Organ-specific metabolism during freezing and thawing in a freeze-tolerant frog.

Freeze-tolerant wood frogs, Rana sylvatica, were exposed to three consecutive freeze-thaw cycles. Each 2-day freezing exposure resulted in the breakdown of liver glycogen and an accumulation of high quantities of glucose in all 10 tissues tested; during each 2-day thaw glucose was restored as liver glycogen. The data suggest that frogs do not maintain cryoprotectants throughout the winter in anticipation of freezing but only synthesize and/or maintain glucose during actual freezing episodes. The pattern of glucose accumulation during freezing suggests a peripheral vasoconstriction as freezing progresses, leaving circulation open to central organs (liver, heart, and brain) for as long as possible. Lactate and alanine contents in tissues rose during each freezing exposure and were reduced during each thaw. Based on anaerobic end-product accumulation, organs appeared to vary up to 10-fold in anaerobic energy requirements in the frozen state. Levels of adenylates and fructose-2,6-biphosphate were measured in liver and muscle and provided additional evidence of tissue-specific differences in metabolism in the frozen state.