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. Ca(2+) then diffuses to the thin filaments, where Ca(2+) binds to the Ca(2+) regulatory sites on troponin to activate muscle contraction. Quantitative studies of these events in intact muscle preparations have relied heavily on Ca(2+)-indicator dyes to measure the change in the spatially-averaged myoplasmic free Ca(2+) concentration (Δ[Ca(2+)]) that results from the release of SR Ca(2+). In normal fibres stimulated by an action potential, Δ[Ca(2+)] is large and brief, requiring that an accurate measurement of Δ[Ca(2+)] be made with a low-affinity rapidly-responding indicator. Some low-affinity Ca(2+) indicators monitor Δ[Ca(2+)] 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[Ca(2+)] in mouse fast-twitch fibres. The simulations use a multi-compartment model of the sarcomere that takes into account Ca(2+)'s release from the SR, its diffusion and binding within the myoplasm, and its re-sequestration by the SR Ca(2+) pump. The simulations are quantitatively consistent with the measurements and appear to provide a satisfactory picture of the underlying Ca(2+) movements.
Baylor
Stephen M
SM
Department of Physiology, University of Pennsylvania School of Medicine, Philadelphia, PA 19104-6085, USA. baylor@mail.med.upenn.edu
Hollingworth
Stephen
S
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D004396:Coloring Agents / Q000378:metabolism*; D056966:Excitation Contraction Coupling; D008954:Models, Biological; D018485:Muscle Fibers, Skeletal / Q000166:cytology* / Q000378:metabolism* / Q000502:physiology; D012518:Sarcomeres / Q000378:metabolism
D004396:Coloring Agents; D002118:Calcium