Abstract
The ryanodine receptor Ca2+ channel (RyRC) constitutes the Ca2+-release pathway in sarcoplasmic reticulum (SR) of cardiac muscle. A direct mechanical and a Ca2+-triggered mechanism (Ca2+-induced Ca2+ release) have been proposed to explain the in situ activation of Ca2+ release in cardiac muscle. A variety of chemical oxidants have been shown to activate RyRC; however, the role of modification induced by oxygen-derived free radicals in pathological states of the muscle remains to be elucidated. It has been hypothesized that oxygen-derived free radicals initiate Ca2+-mediated functional changes in or damage to cardiac muscle by acting on the SR and promoting an increase in Ca2+ release. We confirmed that superoxide anion radical (O2⨪) generated from hypoxanthine-xanthine oxidase reaction decreases calmodulin content and increases 45Ca2+ efflux from the heavy fraction of canine cardiac SR vesicles; hypoxanthine-xanthine oxidase also decreases Ca2+ free within the intravesicular space of the SR with no effect on Ca2+-ATPase activity. Current fluctuations through single Ca2+-release channels have been monitored after incorporation into planar phospholipid bilayers. We demonstrate that activation of the channel by O2⨪ is dependent of the presence of calmodulin and identified calmodulin as a functional mediator of O2⨪-triggered Ca2+ release through the RyRC. For the first time, we show that O2⨪ stimulates Ca2+ release from heavy SR vesicles and suggest the importance of accessory proteins such as calmodulin in modulating the effect of O2⨪. The decreased calmodulin content induced by oxygen-derived free radicals, especially O2⨪, is a likely mechanism of accumulation of cytosolic Ca2+ (due to increased Ca2+ release from SR) after reperfusion of the ischemic heart.
Footnotes
- Received July 14, 1997.
- Accepted October 31, 1997.
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Send reprint requests to: Dr. Eiichiro Okabe, D.D.S., Ph.D., Professor & Chairman, Department of Pharmacology and ESR Laboratory, Kanagawa Dental College, 82 Inaoka-Cho, Yokosuka, Kanagawa 238, Japan. E-mail: okabe{at}kdcnet.ac.jp
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This work was supported by Grants 09877356 (E.O.) and 07557119 (E.O.) from Scientific Research Fund of The Ministry of Education, Science, Sports and Culture of Japan and by a grant from the Research Fund of JEOL Ltd., Tokyo, Japan.
- The American Society for Pharmacology and Experimental Therapeutics
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