Ryanodine binding to sarcoplasmic reticulum membrane; comparison between cardiac and skeletal muscle

[³H]Ryanodine binding to skeletal muscle and cardic sarcoplasmic reticulum (SR) vesicles was compared under experimental conditions known to inhibit or stimulate Ca²⁺ release. In the skeletal muscle SR, ryanodine binds to a single class of high-affinity sites (K_d of 11.3 nM). In cardiac SR vesicles, more than one class of binding sites is observed (_d values of 3.6 and 28.1 nM). Ryanodine binding to skeletal muscle SR vesicles requires high concentrations of NaCl, whereas binding of the drug to cardiac SR is only slightly influenced by ionic strength. In the presence of 5′-adenylyl imidodiphosphate (p[NH]ppA), increased pH, and micromolar concentration of Ca²⁺ (which all induce Ca²⁺ release from SR) binding of ryanodine to SR is significantly increased in skeletal muscle, while being unchanged in cardiac muscle. Ryanodine binding to skeletal but not to cardiac muscle SR is inhibited in the presence of high Ca²⁺ or Mg²⁺ concentrations (all known to inhibit Ca²⁺ release from skeletal muscle SR). Ruthenium red or dicyclohexylcarbodiimide modification of cardiac and skeletal muscle SR inhibit Ca²⁺ release and ryanodine binding in both skeletal and cardiac membranes. These results indicate that significant differences exist in the properties of ryanodine binding to skeletal or cardiac muscle SR. Our data suggest that ryanodine binds preferably to site(s) which are accessible only when the Ca²⁺ release channel is in the open state.