Mc. Mckenna et al., REGULATION OF ENERGY-METABOLISM IN SYNAPTIC TERMINALS AND CULTURED RAT-BRAIN ASTROCYTES - DIFFERENCES REVEALED USING AMINOOXYACETATE, Developmental neuroscience, 15(3-5), 1993, pp. 320-329
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.