Ascorbate/Iron Activates Ca 2+-Release Channels of Skeletal Sarcoplasmic Reticulum Vesicles Reconstituted in Lipid Bilayers

Abstract

Ca 2+-release channel or ryanodine receptor is known to be involved in physiologic Ca 2+-release from sarcoplasmic reticulum in skeletal and cardiac muscle. A variety of chemical oxidants and in particular SH-oxidizing reagents have been shown to activate Ca 2+ release. However, the role of the oxidative modification of the channel in the physiologic mechanism(s) of Ca 2+ release and in pathologic states of the muscle remains to be elucidated. Ascorbate/iron redox couple is known to be an efficient generator of oxygen radicals and semidehydroascorbyl radicals. Ascorbate/iron was shown to be released from cardiomyocytes during ischemia-reperfusion and was suggested to be involved in the ischemia-reperfusion injury and cardiomyocyte death. To understand the potential contribution of ascorbate/iron to Ca 2+ release mechanism(s), calcium release channels from skeletal sarcoplasmic reticulum (SR) were reconstituted in artificial planar bilayers to examine the effects of this redox couple on the channel activity. Ascorbate elicited a transient (about 2 min) but dramatic increase of open-time probability of the channel. At pCa = 7.0, the presence of EGTA blocked ascorbate induced activation of release channels. However, when exogenous iron was added, ascorbate activated Ca 2+ release channels, even in the presence of EGTA. ESR measurements demonstrated that semidehydroascorbyl radicals were generated from ascorbate in the absence of EGTA. The semidehydroascorbyl radical ESR signal was quenched by EGTA in the absence (but not in the presence) of exogenous iron. Thus, the production of ascorbyl radicals was associated with increased channel activity. In the presence of heparin, ascorbate plus iron elicited a long-lasting activation of the channel which had conductance g Ca 2+ = 100 pS characteristic for the ryanodine receptor and which could be blocked by the ryanodine channel inhibitor, ruthenium red. In conclusion the physiologically relevant redox couple - ascorbate/iron - at physiologic concentrations can activate Ca 2+ channels in sarcoplasmic reticulum vesicles.