Ryanodine is a naturally occurring poisonous alkaloid that acts as a potent inhibitor of Ca²⁺ release from sarcoplasmic reticulum (IC₅₀s = 10 and 2.8 nM in skeletal and cardiac muscle, respectively). At micromolar (~100 μM) concentrations, ryanodine irreversibly inhibits ryanodine receptor channel opening, which results in paralysis.