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

Wood frogs, Rana sylvatica, tolerate the loss of 50-60% of total body water during experimental dehydration. The rate of water loss for unpr otected frogs is the same whether animals are frozen (at -2-degrees-C) or unfrozen (at 1-degrees-C) but is greatly reduced when frogs are fr ozen under a protective layer of moss. Dehydrational death could occur in as little as 7-9 days for unprotected animals; this indicates the importance for winter survival of selecting well-protected and damp hi bernation sites. Prior dehydration affected the cooling and freezing p roperties of frogs, reducing supercooling point and the amount of ice formed after 24 h at -2-degrees-C and acting synergistically with free zing exposure in stimulating cryoprotectant synthesis. Analysis of the effects of controlled dehydration at 5-degrees-C showed that changes in body water content alone (without freezing) stimulated liver glycog enolysis and the export of high concentrations of glucose into blood a nd other organs. Autumn-collected frogs dehydrated to 50% of total bod y water lost showed glucose levels of 165-1,409 nmol/mg protein in dif ferent organs, increases of 9- to 313-fold compared with control value s and reaching final levels very similar to those induced by freezing exposure. The data support the proposal that various adaptations for n atural freeze tolerance may have been derived from preexisting mechani sms for dealing with water stress in amphibians and that cell volume c hange may be one of the signals involved in triggering and sustaining molecular adaptations (e.g., cryoprotectant output) that support freez ing survival.