Characterization of increased Ca2+ efflux by quercetin from the sarcoplasmic reticulum in frog skinned skeletal muscle fibres

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Abstract
To clarify further the characteristics of the Ca-releasing action of quercetin, we examined the effect of quercetin on Ca2+ efflux from the sarcoplasmic reticulum without Ca uptake activity by omission of ATP in the presence of a high concentration (10 mm EGTA) of Ca2+ buffer, using mechanically skinned fibres from frog skeletal muscles. Quercetin increases the rate of Ca release in the presence of Ca2+ and shifts the relationship between Ca2+ concentration and the rate of Ca release to a lower range of Ca2+ concentrations. AMP potentiates the effect of quercetin. Mg2+ and procaine decrease the rate of Ca release in the presence of quercetin. These findings indicate that quercetin is very similar to caffeine; in fact, experiments confirmed that quercetin shares the site(s) of action with caffeine. Subsequent findings, however, suggest that the mechanism of the drug-induced Ca release is not as simple as expected with caffeine which may increase the affinity for Ca2+ of the ‘Ca2+-induced Ca-release’ mechanism, and that drugs, Ca2+ and nucleotide interact with one another in a more complex way to open the gate for Ca release. In the presence of 10 mm EGTA quercetin also causes Ca release in the virtual absence of Ca2+ if a sufficient amount of AMP is present, and the rate of Ca release in the presence of 1.55 μm Ca2+ showed different dependence on AMP concentration with and without quercetin. A higher concentration of Mg2+ is required in the presence of AMP than in the absence of AMP in order to eliminate Ca release by quercetin.