Protein Synthesis Increases during Hibernation

As mentioned above, black bears are able to go through their entire dormancy without defecation or urination, which under normal physiological functioning causes urea to build up and eventually would poison the bear. However, winter sleep in bears is anything but normal. Along with maintaining constant lean body mass, black bears also keep amino acid, protein, and urea concentrations constant (Nelson, 1980). While these observations may lead to the hypothesis that hibernating black bears reduce protein activity, the opposite was shown in studies looking at albumin and leucine as indicators of protein metabolism. In fact, protein metabolism is 3-5 times greater in hibernating than non-hibernating black bears (Lundberg et al., 1976).

Table 3. As referenced above, protein anabolism was measured by looking at albumin and other amino acids. This table shows that in this study, the concentration of albumin in the blood of black bears remained constant or increased (Adapted from Lundberg et al., 1976).

Black Bears Neutralize Urea while Hibernating

Urea is produced in animals as a waste product from breaking down protein. While protein turnover increases in black bears, formation of urea decreased and fewer amino acids enter into the urea cycle. This occurs when black bears stop ingesting amino acids in their diets. Humans can experience this also when they have protein deficiencies in their food consumption, but the unique aspect of bear’s dormancy is that they can achieve this without a loss of lean protein mass. However, when urea was injected into bears during hibernation, they lost water in their cells and were disturbed. These observations lead scientists to believe that urea became hydrolyzed and moved into the gut, where bacteria could break it down using urease. Ammonia and carbon dioxide would be made and could be reabsorbed into the blood, where the carbon dioxide would be exhaled and the ammonia could be synthesized with glycerol into alanine or other amino acids, as detailed in Figure 3 on the previous page (Nelson, 1980).

Protein Turnover Increase in Dormant Black Bears

Scientists believe that the activity of protein in bears increases for a number of reasons during hibernation. The first is to generate more enzymes that can aid in breaking down fats, which the bear uses as its primary energy source during the overwintering period. Bears must also make more enzymes to be able to provide for their increased protein metabolism. Also, protein activity and breakdown releases energy in the form of heat, which may contribute to the higher body temperature of bears compared to other hibernators (Nelson, 1980).

Summing Up Protein Metabolism and Urea Breakdown

The synthesis of proteins and neutralization of urea are two of the most unique and important aspects of bear overwintering. There are two ways that they control the urea in their bodies: by limiting the breakdown of proteins into urea and reincorporating nitrogen from catabolized urea and amino acids back into proteins in the bear’s body. This is reinforced by the fact that in hibernation, studies have shown that anabolism of at least two amino acids, leucine and alanine, increases, thereby lessening the amount available to enter the urea cycle (Nelson 1978). The latter mechanism for controlling urea ties protein metabolism into fat metabolism, since glycerol and other products of fat catabolism can be used to combine with the nitrogen from urea to form new proteins (Nelson 1980). However, this mechanism is only possible when black bears are in their winter sleep; while active, starvation causes them to become dehydrated and build up urea like other mammals (Nelson 1978).