Role of antioxidant defenses in the tolerance of severe dehydration by anurans. The case of the leopard frog Rana pipiens.

Many anurans have excellent dehydration tolerance that allows endurance of the loss of up to 50-60% of total body water. One of the effects of severe dehydration is circulatory impairment due the reduced volume and increased viscosity of blood, which leads to organ hypoxia. The rehydration situation, therefore, involves a reoxygenation of tissues that may include elements of oxidative stress that resemble the injury in post-ischemic reperfusion of mammalian organs. The role of endogenous defenses against oxygen radicals in the tolerance of severe dehydration by leopard frogs, Rana pipiens, was investigated by monitoring the activities of antioxidant enzymes and glutathione levels (reduced GSH and oxidized GSSG) in leg muscle and liver of control, 50%-dehydrated, and fully rehydrated frogs. The maximal activities of muscle catalase and liver glutathione peroxidase, measured per mg soluble protein, increased significantly by 52 and 74%, respectively, after dehydration whereas muscle superoxide dismutase and glutathione reductase activities responded oppositely, decreasing by 32 and 35%, respectively. Enzyme activities returned to control levels after full rehydration. Hepatic GSH and GSSG increased early in the rehydration process (30% recovery of total body water), but returned to control levels after full recovery. A similar trend was observed for liver GSSG. The elevation of antioxidant defenses against peroxides during dehydration could provide protection against post-hypoxic oxyradical stress during rehydration. Indeed, analysis of one product of lipid peroxidation, thiobarbituric acid reactive substances, in frog tissues gave no indication of oxidative stress during the dehydration/rehydration cycle.