Adaptations of metabolism for freeze tolerance in the gray tree frog, Hyla versicolor

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Abstract
Biochemical adaptations allowing the natural survival of extracellular freezing were examined in the gray tree frog, Hyla versicolor. Laboratory-reared immature adults froze between −1 and −1.5 °C and survived 5 days of freezing at −2 °C as well as repeated rapid bouts of freeze–thaw. Measurements of ice content showed 41.5% of total body water frozen. Glycerol accumulated as the cryoprotectant in sexually mature adult H. versicolor (423 μmol/mL in blood) while both glycerol and glucose accumulated in immature adults (16.3 ± 6.8 and 25.9 ± 11.6 μmol/mL in blood, respectively). Cryoprotectant synthesis was freezing stimulated only and did not occur over long-term cold acclimation at 0 to 1 °C. Cryoprotectant synthesis was correlated with a 203% increase in liver total phosphorylase activity and an increase in phosphorylase a content from 40 to 60%. Activities of 15 other enzymes of intermediary metabolism were determined in liver and leg muscle; activities of most enzymes increased with freezing exposure as did soluble protein content. Survival of freezing depends upon anaerobic mechanisms of energy production in tissues. Frogs frozen at −2 °C accumulated lactate in liver and muscle. Energy charge dropped in both tissues and the creatine phosphate reserves of muscle were depleted.

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