LACTATE TRANSPORT BY CORTICAL SYNAPTOSOMES FROM ADULT-RAT BRAIN - CHARACTERIZATION OF KINETICS AND INHIBITOR SPECIFICITY

Since lactate released by glial cells may be a key substrate for energ y in neurons, the kinetics for the uptake of L-[U-C-14]lactate by cort ical synaptic terminals from 7- to 8-week-old rat brain were determine d. Lactate uptake was temperature-dependent, and increased by 64.9% at pH 6.2, and decreased by 43.4% at pH 8.2 relative to uptake at pH 7.3 , Uptake of monocarboxylic acids was saturable with increasing substra te concentration. Eadie-Hofstee plots of the data gave evidence of two carrier-mediated uptake mechanisms with a high-affinity K-m of 0.66 m M and V-max of 3.66 mM for pyruvate, and a low-affinity system with a K-m of 9.9 mM for both lactate and pyruvate and V-max values of 16.6 a nd 23.1 nmol/30 s/mg protein for lactate and pyruvate, respectively. S aturable uptake was seen in the presence of 10 mM alpha-cyano-4-hydrox ycinnamate. Lactate transport by synaptic terminals was much more sens itive to inhibition by sulfhydryl reagents than transport in astrocyte s. Addition of 0.5 and 2 mM mersalyl decreased the uptake of 1 mM lact ate by synaptic terminals by 59.3 and 66.37%, respectively. Pyruvate m oderately decreased lactate transport, whereas 3-hydroxybutyrate had l ittle effect, Quercetin, an inhibitor of lactate release, had little e ffect on the content of C-14 lactate in synaptic terminals, supporting the concept that the majority of lactate produced within brain is fro m glial cells. Oxidation of L[U-C-14]lactate by synaptosomes was satur able, and yielded a K-m of 1.23 mM and a V-max of 116 nmol/h/mg protei n. Overall the studies show that synaptic terminals from adult brain h ave a high capacity for transport and oxidation of lactate, consistent with the proposed role for this compound in metabolic trafficking in brain. Furthermore, the data provide kinetic evidence of two carrier-m ediated mechanisms for monocarboxylic acid transport by synaptosomes a nd demonstrate that uptake of lactate by synaptic terminals is regulat ed differently than transport by astrocytes. Uptake of lactate by syna ptic terminals also has differences from the systems described for neu rons.