Glucose loading prevents freezing injury in rapidly cooled wood frogs

The wood frog (Rana sylvatica) is the most commonly studied of ten species of freeze-tolerant vertebrates. Under natural (i.e., slow) rates of cooling, freezing initiates the production of the cryoprotectant glucose, which is mobilized from the liver and distributed to tissues throughout the body. Rapid cooling during freezing is injurious to wood frogs, probably because cryoprotectant production and mobilization are inhibited. To test this hypothesis, we investigated whether rapid-cooling injury is reduced if exogenous glucose is experimentally introduced to tissues before freezing. Glucose-loaded and control (saline-injected) wood frogs were rapidly cooled during freezing to -2.5 degrees C and subsequently assayed for injury at both cellular (erythrocyte) and neuromuscular (behavioral reflex) levels. Rapid cooling produced substantial hemolysis in control frogs, but erythrocyte injury was significantly reduced in glucose-loaded frogs. Similarly neuromuscular injury was significantly higher in control frogs than in glucose-loaded frogs. These findings suggest that rapid-cooling injury results from an inadequate production and distribution of endogenous glucose during freezing. Furthermore, the inverse relationship between the degree of freezing injury and the quantity of exogenous glucose present strongly implicates glucose as a cryoprotectant in R. sylvatica.