Phosphatidylcholine composition of pulmonary surfactant from terrestrial and marine diving mammals

Abstract

Marine mammals are repeatedly exposed to elevated extra-thoracic pressure and alveolar collapse during diving and readily experience alveolar expansion upon inhalation - a unique capability as compared to terrestrial mammals. How marine mammal lungs overcome the challenges of frequent alveolar collapse and recruitment remains unknown. Recent studies indicate that pinniped lung surfactant has more anti-adhesive components compared to terrestrial mammals, which would aid in alveolar opening. However, pulmonary surfactant composition has not yet been investigated in odontocetes, whose physiology and diving behavior differ from pinnipeds. The aim of this study was to investigate the phosphatidylcholine (PC) composition of lung surfactants from various marine mammals and compare these to a terrestrial mammal. We found an increase in anti-adhesive PC species in harp seal (Pagophilus groenlandicus) and California sea lion (Zalophus californianus) compared to dog (Canus lupus familiaris), as well as an increase in the fluidizing PCs 16:0/14:0 and 16:0/16:1 in pinnipeds compared to odontocetes. The harbor porpoise (a representative of the odontocetes) did not have higher levels of fluidizing PCs compared to dog. Our preliminary results support previous findings that pinnipeds may have adapted unique surfactant compositions that allow them to dive at high pressures for extended periods without adverse effects. Future studies will need to investigate the differences in other surfactant components to fully assess the surfactant composition in odontocetes.

Figure 1

Effects of sample collection and storage on PC composition. The abundance of specific PC species (left axis) and the total abundance of PCs (right axis) determined from an aliquot of surfactant isolated from newly collected (fresh) or frozen common dolphin pulmonary lavage (n=1). The number above each bar represents the percent difference between fresh and frozen samples. The significance and implications of the difference in total amount of PCs extracted from 1 mL aliquots of the reconstituted surfactant samples (right axis) are discussed in the results.

Figure 2

Stability of PC composition regarding postmortem sampling time. The abundance (left axis) of specific PC species in A) dog (n=2) and B) a California sea lion (n=1) is shown for samples collected immediately after death (day 0) or one or two days following death. There were no significant differences (p > 0.05) between day 0 and day 1 PC species for dog (error bars represent standard deviation of the mean, n=2). The number above each bar represents the percent difference between time points. The right axis shows the total PCs extracted from 1 Ml aliquots of the reconstituted surfactant samples; differences between total amounts are discussed in the results.

Figure 3

PC abundance in terrestrial and diving marine mammals. The abundance of specific PC species is shown for terrestrial (dog, n=7) and diving marine mammals. PC 18:0/18:1 coeluted with PC 16:0/20:1 in harp seal, harbor porpoise, and common dolphin but was undetermined in California sea lion. Subscript “f” indicates that the sample was frozen prior to surfactant isolation. Cartoon animals are courtesy of Sentiel Rommel.

Figure 4

PC abundance in dog (n=7) and harbor porpoise (n=2, frozen; n=1, fresh). Error bars represent standard deviation of the mean. Asterisks represent PC species with statistically significant differences (p < 0.05). PC 18:0/18:1 co-eluted with PC 16:0/20:1 in HP. Subscript “f” indicates that the sample was frozen prior to surfactant isolation. Abbreviations: HP – harbor porpoise.

Figure 5

PC abundance in odontocetes versus pinnipeds. The abundance of A) chromatographically separated PC species and B) grouped PC species – PCs 16:0/16:0 + 16:0/16:1 + 16:0/14:0 and PCs 16:0/16:1 + 16:0/14:0 – are compared between odontocetes (common dolphin and harbor porpoise) and pinnipeds (harp seal and California sea lion). PC 18:0/18:1 co-eluted with PC 16:0/20:1 in harp seal, harbor porpoise, and dolphin but was undetermined in CSL. Error bars represent standard deviation of the mean. Harbor porpoise and California sea lion included samples frozen prior to surfactant isolation.

Figure 6

Abundance of saturated and unsaturated PCs among terrestrial and marine mammals. The abundance of saturated (14:0/16:0, 16:0/16:0, and 16:0/18:0), mono-unsaturated, polyunsaturated, and unsaturated PC fatty acids (16:1/16:1, 16:0/16:1, 16:0/18:1, 16:0/18:2, 16:1/18:1, 16:0/20:4, and 18:0/18:1) are shown for dog and marine mammals. Error bars represent standard deviation of the mean. PC 18:0/18:1 co-eluted with PC 16:0/20:1 in HS, HP, and D but was undetermined in CSL. Subscript “f” indicates that the sample was frozen prior to surfactant isolation. Abbreviations: CSL – California sea lion (n=1); D – common dolphin (n=1); HP – harbor porpoise (n=2, frozen; n=1, fresh); HS – harp seal (n=1).

Figure 7

Relative quantitation of two PC species. Differences in the relative abundance of A) PC 16:0/20:5 and B) PC 18:1/18:2 are shown for dog and marine mammal samples. Subscript “f” indicates that the sample was frozen prior to surfactant isolation. Error bars represent standard deviation of the mean. Asterisks indicate PC species with statistically significant differences (p < 0.005) relative to dog. Abbreviations: CSL – California sea lion (n=1); D – common dolphin (n=1); DPPC – dipalmitoylphosphatidylcholine; HP – harbor porpoise (n=2, frozen; n=1, fresh); HS – harp seal (n=1).