Ryanodine receptors: how many, where and why?

Ryanodine receptors are intracellular Ca2+ channels that have been known for more than a decade to have a role in releasing Ca2+ from the sarcoplasmic reticulum to regulate contraction in skeletal and cardiac muscle fibres. Vincenzo Sorrentino and Pompeo Volpe review some recent developments: the ryanodine receptor channels have now been found to be expressed in the central nervous system, and the cloning of a third ryanodine receptor gene (RYR3) has revealed that this new isoform is widely expressed in several tissues and cells. In consequence, the view of ryanodine receptors as Ca2+ channels of muscle cells is rapidly changing, and these channels seem set to take a more central position on the stage of intracellular Ca2+ signalling.