The cryoprotectant system of Cope’s gray treefrog, Dryophytes chrysoscelis: responses to cold acclimation, freezing, and thawing
Cope’s gray treefrog (Dryophytes chrysoscelis) is one of few freeze-tolerant frogs that mobilize glycerol as a cryoprotectant, yet cold and freezing-induced accumulation of this and other osmolytes has received little attention in this species. This study investigated the development of freeze tolerance in D. chrysoscelis, analyzing the response of the cryoprotectant system to cold acclimation, freezing, and thawing. Glycerol production was low and unresponsive to acclimation temperature, or duration of acclimation to 5 °C, except for one cold-acclimated frog that presented elevated glycerol in plasma, liver, and skeletal muscle. Curiously, glycerol concentration in skeletal muscle was higher than that of plasma and liver, in both warm- and cold-acclimated frogs, suggesting glycerol synthesis in muscle. Urea concentration in plasma doubled in response to cold acclimation but did not change during freezing. Freezing induced hepatic glycogen catabolism and an increase in glycerol and glucose in several tissues, although the mobilization dynamics differed between these cryoprotectants, possibly as a result of different transport mechanisms. Although hepatic glucose mobilization was of considerable magnitude, glucose accumulation in peripheral tissues was low and was surpassed by that of glycerol and urea. The muscle production of glycerol and the cold-induced accumulation of urea imply a role for skeletal muscle metabolism in the mobilization of cryoprotective solutes in D. chrysoscelis. The cryoprotectant system of D. chrysoscelis is complex, highly variable, and unique, with glycerol, glucose, and likely urea serving as cryoprotectants.