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The metabolic biochemistry of the wandering shrew (Sorex vagrans) Emmett, Brian


The comparative examination of the metabolism of the shrew, Sorex vagrans, at the ultrastructural, mitochondrial and enzymatic level has revealed a number of consistent factors which correlate with the high basal metabolic rate of these mammals. The metabolic picture which emerges is of an organism highly dependent on aerobic metabolism, utilizing lipid or fatty acids as a primary energy source. This appears particularly true for skeletal muscle which, in larger mammals, may be fueled primarily by glycogen and depend more upon anaerobic metabolism at high work loads. Ultrastructural studies reveal that the diaphragm and gastrocnemius muscle are composed of small diameter fibers, associated with an abundance of peripheral and interfibrillar mitochondria. These typical red fibers are considered to operate aerobically, either in a fast or slow twitch fashion. Levels of oxidative (citrate synthase, fumarase β-OH butyrylCoA dehydrogenase) enzymes are elevated with respect to the rat in heart, liver and gastrocnemius muscle while glycolytic enzymes (glycogen phos-phorylase, PK, LDH) are depressed in all tissues but heart which has PK and LDH activities comparable to rat heart. No large differences were observed in the absolute rates or substrate preferences of shrew and rat cardiac mitochondria but enzymatic profiles of these organelles ..show increased activities of citrate synthase and β-OH butyrylCoA dehydrogenase in shrew mitochondria. This may indicate intrinsic differences between shrew and rat cardiac mitochondria, the physiological consequences of which remain to be elucidated. Glycogen is relatively scarce in shrew tissues and, although present in liver, does not form typical mammalian liver α-rosettes. The low levels of phosphorylase in the tissues assayed indicate that the shrew has a reduced ability to metabolize this substrate. In contrast lipid stores appear abundant in all tissues examined. In the gastrocnemius muscle of a series of seven mammals ranging in size from 0.004 to 400 kg., the activities of oxidative enzymes scale with respect to body mass with similar exponents as the scaling of maximal oxygen consumption (VO₂max/M[sub b]). In contrast levels of glycolytic enzymes increase as body size increases, indicating that burst anaerobic work is functionally more important in larger mammals.

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