SUPEROXIDE ANION RADICAL-TRIGGERED CA2-RETICULUM THROUGH RYANODINE RECEPTOR CA2+ CHANNEL( RELEASE FROM CARDIAC SARCOPLASMIC)

The ryanodine receptor Ca2+ channel (RyRC) constitutes the Ca2+-releas e pathway in sarcoplasmic reticulum (SR) of cardiac muscle, A direct m echanical and a Ca2+-triggered mechanism (Ca2+-induced Ca2+ release) h ave been proposed to explain the in situ activation of Ca2+ release in cardiac muscle. A variety of chemical oxidants have been shown to act ivate RyRC; however, the role of modification induced by oxygen-derive d free radicals in pathological states of the muscle remains to be elu cidated. It has been hypothesized that oxygen-derived free radicals in itiate Ca2+-mediated functional changes in or damage to cardiac muscle by acting on the SR and promoting an increase in Ca2+ release. We con firmed that superoxide anion radical (O-2(radical anion)) generated fr om hypoxanthine-xanthine oxidase reaction decreases calmodulin content and increases Ca-45(2+) efflux from the heavy fraction of canine card iac SR vesicles; hypoxanthine-xanthine oxidase also decreases Ca2+ fre e within the intravesicular space of the SR with no effect on Ca2+-ATP ase activity. Current fluctuations through single Ca2+-release channel s have been monitored after incorporation into planar phospholipid bil ayers. We demonstrate that activation of the channel by O-2(radical an ion) is dependent of the presence of calmodulin and identified calmodu lin as a functional mediator of O-2(radical anion)-triggered Ca2+ rele ase through the RyRC. For the first time, we show that O-2(radical ani on); stimulates Ca2+ release from heavy SR vesicles and suggest the im portance of accessory proteins such as calmodulin in modulating the ef fect of O-2(radical anion). The decreased calmodulin content induced b y oxygen-derived free radicals, especially O-2(radical anion), is a li kely mechanism of accumulation of cytosolic Ca2+ (due to increased Ca2 + release from SR) after reperfusion of the ischemic heart.