REGULATION OF ENERGY-METABOLISM IN SYNAPTIC TERMINALS AND CULTURED RAT-BRAIN ASTROCYTES - DIFFERENCES REVEALED USING AMINOOXYACETATE

Several recent studies have demonstrated that the metabolism of energy substrates takes place in multiple compartments in both astrocytes an d synaptic terminals from brain. There are a number of differences in the metabolism of astrocytes and synaptic terminals primarily due to t he localization of key enzymes such as pyruvate carboxylase and glutam ine synthetase in astrocytes. The present study determined the rates o f (CO2)-C-14 production from several energy substrates by primary cult ures of astrocytes and cortical synaptic terminals from rat brain. The rates of (CO2)-C-14 production from labelled substrates by astrocytes were 0.96 +/- 0.13, 11.13 +/- 0.67, 10.51 +/- 0.35, 24.92 +/- 1.66 an d 4.80 +/- 0.50 for D-[6-C-14]glucose, L-[U-C-14]lactate, D-3-hydroxy[ 3-C-14]butyrate, L-[U-C-14]glutamine and L-[U-C-14]ma-late, respective ly. The rates of (CO2)-C-14 production were also measured in the prese nce of 5 mM aminooxyacetate (AOAA) to determine the effect of inhibiti ng the malate-aspartate shuttle and other transaminase reactions on th e oxidation of energy substrates. In astrocytes the addition of AOAA d ecreased the rate of glutamine oxidation 5-fold, consistent with other studies showing that glutamine enters the TCA cycle via transaminatio n. AOAA increased the rate of (CO2)-C-14 production from labelled gluc ose 4-fold, suggesting that inhibition of alanine biosynthesis profoun dly alters the utilization of glucose by astrocytes. AOAA also increas ed the oxidation of lactate and 3-hydroxybutyrate 36 and 58%, respecti vely. The rates of (CO2)-C-14 production from labelled substrates by s ynaptic terminals were 13.12 +/- 1.05, 35.29 +/- 3.58, 17.66 +/- 1.95, 30.18 +/- 1.10 and 9.95 +/- 1.29, respectively, for glucose, lactate, 3-hydroxybutyrate, glutamine and malate, demonstrating that all subst rates were oxidized at a higher rate by synaptic terminals than by ast rocytes. The addition of AOAA decreased the rate of (CO2)-C-14 product ion from labelled lactate by 57% suggesting that the use of lactate fo r energy in synaptic terminals is tightly coupled to the activity of t he malate-aspartate shuttle. AOAA had no effect on the rate of (CO2)-C -14 production from labelled glutamine, demonstrating that exogenous g lutamine enters the TCA cycle in synaptic terminals via glutamate dehy drogenase, not via transamination as is the case with astrocytes. AOAA had no significant effect on the rates of oxidation of glucose, 3-hyd roxybutyrate and malate by synaptic terminals. These findings demonstr ate that inhibiting transamination with AOAA had very different effect s on the oxidation of energy substrates in the two preparations, sugge sting that the regulation of metabolism is quite different in astrocyt es and synaptic terminals. These studies also underscore the importanc e of utilizing multiple energy substrates since the presence of AOAA a ltered energy metabolism in some, but not all, compartments of TCA cyc le activity in both astrocytes and synaptic terminals.