Metabolic responses of glycerol-producing osmerid fishes to cold temperature

Some osmerid fishes are known to produce glycerol in winter to protect against freezing. One such species, the rainbow smelt Osmerus mordax, was acclimated to low temperatures to elucidate the glycerol-producing mechanism. When non-glycerol-producing fall-caught smelt were acclimated to +1 and -1 degrees C in the absence of feeding, both groups synthesized large amounts of glycerol. Liver and muscle glycogen levels were low in both groups, and were also lower in the -1 degrees C group than in the +1 degrees C group, suggesting that glycogen was a source of glycerol. A similar result was obtained with acclimation of winter-caught smelt to +1 and -1 degrees C. Glycogen levels in summer- and winter-acclimatized surf smelt Hypomesus pretiosus also indicated a high glycogen demand in winter. Triglycerides did not appear to be a source of glycerol, as triglyceride levels in both species did not show an inverse relationship with glycerol levels. Because glycerol levels remained relatively high in the acclimated rainbow smelt despite a considerable glycerol efflux and low glycogen levels, glycogen does not appear to be the only source of glycerol in these fishes.