Soleus muscle stability in wild hibernating black bears

Abstract

Based on studies of fast skeletal muscles, hibernating black and brown bears resist skeletal muscle atrophy during months of reduced physical activity and not feeding. The present study examined atrophy sparing in the slow soleus muscle, known to be highly prone to disuse atrophy in humans and other mammals. We demonstrated histochemically that the black bear soleus is rich in slow fibers, averaging 84.0 ± 6.6%. The percentages of slow fibers in fall (87.3 ± 4.9%) and during hibernation (87.1 ± 5.6%) did not differ ( P = 0.3152) from summer. The average fiber cross-sectional area to body mass ratio (48.6 ± 11.7 µm/kg) in winter hibernating bears was not significantly different from that of summer (54.1 ± 11.8 µm/kg, P = 0.4186) and fall (47.0 ± 9.7 µm/kg, P = 0.9410) animals. The percentage of single hybrid fibers containing both slow and fast myosin heavy chains, detected biochemically, increased from 2.6 ± 3.8% in summer to 24.4 ± 24.4% ( P = 0.0244) during hibernation. The shortening velocities of individual hybrid fibers remained unchanged from that of pure slow and fast fibers, indicating low content of the minority myosins. Slow and fast fibers in winter bears exhibited elevated specific tension (kN/m; 22%, P = 0.0161 and 11%, P = 0.0404, respectively) and maintained normalized power. The relative stability of fiber type percentage and size, fiber size-to-body mass ratio, myosin heavy chain isoform content, shortening velocity, power output, and elevated specific tension during hibernation validates the ability of the black bear to preserve the biochemical and performance characteristics of the soleus muscle during prolonged hibernation.

Metabolic reprogramming involving glycolysis in the hibernating brown bear skeletal muscle

“…We actually found higher glycogen levels in the muscles of brown bears during hibernation than in summer (Fig. 5), similar to a recent report on American black bears [58], whereas only a non-significant trend toward such a seasonal change was observed in hibernating ground squirrels [59]. Although we cannot determine exactly when glycogen deposits are made into muscle, this could provide an easy source of glucose for muscle glycolysis during hibernation.…”

Mixing it up: the biological significance of hybrid skeletal muscle fibers

“…However, there is evidence that their muscles may experience a shift towards the expression of faster MHC isoforms. This is evident from a recent study of the soleus muscles of hibernating bears, in which the proportion of hybrid fibers coexpressing MHC I and IIA increased from 2% in the summer to 24% during hibernation (sampled in February and March) (Table S1; Riley et al, 2018). Most of these fibers appear to contain mostly slow MHC, suggesting that the inactivity inherent in hibernation is associated with an upregulation of the expression of fast MHC isoforms within these slow fibers.…”

“…Disuse can take a range of forms, including simple inactivity, space flight (Fitts et al, 2000), bed rest (Andersen et al, 1999a;Borina et al, 2010;Gallagher et al, 2005), spinal cord injuries (Talmadge, 2000;Talmadge et al, 1995) and animal hibernation (Cotton, 2016;Riley et al, 2018). Collectively, these various types of muscle disuse drive the transformation towards faster fiber types (Blaauw et al, 2013;Talmadge, 2000).…”

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“…Hibernating black bears do not experience skeletal muscle atrophy despite the absence of weight bearing and physical activity. Riley and colleagues (13) examined the mechanisms underlying this resistance to disuse atrophy to gain insight into potential novel approaches to treat or prevent such atrophy in humans; for instance, those confined to bed rest. They examined the soleus muscle, a load bearing, slow-twitch muscle and found no observable histological differences between hibernating and nonhibernating states.…”