Liver Glycogen and Freezing Survival in the Chorus Frog (Pseudacris triseriata)

JENKINS, J.L.*; SWANSON, D.L.: Liver Glycogen and Freezing Survival in the Chorus Frog (Pseudacris triseriata).

In freeze tolerant frogs, a principal component of freezing survival is the production and distribution of cryoprotectants (glucose or glycerol) from liver glycogen stores. The size of these stores limits the absolute amount of cryoprotectant produced during a freezing bout, consequently impacting freeze-tolerance. Additionally, in R. sylvatica, liver glycogen is depleted as a result of multiple freeze-thaw cycles. We froze chorus frogs for 2-3 24h bouts over the winter period of 1998 and 1999 to determine the energetic cost of freeze-thaw cycles in this species. Frogs surviving the initial freeze-thaw bout survived subsequent freezing bouts. However, in contrast to R. sylvatica, multiple freeze-thaw cycles failed to significantly reduce liver glycogen stores or change glycogen phosphorylase activity. Freezing survival was unexpectedly low, thus prompting a secondary investigation into the relationship between hepatic glycogen reserves, glucose mobilization and freezing survival in chorus frogs. In all cases freezing in P. triseriata resulted in significant increases in the levels of glucose in liver and muscle tissue. However, levels of cryoprotectant and liver glycogen of animals in the current study were significantly lower than in previous studies where freezing survival was better. Low levels of liver glycogen may be the explanatory factor for the decreased cryoprotectant levels and increased mortality. The frogs in this study showed a positive correlation between liver mass and liver glycogen. Currently, it is unclear if this correlation can be extended to body size, but such a relationship seems likely.

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