Hydrostatic-pressure and temperature effects on the molecular order of erythrocyte membranes from deep-, shallow-, and non-diving mammals

Little is known about the cellular mechanisms involved in the tolerance of deep-diving marine mammals to hydrostatic pressures that cause serious path ologies when experienced by other mammals. We compared fatty-acid compositi on, cholesterol content, and the effects of pressure on the molecular order of erythrocyte membranes from deep-, shallow-, and non-diving mammals to d etermine how these properties may be related to diving performance. Erythro cytes were collected from two deep-diving phocid pinnipeds (northern elepha nt seal (Mirounga angustirostris) and harbor seal (Phoca vitulina)), a rela tively shallow-diving otariid pinniped (northern fur seal (Callorhinus ursi nus)), and several nondiving terrestrial mammals (dog (Canis familiaris), h orse (Equus caballus), and cow (Bos taurus)). Fatty-acid composition clearl y distinguished the phocids from the other species. The monoene content of erythrocyte membranes was substantially lower (3 vs. approximately 20%), wh ereas the lipid unsaturation indices, the ratio of alpha- to gamma-linoleni c acids, and the proportions of long-chain polyunsaturated fatty acids were substantially higher in the phocids. The cell-membrane cholesterol content was also significantly lower in erythrocytes from the deep-diving phocids (cholesterol:phospholipid ratios 0.2-0.3) than from most other mammals (1.0 ). Membranes from the phocids were more ordered than those from the shallow - and non-divers, and were also more sensitive to changes in pressure and t emperature. The physiological significance of these differences in cell-mem brane structure, which affect the order and sensitivity of cell membranes t o hydrostatic pressure, is unknown, but they may be important adaptations t hat allow repeated and prolonged exposure to great hydrostatic pressure.