AJP - Regulatory, Integrative and Comparative Physiology, Vol 265, Issue 6 1324-R1332, Copyright © 1993 by American Physiological Society
Dehydration tolerance in wood frogs: a new perspective on development of amphibian freeze tolerance
T. A. Churchill and K. B. Storey
Institute of Biochemistry, Carleton University, Ottawa, Ontario, Canada.
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 -2 degrees C) or unfrozen
(at 1 degrees C) but is greatly reduced when frogs are frozen 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 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 -2 degrees C
and acting synergistically with freezing 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 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
165-1,409 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
preexisting mechanisms for dealing with water stress in amphibians 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.
