%0 Journal Article
%T Seasonality of Freeze Tolerance in a Subarctic Population of the Wood Frog, Rana sylvatica
%A Jon P. Costanzo
%A M. Clara F. do Amaral
%A Andrew J. Rosendale
%A Richard E. Lee
%J International Journal of Zoology
%D 2014
%I Hindawi Publishing Corporation
%R 10.1155/2014/750153
%X We compared physiological characteristics and responses to experimental freezing and thawing in winter and spring samples of the wood frog, Rana sylvatica, indigenous to Interior Alaska, USA. Whereas winter frogs can survive freezing at temperatures at least as low as £¿16¡ãC, the lower limit of tolerance for spring frogs was between £¿2.5¡ãC and £¿5¡ãC. Spring frogs had comparatively low levels of the urea in blood plasma, liver, heart, brain, and skeletal muscle, as well as a smaller hepatic reserve of glycogen, which is converted to glucose after freezing begins. Consequently, following freezing (£¿2.5¡ãC, 48£¿h) tissue concentrations of these cryoprotective osmolytes were 44¨C88% lower than those measured in winter frogs. Spring frogs formed much more ice and incurred extensive cryohemolysis and lactate accrual, indicating that they had suffered marked cell damage and hypoxic stress during freezing. Multiple, interactive stresses, in addition to diminished cryoprotectant levels, contribute to the reduced capacity for freeze tolerance in posthibernal frogs. 1. Introduction Among temperate ectotherms, cold hardiness in its various forms is most strongly expressed during the winter months, coincident with the greatest need for protection from severe cold. Although seasonal variation in this trait is often pronounced, its physiological basis remains incompletely understood. Recent studies, particularly those using various ¡°-omics¡± approaches [1], attest that the underpinnings are complex and involve a host of adaptations at multiple levels of biological organization. Elucidation of these mechanisms will require comprehensive study of organisms for which the fundamental adaptations of freeze tolerance are reasonably well known. The relatively robust freeze tolerance exhibited by certain woodland frogs is associated with their ability to accrue high concentrations of the cryoprotectants, glucose, and/or glycerol, which during freezing are mobilized from glycogen in the liver. These compounds limit freezing injury by colligatively lowering the equilibrium freezing/melting point ( ) of body fluids and, hence, reducing ice formation, and also by preserving the integrity of membranes and macromolecules, among other things [2, 3]. Because the hepatic glycogen store is substantially reduced following hibernation and mating, spring frogs can accrue only modest amounts of these agents, and this difference purportedly is the cause of their reduced freeze tolerance [4¨C8]. It was recently reported [9] that some freeze-tolerant frogs also use urea as a cryoprotectant, but
%U http://www.hindawi.com/journals/ijz/2014/750153/