Interrelationships (Between Glucose Metabolism, Energy State, and the Cytosolic Free Calcium Concentration in Cortical Synjaptosomes from the Guinea Pig

The stoichiometrics of glycolysis and pyruvate oxidation were determined in cortical synaptpsomes under varying rates of ATP consumption. Glycolysis was measured by using D-3-[³H]glucose as a marker and pyrujvate oxidation by using D-3,4-[¹⁴C]glucose, which has to be metabolized to l-[¹⁴C]pyruvate before being decarboxylated by the pyruvate dehydrogenase complex of intrasynaptosomal mitochondria. Cytosolic free Ca²⁺ concentration ([Ca²⁺]_c) wis determined in parallel and was manipulated by using EGTA in the incubation. The results show that in nonstimulated synapto-somes glycolysis and pyruvate oxidation are tightly coupled and stoichiometric. In the absence of Ca²⁺, when [Ca²⁺]_c drops from 260 nM to 40 nM, glucose utilization increases, following the increase in energy demand, which has been shown to be due to elevated Na+ cycling. KC1 depolarization, ve-ratridine, and a mitochondrial uncoupler, carbonyl cyanide m-chlorophenylhydrazone, all stimulate glycolysis and pyruvate oxidation stoichiometrically, independently of the presence of external Ca²⁺. A rise in [Ca²⁺]_c, therefore, is not required to regulate mitochondrial pyruvate metabolism. It is concluded that synaptosomes exhibit a high degree of respiratory control, that they rely on glucose oxidation for their energetics, and that stimulation of energy production can be achieved independently of changes in [Ca^2+]_c.