Body size, cell membranes and tissue metabolism in mammals

The relationship between body size and the rate of basal metabolism in mammals is the holistic basis of this thesis. In particular, the work presented investigates the relationships between body size, cell membrane phospholipid fatty acid composition, tissue metabolism and sodium pump activity. The species chosen were mouse, rat, rabbit, sheep and cattle, representing a 12 000-fold difference in body weight. Heart, skeletal muscle and kidney cortex of larger mammals had less polyunsaturated membrane phospholipids than those of smaller mammals. The proportions of the various polyunsaturated fatty acids in heart, skeletal muscle and kidney cortex allowed to propose that the tissues of larger mammals generally display lower desaturase and elongase activities. A noteworthy exception is in brain, where membrane fatty acid composition remained remarkably constant in mammals of all size. The liver of larger mammals was less polyunsaturated than that of smaller mammals but rabbits showed the lowest extent of polyunsaturation of all species in this tissue. The proportions of saturated to unsaturated fatty acids in tissue phospholipids did not vary with body size. Physiological measurements with liver and kidney cortex slices confirmed that the tissues of larger mammals are metabolically less active. A significant direct relationship between the rates of oxygen consumption and of potassium uptake (an indicator of sodium pump activity) was found. This work provided evidence that body size, tissue metabolic rate, sodium pump activity and membrane polyunsaturation are related parameters.