Ontogenetic Changes in Skeletal Muscle Fiber Type, Fiber Diameter and Myoglobin Concentration in the Northern Elephant Seal (Mirounga angustirostris)

This Document is Protected by copyright and was first published by Frontiers. All rights reserved. It is reproduced with permission. The definitive version of this article is available at: http://www.frontiersin.org/physiologyNorthern elephant seals (Mirounga angustirostris) (NES) are known to be deep,\ud long-duration divers and to sustain long-repeated patterns of breath-hold, or apnea. Some\ud phocid dives remain within the bounds of aerobic metabolism, accompanied by\ud physiological responses inducing lung compression, bradycardia and peripheral\ud vasoconstriction. Current data suggest an absence of type IIb fibers in pinniped\ud locomotory musculature. To date, no fiber type data exist for NES, a consummate deep\ud diver. In this study, NES were biopsied in the wild. Ontogenetic changes in skeletal\ud muscle were revealed through succinate dehydrogenase (SDH) based fiber typing.\ud Results indicated a predominance of uniformly shaped, large type I fibers and elevated\ud myoglobin (Mb) concentrations in the longissimus dorsi (LD) muscle of adults. No type\ud II muscle fibers were detected in any adult sampled. This was in contrast to the juvenile\ud animals that demonstrated type II myosin in Western Blot analysis, indicative of an\ud ontogenetic change in skeletal muscle with maturation. These data support previous\ud hypotheses that the absence of type II fibers indicates reliance on aerobic metabolism\ud during dives, as well as a depressed metabolic rate and low energy locomotion. We also\ud suggest that the lack of type IIb fibers (adults) may provide a protection against ischemia\ud reperfusion (IR) injury in vasoconstricted peripheral skeletal muscle.Funding was provided by the Baylor University Faculty Research Investment Program (SJT)