Simulation of Ca(2+) Movements within the Sarcomere of Fast-Twitch Mouse Fibers Stimulated by Action Potentials

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 Δf(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 μM/ms and a FDHM of ∼1.6 ms (16°C). Δf(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 Δf(CaD) during a five-shock train is a progressive decline in the rate of decay from the individual peaks of Δf(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.