Churchill, T.A. and Storey, K.B. 1993. Dehydration tolerance in wood frogs: a new perspective on the development of amphibian freeze tolerance. Am. J. Physiol. 265, R1324- R1332.

DEHYDRATION TOLERANCE IN WOOD FROGS: A NEW PERSPECTIVE ON THE DEVELOPMENT OF AMPHIBIAN FREEZE TOLERANCE


Thomas A. Churchill and Kenneth B. Storey


Abstract
Wood frogs, Rana sylvatica, tolerate the loss of 50-60 % of total body water during experimental dehydration. The rate of water loss for unprotected frogs is the same whether animals are frozen (at -2C) or unfrozen (at 1C) but is greatly reduced when frogs are frozen under a protective layer of moss. Dehydrational death could occur in as little as 7-9 d for unprotected animals and this indicates the importance for winter survival of selecting well-protected and damp hibernation sites. Prior dehydration affected the cooling and freezing properties of frogs, reducing supercooling point and the amount of ice formed after 24 h at -2C, and acting synergistically with freezing exposure in stimulating cryoprotectant synthesis. Analysis of the effects of controlled dehydration at 5C showed that changes in body water content alone (without freezing) stimulated liver glycogenolysis and the export of high concentrations of glucose into blood and other organs. Autumn-collected frogs dehydrated to 50 % of total body water lost showed glucose levels of 165-1409 nmol/mg protein in different organs, increases of 9 to 313-fold compared with control values and reaching final levels very similar to those induced by freezing exposure. The data support the proposal that various adaptations for natural freeze tolerance may have been derived from pre-existing mechanisms for dealing with water stress in the Amphibia and that cell volume change may be one of the signals involved in triggering and sustaining molecular adaptations (e.g. cryoprotectant output) that support freezing survival.