182 Circulation Research July 7 , 2017 Structural Considerations : The Dyad

Circulation Research is available at http://circres.ahajournals. org DOI: 10.1161/CIRCRESAHA.117. 310230 The process of excitation–contraction (E–C) coupling links the electric excitation of the surface membrane (action potential) to contraction. Since the initial measurements in cardiac muscle, an enormous amount of work has shown the underlying changes of cytoplasmic calcium concentration ([Ca] i ). Ca binds to troponin resulting in sliding of the thick and thin filaments, cell shortening, and thence the development of pressure within the ventricle and ejection of blood. Force, therefore, depends on the amount of Ca bound to troponin. This will be a function of both the magnitude and duration of the rise of [Ca] i . It will also depend on the strength of Ca binding, a factor that can be altered genetically, is controlled by factors such as phosphorylation and may form the basis of therapeutic interventions. Nevertheless, the major factor that regulates contraction is the level of intracellular Ca. As well as focusing on the increase of [Ca] i during systole, it is important to remember that proper cardiac function requires that force and [Ca] i relax quickly to low enough levels such that the heart can refill with blood. Therefore, both diastolic and systolic [Ca] i must be tightly regulated; this regulation is the subject of the current article. The events that occur in E–C coupling are now well established (Figure 1). The process depends not only on a combination of the properties of Ca channels and transporters but, equally importantly, also on their precise locations and spatial arrangement. The depolarization produced by the action potential opens L-type Ca channels situated in the surface membrane and transverse tubules. The resulting entry of a small amount of Ca results in a large increase of [Ca] i in the dyadic space (the region bounded by the t-tubule and sarcoplasmic reticulum [SR]). This increase of [Ca] i makes the SR Ca release channels (ryanodine receptors [RyR]) open thereby releasing a much larger amount of Ca from the SR in a process termed calcium-induced calcium release. The magnitude of the rise of [Ca] i depends not only on the structures mentioned above but also on Ca binding to buffers and uptake into organelles including mitochondria. For relaxation to occur, Ca must be removed from the cytoplasm. This requires that the RyRs close and then that Ca is pumped (1) back into the SR, by the SERCA (SR CaATPase) and (2) out of the cell, largely by the sodium–calcium exchange (NCX). The remainder of this review is in 3 sections. In the first, we discuss recent studies on the spatial organization of the structures responsible for calcium cycling. The second addresses the general principles that determine how the amplitude of the Ca transient is controlled, with particular reference to understanding the importance of Ca flux balance. In the final section, we consider specific steps in Ca signaling. Abstract: Cardiac contractility is regulated by changes in intracellular Ca concentration ([Ca]i). Normal function requires that [Ca]i be sufficiently high in systole and low in diastole. Much of the Ca needed for contraction comes from the sarcoplasmic reticulum and is released by the process of calcium-induced calcium release. The factors that regulate and fine-tune the initiation and termination of release are reviewed. The precise control of intracellular Ca cycling depends on the relationships between the various channels and pumps that are involved. We consider 2 aspects: (1) structural coupling: the transporters are organized within the dyad, linking the transverse tubule and sarcoplasmic reticulum and ensuring close proximity of Ca entry to sites of release. (2) Functional coupling: where the fluxes across all membranes must be balanced such that, in the steady state, Ca influx equals Ca efflux on every beat. The remainder of the review considers specific aspects of Ca signaling, including the role of Ca buffers, mitochondria, Ca leak, and regulation of diastolic [Ca]i. (Circ Res. 2017;121:181-195. DOI: 10.1161/CIRCRESAHA.117.310230.)