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The influence of quercetin on glycogen catabolism and related parameters was investigated in the isolated perfused rat liver and subcellular systems.Quercetin stimulated glycogenolysis (glucose release). This effect was already evident at a concentration of 50 μM maximal at 300 μM and declined at higher concentrations. Quercetin also stimulated oxygen consumption, with a similar concentration dependence.Lactate production from endogenous glycogen (glycolysis) was diminished by quercetin without significant changes in pyruvate production.Quercetin did not inhibit glucose transport into cells but decreased intracellular sequestration of [5-3H]glucose under conditions of net glucose release.In isolated mitochondria, quercetin diminished the energy transduction efficiency. It also inhibited several enzymatic activities, e.g. the K+-ATPase/Na+-ATPase of plasma membrane vesicles and the glucose 6-phosphatase of isolated microsomes.No significant changes of the cellular contents of AMP, ADP and ATP were found. The cellular content of glucose 6-phosphate, however, was increased (3.12-fold).Some of the effects of quercetin (glycogenolysis stimulation) can be attributed to its action on mitochondrial energy metabolism, as, for example, uncoupling of oxidative phosphorylation. However, the multiplicity of the effects on several enzymatic systems certainly produces an intricate interplay that also generates complex and apparently contradictory effects.