Dantrolene Requires Mg²⁺ and ATP To Inhibit the Ryanodine Receptor

Dantrolene is a ryanodine receptor (RyR) inhibitor, which is used to relax muscles in malignant hyperthermia syndrome. Although dantrolene binds to the RyR protein, its mechanism of action is unknown, mainly because of the controversial results showing that dantrolene inhibited Ca²⁺ release from intact fibers and sarcoplasmic reticulum (SR) vesicles, but failed to inhibit single RyR channel currents in bilayers. Accordingly, it was concluded that an important factor for dantrolene's action was lost during the purification procedure of RyR. Recently, Mg²⁺ was demonstrated to be the essential factor for dantrolene to inhibit Ca²⁺ release in skinned muscle fibers. The aim of the present study was to confirm these results in Ca²⁺ release and bilayer experiments, using SR vesicles and solubilized channels, respectively. Our Ca²⁺ release experiments demonstrated that the effect of dantrolene and Mg²⁺ was cooperative and that ATP enhanced the inhibiting effect of dantrolene. Namely, 10 µM dantrolene reduced RyR channel open probability by ∼50% in the presence of 3 mM free Mg²⁺ and 1 mM ATP, whereas channel activity further decreased to ∼20% of control when [ATP] was increased to 2 mM. Our data provide important complementary information that supports the direct, Mg²⁺-dependent mechanism of dantrolene's action and suggests that dantrolene also requires ATP to inhibit RyR.