Protein Use and Muscle‐Fiber Changes in Free‐Ranging, Hibernating Black Bears

Abstract

Studies of the metabolic and physiological changes that bears undergo during hibernation have, for the most part, supported the paradigm that bears use only fatty tissues as a metabolic substrate during hibernation. This study was performed to document the extent of protein loss and alteration of muscle-fiber characteristics of selected muscles in black bears during winter dormancy. Muscle biopsies were removed from the gastrocnemius and biceps femoris from seven free-ranging female black bears on the Uncompahgre Plateau in west-central Colorado. Six of the seven bears produced cubs during the hibernating season. Muscle samples were collected from the left hind limb shortly after bears entered their dens (fall), and additional samples were collected from the right hind limb just prior to bears leaving their dens (spring). Protein concentration, fast- and slow-twitch muscle-fiber ratios and muscle-fiber cross-sectional areas, and citrate synthase activity were measured in the laboratory. While protein concentration decreased in both muscles during the hibernation period, it was lower than predicted for lactating females. In addition, muscle-fiber number and cross-sectional area were unchanged in these muscles, suggesting only limited muscle atrophy. In support of these observations, there was a moderate but significant increase in the proportion of fast-twitch fibers only in the biceps femoris, with a concomitant decrease in citrate synthase activity, but no alteration of the fiber ratio in the gastrocnemius during hibernation. These findings suggest that hibernating bears, particularly lactating females, do use some protein, in concert with fat catabolism, as a metabolic substrate and as a source of water. However, the extent of this protein use is moderate and is associated with limited alteration of muscle structure, characteristic of disuse atrophy.

Elevated expression of protein biosynthesis genes in liver and muscle of hibernating black bears (Ursus americanus)

“…While skeletal muscle mass in ground squirrels and bats is reduced by 14 -65% during hibernation (7), no significant loss in lean body mass was observed in adult black bears during hibernation (25,29), except for 4 -10% reduction in muscle protein reported specifically for lactating females (38). Hibernating brown and black bears show a net increase in plasma protein concentration (17,27), and this is consistent with elevated protein synthesis in liver during hibernation.…”

Extreme Respiratory Sinus Arrhythmia Enables Overwintering Black Bear Survival—Physiological Insights and Applications to Human Medicine

“…During early winter 1999 and 2000, radio-collared wild bears were located in their winter dens, anesthetized with Telazol®, and temporarily extricated from their dens [12,37]. Customized IDRs and sensing leads (model 6942; Medtronic, Inc., Minneapolis, MN) were surgically implanted in early winter using aseptic techniques.…”

“…Clinical applications of a reduced metabolic state typical of overwintering bears include treatment for human heart failure, stroke, myocardial infarction, and extending the duration of whole organ ischemia prior to transplantation [4]. We previously demonstrated that bears have limited loss of skeletal muscle protein and strength during winter, which has applications in preventing muscle disuse atrophy [12,23,37]. It is also advantageous to conserve cardiac muscle to maintain sufficient perfusion of all body tissues during winter and to enable a response to predatory threats.…”

A Low Temperature Environment Delays the Changes in Muscle Fiber Type Composition Induced by Unloading

“…On the other hand, hibernating animals, such as black bears and goldenmantled ground squirrels, develop less serious muscle atrophy than non-hibernating animals, despite their inactivity during hibernation [26,27]. A study carried out in Syrian hamsters has shown that exposure to low temperature (LT) had a short-term preventive effect on hindlimb-unloaded (HU) muscle atrophy [12].…”