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 pathologies when experienced by other mammals. We compared fatty-acid composition, cholesterol content, and the effects of pressure on the molecular order of erythrocyte membranes from deep-, shallow-, and non-diving mammals to determine how these properties may be related to diving performance. Erythrocytes were collected from two deep-diving phocid pinnipeds (northern elephant seal (Mirounga angustirostris) and harbor seal (Phoca vitulina)), a relatively shallow-diving otariid pinniped (northern fur seal (Callorhinus ursinus)), and several nondiving terrestrial mammals (dog (Canis familiaris), horse (Equus caballus), and cow (Bos taurus)). Fatty-acid composition clearly distinguished the phocids from the other species. The monoene content of erythrocyte membranes was substantially lower (3 vs. approximately 20%), whereas the lipid unsaturation indices, the ratio of alpha- to gamma-linolenic 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 temperature. The physiological significance of these differences in cell-membrane structure, which affect the order and sensitivity of cell membranes to hydrostatic pressure, is unknown, but they may be important adaptations that allow repeated and prolonged exposure to great hydrostatic pressure.