Comprehensive Simulation of Ca2+ Transients in the Continuum of Mouse Skeletal Muscle Fiber Types

Abstract

Mag-Fluo-4 has revealed differences in the kinetics of the Ca2+ transients of mammalian fiber types (I, IIA, IIX, and IIB). We simulated the changes in [Ca2+] through the sarcomere of these four fiber types, considering classical (troponin –Tn–, parvalbumin –Pv–, adenosine triphosphate –ATP–, sarcoplasmic reticulum Ca2+ pump –SERCA–, and dye) and new (mitochondria –MITO–, Na+/Ca2+ exchanger –NCX–, and store-operated calcium entry –SOCE–) Ca2+ binding sites, during single and tetanic stimulation. We found that during a single twitch, the sarcoplasmic peak [Ca2+] for fibers type IIB and IIX was around 16 µM, and for fibers type I and IIA reached 10–13 µM. The release rate in fibers type I, IIA, IIX, and IIB was 64.8, 153.6, 238.8, and 244.5 µM ms−1, respectively. Both the pattern of change and the peak concentrations of the Ca2+-bound species in the sarcoplasm (Tn, PV, ATP, and dye), the sarcolemma (NCX, SOCE), and the SR (SERCA) showed the order IIB ≥ IIX > IIA > I. The capacity of the NCX was 2.5, 1.3, 0.9, and 0.8% of the capacity of SERCA, for fibers type I, IIA, IIX, and IIB, respectively. MITO peak [Ca2+] ranged from 0.93 to 0.23 µM, in fibers type I and IIB, respectively, while intermediate values were obtained in fibers IIA and IIX. The latter numbers doubled during tetanic stimulation. In conclusion, we presented a comprehensive mathematical model of the excitation–contraction coupling that integrated most classical and novel Ca2+ handling mechanisms, overcoming the limitations of the fast- vs. slow-fibers dichotomy and the use of slow dyes.

A controversial issue: Can mitochondria modulate cytosolic calcium and contraction of skeletal muscle fibers?

“…Although very high, such value is compatible with the determinations in liver, brain, and heart (see Chalmers and Nicholls, 2003 ; Bazil et al, 2013 ) and requires a high buffering power. At the best of our knowledge, the few attempts to simulate the amount of calcium accumulated in the mitochondria during contractile activity of fast skeletal muscle fibers have assumed a much lower buffering power, ∼200:1 ( Marcucci et al, 2018 ; Rincón et al, 2021 ), similar to the value proposed by Wust et al (2017) for cardiomyocytes.…”

“…1 . The sarcolemma depolarization is followed by a release via RyR estimated at 340–350 μmoles/L fiber ( Baylor and Hollingworth, 2007 ; Baylor and Hollingworth, 2012 ; Holash and MacIntosh, 2019 ; Rincón et al, 2021 ) for a twitch triggered by a single pulse, >550 μmoles/L fiber for a train of five pulses at 67 Hz ( Baylor and Hollingworth, 2007 ; Baylor and Hollingworth, 2012 ; Rincón et al, 2021 ) and 1,350 μmoles/L fiber with a 400-ms voltage clamp at +30 mV ( Manno et al, 2013 ). The amount bound by troponin C fast isoform with two calcium-binding sites can reach 240 μmoles/L fiber , while parvalbumin (PVA) with a concentration of calcium binding site of 1,260 μmoles/L fiber can bind more than 300 μmoles/L fiber ( Baylor and Hollingworth, 2007 ) and other minor cytosolic buffers as ATP can bind between 5 and 30 μmoles/L fiber ( Baylor and Hollingworth, 1998 ).…”

“…This represents a precondition to contribute to the modulation of cytosolic Ca 2+ transients. The available model estimations adopt the following parameters: Marcucci et al, 2018 : k on = 0.8 μM −1 s −1 , k off 0.19 s −1 , k d = 0.24 μM; Rincón et al, 2021 : k on = 0.6 μM −1 s −1 , k off 0.156 s −1 , k d = 0.26 μM. For comparison, the k d calculated in Bazil et al (2013) for cardiac mitochondria are 2 and 1.7 μM, respectively, for the two classes of buffers, while the kinetic parameters are assumed to be fast enough to rapidly reach the equilibrium, at least in the time scale of their experimental data, i.e., in the order of several minutes.…”

mTORC1-dependent SOCE activity regulates synaptic gene expression and muscle response to denervation

“…Data from the literature suggest that SOCE is involved in the maintenance of SR Ca 2+ stores in muscle fibers, thereby limiting muscle fatigue. However, the amount of Ca 2+ mobilized by SOCE has been shown to be relatively small as compared to intracellular stores and to the amount of Ca 2+ released upon excitation/contraction, interrogating on the role of these fluxes in muscle physiology 24–29, 57 . Here, we provide a new signaling role for SOCE in myofibers.…”