Rainbow smelt, Osmerus mordax, maintain high glycerol levels in winter to avoid freezing. After intramuscular injection of 14C-labeled glucose, [14C]glycerol was found in the blood, liver and muscle, indicating that glycogen is a source of glycerol. Levels of both the active and inactive forms of glycogen phosphorylase were higher in muscle in winter than in autumn, although the fraction in the active form did not change significantly. More of the phosphorylase was in the active form in the liver than in the muscle. Short-term starvation resulted in a significant decrease in the level of glycogen soon after the stomachs were emptied, presumably to replace glycerol lost to the water. However, tissue glycerol levels remained relatively high, despite a near depletion of glycogen reserves. Triglyceride levels increased slightly during starvation, indicating that triglycerides were not involved in glycerol synthesis. After intramuscular injection of 14C-labeled pyruvate, [14C]glycerol was found in the blood, liver and muscle, indicating a second route, presumably from muscle protein, to glycerol synthesis. Liver phosphoenolpyruvate carboxykinase activity was slightly higher in winter, possibly to assist in the conversion of pyruvate to glycerol.

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