ANTIOXIDANT DEFENSES IN THE TOLERANCE OF FREEZING AND ANOXIA BY GARTER SNAKES

The garter snake Thamnophis sirtalis parietalis can readily tolerate s everal hours of freezing or anoxia exposure. Both stresses halt oxygen availability to tissues and to endure these stresses snakes must cope with potential oxidative stress arising as a result of the ischemic/a noxic condition followed by reperfusion of aerated blood during recove ry. To determine whether antioxidant defenses are important for freezi ng and anoxia survival, we monitored the activities of antioxidant enz ymes and the levels of glutathione (GSH and GSSG) during freezing (5 h at -2.5-degrees-C) and anoxia (10 h under N2 gas at 5-degrees-C) expo sures in three organs (muscle, liver, and lung) of snakes. Freezing re sulted in a significant rise in the activity of muscle and lung catala se (by 183 and 63%) and in muscle glutathione peroxidase (52%). Anoxia enhanced muscle and liver superoxide dismutase activities (by 59 and 118%) and also caused a 57% increase in muscle GSH levels. The increas e in muscle GSH concentration in anoxia (from 0.45 to 0.71 mM) could a lso stimulate muscle glutathione peroxidase activity in vivo by 1.5-fo ld because of its low affinity for GSH (K(m) = 11 mM). The ratio of GS SG/GSH was not affected by experimental state in any tissue, suggestin g that oxidative stress did not occur during the freezing or anoxic ex posure. Rather, H2O2- and O2--detoxification systems may be activated in preparation for possible oxygen free radical overgeneration during thawing or reoxygenation. Antioxidant defenses appear to be part of th e adaptive machinery for reptilian tolerance of freezing and anoxia.