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Quercetin Inhibits Ca2+ Uptake but not Ca2+ Release by Sarcoplasmic Reticulum in Skinned Muscle Fibers
Varda Shoshan, Kevin P. Campbell, David H. MacLennan, Wanda Frodis and Beverley A. Britt
Proceedings of the National Academy of Sciences of the United States of America
Vol. 77, No. 8, [Part 2: Biological Sciences] (Aug., 1980), pp. 4435-4438
Published by: National Academy of Sciences
Stable URL: http://www.jstor.org/stable/9118
Page Count: 4
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Quercetin inhibited Ca2+-dependent ATP hydrolysis, ATP-dependent Ca2+ uptake, chelator-induced [ethylene glycol bis(β -aminoethyl ether)-N,N,N′,N′-tetraacetic acid] Ca2+ release, and ATP synthesis coupled to Ca2+ release in isolated vesicles of sarcoplasmic reticulum. Use of this inhibitor permitted evaluation of whether Ca2+ release from sarcoplasmic reticulum in situ occurs through a reversal of the uptake pathway. Release of Ca2+ from the sarcoplasmic reticulum of skinned muscle fibers can be detected by the measurement of tension in the fiber. If the sarcoplasmic reticulum of these preparations is first allowed to accumulate Ca2+, tension development may be induced by the addition of Ca2+ itself or of caffeine to the bathing medium or by depolarization with Ca-. The presence of quercetin during the loading phase inhibited Ca2+ uptake by sarcoplasmic reticulum in situ. When quercetin was added together with initiators of tension development, however, the rate of tension development was enhanced 4- to 7-fold and the relaxation rate of the fibers was greatly inhibited. These results suggest that quercetin had no effect on Ca2+ release in skinned fiber; its effect on Ca2+ reuptake could account for the apparent enhancement of the release rate and for the prolonged relaxation time. These observations rule out reversal of the Ca2+ pump as the mechanism of Ca2+ release in situ.
Proceedings of the National Academy of Sciences of the United States of America © 1980 National Academy of Sciences