A MICROCALORIMETRIC STUDY OF TURTLE CORTICAL SLICES - INSIGHTS INTO BRAIN METABOLIC DEPRESSION

In previous papers, we have examined turtle cortical neurons in vitro for mechanisms of anoxic metabolic depression ('channel arrest' and ch anges in electrical parameters). Negative results prompted the current study with the aim of examining more closely the energy profile and m etabolism of turtle cortical slices. Calorimetry is used to measure he at dissipation during normoxia and nitrogen perfusion (120 min) and th e results are converted into an ATP utilization rate. These indicate t hat the control rate of ATP utilization (1.72 mu mol ATP g(-1) min(-1) ) agrees closely with in vivo whole-brain metabolic measurements. Both nitrogen perfusion and pharmacologically induced anoxic (cyanide+N-2) groups depressed heat dissipation considerably compared with the cont rol value (nitrogen 37%; pharmacological anoxia 49%). The resulting AT P utilization estimates indicate metabolic depressions of 30% (nitroge n) and 42% (pharmacological anoxia). The slice preparation did not exh ibit a change in any measured adenylate parameter for up to 120 min of anoxia or pharmacological anoxia. Significant changes did occur in [A DP], ATP/ADP ratio and energy charge after 240 min of exposure to anox ic conditions. These results support the idea that the turtle cortical slice preparation has a profound resistance to anoxia, with both nitr ogen perfusion and pharmacological anoxia causing a rapid decline in h eat dissipation and metabolism.