The cardiac sarcoplasmic reticulum calcium-release channel: modulation of ryanodine binding and single-channel activity

[3H]Ryanodine binding to a preparation of isolated cardiac sarcoplasmic reticulum has been investigated. A method is reported which produces a very high level of specific binding. Scatchard analysis of binding up to 50 nM ryanodine yields data which infer a single class of binding sites with a Kd of 1.4 nM and a Bmax of 9.7 pmol/mg protein. Micromolar calcium is the principal activating ligand and its effects on binding are modulated by ligands which similarly affect the activity of single calcium-release channels incorporated into artificial planar phospholipid bilayers. The benzimidazole drug, sulmazole, is able to stimulate ryanodine binding in the presence of sub-activating calcium concentrations. Ryanodine binds to the native channel only when it is in its open state and stimulation of maximal ryanodine binding is achieved by ligands which are insufficient to produce full single-channel activation. A model is proposed which relates the modulation of ryanodine binding to the behaviour of single channels.