EDWARDS, J.R.; JENKINS, J.J.; SWANSON, D.L.*; Univ. of South Dakota: Effects of dehydration on glucose mobilization in freeze-tolerant chorus frogs and freeze-intolerant toads

Freeze-tolerance in anurans is thought to have evolved from a predisposition for dehydration-tolerance. To test this hypothesis, we dehydrated summer/fall-collected and winter acclimated freeze-tolerant chorus frogs (Pseudacris triseriata) and dehydration-tolerant, but freeze-intolerant, Woodhouse�s (Bufo woodhousii) and Great Plains (Bufo cognatus) toads to 25 and 50% body water loss (BWL). Following treatments, we measured glucose, glycogen, and glycogen phosphorylase and glycogen synthetase (summer/fall only) activities in liver and leg muscle. Hepatic glucose levels were not significantly altered by dehydration in summer/fall for either frogs or toads, suggesting the absence of a large-scale mobilization of glucose from hepatic glycogen. However, winter acclimated frogs did show an increment (2.9-fold) in hepatic glucose with dehydration, accompanied by a reduction in hepatic glycogen levels. This mobilization of glucose did not result from elevated activity of glycogen phosphorylase, as phosphorylase activity in the liver actually decreased with dehydration. Winter acclimated toads did not mobilize hepatic glucose in response to dehydration. The pattern of hepatic glucose mobilization with dehydration in winter frogs is consistent with that in other freeze-tolerant frog species, and provides additional support for the hypothesis that freezing tolerance evolved from a capacity for dehydration tolerance. However, the lack of hepatic glucose mobilization in response to dehydration in fall frogs suggests that season influences dehydration-induced regulation of glucose metabolism in chorus frogs. Furthermore, the absence of a dehydration-induced mobilization of hepatic glucose at both seasons in toads suggests that this dehydration response is not universal for terrestrial anurans.