Essential Role of Calmodulin in RyR Inhibition by Dantrolene s

Dantrolene is the first line therapy of malignant hyperther-mia. Animal studies suggest that dantrolene also protects against heart failure and arrhythmias caused by spontaneous Ca21 release. Although dantrolene inhibits Ca21 release from the sarcoplasmic reticulum of skeletal and cardiac muscle prepa-rations, its mechanism of action has remained controversial, because dantrolene does not inhibit single ryanodine receptor (RyR) Ca21 release channels in lipid bilayers. Here we test the hypothesis that calmodulin (CaM), a physiologic RyR binding partner that is lost during incorporation into lipid bilayers, is required for dantrolene inhibition of RyR channels. In single channel recordings (100 nM cytoplasmic [Ca21] 1 2 mM ATP), dantrolene caused inhibition of RyR1 (rabbit skeletal muscle) and RyR2 (sheep) with a maximal inhibition of Po (Emax) to 52 6 4 % of control only after adding physiologic [CaM] 5 100 nM. Dantrolene inhibited RyR2 with an IC50 of 0.16 6 0.03 mM. Mutant N98S-CaM facilitated dantrolene inhibition with an IC50 5 5.9 6 0.3 nM. In mouse cardiomyo-cytes, dantrolene had no effect on cardiac Ca21 release in the absence of CaM, but reduced Ca21 wave frequency (IC50 5 0.426 0.18 mM, Emax5 476 4%) and amplitude (IC505 0.196 0.04 mM, Emax5 666 4%) in the presence of 100 nM CaM. We conclude that CaM is essential for dantrolene inhibition of RyR1 and RyR2. Its absence explains why dantrolene inhibition of single RyR channels has not been previously observed