Experimental study of safety limits in differential hypothermia Part 2. Metabolic effects on normal rabbit brains

Differential hypothermia (D.H.) treatment was performed on normal rabbit brains and the associated changes in cerebral metabolism were observed in series. Under generalized body hypothermia (rectal temperature; 23℃), a portion of normal brain was maintained at normothermia (37±1℃) by heating with microwave irradiation (2.45GHz). Brain tissues were sampled as follows; before treatment, at the beginning, 3, 6 and 10 hours later, and finally 12 and 36 hours after completion of the 10 hour-treatment. Five rabbits were studied in each group to be 35 as a total sum. In each rabbit, two samples; one from the normothermic portion and one from the hypothermic portion, were obtained. By using enzymatic analysis, six glycolytic metabolites (glucose, glucose-6-phosphate, fructose-1,6-diphosphate, dihydroxyacetone phosphate, pyruvate, lactate) and three adenine nucleotides (ATP, ADP, AMP) were investigated quantitatively. From these results, the ratio of lactate and pyruvate (L/P ratio) and energy charge potential (ECP) were calculated. Before the treatment, the L/P ratio was 26.4 and ECP was 0.921. Up to 6 hours after the beginning, the metabolic status in the normothermic portion was relatively higher than the hypothermic portion. Namely, L/P ratio and ECP in the normothermic portion reached 27.6 and 0.953 respectively. On the completion of the 10 hour-treatment, however, glucose decreased considerably. While G6P increased markedly associated with a L/P ratio of 47.5. Similarly ATP was decreased, while ADP and AMP were increased, therefore ECP decreased to 0.873. These data indicated tissue hypoxia. On the other hand, 36 hours after the treatment, the L/P ratio recovered completely to 18.7, ECP also recovered to 0.901. These results indicated that metabolism in the normothermic portion was smooth up to 6 hours duration of treatrent, although it was constantly higher than the hypothermic portion. On the completion of the 10 hour-treatment, increase of glycolysis by tissue hypoxia and failure of energy metabolism were observed. However, these changes were considered still reversible because the results obtained after the treatment indicated reasonable recovery. Therefore it was suggested that 10 hour-differential hypothermia treatment on the normal rabbit brains was safe from a viewpoint of glycolytic and energy metabolism.