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COMPARATIVE AND EVOLUTIONARY PHYSIOLOGY

Relationship between body size, Na⁺-K⁺-ATPase activity, and membrane lipid composition in mammal and bird kidney

Metabolic Research Centre and Departments of ¹Biomedical and ²Biological Sciences, University of Wollongong, Wollongong, New South Wales, Australia

Submitted 5 May 2004 ; accepted in final form 22 September 2004

We investigated the relationship between body size, Na⁺-K⁺-ATPase molecular activity, and membrane lipid composition in the kidney of five mammalian and eight avian species ranging from 30-g mice to 280-kg cattle and 13-g zebra finches to 35-kg emus, respectively. Na⁺-K⁺-ATPase activity was found to be higher in the smaller species of both groups. In small mammals, the higher Na⁺-K⁺-ATPase activity was primarily the result of an increase in the molecular activity (turnover rate) of individual enzymes, whereas in small birds the higher Na⁺-K⁺-ATPase activity was the result of an increased enzyme concentration. Phospholipids from both mammals and birds contained a relatively constant percentage of unsaturated fatty acids; however, phospholipids from the smaller species were generally more polyunsaturated, and a complementary significant allometric increase in monounsaturate content was observed in the larger species. In particular, the relative content of the highly polyunsaturated docosahexaenoic acid [22:6(n-3)] displayed the greatest variation with body mass, scaling with allometric exponents of –0.21 and –0.26 in the mammals and birds, respectively. This allometric variation in fatty acid composition was correlated with Na⁺-K⁺-ATPase molecular activity in mammals, whereas in birds molecular activity only correlated with membrane cholesterol content. These relationships are discussed with respect to the metabolic intensity of different-sized animals.

allometry; basal metabolic rate; fatty acids; phospholipids; sodium-potassium-adenosinetriphosphatase

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