Ryanodine

Ryanodine is a poisonous alkaloid found in the South American plant Ryania speciosa.

The compound has extremely high affinity to the ryanodine receptor, a group of calcium channels found in skeletal and heart muscle cells. It binds with such high affinity to the receptor that it was used as a label for the first purification of that class of ion channels and gave its name to it.

At nanomolar concentrations, ryanodine locks the receptor in a half-open state, whereas it fully closes them at micromolar concentration. The effect of the nanomolar-level binding is that ryanodine causes release of calcium from calcium stores in the sarcoplasmic reticulum leading to massive muscular contractions.