Quercetin had a biphasic effect on Ca 2؉ uptake and calcium-stimulated ATP hydrolysis in isolated cardiac sarcoplasmic reticulum (SR). Stimulation of Ca
2؉ATPase was observed at low quercetin concentrations (<25 M) followed by inhibition at higher concentrations. The effects were dependent upon the SR protein concentration, the MgATP concentration, and intact phospholamban regulation of cardiac Ca
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