AJP - Regulatory, Integrative and Comparative Physiology, Vol 265, Issue 4 721-R725, Copyright © 1993 by American Physiological Society
Physiological responses of freeze-tolerant and -intolerant frogs: clues to evolution of anuran freeze tolerance
J. P. Costanzo, R. E. Lee Jr and P. H. Lortz
Department of Zoology, Miami University, Oxford, Ohio 45056.
Freeze tolerance in the wood frog, Rana sylvatica, is promoted by multiple,
integrated physiological responses to ice forming within body tissues. By
analyzing the freezing responses of the sympatric, but freeze intolerant,
leopard frog (R. pipiens), we sought clues to the evolution of anuran
freeze tolerance. Physiological responses critical to R. sylvatica's freeze
tolerance, such as the synthesis and distribution of the cryoprotectant
glucose, protective dehydration of organs, and deferred cardiac failure,
were present, but comparatively less pronounced, in R. pipiens. Both
species were innately tolerant of hyperglycemia. Glucose supplements did
not enhance the freezing viability of R. pipiens, although in vitro tests
of cryoprotectant efficacy revealed that glucose and glycerol provided
comparable protection to erythrocytes of both species. We conclude that the
evolution of freeze tolerance in R. sylvatica is not only promoted by its
desiccation tolerance and the fortuitous biophysical consequences of
freezing (e.g., exothermic induction of cardioacceleration and moderation
of cooling rate) but also involves a progressive enhancement of fundamental
physiological stress responses.
