Abstract

Northern Elephant Seals (NES) are routinely subjected to high pressures and cold temperatures, yet, unlike humans, do not suffer from the thrombotic effects of platelet activation associated with rapid decompression.^1,2,3,4 High pressures and cold temperatures have been shown to increase membrane order,⁵ thus we determined the iochemical and biophysical characteristics of NES platelets.

NES platelets undergo cold-induced shape change as they are chilled from 37°C to 4°C. Below 20°C there is a sharp increase in the number of activated cells, as defined by the presence of multiple filopodia and/or a spherical shape. This cold-induced shape change correlates with the main membrane phospholipid phase transition, which was determined using Fourier Transform Infrared Spectroscopy (FTIR).

FTIR analysis demonstrates that NES platelets have a very broad phase transition (T_m), with two transitions centered at 21°C and 8-10°C. Removal of cholesterol, using 10mM Methyl-β-cyclodextrin, resulted in more sharply defined transitions, with an increase in membrane fluidity above the main T_m (8-10°C), and a decrease in fluidity below T_m. This is consistent with the proposed role of cholesterol as a regulator of membrane phospholipid order.

Cholesterol-rich membrane microdomains, or rafts, have been demonstrated to play a key role in human platelet intracellular signaling.⁶ We labeled NES platelets with the lipophilic dye, diIC₁₈ (1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanineperchlorate), which preferentially inserts into ordered membrane domains, and observed that NES platelets form raft aggregates after incubation at 4^oC. Removal of cholesterol results in disruption of raft aggregates.

NES platelets were pressurized to 160 atmospheres, held at high pressure for one hour, then decompressed rapidly (within 5 minutes) and fixed immediately for morphologic analysis. Three hundred cells were counted for each condition, and human platelets were subjected to identical conditions for comparison. The number of activated NES platelets increased by 45%, however, human platelet activation increased by 310% following this pressure excursion. These results suggest that NES platelets are more resistant to decompression-induced platelet activation than human platelets.

Acknowledgements

We thank Frances Gulland and Martin Haulena and staff of the The Marine Mammal Center (Sausalito, CA), Dan Crocker (Sonoma State University), and Burney LeBoeuf (University of California- Santa Cruz) for their assistance in our obtaining elephant seal blood. This work was supported by NIH grant ROHL57810 (F. Tablin).

References

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