ORGANIC SOLUTES IN FREEZING TOLERANCE

The accumulation of high levels of low-molecular-weight solutes (polyh ydric alcohols, saccharides) provides cryoprotection to freeze-toleran t animals by minimizing, via colligative effects, the percentage of bo dy water converted to extracellular ice and the extent of cell volume reduction. Many freeze-tolerant insects accumulate high levels of poly ols during autumn cold hardening, whereas freeze-tolerant frogs respon d to ice formation in peripheral tissues by synthesizing large amounts of glucose in the liver and rapidly distributing the sugar throughout the body. Seasonal patterns of enzymatic change occur in cold-hardy i nsects; activities associated with cryoprotectant synthesis rise in th e fall, whereas enzymes associated with polyol degradation dominate in the spring. Enzyme profiles also revealed the route of glycerol degra dation via polyol dehydrogenase and the novel enzyme, glyceraldehyde k inase. Proton magnetic resonance imaging of freezing and thawing in wh ole frogs showed a new adaptive effect of the very high glucose levels in core organs; during thawing, organs such as liver and heart melted first, allowing recovery of their vital functions to begin while the rest of the frog thawed. New studies have examined signal transduction in the stimulation of glucose production by wood frog liver, revealin g the key role of beta-adrenergic receptors and cAMP-mediated activati on of glycogenolysis for cryoprotectant synthesis. The seasonal elevat ion of plasma membrane glucose transporters was also shown to be key t o cryoprotectant distribution during freezing. Other new work has show n that frog freeze tolerance probably grew out of preexisting mechanis ms of amphibian dehydration tolerance and that both freeze-tolerant an d -intolerant frogs show a hyperglycemic response to desiccation at 5 degrees C. (C) 1997 Elsevier Science Inc.