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First published online September 5, 2008
Journal of Experimental Biology 211, 2969-2975 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.019695
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Urea loading enhances freezing survival and postfreeze recovery in a terrestrially hibernating frog

Jon P. Costanzo* and Richard E. Lee, Jr

Department of Zoology, Miami University, Oxford, OH 45056, USA

* Author for correspondence (e-mail: costanjp{at}muohio.edu)

Accepted 21 July 2008

We tested the hypothesis that urea, an osmolyte accumulated early in hibernation, functions as a cryoprotectant in the freeze-tolerant wood frog, Rana sylvatica. Relative to saline-treated, normouremic (10 µmol ml–1) frogs, individuals rendered hyperuremic (70 µmol ml–1) by administration of an aqueous urea solution exhibited significantly higher survival (100% versus 64%) following freezing at –4°C, a potentially lethal temperature. Hyperuremic frogs also had lower plasma levels of intracellular proteins (lactate dehydrogenase, creatine kinase, hemoglobin), which presumably escaped from damaged cells, and more quickly recovered neurobehavioral functions following thawing. Experimental freezing–thawing did not alter tissue urea concentrations, but did elevate glucose levels in the blood and organs of all frogs. When measured 24 h after thawing commenced, glucose concentrations were markedly higher in urea-loaded frogs as compared to saline-treated ones, possibly because elevated urea retarded glucose clearance. Like other low-molecular-mass cryoprotectants, urea colligatively reduces both the amount of ice forming within the body and the osmotic dehydration of cells. In addition, by virtue of certain non-colligative properties, it may bestow additional protection from freeze–thaw damage not afforded by glucose.

Key words: amphibian, freeze tolerance, osmolyte, hibernation, Rana sylvatica, wood frog, cryoprotection


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