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.
Doxorubicin is an anthracycline antibiotic that is used widely as a chemotherapeutic agent. However, the usefulness of this agent is limited due to its cardiotoxic effects. The mechanisms associated with this cardiotoxicity remain essentially unknown, despite numerous studies describing a range of structural and functional abnormalities. The purpose of the present study was to determine the in vivo and in vitro effects of doxorubicin exposure on sarcoplasmic reticulum (SR) Ca2+-content and contractile protein function. The Ca2+-content of SR is shown to have a biphasic response to in vivo and in vitro doxorubicin exposure that is time- and dose-dependent. In vitro doxorubicin exposure initially reduces the SR Ca2+-content, but the predominant action to block the SR Ca2+-release channel increases SR Ca2+-content within 60 min. Similar results are observed with in vivo doxorubicin exposure: it leads to Ca2+-overload. These data are consistent with the view that doxorubicin acts in a similar manner to ryanodine and results in cardiomyopathy due to Ca2+-overload.
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