dominant phospholipid (60% to 80%); phosphatidylglycerol was virtually
absent; and there was a significant proportion of the combination of
phosphatidylserine and phosphatidylinositol (10%). The Australian lungfish
N. forsteri possessed a surfactant 5 times richer in CHOL and 2 and 3 times
poorer in DSP than the surfactant of the African lungfish P. annectens and
the South American lungfish L. paradoxa, respectively. Hence, the CHOL/DSP
mass ratio of N. forsteri was 12 and 20 times greater than that of P.
annectens and L. paradoxa, respectively. Therefore, the surfactant
composition of the two derived species of lungfish (P. annectens and L.
paradoxa) very closely resembles that of amphibians, whereas surfactant
from the primitive lungfish (N. forsteri) is almost identical to that of
the primitive air-breathing actinopterygiian fish. Thus, it is likely that
pulmonary surfactant had only a single origin, coinciding with that of the
vertebrates. As with most nonmammalian vertebrates, it is possible that
lungfish surfactant functions as an antiglue at low lung volumes or when
the lungs are completely collapsed. Furthermore, it appears that within a
species, an increase in lung development correlates with an increase in the
relative amount of surfactant cholesterol and a decrease in the
phospholipid saturation level.
