Abstract
Northern elephant seals (Mirounga angustirostris) (NES) are known to be deep, long-duration divers and to sustain long-repeated patterns of breath-hold, or apnea. Some phocid dives remain within the bounds of aerobic metabolism, accompanied by physiological responses inducing lung compression, bradycardia, and peripheral vasoconstriction. Current data suggest an absence of type IIb fibers in pinniped locomotory musculature. To date, no fiber type data exist for NES, a consummate deep diver. In this study, NES were biopsied in the wild. Ontogenetic changes in skeletal muscle were revealed through succinate dehydrogenase (SDH) based fiber typing. Results indicated a predominance of uniformly shaped, large type I fibers and elevated myoglobin (Mb) concentrations in the longissimus dorsi (LD) muscle of adults. No type II muscle fibers were detected in any adult sampled. This was in contrast to the juvenile animals that demonstrated type II myosin in Western Blot analysis, indicative of an ontogenetic change in skeletal muscle with maturation. These data support previous hypotheses that the absence of type II fibers indicates reliance on aerobic metabolism during dives, as well as a depressed metabolic rate and low energy locomotion. We also suggest that the lack of type IIb fibers (adults) may provide a protection against ischemia reperfusion (IR) injury in vasoconstricted peripheral skeletal muscle.
Highlights
Kooyman et al (1980) defined the aerobic dive limit (ADL) as the maximum dive duration maintained with aerobic metabolism, while studying Weddell seals (Leptonychotes weddellii)
We determined that the adult NES, a deep-diving phocid (Le Boeuf et al, 2000; Kuhn et al, 2009; Robinson et al, 2012), has uniformly large and metabolically uniform (SDH) type I fibers in the fiber bundles of the longissimus dorsi (LD) muscle investigated in this study
Western Blot analysis revealed pup muscle has binding for type IIa and IIb myosin heavy chain, demonstrating ontogenetic changes in fiber type
Summary
Kooyman et al (1980) defined the aerobic dive limit (ADL) as the maximum dive duration maintained with aerobic metabolism, while studying Weddell seals (Leptonychotes weddellii) These authors discovered that this pinniped species maintains oxygen stores in relation to metabolic rate (oxygen consumption) during diving. Deep diving pinniped species display physiological adaptations of their locomotor muscle that lengthen ADL, including elevated concentrations of Mb and aerobic enzymes, reliance on lipids for fuel and a prevalence of type I, slow twitch, oxidative muscle fibers (Lenfant et al, 1970; Hochachka and Foreman, 1993; Kanatous et al, 1999, 2002; Trumble and Kanatous, 2012). California sea lions (CSL) (Zalophus californianus) are relatively shallow divers (Feldkamp et al, 1989; Weise et al, 2006) and have a more heterogeneous fiber type distribution in the muscle given different dive durations and rates of muscular activity, all of which could determine recruitment of glycolytic fibers (Ponganis and Pierce, 1978)
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