Calcium indicators and calcium signalling in skeletal muscle fibres during excitation–contraction coupling

During excitation–contraction coupling in skeletal muscle, calcium ions are released into the myoplasm by the sarcoplasmic reticulum (SR) in response to depolarization of the fibre’s exterior membranes. Ca2+ then diffuses to the thin filaments, where Ca2+ binds to the Ca2+ regulatory sites on troponin to activate muscle contraction. Quantitative studies of these events in intact muscle preparations have relied heavily on Ca2+-indicator dyes to measure the change in the spatially-averaged myoplasmic free Ca2+ concentration (Δ[Ca2+]) that results from the release of SR Ca2+. In normal fibres stimulated by an action potential, Δ[Ca2+] is large and brief, requiring that an accurate measurement of Δ[Ca2+] be made with a low-affinity rapidly-responding indicator. Some low-affinity Ca2+ indicators monitor Δ[Ca2+] much more accurately than others, however, as reviewed here in measurements in frog twitch fibres with sixteen low-affinity indicators. This article also examines measurements and simulations of Δ[Ca2+] in mouse fast-twitch fibres. The simulations use a multi-compartment model of the sarcomere that takes into account Ca2+’s release from the SR, its diffusion and binding within the myoplasm, and its re-sequestration by the SR Ca2+ pump. The simulations are quantitatively consistent with the measurements and appear to provide a satisfactory picture of the underlying Ca2+ movements.