Tetanic Ca2+ transient differences between slow- and fast-twitch mouse skeletal muscle fibres: a comprehensive experimental approach

Abstract

One hundred and eighty six enzymatically dissociated murine muscle fibres were loaded with Mag-Fluo-4 AM, and adhered to laminin, to evaluate the effect of modulating cytosolic Ca(2+) buffers and sarcoendoplasmic reticulum Ca(2+) ATPase (SERCA), mitochondria, and Na(+)/Ca(2+) exchanger (NCX) on the differential tetanic Ca(2+) transient kinetics found in different fibre types. Tetanic Ca(2+) transients were classified as morphology type I (MT-I) or type II (MT-II) according to their shape. The first peak of the MT-I (n = 25) and MT-II (n = 23) tetanic Ca(2+) transients had an amplitude (∆F/F) of 0.41 ± 0.03 and 0.83 ± 0.05 and a rise time (ms) of 1.35 and 0.98, respectively. MT-I signals had a time constant of decay (τ1, ms) of 75.9 ± 4.2 while MT-II transients showed a double exponential decay with time constants of decay (τ1 and τ2, ms) of 18.3 ± 1.4 and 742.2 ± 130.3. Sarcoendoplasmic reticulum Ca(2+) ATPase inhibition demonstrated that the decay phase of the tetanic transients mostly rely on SERCA function. Adding Ca(2+) chelators in the AM form to MT-I fibres changed the morphology of the initial five peaks to a MT-II one, modifying the decay phase of the signal in a dose-dependent manner. Mitochondria and NCX function have a minor role in explaining differences in tetanic Ca(2+) transients among fibre types but still help in removing Ca(2+) from the cytosol in both MT-I and MT-II fibres. Cytoplasmic Ca(2+) buffering capacity and SERCA function explain most of the different kinetics found in tetanic Ca(2+) transients from different fibre types, but mitochondria and NCX have a measurable role in shaping tetanic Ca(2+) responses in both slow and fast-twitch muscle fibre types. We provided experimental evidence on the mechanisms that help understand the kinetics of tetanic Ca(2+) transients themselves and explain kinetic differences found among fibre types.

Myosin-based regulation of twitch and tetanic contractions in mammalian skeletal muscle

“…Another, perhaps more likely explanation, would be that return of the thick filament to the helical array after a tetanus may be slow because the thin filament is not fully off at this time. Several previous studies have reported a slow tail in intracellular free calcium transients following a short tetanus, in both mammalian (Baylor and Hollingworth, 2003;Calderón et al, 2014;Hollingworth et al, 1996) and amphibian fast-twitch muscle fibers (Caputo et al, 1994;Konishi, 1998). Fast-twitch fibers from both amphibian and mammalian muscle contain millimolar concentrations of the Ca 2+ /Mg 2+ exchange buffer parvalbumin, which binds Mg 2+ in resting muscle.…”

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

“…The ECC is becoming more complex [1] and experimentally addressing some questions is challenging. For instance, important papers have investigated Ca 2+ kinetics in MITO and SR, as well as SOCE function in fast fibers [33,47,48], but differences among all fiber types are infrequently [12] or never studied. The mathematical model presented here helps address these limitations.…”

“…Furthermore, a major limitation of several models is their dichotomic approach (slow and fast), yet there are experimentally measured Ca 2+ transients of at least four fiber types: I, IIA, IIX, and IIB [6]. Some models have not included important mechanisms dealing with Ca 2+ , such as the mitochondria or the NCX, despite their importance in shaping the Ca 2+ transients in different fiber types [12,13].…”

Doublet stimulation increases Ca2+ binding to troponin C to ensure rapid force development in skeletal muscle

“…To ensure only fast fibers were used in this study, only fibers exhibiting the Ca 2+ transient shape characteristic for type IIb/IIx fibers were selected for analysis (Calderón et al, 2009(Calderón et al, , 2014.…”