Glycerol metabolism in the hibernating black bear

Summary

The contribution of glycerol to protein, carbohydrate, and lipid metabolism was studied in black bears. Special attention was directed at the role of glycerol in preventing uremia. In summer and winter, U-¹⁴C glycerol, as well asl(U¹⁴C)-alanine andd(U-¹⁴C)-glucose were injected intravenously; timed sampling of venous blood and expiratory gases was made.

In winter, during hibernation,¹⁴C-label from glycerol was found in alanine and other free amino acids and plasma proteins, pyruvate, lactate, glucose and lipid esters. After 48 h, most of the¹⁴C-label in plasma was found in proteins. However, during the four days of study, no label was found in serum urea. Similar results occurred in summer except, in marked contrast to winter,¹⁴C-labeled urea was continually detected in blood. Expired¹⁴C-CO₂ was considerably lower in winter than summer and winter respiratory quotient was 0.69.

In both summer and winter,¹⁴C-labeled alanine also entered plasma proteins and generated pyruvate, lactate and glucose. Once again the marked difference between active and dormant phases was demonstrated: There was no labeled plasma urea in winter while it was continually detected in summer.

¹⁴C-labeled glucose experiments revealed very slow carbohydrate metabolism in winter.

These findings suggest that in winter, glycerol helps prevent uremia by serving as a carbon source for amino acid formation. The nitrogen involved in these reactions is thus diverted from urea synthesis into protein synthetic pathways. Further, glycerol also appears to serve as an active substrate for gluconeogenesis and lipogenesis in hibernation.