Whole-body urea cycling and protein turnover during hyperphagia and dormancy in growing bears (Ursus americanusandU.arctos)

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Abstract
Subadult bears were studied during their autumn hyperphagia (n = 3) and winter dormancy (n = 6). Urea kinetics were measured with14C- and15N-urea, protein turnover was estimated with15N-glycine, and body composition was assessed with3H-water. Reduced amino acid degradation in winter was indicated by declines in plasma urea and aminotransferase activities, and lower urea production than in autumn (4.7 vs. 27.5 mmol urea-N∙kg−0.75∙d−1). Only 7.5% of urea produced in hyperphagic bears was degraded and just 1.1% of the degraded N reutilized as amino-N. Dormant bears reutilized 99.7% of urea produced, indicating thorough microbial ureolysis and urea-N resorption. Low rates of body N loss during dormancy suggested losses of non-urea N as creatinine. Protein turnover rates (15.2–21.5 g∙kg−0.75∙d−1) were similar between seasons and reflected the apparent maintenance of hepatic, intestinal, and muscular functions through dormancy. Protein synthesis accounted for 32% of energy expended in dormancy, which was mainly (91.5%) derived from fat oxidation. Consistent organ function and body temperature in dormant bears enables recycling of urea-N, which minimizes body protein loss and conserves mobility. In comparison with heterothermic hibernation, ursid dormancy would provide greater flexibility during winter and facilitate rapid resumption of foraging and growth in spring.