PROTEIN USE AND MUSCLE-FIBER CHANGES IN FREE-RANGING, HIBERNATING BLACK BEARS

Studies of the metabolic and physiological changes that bears undergo during hibernation have, for the most part, supported the paradigm tha t bears use only fatty tissues as a metabolic substrate during hiberna tion. 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 gas trocnemius and biceps femoris from seven free-ranging female b lack bears on the Uncompahgre Plateau in west-central Colorado. Six of the seven bears produced cubs during the hibernating season. Muscle s amples were collected from the left hind limb shortly after bears ente red their dens (fall), and additional samples were collected from the right hind limb just prior to bears leaving their dens (spring). Prote in concentration, fast- and slow-twitch muscle-fiber ratios and muscle -fiber cross-sectional areas, and citrate synthase activity were measu red in the laboratory. While protein concentration decreased in both m uscles during the hibernation period, it was lower than predicted for lactating females. In addition, muscle-fiber number and cross-sectiona l area were unchanged in these muscles, suggesting only limited muscle atrophy. In support of these observations, there was a moderate but s ignificant increase in the proportion of fast-twitch fibers only in th e biceps femoris, with a concomitant decrease in citrate synthase acti vity, but no alteration of the fiber ratio in the gastrocnemius during hibernation. These findings suggest that hibernating bears, particula rly lactating females, do use some protein, in concert with fat catabo lism, 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.