Inactivation of Ca2+ transients in amphibian and mammalian muscle fibres

Understanding altered contractile properties in advanced age: insights from a systematic muscle modelling approach

Age-related alterations of skeletal muscle are numerous and present inconsistently, and the effect of their interaction on contractile performance can be nonintuitive. Hill-type muscle models predict muscle force according to well-characterised contractile phenomena. Coupled with simple, yet reasonably realistic activation dynamics, such models consist of parameters that are meaningfully linked to fundamental aspects of muscle excitation and contraction. We aimed to illustrate the utility of a muscle model for elucidating relevant mechanisms and predicting changes in output by simulating the individual and combined effects on isometric force of several known ageing-related adaptations. Simulating literature-informed reductions in free Ca2+ concentration and Ca2+ sensitivity generated predictions at odds qualitatively with the characteristic slowing of contraction speed. Conversely, incorporating slower Ca2+ removal or a fractional increase in type I fibre area emulated expected changes; the former was required to simulate slowing of the twitch measured experimentally. Slower Ca2+ removal more than compensated for force loss arising from a large reduction in Ca2+ sensitivity or moderate reduction in Ca2+ release, producing realistic age-related shifts in the force-frequency relationship. Consistent with empirical data, reductions in free Ca2+ concentration and Ca2+ sensitivity reduced maximum tetanic force only slightly, even when acting in concert, suggesting a modest contribution to lower specific force. Lower tendon stiffness and slower intrinsic shortening speed slowed and prolonged force development in a compliance-dependent manner without affecting force decay. This work demonstrates the advantages of muscle modelling for exploring sources of variation and identifying mechanisms underpinning the altered contractile properties of aged muscle.

Additional in-series compliance reduces muscle force summation and alters the time course of force relaxation during fixed-end contractions

There are high mechanical demands placed on skeletal muscles in movements requiring rapid acceleration of the body or its limbs. Tendons are responsible for transmitting muscle forces, but, because of their elasticity, can manipulate the mechanics of the internal contractile apparatus. Shortening of the contractile apparatus against the stretch of tendon affects force generation according to known mechanical properties; however, the extent to which differences in tendon compliance alter force development in response to a burst of electrical impulses is unclear. To establish the influence of series compliance on force summation, we studied electrically evoked doublet contractions in the cane toad peroneus muscle in the presence and absence of a compliant artificial tendon. Additional series compliance reduced tetanic force by two-thirds, a finding predicted based on the force-length property of skeletal muscle. Doublet force and force-time integral expressed relative to the twitch were also reduced by additional series compliance. Active shortening over a larger range of the ascending limb of the force-length curve and at a higher velocity, leading to a progressive reduction in force-generating potential, could be responsible. Muscle-tendon interaction may also explain the accelerated time course of force relaxation in the presence of additional compliance. Our findings suggest that a compliant tendon limits force summation under constant-length conditions. However, high series compliance can be mechanically advantageous when a muscle-tendon unit is actively stretched, permitting muscle fibres to generate force almost isometrically, as shown during stretch-shorten cycles in locomotor activities. Restricting active shortening would likely favour rapid force development.

Differential effects of contractile potentiators on action potential-induced Ca2+ transients of frog and mouse skeletal muscle fibres

Muscle fibres, isolated from frog tibialis anterior and mouse flexor digitorum brevis (FDB) were loaded with the fast dye MagFluo-4 to study the effects of potentiators caffeine, nitrate, Zn²⁺ and perchlorate on Ca²⁺ transients elicited by single action potentials. Overall, the potentiators doubled the transients amplitude and prolonged by about 1.5-fold their decay time. In contrast, as shown here for the first time, nitrate and Zn²⁺, but not caffeine, activated a late, secondary component of the transient rising phase of frog but not mouse, fibres. The rise time was increased from 1.9 ms in normal solution (NR) to 3.3 ms (nitrate) and 4.4 ms (Zn²⁺). In NR, a single exponential, fitted the rising phase of calcium transients of frog (τ₁ = 0.47 ms) and mouse (τ₁ = 0.28 ms). In nitrate and Zn²⁺ only frog transients showed a secondary exponential component, τ₂ = 0.72 ms (nitrate) and 0.94 ms, (Zn²⁺⁾. We suggest that nitrate and Zn²⁺ activate a late slower component of the ΔF/F signals of frog but not of mouse fibres, possibly promoting Ca²⁺ induced Ca²⁺ release at level of the RyR3, that in frog muscle fibres are localized in the para-junctional region of the triads and are absent in mouse FDB muscle fibres.

Juxtaposition of the changes in intracellular calcium and force during staircase potentiation at 30 and 37°C

Temperature-dependent changes in basal calcium and in the calcium transient contribute to force potentiation during repetitive stimulation.

Ca²⁺ entry during the action potential stimulates muscle contraction. During repetitive low frequency stimulation, skeletal muscle undergoes staircase potentiation (SP), a progressive increase in the peak twitch force induced by each successive stimulus. Multiple mechanisms, including myosin regulatory light chain phosphorylation, likely contribute to SP, a temperature-dependent process. Here, we used the Ca²⁺-sensitive fluorescence indicators acetoxymethyl (AM)-furaptra and AM-fura-2 to examine the intracellular Ca²⁺ transient (ICT) and the baseline Ca²⁺ level at the onset of each ICT during SP at 30 and 37°C in mouse lumbrical muscle. The stimulation protocol, 8 Hz for 8 s, resulted in a 27 ± 3% increase in twitch force at 37°C and a 7 ± 2% decrease in twitch force at 30°C (P < 0.05). Regardless of temperature, the peak rate of force production (+df/dt) was higher in all twitches relative to the first twitch (P < 0.05). Consistent with the differential effects of stimulation on twitch force at the two temperatures, raw ICT amplitude decreased during repetitive stimulation at 30°C (P < 0.05) but not at 37°C. Cytosolic Ca²⁺ accumulated during SP such that baseline Ca²⁺ at the onset of ICTs occurring late in the train was higher (P < 0.05) than that of those occurring early in the train. ICT duration increased progressively at both temperatures. This effect was not entirely proportional to the changes in twitch duration, as twitch duration characteristically decreased before increasing late in the protocol. This is the first study identifying a changing ICT as an important, and temperature-sensitive, modulator of muscle force during repetitive stimulation. Moreover, we extend previous observations by demonstrating that contraction-induced increases in baseline Ca²⁺ coincide with greater +df/dt but not necessarily with higher twitch force.

The excitation-contraction coupling mechanism in skeletal muscle

First coined by Alexander Sandow in 1952, the term excitation-contraction coupling (ECC) describes the rapid communication between electrical events occurring in the plasma membrane of skeletal muscle fibres and Ca2+ release from the SR, which leads to contraction. The sequence of events in twitch skeletal muscle involves: (1) initiation and propagation of an action potential along the plasma membrane, (2) spread of the potential throughout the transverse tubule system (T-tubule system), (3) dihydropyridine receptors (DHPR)-mediated detection of changes in membrane potential, (4) allosteric interaction between DHPR and sarcoplasmic reticulum (SR) ryanodine receptors (RyR), (5) release of Ca2+ from the SR and transient increase of Ca2+ concentration in the myoplasm, (6) activation of the myoplasmic Ca2+ buffering system and the contractile apparatus, followed by (7) Ca2+ disappearance from the myoplasm mediated mainly by its reuptake by the SR through the SR Ca2+ adenosine triphosphatase (SERCA), and under several conditions movement to the mitochondria and extrusion by the Na+/Ca2+ exchanger (NCX). In this text, we review the basics of ECC in skeletal muscle and the techniques used to study it. Moreover, we highlight some recent advances and point out gaps in knowledge on particular issues related to ECC such as (1) DHPR-RyR molecular interaction, (2) differences regarding fibre types, (3) its alteration during muscle fatigue, (4) the role of mitochondria and store-operated Ca2+ entry in the general ECC sequence, (5) contractile potentiators, and (6) Ca2+ sparks.

Quantifying SOCE fluorescence measurements in mammalian muscle fibres. The effects of ryanodine and osmotic shocks

We have quantified Ca(2+) entry through store operated calcium channels in mice muscle fibres, measuring the rates of change of myoplasmic [Ca(2+)], d[Ca(2+)](myo)/dt, and of Ca(2+) removal, d[Ca(2+)](Removal)/dt, turning store operated calcium entry (SOCE) ON, and OFF, by switching on or off external Ca(2+). In depleted fibres, poisoned with 10 μM cyclopiazonic acid SOCE influx was about 3 μM/s. Ryanodine (50 μM) caused a robust, nifedipine (50 μM) independent, increase in SOCE activation to 8.6 μM/s. Decreasing medium osmolarity from 300 to 220 mOsm/L, decreased SOCE to 0.9 μM/s, while increasing osmolarity from 220 to 400 mOsm/L potentiated SOCE to 43.6 μM/s. Ryanodine inhibited the effects of hypotonicity. Experiments using 2-aminoethoxydiphenyl borate, nifedipine, or Mn(2+) quenching, strongly suggest that the increased [Ca(2+)](myo) by ryanodine or hypertonic shock is mediated by potentiated SOCE activation. The Ca(2+) response decay, quantified by d[Ca(2+)](Removal)/dt, indicates a robust residual Ca(2+) removal mechanism in sarco-endoplasmic reticulum calcium ATPase poisoned fibres. SOCE high sensitivity to osmotic shocks, or to ryanodine receptor (RyR) binding, suggests its high dependency on the structural relationship between its molecular constituents, Orai1 and stromal interaction molecule and the sarcoplasmic reticulum and plasma membranes, in the triadic junctional region, where RyRs, are conspicuously present. This study demonstrates that SOCE machinery is highly sensitive to structural changes caused by binding of an agonist to its receptor or by imposed osmotical volume changes.

Quantifying Ca2+ release and inactivation of Ca2+ release in fast- and slow-twitch muscles

The aims of this study were to quantify the Ca(2+) release underlying twitch contractions of mammalian fast- and slow-twitch muscle and to comprehensively describe the transient inactivation of Ca(2+) release following a stimulus. Experiments were performed using bundles of fibres from mouse extensor digitorum longus (EDL) and soleus muscles. Ca(2+) release was quantified from the amount of ATP used to remove Ca(2+) from the myoplasm following stimulation. ATP turnover by crossbridges was blocked pharmacologically (N-benzyl-p-toluenesulphonamide for EDL, blebbistatin for soleus) and muscle heat production was used as an index of Ca(2+) pump ATP turnover. At 20°C, Ca(2+) release in response to a single stimulus was 34 and 84 μmol (kg muscle)(-1) for soleus and EDL, respectively, and increased with temperature (30°C: soleus, 61 μmol kg(-1); EDL, 168 μmol kg(-1)). Delivery of another stimulus within 100 ms of the first produced a smaller Ca(2+) release. The maximum magnitude of the decrease in Ca(2+) release was greater in EDL than soleus. Ca(2+) release recovered with an exponential time course which was faster in EDL (mean time constant at 20°C, 32.1 ms) than soleus (65.6 ms) and faster at 30°C than at 20°C. The amounts of Ca(2+) released and crossbridge cycles performed are consistent with a scheme in which Ca(2+) binding to troponin-C allowed an average of ∼1.7 crossbridge cycles in the two muscles.

Kinetic changes in tetanic Ca²⁺ transients in enzymatically dissociated muscle fibres under repetitive stimulation

We used enzymatically dissociated flexor digitorum brevis (FDB) and soleus fibres loaded with the fast Ca(2+) dye Magfluo-4 AM, and adhered to Laminin, to test whether repetitive stimulation induces progressive changes in the kinetics of Ca(2+) release and reuptake in a fibre-type-dependent fashion. We applied a protocol of tetani of 350 ms, 100 Hz, every 4 s to reach a mean amplitude reduction of 25% of the first peak. Morphology type I (MT-I) and morphology type II (MT-II) fibres underwent a total of 96 and 52.8 tetani (P < 0.01 between groups), respectively. The MT-II fibres (n = 18) showed significant reductions of the amplitude (19%), an increase in rise time (8.5%) and a further reduction of the amplitude/rise time ratio (25.5%) of the first peak of the tetanic transient after 40 tetani, while MT-I fibres (n = 5) did not show any of these changes. However, both fibre types showed significant reductions in the maximum rate of rise of the first peak after 40 tetani. Two subpopulations among the MT-II fibres could be distinguished according to Ca(2+) reuptake changes. Fast-fatigable MT-II fibres (fMT-II) showed an increase of 32.2% in the half-width value of the first peak, while for fatigue-resistant MT-II fibres (rMT-II), the increase amounted to 6.9%, both after 40 tetani. Significant and non-significant increases of 36.4% and 11.9% in the first time constant of decay (t(1)) values were seen after 40 tetani in fMT-II and rMT-II fibres, respectively. MT-I fibres did not show kinetic changes in any of the Ca(2+) reuptake variables. All changes were reversed after an average recovery of 7.5 and 15.4 min for MT-I and MT-II fibres, respectively. Further experiments ruled out the possibility that the differences in the kinetic changes of the first peak of the Ca(2+) transients between fibres MT-I and MT-II could be related to the inactivation of Ca(2+) release mechanism. In conclusion, we established a model of enzymatically dissociated fibres, loaded with Magfluo-4 and adhered to Laminin, to study muscle fatigue and demonstrated fibre-type-dependent, fatigue-induced kinetic changes in both Ca(2+) release and reuptake.

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.

Myosin heavy chain isoform composition and Ca(2+) transients in fibres from enzymatically dissociated murine soleus and extensor digitorum longus muscles

Electrically elicited Ca(2+) transients reported with the fast Ca(2+) dye MagFluo-4 AM and myosin heavy chain (MHC) electrophoretic patterns were obtained in intact, enzymatically dissociated fibres from adult mice extensor digitorum longus (EDL) and soleus muscles. Thirty nine fibres (23 from soleus and 16 from EDL) were analysed by both fluorescence microscopy and electrophoresis. These fibres were grouped as follows: group 1 included 13 type I and 4 type IC fibres; group 2 included 2 type IIC, 3 IIA and 1 I/IIA/IIX fibres; group 3 included 4 type IIX and 1 type IIX/IIB fibres; group 4 included 2 type IIB/IIX and 9 type IIB fibres. Ca(2+) transients obtained in groups 1, 2, 3 and 4 had the following kinetic parameters (mean +/- s.e.m.): amplitude (F/F): 0.61 +/- 0.05, 0.53 +/- 0.08, 0.61 +/- 0.06 and 0.61 +/- 0.03; rise time (ms): 1.64 +/- 0.05, 1.35 +/- 0.05, 1.18 +/- 0.06 and 1.14 +/- 0.04; half-amplitude width (ms): 19.12 +/- 1.85, 11.86 +/- 3.03, 4.62 +/- 0.31 and 4.23 +/- 0.37; and time constants of decay (tau(1) and tau(2), ms): 3.33 +/- 0.13 and 52.48 +/- 3.93, 2.69 +/- 0.22 and 41.06 +/- 9.13, 1.74 +/- 0.06 and 12.88 +/- 1.93, and 1.56 +/- 0.11 and 9.45 +/- 1.03, respectively. The statistical differences between the four groups and the analysis of the distribution of the parameters of Ca(2+) release and clearance show that there is a continuum from slow to fast, that parallels the MHC continuum from pure type I to pure IIB. However, type IIA fibres behave more like IIX and IIB fibres regarding Ca(2+) release but closer to type I fibres regarding Ca(2+) clearance. In conclusion, we show for the first time the diversity of Ca(2+) transients for the whole continuum of fibre types and correlate this functional diversity with the structural and biochemical diversity of the skeletal muscle fibres.

Low-affinity Ca2+ indicators compared in measurements of skeletal muscle Ca2+ transients

The low-affinity fluorescent Ca(2+) indicators OGB-5N, Fluo-5N, fura-5N, Rhod-5N, and Mag-fluo-4 were evaluated for their ability to accurately track the kinetics of the spatially averaged free Ca(2+) transient (Delta[Ca(2+)]) in skeletal muscle. Frog single fibers were injected with one of the above indicators and, usually, furaptra (previously shown to rapidly track Delta[Ca(2+)]). In response to an action potential, the full duration at half-maximum of the indicator's fluorescence change (DeltaF) was found to be larger with OGB-5N, Fluo-5N, fura-5N, and Rhod-5N than with furaptra; thus, these indicators do not track Delta[Ca(2+)] with kinetic fidelity. In contrast, the DeltaF time course of Mag-fluo-4 was identical to furaptra's; thus, Mag-fluo-4 also yields reliable kinetic information about Delta[Ca(2+)]. Mag-fluo-4's DeltaF has a larger signal/noise ratio than furaptra's (for similar indicator concentrations), and should thus be more useful for tracking Delta[Ca(2+)] in small cell volumes. However, because the resting fluorescence of Mag-fluo-4 probably arises largely from indicator that is bound with Mg(2+), the amplitude of the Mag-fluo-4 signal, and its calibration in Delta[Ca(2+)] units, is likely to be more sensitive to variations in [Mg(2+)] than furaptra's.

Amyloid-β protein impairs Ca2+ release and contractility in skeletal muscle

Inclusion body myositis (IBM), the most common muscle disorder in the elderly, is partly characterized by dysregulation of β-amyloid precursor protein (βAPP) expression and abnormal, intracellular accumulation of full-length βAPP and β-amyloid epitopes. The present study examined the effects of β-amyloid accumulation on force generation and Ca(2+) release in skeletal muscle from transgenic mice harboring human βAPP and assessed the consequence of Aβ(1-42) modulation of the ryanodine receptor Ca(2+) release channels (RyRs). β-Amyloid laden muscle produced less peak force and exhibited Ca(2+) transients with smaller amplitude. To determine whether modification of RyRs by β-amyloid underlie the effects observed in muscle, in vitro Ca(2+) release assays and RyR reconstituted in planar lipid bilayer experiments were conducted in the presence of Aβ(1-42). Application of Aβ(1-42) to RyRs in bilayers resulted in an increased channel open probability and changes in gating kinetics, while addition of Aβ(1-42) to the rabbit SR vesicles resulted in RyR-mediated Ca(2+) release. These data may relate altered βAPP metabolism in IBM to reductions in RyR-mediated Ca(2+) release and muscle contractility.

The use of CalciumOrange-5N as a specific marker of mitochondrial Ca2+ in mouse skeletal muscle fibers

We report the use of the fluorescent dye CalciumOrange-5N (CaOr-5N) as a specific mitochondria Ca(2+) marker in enzymatically dissociated mouse FBD muscle fibers. Using laser scanning confocal microscopy and the dyes Mitotracker Green (MTG), di-8-ANEPPS and endoplasmic reticulum tracker green (ERTG), we determined the relative position of mitochondria, transverse tubules and sarcoplasmic reticulum in the sarcomere. Comparison with electron micrographies showed that mitochondria are mostly present at both sides of Z lines and near the triads located at the A-I band border. CaOr-5N fluorescence was mainly distributed in mitochondria, highly co-localised with MTG and basically excluded from the A band space. ERTG localised mostly between the two t-tubules present in each sarcomere. We studied the effect of the protonophore FCCP using CaOr-5N to measure mitochondrial Ca(2+) and JC-1 dye to measure mitochondria inner membrane potential (DeltaPsi(m)). After FCCP treatment, the CaOr-5N fluorescence diminished by about 33% in 80 s, while JC-1 fluorescence diminished by 36% in 200 s. Our results show the loss of Ca(2+) from mitochondria when DeltaPsi(m) is depolarised and demonstrate the usefulness of CaOr-5N to mark mitochondrial [Ca(2+)](m).

Effect of mitochondria poisoning by FCCP on Ca2+ signaling in mouse skeletal muscle fibers

We have studied the effects of mitochondria poisoning by carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone (FCCP) on Ca(2+) signaling in enzymatically dissociated mouse flexor digitorum brevis (FDB) muscle fibers. We used Fura-2AM to measure resting [Ca(2+)](i) and MagFluo-4AM to measure Ca(2+) transients. Exposure to FCCP (2 microM, 2 min) caused a continuous increase in [Ca(2+)](i) at a rate of 0.60 nM/s and a drastic reduction of electrically elicited Ca(2+) transients without much effect on their decay phase. Half of the maximal effect occurred at [Ca(2+)](i) = 220 nM. This effect was partially reversible after long recuperation and was not diminished by Tiron, a reactive oxygen species (ROS) scavenger. FCCP had no effects on fiber excitability as shown by the generation of action potentials. 4CmC, an agonist of ryanodine receptors, induced a massive Ca(2+) release. FCCP diminished the rate but not the amount of Ca(2+) released, indicating that depletion of Ca(2+) stores did not cause the decrease in Ca(2+) transient amplitude. Ca(2+) transient amplitude could also be diminished, but to a lesser degree, by increases in [Ca(2+)](i) induced by repetitive stimulation of fibers treated with ciclopiazonic acid. This suggests an important role for Ca(2+) in the FCCP effect on transient amplitude.

Calcium transients in developing mouse skeletal muscle fibres

Ca(2)(+) transients elicited by action potentials were measured using MagFluo-4, at 20-22 degrees C, in intact muscle fibres enzymatically dissociated from mice of different ages (7, 10, 15 and 42 days). The rise time of the transient (time from 10 to 90% of the peak) was 2.4 and 1.1 ms in fibres of 7- and 42-day-old mice, respectively. The decay of the transient was described by a double exponential function, with time constants of 1.8 and 16.4 ms in adult, and of 4.6 and 105 ms in 7-day-old animals. The fractional recovery of the transient peak amplitude after 10 ms, F(2(10))/F(1), determined using twin pulses, was 0.53 for adult fibres and ranged between 0.03 and 0.60 in fibres of 7-day-old animals This large variance may indicate differences in the extent of inactivation of Ca(2)(+) release, possibly related to the difference in ryanodine receptor composition between young and old fibres. At the 7 and 10 day stages, fibres responded to Ca(2)(+)-free solutions with a larger decrease in the transient peak amplitude (25% versus 11% in adult fibres), possibly indicating a contribution of Ca(2)(+) influx to the Ca(2)(+) transient in younger animals. Cyclopiazonic acid (1 mum), an inhibitor of the sarcoplasmic reticulum (SR) Ca(2)(+)-ATPase, abolished the Ca(2)(+) transient decay in fibres of 7- and 10-day-old animals and significantly reduced its rate in older animals. Analysis of the transients with a Ca(2)(+) removal model showed that the results are consistent with a larger relative contribution of the SR Ca(2)(+) pump and a lower expression of myoplasmic Ca(2)(+) buffers in fibres of young versus old animals.