Ca²⁺ Release in Muscle Cells

Abstract

It is well established that Ca²⁺ release from intracellular stores is required for muscle contraction. This process involves Ca²⁺ release channels such as the ryanodine receptor (RYR) and the inositol 1,4,5-trisphosphate receptor (InsP₃R). Confocal imaging has recently made possible the visualization of localized Ca²⁺ release through RYRs and InsP₃Rs leading to the existence of a hierarchy of Ca²⁺ signals from the smallest Ca²⁺ response corresponding to the opening of a single channel to the propagating Ca²⁺ wave. Both RYRs and InsP₃Rs show some structural and functional similarities (both are tetramers regulated by Ca²⁺) but they also display discrepancies and distinct tissue distribution. Although both channel families are activated by Ca²⁺ in a mechanism known as “Ca²⁺-induced Ca²⁺ release” (CICR), the different classes and isoforms display distinct Ca²⁺-dependencies. Some structure/function relationships underlying these differential regulation by Ca²⁺ ions have been reported elsewhere (Ehrlich, 1995) and in this book (see the chapter by Parys et al., this book). Here, it is our intention to review the different isoforms of RYRs and InsP₃Rs and their role in inducing localized Ca²⁺ responses and propagated Ca²⁺ waves in muscle cells. In addition, we will report how the local control of excitation-contraction coupling may be used to propose new subcellular mechanisms to explain modifications of the contractility in physiopathological conditions.