Regulation of the purified Ca2+ release channel/ryanodine receptor complex of skeletal muscle sarcoplasmic reticulum by luminal calcium

Ca-45(2+) flux and single channel measurements have revealed that the Ca2+ release channel/ryanodine receptor complex of striated muscle is regulated by micromolar cytoplasmic Ca2+. The effect of luminal Ca2+, however, remains controversial. In the experiments presented here, we reconstituted the isolated Ca2+ release channel of rabbit skeletal muscle sarcoplasmic reticulum into planar lipid bilayers in the presence of symmetrical K+ solutions. Using K+ as the charge carrier, we were able to examine the effect of changes in luminal calcium in the micro- to millimolar range. In the presence of activating cytoplasmic Ca2+, the release channel was activated and inactivated in a concentration-dependent manner by luminal Ca2+. Since increasing cytoplasmic EGTA concentrations shifted the dependence of channel open probability on luminal Ca2+ to higher Ca2+ concentrations, ill is suggested that luminal Ca2+ exerts its regulating effect by acting on Ca2+ binding sites accessible from the cytoplasmic side of the channel.