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

Abstract

⁴⁵Ca²⁺ 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, it is suggested that luminal Ca²⁺ exerts its regulating effect by acting on Ca²⁺ binding sites accessible from the cytoplasmic side of the channel.