Comparison of myoplasmic calcium movements during excitation-contraction coupling in frog twitch and mouse fast-twitch muscle fibers

Voltage sensor current, SR Ca2+ release, and Ca2+ channel current during trains of action potential-like depolarizations of skeletal muscle fibers

In skeletal muscle, CaV 1.1 serves as the voltage sensor for both excitation-contraction coupling (ECC) and L-type Ca2+ channel activation. We have recently adapted the technique of action potential (AP) voltage clamp (APVC) to monitor the current generated by the movement of intramembrane voltage sensors (IQ ) during single imposed transverse tubular AP-like depolarization waveforms (IQAP ). We now extend this procedure to monitoring IQAP , and Ca2+ currents during trains of tubular AP-like waveforms in adult murine skeletal muscle fibers, and compare them with the trajectories of APs and AP-induced Ca2+ release measured in other fibers using field stimulation and optical probes. The AP waveform remains relatively constant during brief trains (<1 sec) for propagating APs in non-V clamped fibers. Trains of 10 AP-like depolarizations at 10 Hz (900 ms), 50 Hz (180 ms), or 100 Hz (90 ms) did not alter IQAP amplitude or kinetics, consistent with previous findings in isolated muscle fibers where negligible charge immobilization occurred during 100 ms step depolarizations. Using field stimulation, Ca2+ release did exhibit a considerable decline from pulse to pulse during the train, also consistent with previous findings, indicating that the decline of Ca2+ release during a short train of APs is not correlated to modification of charge movement. Ca2+ currents during single or 10 Hz trains of AP-like depolarizations were hardly detectable, were minimal during 50 Hz trains, and became more evident during 100 Hz trains in some fibers. Our results verify predictions on the behavior of the ECC machinery in response to AP-like depolarizations and provide a direct demonstration that Ca2+ currents elicited by single AP-like waveforms are negligible, but can become more prominent in some fibers during short high-frequency train stimulation that elicits maximal isometric force.

Superfast excitation-contraction coupling in adult zebrafish skeletal muscle fibers

The zebrafish has emerged as a very relevant animal model for probing the pathophysiology of human skeletal muscle disorders. This vertebrate animal model displays a startle response characterized by high-frequency swimming activity powered by contraction of fast skeletal muscle fibers excited at extremely high frequencies, critical for escaping predators and capturing prey. Such intense muscle performance requires extremely fast properties of the contractile machinery but also of excitation-contraction coupling, the process by which an action potential spreading along the sarcolemma induces a change in configuration of the dihydropyridine receptors, resulting in intramembrane charge movements, which in turn triggers the release of Ca2+ from the sarcoplasmic reticulum. However, thus far, the fastest Ca2+ transients evoked by vertebrate muscle fibers has been described in muscles used to produce sounds, such as those in the toadfish swim bladder, but not in muscles used for locomotion. By performing intracellular Ca2+ measurements under voltage control in isolated fast skeletal muscle fibers from adult zebrafish and mouse, we demonstrate that fish fast muscle fibers display superfast kinetics of action potentials, intramembrane charge movements, and action potential-evoked Ca2+ transient, allowing fusion and fused sustained Ca2+ transients at frequencies of excitation much higher than in mouse fast skeletal muscle fibers and comparable to those recorded in muscles producing sounds. The present study is the first demonstration of superfast kinetics of excitation-contraction coupling in skeletal muscle allowing superfast locomotor behaviors in a vertebrate.

From α1s splicing to γ1 function: A new twist in subunit modulation of the skeletal muscle L-type Ca2+ channel

Melzer discusses a recent JGP study showing that alternative splicing of the skeletal muscle L-type calcium channel impacts on a modulatory effect of its γ subunit.

Detection of Ca2+ transients near ryanodine receptors by targeting fluorescent Ca2+ sensors to the triad

In intact muscle fibers, functional properties of ryanodine receptor (RYR)–mediated sarcoplasmic reticulum (SR) Ca²⁺ release triggered by activation of the voltage sensor Ca_V1.1 have so far essentially been addressed with diffusible Ca²⁺-sensitive dyes. Here, we used a domain (T306) of the protein triadin to target the Ca²⁺-sensitive probe GCaMP6f to the junctional SR membrane, in the immediate vicinity of RYR channels, within the triad region. Fluorescence of untargeted GCaMP6f was distributed throughout the muscle fibers and experienced large Ca²⁺-dependent changes, with obvious kinetic delays, upon application of voltage-clamp depolarizing pulses. Conversely, T306-GCaMP6f localized to the triad and generated Ca²⁺-dependent fluorescence transients of lower amplitude and faster kinetics for low and intermediate levels of Ca²⁺ release than those of untargeted GCaMP6f. By contrast, model simulation of the spatial gradients of Ca²⁺ following Ca²⁺ release predicted limited kinetic differences under the assumptions that the two probes were present at the same concentration and suffered from identical kinetic limitations. At the spatial level, T306-GCaMP6f transients within distinct regions of a same fiber yielded a uniform time course, even at low levels of Ca²⁺ release activation. Similar observations were made using GCaMP6f fused to the γ1 auxiliary subunit of Ca_V1.1. Despite the probe's limitations, our results point out the remarkable synchronicity of voltage-dependent Ca²⁺ release activation and termination among individual triads and highlight the potential of the approach to visualize activation or closure of single groups of RYR channels. We anticipate targeting of improved Ca²⁺ sensors to the triad will provide illuminating insights into physiological normal RYR function and its dysfunction under stress or pathological conditions.

Components of activation heat in skeletal muscle

Activation heat (q_A) production by muscle is the thermal accompaniment of the release of Ca²⁺ from the sarcoplasmic reticulum (SR) into the cytoplasm, its interactions with regulatory proteins and other cytoplasmic Ca²⁺ buffers and its return to the SR. The contribution of different Ca²⁺-related reactions to q_A is difficult to determine empirically and therefore, for this study, a mathematical model was developed to describe Ca²⁺ movements and accompanying thermal changes in muscle fibres in response to stimulation. The major sources of heat within a few milliseconds of the initiation of Ca²⁺ release are Ca²⁺ binding to Tn and Pv. Ca²⁺ binding to ATP produces a relatively small amount of heat. Ca²⁺ dissociation from ATP and Tn, with heat absorption, are of similar time course to the decline of force. In muscle lacking Pv (e.g. mouse soleus), Ca²⁺ is then rapidly pumped into the SR. In muscles with Pv, Ca²⁺ that dissociates from Tn and ATP binds to Pv and then dissociates slowly (over 10 s of seconds) and is then pumped into the SR; the net effect of these two processes is heat absorption. It is proposed that this underlies Hill's "negative delayed heat". After all the Ca²⁺ is returned to the SR, q_A is proportional to the amount of Ca²⁺ released into the cytoplasm. In muscles with Pv this is 20-60 s after Ca²⁺ release; in muscles without Pv, all Ca²⁺ is returned to the SR soon after the end of force relaxation.

Ca2+-induced sarcoplasmic reticulum Ca2+ release in myotubularin-deficient muscle fibers

Skeletal muscle deficiency in the 3-phosphoinositide (PtdInsP) phosphatase myotubularin (MTM1) causes myotubular myopathy which is associated with severe depression of voltage-activated sarcoplasmic reticulum Ca²⁺ release through ryanodine receptors. In the present study we aimed at further understanding how Ca²⁺ release is altered in MTM1-deficient muscle fibers, at rest and during activation. While in wild-type muscle fibers, SR Ca²⁺ release exhibits fast stereotyped kinetics of activation and decay throughout the voltage range of activation, Ca²⁺ release in MTM1-deficient muscle fibers exhibits slow and unconventional kinetics at intermediate voltages, suggestive of partial loss of the normal control of ryanodine receptor Ca²⁺ channel activity. In addition, the diseased muscle fibers at rest exhibit spontaneous elementary Ca²⁺ release events at a frequency 30 times greater than that of control fibers. Eighty percent of the events have spatiotemporal properties of archetypal Ca²⁺ sparks while the rest take either the form of lower amplitude, longer duration Ca²⁺ release events or of a combination thereof. The events occur at preferred locations in the fibers, indicating spatially uneven distribution of the parameters determining spontaneous ryanodine receptor 1 opening. Spatially large Ca²⁺ release sources were obviously involved in some of these events, suggesting that opening of ryanodine receptors in one cluster can activate opening of ryanodine receptors in a neighboring one. Overall results demonstrate that opening of Ca²⁺-activated ryanodine receptors is promoted both at rest and during excitation-contraction coupling in MTM1-deficient muscle fibers. Because access to this activation mode is denied to ryanodine receptors in healthy skeletal muscle, this may play an important role in the associated disease situation.

Modeling Ca2+-Bound Troponin in Excitation Contraction Coupling

To explain disparate decay rates of cytosolic Ca²⁺ and structural changes in the thin filaments during a twitch, we model the time course of Ca²⁺-bound troponin (Tn) resulting from the free Ca²⁺ transient of fast skeletal muscle. In fibers stretched beyond overlap, the decay of Ca²⁺ as measured by a change in fluo-3 fluorescence is significantly slower than the intensity decay of the meridional 1/38.5 nm⁻¹ reflection of Tn; this is not simply explained by considering only the Ca²⁺ binding properties of Tn alone (Matsuo et al., 2010). We apply a comprehensive model that includes the known Ca²⁺ binding properties of Tn in the context of the thin filament with and without cycling crossbridges. Calculations based on the model predict that the transient of Ca²⁺-bound Tn correlates with either the fluo-3 time course in muscle with overlapping thin and thick filaments or the intensity of the meridional 1/38.5 nm⁻¹ reflection in overstretched muscle. Hence, cycling crossbridges delay the dissociation of Ca²⁺ from Tn. Correlation with the fluo-3 fluorescence change is not causal given that the transient of Ca²⁺-bound Tn depends on sarcomere length, whereas the fluo-3 fluorescence change does not. Transient positions of tropomyosin calculated from the time course of Ca²⁺-bound Tn are in reasonable agreement with the transient of measured perturbations of the Tn repeat in overlap and non-overlap muscle preparations.

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.

Structural and functional properties of ryanodine receptor type 3 in zebrafish tail muscle

The ryanodine receptor (RyR)1 isoform of the sarcoplasmic reticulum (SR) Ca(2+) release channel is an essential component of all skeletal muscle fibers. RyR1s are detectable as "junctional feet" (JF) in the gap between the SR and the plasmalemma or T-tubules, and they are required for excitation-contraction (EC) coupling and differentiation. A second isoform, RyR3, does not sustain EC coupling and differentiation in the absence of RyR1 and is expressed at highly variable levels. Anatomically, RyR3 expression correlates with the presence of parajunctional feet (PJF), which are located on the sides of the SR junctional cisternae in an arrangement found only in fibers expressing RyR3. In frog muscle fibers, the presence of RyR3 and PJF correlates with the occurrence of Ca(2+) sparks, which are elementary SR Ca(2+) release events of the EC coupling machinery. Here, we explored the structural and functional roles of RyR3 by injecting zebrafish (Danio rerio) one-cell stage embryos with a morpholino designed to specifically silence RyR3 expression. In zebrafish larvae at 72 h postfertilization, fast-twitch fibers from wild-type (WT) tail muscles had abundant PJF. Silencing resulted in a drop of the PJF/JF ratio, from 0.79 in WT fibers to 0.03 in the morphants. The frequency with which Ca(2+) sparks were detected dropped correspondingly, from 0.083 to 0.001 sarcomere(-1) s(-1). The few Ca(2+) sparks detected in morphant fibers were smaller in amplitude, duration, and spatial extent compared with those in WT fibers. Despite the almost complete disappearance of PJF and Ca(2+) sparks in morphant fibers, these fibers looked structurally normal and the swimming behavior of the larvae was not affected. This paper provides important evidence that RyR3 is the main constituent of the PJF and is the main contributor to the SR Ca(2+) flux underlying Ca(2+) sparks detected in fully differentiated frog and fish fibers.

Intracellular calcium movements during relaxation and recovery of superfast muscle fibers of the toadfish swimbladder

Ca²⁺ movements during intercall intervals in superfast toadfish swimbladder muscle fibers depend on the accumulation of Ca²⁺ on parvalbumin and a consequent slow rate of Ca²⁺ pumping.

The mating call of the Atlantic toadfish is generated by bursts of high-frequency twitches of the superfast twitch fibers that surround the swimbladder. At 16°C, a calling period can last several hours, with individual 80–100-Hz calls lasting ∼500 ms interleaved with silent periods (intercall intervals) lasting ∼10 s. To understand the intracellular movements of Ca²⁺ during the intercall intervals, superfast fibers were microinjected with fluo-4, a high-affinity fluorescent Ca²⁺ indicator, and stimulated by trains of 40 action potentials at 83 Hz, which mimics fiber activity during calling. The fluo-4 fluorescence signal was measured during and after the stimulus trains; the signal was also simulated with a kinetic model of the underlying myoplasmic Ca²⁺ movements, including the binding and transport of Ca²⁺ by the sarcoplasmic reticulum (SR) Ca²⁺ pumps. The estimated total amount of Ca²⁺ released from the SR during a first stimulus train is ∼6.5 mM (concentration referred to the myoplasmic water volume). At 40 ms after cessation of stimulation, the myoplasmic free Ca²⁺ concentration ([Ca²⁺]) is below the threshold for force generation (∼3 µM), yet the estimated concentration of released Ca²⁺ remaining in the myoplasm (Δ[Ca_M]) is large, ∼5 mM, with ∼80% bound to parvalbumin. At 10 s after stimulation, [Ca²⁺] is ∼90 nM (three times the assumed resting level) and Δ[Ca_M] is ∼1.3 mM, with 97% bound to parvalbumin. Ca²⁺ movements during the intercall interval thus appear to be strongly influenced by (a) the accumulation of Ca²⁺ on parvalbumin and (b) the slow rate of Ca²⁺ pumping that ensues when parvalbumin lowers [Ca²⁺] near the resting level. With repetitive stimulus trains initiated at 10-s intervals, Ca²⁺ release and pumping come quickly into balance as a result of the stability (negative feedback) supplied by the increased rate of Ca²⁺ pumping at higher [Ca²⁺].