Ryanodine, a diterpenoid poison derived from Ryania speciosa, acts as a modulator of the ryanodine receptor, which is permeable to cells. Depending on its concentration, ryanodine can either stimulate or inhibit Ca2+ release mediated by these receptors.

At concentrations above 250 nM, ryanodine induces a slowly developing, dose dependent contracture which could not be blocked by 5 mMCo2+[1].Ryanodine (100-5000 nM, 30-120 mintutes) irreversibly depresses twitch and tetanic tension of both fast and slow muscle in a dose-related manner[1].