title: Simulation of Ca2+ movements within the sarcomere of fast-twitch mouse fibers stimulated by action potentials.
abstract: Ca(2+) release from the sarcoplasmic reticulum (SR) of skeletal muscle takes place at the triadic junctions; following release, Ca(2+) spreads within the sarcomere by diffusion. Here, we report multicompartment simulations of changes in sarcomeric Ca(2+) evoked by action potentials (APs) in fast-twitch fibers of adult mice. The simulations include Ca(2+) complexation reactions with ATP, troponin, parvalbumin, and the SR Ca(2+) pump, as well as Ca(2+) transport by the pump. Results are compared with spatially averaged Ca(2+) transients measured in mouse fibers with furaptra, a low-affinity, rapidly responding Ca(2+) indicator. The furaptra Deltaf(CaD) signal (change in the fraction of the indicator in the Ca(2+)-bound form) evoked by one AP is well simulated under the assumption that SR Ca(2+) release has a peak of 200-225 microM/ms and a FDHM of approximately 1.6 ms (16 degrees C). Deltaf(CaD) elicited by a five-shock, 67-Hz train of APs is well simulated under the assumption that in response to APs 2-5, Ca(2+) release decreases progressively from 0.25 to 0.15 times that elicited by the first AP, a reduction likely due to Ca(2+) inactivation of Ca(2+) release. Recovery from inactivation was studied with a two-AP protocol; the amplitude of the second release recovered to >0.9 times that of the first with a rate constant of 7 s(-1). An obvious feature of Deltaf(CaD) during a five-shock train is a progressive decline in the rate of decay from the individual peaks of Deltaf(CaD). According to the simulations, this decline is due to a reduction in available Ca(2+) binding sites on troponin and parvalbumin. The effects of sarcomere length, the location of the triadic junctions, resting [Ca(2+)], the parvalbumin concentration, and possible uptake of Ca(2+) by mitochondria were also investigated. Overall, the simulations indicate that this reaction-diffusion model, which was originally developed for Ca(2+) sparks in frog fibers, works well when adapted to mouse fast-twitch fibers stimulated by APs.
authors.affiliation: Department of Physiology, University of Pennsylvania School of Medicine, Philadelphia, PA 19104, USA.
authors.forename: Stephen M
authors.initials: SM
authors.lastname: Baylor
authors.forename: Stephen
authors.initials: S
authors.lastname: Hollingworth
mesh_terms: D000200:Action Potentials; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001665:Binding Sites; D020013:Calcium Signaling; D003198:Computer Simulation; D004058:Diffusion; D004558:Electric Stimulation; D005456:Fluorescent Dyes; D016257:Fura-2 / Q000031:analogs & derivatives; D066298:In Vitro Techniques; D007700:Kinetics; D051379:Mice; D008931:Mitochondria, Muscle / Q000378:metabolism; D008954:Models, Biological; D018656:Muscle Fibers, Fast-Twitch / Q000201:enzymology / Q000378:metabolism*; D010320:Parvalbumins / Q000378:metabolism; D011485:Protein Binding; D015203:Reproducibility of Results; D012518:Sarcomeres / Q000378:metabolism*; D012519:Sarcoplasmic Reticulum / Q000201:enzymology / Q000378:metabolism*; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases / Q000378:metabolism; D014336:Troponin / Q000378:metabolism
chemical_list: D005456:Fluorescent Dyes; D010320:Parvalbumins; D014336:Troponin; C058776:2-(2-(5-carboxy)oxazole)-5-hydroxy-6-aminobenzofuran-N,N,O-triacetic acid; D000255:Adenosine Triphosphate; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D016257:Fura-2