Ryanodine receptor Ca2+ release channels: Does diversity in form equal diversity in function?

Complexities in calcium signaling in eukaryotic cells require diversity in the proteins involved in generating these signals. Ln this review, we consider the ryanodine receptor (RyR) family of intracellular calcium release channels. This includes species, tissue, and cellular distributions of the RyRs and mechanisms of activation, deactivation, and inactivation of RyR calcium release events. In addition, as first observed in nonmammalian vertebrate skeletal muscles, it is now clear that more than one RyR isoform is frequently coexpressed within many cell types. How multiple ryanodine receptor release channels are used to generate intracellular calcium transients is unknown. Therefore, a primary focus of this review is why more than one RyR is required for this purpose, particularly in a tissue, such as vertebrate fast-twitch skeletal muscles, where a relatively simple and straightforward change in calcium would appear to be required to elicit contraction. Finally, the roles of the RyR isoforms and the calcium release events they mediate in the development of embryonic skeletal muscle are considered.