ANTIOXIDANT DEFENSES AND METABOLIC DEPRESSION - THE HYPOTHESIS OF PREPARATION FOR OXIDATIVE STRESS IN LAND SNAILS

The roles of enzymatic antioxidant defenses in the natural tolerance o f environmental stresses that impose changes in oxygen availability an d oxygen consumption on animals is discussed with a particular focus o n the biochemistry of estivation and metabolic depression in pulmonate land snails. Despite reduced oxygen consumption and P-O2 during estiv ation, which should also mean reduced production of oxyradicals, the a ctivities of antioxidant enzymes, such as superoxide dismutase and cat alase, increased in 30 day-estivating snails. This appears to be an ad aptation that allows the snails to deal with oxidative stress that tak es place during arousal when P-O2 and oxygen consumption rise rapidly. Indeed, oxidative stress was indicated by increased levels of lipid p eroxidation damage products accumulating in hepatopancreas within minu tes after arousal was initiated. The various metabolic sites responsib le for free radical generation during arousal are still unknown but it seems unlikely that the enzyme xanthine oxidase plays any substantial role in this despite being implicated in oxidative stress in mammalia n models of ischemia/reperfusion. We propose that the activation of an tioxidant defenses in the organs of Otala lactea during estivation is a preparative mechanism against oxidative stress during arousal. Incre ased activities of antioxidant enzymes have also observed under other stress situations in which the actual production of oxyradicals should decrease. For example, antioxidant defenses are enhanced during anoxi a exposure in garter snakes Thamnophis sirtalis parietalis (10 h at 5 degrees C) and leopard frogs Rana pipiens (30 h at 5 degrees C) and du ring freezing exposure tan ischemic condition due to plasma freezing) in T. sirtalis parietalis and wood frogs Rana sylvatica. II seems that enhancement of antioxidant enzymes during either anoxia or freezing i s used as a preparatory mechanism to deal with a physiological oxidati ve stress that occurs rapidly within the early minutes of recovery dur ing reoxygenation or thawing. Thus, a wide range of stress tolerant an imals display coordinated changes in antioxidant defenses that allow t hem to deal with oxidative stress that occurs as part of natural cycle s of stress/recovery that alter oxygen levels in tissues. The molecula r mechanisms that trigger and regulate changes in antioxidant enzyme a ctivities in these species are still unknown but could prove to have k ey relevance for the development of new intervention strategies in the treatment of cardiovascular ischemia/reperfusion injuries in humans. (C) 1998 Elsevier Science Inc. All rights reserved.