HYPOTHERMIA, METABOLIC STRESS, AND NMDA-MEDIATED EXCITOTOXICITY

Isolated embryonic retinas were metabolically stressed by inhibition o f glycolysis either with iodoacetate (IOA) or by glucose withdrawal pl us 10 mM 2-deoxy-D-glucose, and the effects of hypothermia were examin ed. Incubation at 30 versus 37-degrees-C during 30 min of hypoglycemia with IOA completely reduced the rapid swelling-related GABA release [ 6 +/- 2 vs. 68 +/- 10 nmol/100 mg of protein (mean +/- SEM) for 30 and 37-degrees-C, respectively]. Histology of the retina immediately foll owing 30 min of metabolic stress at 30-degrees-C appeared normal, wher eas that at 37-degrees-C showed a pattern of acute edema, characterist ic of NMDA-mediated acute excitotoxicity. Coincubation with a competit ive or noncompetitive NMDA antagonist, respectively, CGS-19755 (10 muM ) or MK-801 (1 muM), during 30 min of hypoglycemia at 37-degrees-C com pletely prevented tissue swelling, whereas extracellular GABA content remained at basal levels, indicating that the cytotoxic effects of IOA treatment for 30 min at 37-degrees-C were NMDA receptor mediated. Lon ger periods of hypoglycemia at 37-degrees-C produced acute toxicity th at was only partially N MDA receptor mediated. Hypothermia delayed the onset of NMDA-mediated toxicity by 30-60 min. At 30-degrees-C, the ra te of loss of ATP was slowed during the first several minutes of hypog lycemia (82 and 58% of maximal tissue levels at 30 and 37-degrees-C, r espectively, at 5 min), but by 10 min, ATP levels were comparably redu ced. After a transient exposure of retina to 50 muM NMDA in Mg2+-free medium, hypothermia significantly attenuated acute GABA release by 30% . At 24 h of recovery, lactate dehydrogenase release was decreased by 37%. Hypothermia had no effect when the exposure was done in medium co ntaining physiological concentrations of Mg2+. The above results sugge st that the protective effect of hypothermia during the metabolic insu lt is predominately directed at the cellular events that lead up to NM DA receptor involvement. Reduction in the rate of loss of ATP, however , does not fully account for the delay in involvement of NMDA receptor s during metabolic stress at 30-degrees-C. The attenuation of direct N MDA-mediated toxicity in Mg2+-free medium further suggests that decrea sed temperature may result in altered channel properties during situat ions when the Mg2+ block is lifted.