LIMITATION OF GLYCOLYSIS BY HEXOKINASE IN RAT-BRAIN SYNAPTOSOMES DURING INTENSE ION PUMPING

Incubation of rat brain synaptosomes under conditions of either increa sed energy utilization (addition of Na+ channel opener, veratridine, o r ionophores, monensin and nigericin) or inhibition of oxidative phosp horylation (addition of rotenone), or a combination thereof, decreased [ATP], increased [ADP] and stimulated glycolysis. The rates of lactat e generation were linear over a 15-min interval in the presence of rot enone alone but decreased in the other two conditions. During the firs t 5 min, the amount of lactate formed with veratridine, monensin or ni gericin was as high or higher than with rotenone, but it was lower in the last 10 min. With a combination of one of the stimulators of ion m ovements and rotenone the rate of glycolysis was always markedly lower than with each compound added singly. The stimulated rates of lactate formation correlated positively with the synaptosomal content of [ATP ]. After 15 min, [ATP] was 0.9-1.0 nmol/mg with rotenone, 0.5-0.9 nmol /mg with veratridine (or ionophores), and < 0.3 nmol/mg with a combina tion of the two. Under the conditions used, calcium did not affect gly colytic activity directly. The Lineweaver-Burk plot of the rate of lac tate formation against [ATP] yielded a straight line with a K-m for AT P of about 0.1 mM, which is very similar to the K-m for this nucleotid e of brain hexokinase bound to mitochondria. In C6 cells glycolytic ra te measured with a combination of an ionophore and rotenone was higher than with each of these compounds added singly while [ATP] never decl ined below about 9 nmol/mg prot. It is concluded that in synaptosomes, the high rate of energy utilization required for intense ion movement s decreases [ATP] to a level that limits hexokinase activity kinetical ly. This may contribute to a reduction in the rate of glycolysis and h ence energy production in brain hypoxia and ischemia.