Immunochemical quantification of sarcoplasmic reticulum Ca-ATPase, of calsequestrin and of parvalbumin in rabbit skeletal muscles of defined fiber composition

In vivo heat production dynamics during a contraction-relaxation cycle in rat single skeletal muscle fibers

Skeletal muscle generates heat via contraction-dependent (shivering) and independent (nonshivering) mechanisms. While this thermogenic capacity of skeletal muscle undoubtedly contributes to the body temperature homeostasis of animals and impacts various cellular functions, the intracellular temperature and its dynamics in skeletal muscle in vivo remain elusive. We aimed to determine the intracellular temperature and its changes within skeletal muscle in vivo during contraction and following relaxation. In addition, we tested the hypothesis that sarcoplasmic reticulum Ca2+ ATPase (SERCA) generates heat and increases the myocyte temperature during a transitory Ca2+-induced contraction-relaxation cycle. The intact spinotrapezius muscle of anesthetized adult male Wistar rats (n = 18) was exteriorized and loaded with the fluorescent probe Cellular Thermoprobe for Fluorescence Ratio (49.3 μM) by microinjection over 1 s. The fluorescence ratio (i.e., 580 nm/515 nm) was measured in vivo during 1) temperature increases induced by means of an external heater, and 2) Ca2+ injection (3.9 nL, 2.0 mM). The fluorescence ratio increased as a linear function of muscle surface temperature from 25 °C to 40 °C (r2 = 0.97, P < 0.01). Ca2+ injection (3.9 nL, 2.0 mM) significantly increased myocyte intracellular temperature: An effect that was suppressed by SERCA inhibition with cyclopiazonic acid (CPA, Ca2+: 38.3 ± 1.4 °C vs Ca2++CPA: 28.3 ± 2.8 °C, P < 0.01 at 1 min following injection). While muscle shortening occurred immediately after the Ca2+ injection, the increased muscle temperature was maintained during the relaxation phase. In this investigation, we demonstrated a novel model for measuring the intracellular temperature of skeletal muscle in vivo and further that heat generation occurs concomitant principally with SERCA functioning and muscle relaxation.

Constitutive assembly of Ca2+ entry units in soleus muscle from calsequestrin knockout mice

Calcium (Ca2+) entry units (CEUs) are junctions within the I band of the sarcomere between stacks of sarcoplasmic reticulum (SR) cisternae and extensions of the transverse (T)-tubule. CEUs contain STIM1 and Orai1 proteins, the molecular machinery of store-operated Ca2+ entry (SOCE). In extensor digitorum longus (EDL) fibers of wild-type (WT) mice, CEUs transiently assemble during acute exercise and disassemble several hours thereafter. By contrast, calsequestrin-1 (CASQ1) ablation induces a compensatory constitutive assembly of CEUs in EDL fibers, resulting in enhanced constitutive and maximum SOCE that counteracts SR Ca2+ depletion during repetitive activity. However, whether CEUs form in slow-twitch fibers, which express both the skeletal CASQ1 and the cardiac CASQ2 isoforms, is unknown. Herein, we compared the structure and function of soleus muscles from WT and knockout mice that lack either CASQ1 (CASQ1-null) or both CASQs (dCASQ-null). Ultrastructural analyses showed that SR/T-tubule junctions at the I band, virtually identical to CEUs in EDL muscle, were present and more frequent in CASQ1-null than WT mice, with dCASQ-null exhibiting the highest incidence. The greater incidence of CEUs in soleus from dCASQ-null mice correlated with increased specific force production during repetitive, high-frequency stimulation, which depended on Ca2+ entry. Consistent with this, Orai1 expression was significantly increased in soleus of CASQ1-null mice, but even more in dCASQ-null mice, compared with WT. Together, these results strengthen the concept that CEU assembly strongly depends on CASQ expression and provides an alternative source of Ca2+ needed to refill SR Ca2+ stores to maintain specific force production during sustained muscle activity.

Excitation-contraction coupling in mammalian skeletal muscle: Blending old and last-decade research

The excitation–contraction coupling (ECC) in skeletal muscle refers to the Ca²⁺-mediated link between the membrane excitation and the mechanical contraction. The initiation and propagation of an action potential through the membranous system of the sarcolemma and the tubular network lead to the activation of the Ca²⁺-release units (CRU): tightly coupled dihydropyridine and ryanodine (RyR) receptors. The RyR gating allows a rapid, massive, and highly regulated release of Ca²⁺ from the sarcoplasmic reticulum (SR). The release from triadic places generates a sarcomeric gradient of Ca²⁺ concentrations ([Ca²⁺]) depending on the distance of a subcellular region from the CRU. Upon release, the diffusing Ca²⁺ has multiple fates: binds to troponin C thus activating the contractile machinery, binds to classical sarcoplasmic Ca²⁺ buffers such as parvalbumin, adenosine triphosphate and, experimentally, fluorescent dyes, enters the mitochondria and the SR, or is recycled through the Na⁺/Ca²⁺ exchanger and store-operated Ca²⁺ entry (SOCE) mechanisms. To commemorate the 7^th decade after being coined, we comprehensively and critically reviewed “old”, historical landmarks and well-established concepts, and blended them with recent advances to have a complete, quantitative-focused landscape of the ECC. We discuss the: 1) elucidation of the CRU structures at near-atomic resolution and its implications for functional coupling; 2) reliable quantification of peak sarcoplasmic [Ca²⁺] using fast, low affinity Ca²⁺ dyes and the relative contributions of the Ca²⁺-binding mechanisms to the whole concert of Ca²⁺ fluxes inside the fibre; 3) articulation of this novel quantitative information with the unveiled structural details of the molecular machinery involved in mitochondrial Ca²⁺ handing to understand how and how much Ca²⁺ enters the mitochondria; 4) presence of the SOCE machinery and its different modes of activation, which awaits understanding of its magnitude and relevance in situ; 5) pharmacology of the ECC, and 6) emerging topics such as the use and potential applications of super-resolution and induced pluripotent stem cells (iPSC) in ECC. Blending the old with the new works better!

What Is Parvalbumin for?

Parvalbumin (PA) is a small, acidic, mostly cytosolic Ca2+-binding protein of the EF-hand superfamily. Structural and physical properties of PA are well studied but recently two highly conserved structural motifs consisting of three amino acids each (clusters I and II), which contribute to the hydrophobic core of the EF-hand domains, have been revealed. Despite several decades of studies, physiological functions of PA are still poorly known. Since no target proteins have been revealed for PA so far, it is believed that PA acts as a slow calcium buffer. Numerous experiments on various muscle systems have shown that PA accelerates the relaxation of fast skeletal muscles. It has been found that oxidation of PA by reactive oxygen species (ROS) is conformation-dependent and one more physiological function of PA in fast muscles could be a protection of these cells from ROS. PA is thought to regulate calcium-dependent metabolic and electric processes within the population of gamma-aminobutyric acid (GABA) neurons. Genetic elimination of PA results in changes in GABAergic synaptic transmission. Mammalian oncomodulin (OM), the β isoform of PA, is expressed mostly in cochlear outer hair cells and in vestibular hair cells. OM knockout mice lose their hearing after 3–4 months. It was suggested that, in sensory cells, OM maintains auditory function, most likely affecting outer hair cells’ motility mechanisms.

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

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.

Energy metabolism design of the striated muscle cell

The design of the energy metabolism system in striated muscle remains a major area of investigation. Here, we review our current understanding and emerging hypotheses regarding the metabolic support of muscle contraction. Maintenance of ATP free energy, so called energy homeostasis, via mitochondrial oxidative phosphorylation is critical to sustained contractile activity, and this major design criterion is the focus of this review. Cell volume invested in mitochondria reduces the space available for generating contractile force, and this spatial balance between mitochondria acontractile elements to meet the varying sustained power demands across muscle types is another important design criterion. This is accomplished with remarkably similar mass-specific mitochondrial protein composition across muscle types, implying that it is the organization of mitochondria within the muscle cell that is critical to supporting sustained muscle function. Beyond the production of ATP, ubiquitous distribution of ATPases throughout the muscle requires rapid distribution of potential energy across these large cells. Distribution of potential energy has long been thought to occur primarily through facilitated metabolite diffusion, but recent analysis has questioned the importance of this process under normal physiological conditions. Recent structural and functional studies have supported the hypothesis that the mitochondrial reticulum provides a rapid energy distribution system via the conduction of the mitochondrial membrane potential to maintain metabolic homeostasis during contractile activity. We extensively review this aspect of the energy metabolism design contrasting it with metabolite diffusion models and how mitochondrial structure can play a role in the delivery of energy in the striated muscle.

Sarcolipin Exhibits Abundant RNA Transcription and Minimal Protein Expression in Horse Gluteal Muscle

Ca²⁺ regulation in equine muscle is important for horse performance, yet little is known about this species-specific regulation. We reported recently that horse encode unique gene and protein sequences for the sarcoplasmic reticulum (SR) Ca²⁺-transporting ATPase (SERCA) and the regulatory subunit sarcolipin (SLN). Here we quantified gene transcription and protein expression of SERCA and its inhibitory peptides in horse gluteus, as compared to commonly-studied rabbit skeletal muscle. RNA sequencing and protein immunoblotting determined that horse gluteus expresses the ATP2A1 gene (SERCA1) as the predominant SR Ca²⁺-ATPase isoform and the SLN gene as the most-abundant SERCA inhibitory peptide, as also found in rabbit skeletal muscle. Equine muscle expresses an insignificant level of phospholamban (PLN), another key SERCA inhibitory peptide expressed commonly in a variety of mammalian striated muscles. Surprisingly in horse, the RNA transcript ratio of SLN-to-ATP2A1 is an order of magnitude higher than in rabbit, while the corresponding protein expression ratio is an order of magnitude lower than in rabbit. Thus, SLN is not efficiently translated or maintained as a stable protein in horse muscle, suggesting a non-coding role for supra-abundant SLN mRNA. We propose that the lack of SLN and PLN inhibition of SERCA activity in equine muscle is an evolutionary adaptation that potentiates Ca²⁺ cycling and muscle contractility in a prey species domestically selected for speed.

Purification of sarcoplasmic reticulum vesicles from horse gluteal muscle

We have analyzed protein expression and enzyme activity of the sarcoplasmic reticulum Ca²⁺-transporting ATPase (SERCA) in horse gluteal muscle. Horses exhibit a high incidence of recurrent exertional rhabdomyolysis, with myosolic Ca²⁺ proposed, but yet to be established, as the underlying cause. To better assess Ca²⁺ regulatory mechanisms, we developed an improved protocol for isolating sarcoplasmic reticulum (SR) vesicles from horse skeletal muscle, based on mechanical homogenization and optimized parameters for differential centrifugation. Immunoblotting identified the peak subcellular fraction containing the SERCA1 protein (fast-twitch isoform). Gel analysis using the Stains-all dye demonstrated that calsequestrin (CASQ) and phospholipids are highly enriched in the SERCA-containing subcellular fraction isolated from horse gluteus. Immunoblotting also demonstrated that these horse SR vesicles show low content of glycogen phosphorylase (GP), which is likely an abundant contaminating protein of traditional horse SR preps. The maximal Ca²⁺-activated ATPase activity (V_max) of SERCA in horse SR vesicles isolated using this protocol is 5‒25-fold greater than previously-reported SERCA activity in SR preps from horse skeletal muscle. We propose that this new protocol for isolating SR vesicles will be useful for determining enzymatic parameters of horse SERCA with high fidelity, plus assessing regulatory effect of SERCA peptide subunit(s) expressed in horse muscle.

Characterization of fast-twitch and slow-twitch skeletal muscles of calsequestrin 2 (CASQ2)-knock out mice: unexpected adaptive changes of fast-twitch muscles only

This study investigates the functional role of calsequestrin 2 (CASQ2) in both fast-twitch and slow-twitch skeletal muscles by using CASQ2-/- mice; CASQ2 is expressed throughout life in slow-twitch muscles, but only in the developmental and neonatal stages in fast-twitch muscles. CASQ2-/- causes increase in calsequestrin 1 (CASQ1) expression, but without functional changes in both muscle types. CASQ2-/- mice have ultrastructural changes in fast-twitch muscles only, i.e., formation of pentads and stacks in the sarcoplasmic reticulum.

Length-dependent Ca2+ activation in skeletal muscle fibers from mammalians

We tested the hypotheses that 1) a decrease in activation of skeletal muscles at short sarcomere lengths (SLs) is caused by an inhibition of Ca(2+) release from the sarcoplasmic reticulum (SR), and 2) the decrease in Ca(2+) would be caused by an inhibition of action potential conduction from the periphery to the core of the fibers. Intact, single fibers dissected from the flexor digitorum brevis from mice were activated at different SLs, and intracellular Ca(2+) was imaged with confocal microscopy. Force decreased at SLs shorter than 2.1 μm, while Ca(2+) concentration decreased at SLs below 1.9 μm. The concentration of Ca(2+) at short SL was lower at the core than at the peripheries of the fiber. When the external concentration of Na(+) was decreased in the experimental media, impairing action potential conduction, Ca(2+) gradients were observed in all SLs. When caffeine was used in the experimental media, the gradients of Ca(2+) were abolished. We concluded that there is an inhibition of Ca(2+) release from the sarcoplasmic reticulum (SR) at short SLs, which results from a decreased conduction of action potential from the periphery to the core of the fibers.

Ca(2+) leakage out of the sarcoplasmic reticulum is increased in type I skeletal muscle fibres in aged humans

KEY POINTS

The amount of Ca(2+) stored in the sarcoplasmic reticulum (SR) of muscle fibres is decreased in aged individuals, and an important question is whether this results from increased Ca(2+) leakage out through the Ca(2+) release channels (ryanodine receptors; RyRs). The present study examined the effects of blocking the RyRs with Mg(2+), or applying a strong reducing treatment, on net Ca(2+) accumulation by the SR in skinned muscle fibres from Old (∼70 years) and Young (∼24 years) adults. Raising cytoplasmic [Mg(2+)] and reducing treatment increased net SR Ca(2+) accumulation in type I fibres of Old subjects relative to that in Young. The densities of RyRs and dihydropyridine receptors were not significantly changed in the muscle of Old subjects. These findings indicate that oxidative modification of the RyRs causes increased Ca(2+) leakage from the SR in muscle fibres in Old subjects, which probably deleteriously affects normal muscle function both directly and indirectly.

ABSTRACT

The present study examined whether the lower Ca(2+) storage levels in the sarcoplasmic reticulum (SR) in vastus lateralis muscle fibres in Old (70 ± 4 years) relative to Young (24 ± 4 years) human subjects is the result of increased leakage of Ca(2+) out of the SR through the Ca(2+) release channels/ryanodine receptors (RyRs) and due to oxidative modification of the RyRs. SR Ca(2+) accumulation in mechanically skinned muscle fibres was examined in the presence of 1, 3 or 10 mm cytoplasmic Mg(2+) because raising [Mg(2+)] strongly inhibits Ca(2+) efflux through the RyRs. In type I fibres of Old subjects, SR Ca(2+) accumulation in the presence of 1 mm Mg(2+) approached saturation at shorter loading times than in Young subjects, consistent with Ca(2+) leakage limiting net uptake, and raising [Mg(2+)] to 10 mm in such fibres increased maximal SR Ca(2+) accumulation. No significant differences were seen in type II fibres. Treatment with dithiothreitol (10 mm for 5 min), a strong reducing agent, also increased maximal SR Ca(2+) accumulation at 1 mm Mg(2+) in type I fibres of Old subjects but not in other fibres. The densities of dihydropyridine receptors and RyRs were not significantly different in muscles of Old relative to Young subjects. These findings indicate that Ca(2+) leakage from the SR is increased in type I fibres in Old subjects by reversible oxidative modification of the RyRs; this increased SR Ca(2+) leak is expected to have both direct and indirect deleterious effects on Ca(2+) movements and muscle function.

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.

Sarcoplasmic reticulum Ca2+ uptake and leak properties, and SERCA isoform expression, in type I and type II fibres of human skeletal muscle

The Ca(2+) uptake properties of the sarcoplasmic reticulum (SR) were compared between type I and type II fibres of vastus lateralis muscle of young healthy adults. Individual mechanically skinned muscle fibres were exposed to solutions with the free [Ca(2+)] heavily buffered in the pCa range (-log10[Ca(2+)]) 7.3-6.0 for set times and the amount of net SR Ca(2+) accumulation determined from the force response elicited upon emptying the SR of all Ca(2+). Western blotting was used to determine fibre type and the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) isoform present in every fibre examined. Type I fibres contained only SERCA2 and displayed half-maximal Ca(2+) uptake rate at ∼pCa 6.8, whereas type II fibres contained only SERCA1 and displayed half-maximal Ca(2+) uptake rate at ∼pCa 6.6. Maximal Ca(2+) uptake rate was ∼0.18 and ∼0.21 mmol Ca(2+) (l fibre)(-1) s(-1) in type I and type II fibres, respectively, in good accord with previously measured SR ATPase activity. Increasing free [Mg(2+)] from 1 to 3 mM had no significant effect on the net Ca(2+) uptake rate at pCa 6.0, indicating that there was little or no calcium-induced calcium release occurring through the Ca(2+) release channels during uptake in either fibre type. Ca(2+) leakage from the SR at pCa 8.5, which is thought to occur at least in part through the SERCA, was ∼2-fold lower in type II fibres than in type I fibres, and was little affected by the presence of ADP, in marked contrast to the larger SR Ca(2+) leak observed in rat muscle fibres under the same conditions. The higher affinity of Ca(2+) uptake in the type I human fibres can account for the higher relative level of SR Ca(2+) loading observed in type I compared to type II fibres, and the SR Ca(2+) leakage characteristics of the human fibres suggest that the SERCAs are regulated differently from those in rat and contribute comparatively less to resting metabolic rate.

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

Single twitch fibers from frog leg muscles were isolated by dissection and micro-injected with furaptra, a rapidly responding fluorescent Ca(2+) indicator. Indicator resting fluorescence (FR) and the change evoked by an action potential (ΔF) were measured at long sarcomere length (16°C); ΔF/FR was scaled to units of ΔfCaD, the change in fraction of the indicator in the Ca(2+)-bound form. ΔfCaD was simulated with a multicompartment model of the underlying myoplasmic Ca(2+) movements, and the results were compared with previous measurements and analyses in mouse fast-twitch fibers. In frog fibers, sarcoplasmic reticulum (SR) Ca(2+) release evoked by an action potential appears to be the sum of two components. The time course of the first component is similar to that of the entire Ca(2+) release waveform in mouse fibers, whereas that of the second component is severalfold slower; the fractional release amounts are ~0.8 (first component) and ~0.2 (second component). Similar results were obtained in frog simulations with a modified model that permitted competition between Mg(2+) and Ca(2+) for occupancy of the regulatory sites on troponin. An anatomical basis for two release components in frog fibers is the presence of both junctional and parajunctional SR Ca(2+) release channels (ryanodine receptors [RyRs]), whereas mouse fibers (usually) have only junctional RyRs. Also, frog fibers have two RyR isoforms, RyRα and RyRβ, whereas the mouse fibers (usually) have only one, RyR1. Our simulations suggest that the second release component in frog fibers functions to supply extra Ca(2+) to activate troponin, which, in mouse fibers, is not needed because of the more favorable location of their triadic junctions (near the middle of the thin filament). We speculate that, in general, parajunctional RyRs permit increased myofilament activation in fibers whose triadic junctions are located at the z-line.

Intracellular calcium movements during excitation-contraction coupling in mammalian slow-twitch and fast-twitch muscle fibers

In skeletal muscle fibers, action potentials elicit contractions by releasing calcium ions (Ca²⁺) from the sarcoplasmic reticulum. Experiments on individual mouse muscle fibers micro-injected with a rapidly responding fluorescent Ca²⁺ indicator dye reveal that the amount of Ca²⁺ released is three- to fourfold larger in fast-twitch fibers than in slow-twitch fibers, and the proportion of the released Ca²⁺ that binds to troponin to activate contraction is substantially smaller.

Measurement and simulation of myoplasmic calcium transients in mouse slow-twitch muscle fibres

Bundles of intact fibres from soleus muscles of adult mice were isolated by dissection and one fibre within a bundle was micro-injected with either furaptra or mag-fluo-4, two low-affinity rapidly responding Ca(2+) indicators. Fibres were activated by action potentials to elicit changes in indicator fluorescence (ΔF), a monitor of the myoplasmic free Ca(2+) transient ([Ca(2+)]), and changes in fibre tension. All injected fibres appeared to be slow-twitch (type I) fibres as inferred from the time course of their tension responses. The full-duration at half-maximum (FDHM) of ΔF was found to be essentially identical with the two indicators; the mean value was 8.4 ± 0.3 ms (±SEM) at 16°C and 5.1 ± 0.3 ms at 22°C. The value at 22°C is about one-third that reported previously in enzyme-dissociated slow-twitch fibres that had been AM-loaded with mag-fluo-4: 12.4 ± 0.8 ms and 17.2 ± 1.7 ms. We attribute the larger FDHM in enzyme-dissociated fibres either to an alteration of fibre properties due to the enzyme treatment or to some error in the measurement of ΔF associated with AM loading. ΔF in intact fibres was simulated with a multi-compartment reaction-diffusion model that permitted estimation of the amount and time course of Ca(2+) release from the sarcoplasmic reticulum (SR), the binding and diffusion of Ca(2+) in the myoplasm, the re-uptake of Ca(2+) by the SR Ca(2+) pump, and Δ[Ca(2+)] itself. In response to one action potential at 16°C, the following estimates were obtained: 107 μm for the amount of Ca(2+) release; 1.7 ms for the FDHM of the release flux; 7.6 μm and 4.9 ms for the peak and FDHM of spatially averaged Δ[Ca(2+)]. With five action potentials at 67 Hz, the estimated amount of Ca(2+) release is 186 μm. Two important unknown model parameters are the on- and off-rate constants of the reaction between Ca(2+) and the regulatory sites on troponin; values of 0.4 × 10(8) m(-1) s(-1) and 26 s(-1), respectively, were found to be consistent with the ΔF measurements.

Fiber Types in Mammalian Skeletal Muscles

Mammalian skeletal muscle comprises different fiber types, whose identity is first established during embryonic development by intrinsic myogenic control mechanisms and is later modulated by neural and hormonal factors. The relative proportion of the different fiber types varies strikingly between species, and in humans shows significant variability between individuals. Myosin heavy chain isoforms, whose complete inventory and expression pattern are now available, provide a useful marker for fiber types, both for the four major forms present in trunk and limb muscles and the minor forms present in head and neck muscles. However, muscle fiber diversity involves all functional muscle cell compartments, including membrane excitation, excitation-contraction coupling, contractile machinery, cytoskeleton scaffold, and energy supply systems. Variations within each compartment are limited by the need of matching fiber type properties between different compartments. Nerve activity is a major control mechanism of the fiber type profile, and multiple signaling pathways are implicated in activity-dependent changes of muscle fibers. The characterization of these pathways is raising increasing interest in clinical medicine, given the potentially beneficial effects of muscle fiber type switching in the prevention and treatment of metabolic diseases.

Remodeling of calcium handling in skeletal muscle through PGC-1α: impact on force, fatigability, and fiber type

Regular endurance exercise remodels skeletal muscle, largely through the peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α). PGC-1α promotes fiber type switching and resistance to fatigue. Intracellular calcium levels might play a role in both adaptive phenomena, yet a role for PGC-1α in the adaptation of calcium handling in skeletal muscle remains unknown. Using mice with transgenic overexpression of PGC-1α, we now investigated the effect of PGC-1α on calcium handling in skeletal muscle. We demonstrate that PGC-1α induces a quantitative reduction in calcium release from the sarcoplasmic reticulum by diminishing the expression of calcium-releasing molecules. Concomitantly, maximal muscle force is reduced in vivo and ex vivo. In addition, PGC-1α overexpression delays calcium clearance from the myoplasm by interfering with multiple mechanisms involved in calcium removal, leading to higher myoplasmic calcium levels following contraction. During prolonged muscle activity, the delayed calcium clearance might facilitate force production in mice overexpressing PGC-1α. Our results reveal a novel role of PGC-1α in altering the contractile properties of skeletal muscle by modulating calcium handling. Importantly, our findings indicate PGC-1α to be both down- as well as upstream of calcium signaling in this tissue. Overall, our findings suggest that in the adaptation to chronic exercise, PGC-1α reduces maximal force, increases resistance to fatigue, and drives fiber type switching partly through remodeling of calcium transients, in addition to promoting slow-type myofibrillar protein expression and adequate energy supply.

Loss of IL-15 receptor α alters the endurance, fatigability, and metabolic characteristics of mouse fast skeletal muscles

IL-15 receptor α (IL-15Rα) is a component of the heterotrimeric plasma membrane receptor for the pleiotropic cytokine IL-15. However, IL-15Rα is not merely an IL-15 receptor subunit, as mice lacking either IL-15 or IL-15Rα have unique phenotypes. IL-15 and IL-15Rα have been implicated in muscle phenotypes, but a role in muscle physiology has not been defined. Here, we have shown that loss of IL-15Rα induces a functional oxidative shift in fast muscles, substantially increasing fatigue resistance and exercise capacity. IL-15Rα-knockout (IL-15Rα-KO) mice ran greater distances and had greater ambulatory activity than controls. Fast muscles displayed fatigue resistance and a slower contractile phenotype. The molecular signature of these muscles included altered markers of mitochondrial biogenesis and calcium homeostasis. Morphologically, fast muscles had a greater number of muscle fibers, smaller fiber areas, and a greater ratio of nuclei to fiber area. The alterations of physiological properties and increased resistance to fatigue in fast muscles are consistent with a shift toward a slower, more oxidative phenotype. Consistent with a conserved functional role in humans, a genetic association was found between a SNP in the IL15RA gene and endurance in athletes stratified by sport. Therefore, we propose that IL-15Rα has a role in defining the phenotype of fast skeletal muscles in vivo.

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.

Deconstructing calsequestrin. Complex buffering in the calcium store of skeletal muscle

Since its discovery in 1971, calsequestrin has been recognized as the main Ca(2+) binding protein inside the sarcoplasmic reticulum (SR), the organelle that stores and upon demand mobilizes Ca(2+) for contractile activation of muscle. This article reviews the potential roles of calsequestrin in excitation-contraction coupling of skeletal muscle. It first considers the quantitative demands for a structure that binds Ca(2+) inside the SR in view of the amounts of the ion that must be mobilized to elicit muscle contraction. It briefly discusses existing evidence, largely gathered in cardiac muscle, of two roles for calsequestrin: as Ca(2+) reservoir and as modulator of the activity of Ca(2+) release channels, and then considers the results of an incipient body of work that manipulates the cellular endowment of calsequestrin. The observations include evidence that both the Ca(2+) buffering capacity of calsequestrin in solution and that of the SR in intact cells decay as the free Ca(2+) concentration is lowered. Together with puzzling observations of increase of Ca(2+) inside the SR, in cells or vesicular fractions, upon activation of Ca(2+) release, this is interpreted as evidence that the Ca(2+) buffering in the SR is non-linear, and is optimized for support of Ca(2+) release at the physiological levels of SR Ca(2+) concentration. Such non-linearity of buffering is qualitatively explained by a speculation that puts together ideas first proposed by others. The speculation pictures calsequestrin polymers as 'wires' that both bind Ca(2+) and efficiently deliver it near the release channels. In spite of the kinetic changes, the functional studies reveal that cells devoid of calsequestrin are still capable of releasing large amounts of Ca(2+) into the myoplasm, consistent with the long term viability and apparent good health of mice engineered for calsequestrin ablation. The experiments therefore suggest that other molecules are capable of providing sites for reversible binding of large amounts of Ca(2+) inside the sarcoplasmic reticulum.

Calsequestrin content and SERCA determine normal and maximal Ca2+ storage levels in sarcoplasmic reticulum of fast- and slow-twitch fibres of rat

Whilst calsequestrin (CSQ) is widely recognized as the primary Ca2+ buffer in the sarcoplasmic reticulum (SR) in skeletal muscle fibres, its total buffering capacity and importance have come into question. This study quantified the absolute amount of CSQ isoform 1 (CSQ1, the primary isoform) present in rat extensor digitorum longus (EDL) and soleus fibres, and related this to their endogenous and maximal SR Ca2+ content. Using Western blotting, the entire constituents of minute samples of muscle homogenates or segments of individual muscle fibres were compared with known amounts of purified CSQ1. The fidelity of the analysis was proven by examining the relative signal intensity when mixing muscle samples and purified CSQ1. The CSQ1 contents of EDL fibres, almost exclusively type II fibres, and soleus type I fibres [SOL (I)] were, respectively, 36 +/- 2 and 10 +/- 1 micromol (l fibre volume)(-1), quantitatively accounting for the maximal SR Ca2+ content of each. Soleus type II [SOL (II)] fibres (approximately 20% of soleus fibres) had an intermediate amount of CSQ1. Every SOL (I) fibre examined also contained some CSQ isoform 2 (CSQ2), which was absent in every EDL and other type II fibre except for trace amounts in one case. Every EDL and other type II fibre had a high density of SERCA1, the fast-twitch muscle sarco(endo)plasmic reticulum Ca2+-ATPase isoform, whereas there was virtually no SERCA1 in any SOL (I) fibre. Maximal SR Ca2+ content measured in skinned fibres increased with CSQ1 content, and the ratio of endogenous to maximal Ca2+ content was inversely correlated with CSQ1 content. The relative SR Ca2+ content that could be maintained in resting cytoplasmic conditions was found to be much lower in EDL fibres than in SOL (I) fibres (approximately 20 versus >60%). Leakage of Ca2+ from the SR in EDL fibres could be substantially reduced with a SR Ca2+ pump blocker and increased by adding creatine to buffer cytoplasmic [ADP] at a higher level, both results indicating that at least part of the Ca2+ leakage occurred through SERCA. It is concluded that CSQ1 plays an important role in EDL muscle fibres by providing a large total pool of releasable Ca2+ in the SR whilst maintaining free [Ca2+] in the SR at sufficiently low levels that Ca2+ leakage through the high density of SERCA1 pumps does not metabolically compromise muscle function.

Skeletal Muscle Fatigue: Cellular Mechanisms

Repeated, intense use of muscles leads to a decline in performance known as muscle fatigue. Many muscle properties change during fatigue including the action potential, extracellular and intracellular ions, and many intracellular metabolites. A range of mechanisms have been identified that contribute to the decline of performance. The traditional explanation, accumulation of intracellular lactate and hydrogen ions causing impaired function of the contractile proteins, is probably of limited importance in mammals. Alternative explanations that will be considered are the effects of ionic changes on the action potential, failure of SR Ca2+release by various mechanisms, and the effects of reactive oxygen species. Many different activities lead to fatigue, and an important challenge is to identify the various mechanisms that contribute under different circumstances. Most of the mechanistic studies of fatigue are on isolated animal tissues, and another major challenge is to use the knowledge generated in these studies to identify the mechanisms of fatigue in intact animals and particularly in human diseases.

Matching of sarcoplasmic reticulum and contractile properties in rat fast- and slow-twitch muscle fibres

1. The twitch characteristics (fast-twitch or slow-twitch) of skeletal muscle fibres are determined not only by the contractile apparatus properties of the fibre, but also by the time-course of Ca2+ release and re-uptake by the sarcoplasmic reticulum (SR). The present study examined, in individual fibres from non-transforming muscle of the rat, whether particular SR properties are matched to the contractile apparatus properties of the fibre, in particular in the case of fibres with fast-twitch contractile apparatus located in a slow-twitch muscle, namely the soleus. 2. Force was recorded in single, mechanically skinned fibres from extensor digitorum longus (EDL), gastrocnemius, peroneus longus and soleus muscles. Using repeated cycles in which the SR was emptied of all releasable Ca2+ and then reloaded, it was possible to determine the relative amount of Ca2+ present in the SR endogenously, the maximum SR capacity and the rate of Ca2+ loading. The sensitivity of the contractile apparatus to Ca2+ and Sr2+ was used to classify the fibres as fast-twitch (FT), slow-twitch (ST) or mixed (< 3% of the fibres examined) and thereby identify the likely troponin C and myosin heavy chain types present. 3. There was no significant difference in SR properties between the groups of FT fibres obtained from the four different muscles, including soleus. Despite some overlap in the SR properties of individual fibres between the FT and ST groups, the properties of the FT fibres in all four muscles studied were significantly different from those of the ST and mixed fibres. 4. In general, in FT fibres the SR had a larger capacity and the endogenous Ca2+ content was a relatively lower percentage of maximum compared with ST fibres. Importantly, in terms of their SR properties, FT fibres from soleus muscle more closely resembled FT fibres from other muscles than they did ST fibres from soleus muscle.

Reorganized stores and impaired calcium handling in skeletal muscle of mice lacking calsequestrin-1

Calsequestrin (CS), the major Ca(2+)-binding protein in the sarcoplasmic reticulum (SR), is thought to play a dual role in excitation-contraction coupling: buffering free Ca(2+) increasing SR capacity, and modulating the activity of the Ca(2+) release channels (RyRs). In this study, we generated and characterized the first murine model lacking the skeletal CS isoform (CS1). CS1-null mice are viable and fertile, even though skeletal muscles appear slightly atrophic compared to the control mice. No compensatory increase of the cardiac isoform CS2 is detectable in any type of skeletal muscle. CS1-null muscle fibres are characterized by structural and functional changes, which are much more evident in fast-twitch muscles (EDL) in which most fibres express only CS1, than in slow-twitch muscles (soleus), where CS2 is expressed in about 50% of the fibres. In isolated EDL muscle, force development is preserved, but characterized by prolonged time-to-peak and half-relaxation time, probably related to impaired calcium release from and re-uptake by the SR. Ca(2+)-imaging studies show that the amount of Ca(2+) released from the SR and the amplitude of the Ca(2+) transient are significantly reduced. The lack of CS1 also causes significant ultrastructural changes, which include: (i) striking proliferation of SR junctional domains; (ii) increased density of Ca(2+)-release channels (confirmed also by (3)H-ryanodine binding); (iii) decreased SR terminal cisternae volume; (iv) higher density of mitochondria. Taken together these results demonstrate that CS1 is essential for the normal development of the SR and its calcium release units and for the storage and release of appropriate amounts of SR Ca(2+).

RyR isoforms and fibre type-specific expression of proteins controlling intracellular calcium concentration in skeletal muscles

Muscle fibres which shorten with high maximum shortening velocity also exhibit fast kinetics of contraction, i.e. short values of time to peak tension and time to half relaxation. This short review aims to discuss the molecular basis of such correlation, to reach, based on the available literature, an answer to the question whether there is a correlation in expression of proteins determining shortening velocity, myosin isoforms in the first place, and proteins controlling cytosolic calcium concentration and its variations at rest or during contraction. Although the isoforms of RyR, the sarcoplasmic calcium release channels, do not show a tightly coordinated expression with myosin isoforms, other proteins involved in controlling intracellular calcium do. This is likely sufficient to guarantee the correlation between maximum shortening velocity and speed of isometric contraction.

Respiratory muscle fibres: specialisation and plasticity

Skeletal muscles are composed of fibres of different types, each type being identified by the isoform of myosin heavy chain which is expressed as slow 1, fast 2A, fast 2X, and fast 2B. Slow fibres are resistant to fatigue due to their highly oxidative metabolism whereas 2X and 2B fibres are easily fatiguable and fast 2A fibres exhibit intermediate fatigue resistance. Slow fibres and fast fibres are present in equal proportions in the adult human diaphragm while intercostal muscles contain a higher proportion of fast fibres. A small fibre size, abundance of capillaries, and a high aerobic oxidative enzyme activity are typical features of diaphragm fibres and give them the resistance to fatigue required by their continuous activity. Because of their fibre composition, intercostal muscles are less resistant to fatigue. The structural and functional characteristics of respiratory muscle fibres are not fixed, however, and can be modified in response to several physiological and pathological conditions such as training (adaptation to changes in respiratory load), adaptation to hypoxia, age related changes, and changes associated with respiratory diseases. The properties of respiratory muscle fibres can also be modified by pharmacological agents such as beta2 agonists and corticosteroids used for the treatment of respiratory diseases.

Effect of endurance exercise on the Ca2+ pumps from transverse tubule and sarcoplasmic reticulum of rabbit skeletal muscle

The sarcoplasmic reticulum (SR) Ca(2+) pump is the main homeostatic regulatory mechanism in fast skeletal muscle that maintains intracellular Ca(2+) concentration ([Ca(2+)](i)) at the nanomolar level at rest. The transverse tubule (TT) Ca(2+) pump transports cytosolic Ca(2+) to the extracellular space. During prolonged muscular activity, [Ca(2+)](i) may increase. TT and SR isolated microsomal vesicles were highly purified, and the purity was checked by immunoblotting. The present study shows the effects of endurance exercise on the activities and structures of the TT and SR Ca(2+) pumps of fast skeletal muscle from rabbit at rest. The Ca(2+) pump activity increased manifolds in TT but did not change in SR. The protein denaturalization profiles obtained by differential scanning calorimetry showed 1) a shift in the transition temperature and an increase in the enthalpy of the TT Ca(2+) pump and 2) a significant change in the transition temperature of the SR Ca(2+) pump Ca(2+)-binding domain. We conclude that the TT Ca(2+) pump activity was upgraded in association with structural changes to handle the changes in [Ca(2+)](i) and TT lumen Ca(2+) concentration that occur during endurance exercise.

Nandrolone decanoate treatment affects sarcoplasmic reticulum Ca(2+) ATPase function in skinned rat slow- and fast-twitch fibres

The effects of anabolic-androgenic steroid administration on the function of the sarcoplasmic reticulum (SR) pump were investigated in chemically skinned fibres from the extensor digitorum longus (EDL) and soleus muscles of sedentary rats. Twenty male rats were divided into two groups, one group received an intramuscular injection of nandrolone decanoate (15 mg x kg(-1)) weekly for 8 weeks, the second received similar weekly doses of vehicle (sterile peanut oil). Compared with control muscles, nandrolone decanoate treatment reduced SR Ca(2+) loading in EDL and soleus fibres by 49% and 29%, respectively. In control and treated muscles, the rate of Ca(2+) leakage depended on the quantity of Ca(2+) loaded. Furthermore, for similar SR Ca(2+) contents, the Ca(2+) leakage rate was not significantly modified by nandrolone decanoate treatment. Nandrolone decanoate treatment thus affects Ca (2+) uptake by the SR in a fibre-type dependent manner.

ATP utilization for calcium uptake and force production in different types of human skeletal muscle fibres

The contractile properties and ATPase activity of skinned human skeletal muscle fibres from vastus lateralis were examined. Fibre types were resolved from single fibre segments by SDS-polyacrylamide gel electrophoresis. ATPase activity was determined by enzymatic coupling of ATP resynthesis to the oxidation of NADH. The partitioning of ATPase activity into (a) calcium-activated activity due to actomyosin (AM) interaction, (b) calcium-activated activity of the sarcoplasmic reticular (SR) calcium pump, and (c) basal (calcium independent) activity was investigated by comparing ATP utilization before and after exposure of the preparations for 30 min to a solution containing 0.5 % Triton X-100, which effectively abolished the SR ATPase activity. Partitioning of ATPase activity was also determined by measuring ATP utilization and force at different concentrations of butanedione monoxime (BDM), which inhibits AM interaction. The results obtained with Triton X-100 and BDM were similar. At saturating Ca2+ concentrations and 20 degrees C, the AM, SR and basal ATPase activities per litre cell volume (+/- S.E.M.) amounted to 46 +/- 4, 51 +/- 4 and 19 +/- 2 muM s-1 in type I fibres (n = 21), 139 +/- 14, 69 +/- 8 and 30 +/- 3 muM s-1 in type IIA fibres (n = 25), 137 +/- 22, 175 +/- 28 and 26 +/- 8 muM s-1 in type IIA/B fibres (n = 4) and 108 +/- 13, 169 +/- 42 and 32 +/- 8 muM s-1 in type IIB fibres (n = 2). These results indicate that ATP utilization for SR Ca2+ pumping in fast fibres is considerably larger than in slow fibres. The SR ATPase activity in human muscle represents a considerable fraction of the total (AM + SR + basal) ATPase activity.

Human skeletal muscle fibres: molecular and functional diversity

Contractile and energetic properties of human skeletal muscle have been studied for many years in vivo in the body. It has been, however, difficult to identify the specific role of muscle fibres in modulating muscle performance. Recently it has become possible to dissect short segments of single human muscle fibres from biopsy samples and make them work in nearly physiologic conditions in vitro. At the same time, the development of molecular biology has provided a wealth of information on muscle proteins and their genes and new techniques have allowed analysis of the protein isoform composition of the same fibre segments used for functional studies. In this way the histological identification of three main human muscle fibre types (I, IIA and IIX, previously called IIB) has been followed by a precise description of molecular composition and functional and biochemical properties. It has become apparent that the expression of different protein isoforms and therefore the existence of distinct muscle fibre phenotypes is one of the main determinants of the muscle performance in vivo. The present review will first describe the mechanisms through which molecular diversity is generated and how fibre types can be identified on the basis of structural and functional characteristics. Then the molecular and functional diversity will be examined with regard to (1) the myofibrillar apparatus; (2) the sarcolemma and the sarcoplasmic reticulum; and (3) the metabolic systems devoted to producing ATP. The last section of the review will discuss the advantage that fibre diversity can offer in optimizing muscle contractile performance.

Calcium ion in skeletal muscle: its crucial role for muscle function, plasticity, and disease

Mammalian skeletal muscle shows an enormous variability in its functional features such as rate of force production, resistance to fatigue, and energy metabolism, with a wide spectrum from slow aerobic to fast anaerobic physiology. In addition, skeletal muscle exhibits high plasticity that is based on the potential of the muscle fibers to undergo changes of their cytoarchitecture and composition of specific muscle protein isoforms. Adaptive changes of the muscle fibers occur in response to a variety of stimuli such as, e.g., growth and differentition factors, hormones, nerve signals, or exercise. Additionally, the muscle fibers are arranged in compartments that often function as largely independent muscular subunits. All muscle fibers use Ca(2+) as their main regulatory and signaling molecule. Therefore, contractile properties of muscle fibers are dependent on the variable expression of proteins involved in Ca(2+) signaling and handling. Molecular diversity of the main proteins in the Ca(2+) signaling apparatus (the calcium cycle) largely determines the contraction and relaxation properties of a muscle fiber. The Ca(2+) signaling apparatus includes 1) the ryanodine receptor that is the sarcoplasmic reticulum Ca(2+) release channel, 2) the troponin protein complex that mediates the Ca(2+) effect to the myofibrillar structures leading to contraction, 3) the Ca(2+) pump responsible for Ca(2+) reuptake into the sarcoplasmic reticulum, and 4) calsequestrin, the Ca(2+) storage protein in the sarcoplasmic reticulum. In addition, a multitude of Ca(2+)-binding proteins is present in muscle tissue including parvalbumin, calmodulin, S100 proteins, annexins, sorcin, myosin light chains, beta-actinin, calcineurin, and calpain. These Ca(2+)-binding proteins may either exert an important role in Ca(2+)-triggered muscle contraction under certain conditions or modulate other muscle activities such as protein metabolism, differentiation, and growth. Recently, several Ca(2+) signaling and handling molecules have been shown to be altered in muscle diseases. Functional alterations of Ca(2+) handling seem to be responsible for the pathophysiological conditions seen in dystrophinopathies, Brody's disease, and malignant hyperthermia. These also underline the importance of the affected molecules for correct muscle performance.

Deletions or specific substitutions of a few residues in the NH(2)-terminal region (Ala(3) to Thr(9)) of sarcoplasmic reticulum Ca(2+)-ATPase cause inactivation and rapid degradation of the enzyme expressed in COS-1 cells

Amino acid residues in the NH(2)-terminal region (Glu(2) - Ala(14)) of adult fast twitch skeletal muscle sarcoplasmic reticulum Ca(2+)-ATPase (SERCA1a) were deleted or substituted, and the mutants were expressed in COS-1 cells. Deletion of any single residue in the Ala(3)-Ser(6) region or deletion of two or more consecutive residues in the Ala(3)-Thr(9) region caused strongly reduced expression. Substitution mutants A4K, A4D, and H5K also showed very low expression levels. Deletion of any single residue in the Ala(3)-Ser(6) region caused only a small decrease in the specific Ca(2+) transport rate/mg of SERCA1a protein. In contrast, other mutants showing low expression levels had greatly reduced specific Ca(2+) transport rates. In vitro expression experiments indicated that translation, transcription, and integration into the microsomal membranes were not impaired in the mutants that showed very low expression levels in COS-1 cells. Pulse-chase experiments using [(35)S]methionine/cysteine labeling of transfected COS-1 cells demonstrated that degradation of the mutants showing low expression levels was substantially faster than that of the wild type. Lactacystin, a specific inhibitor of proteasome, inhibited the degradation accelerated by single-residue deletion of Ala(3). These results suggest that the NH(2)-terminal region (Ala(3) -Thr(9)) of SERCA1a is sensitive to the endoplasmic reticulum-mediated quality control and is thus critical for either correct folding of the SERCA1a protein or stabilization of the correctly folded SERCA1a protein or both.

Parvalbumin content in striated muscles of the common shrew (Sorex araneus)

The parvalbumin content of mammalian muscles correlates positively with isometric relaxation rate and fiber type IIB frequency of the muscles but negatively with animal size. Since shrews are small-bodied animals with a relatively low number of type IIB fibers, it is of some interest to know how the parvalbumin content of shrew muscle correlates with the above factors. Parvalbumin content in heart, diaphragm, and gastrocnemius muscle of the common shrew, mouse, and rat was determined electrophoretically. Parvalbumin was not found in heart muscle of any species. Shrew diaphragm (0.29 ± 0.04 g/kg) had significantly less parvalbumin than mouse (0.63 ± 0.11 g/kg) or rat (0.54 ± 0.09 g/kg) diaphragm. Similarly, the parvalbumin content of shrew gastrocnemius muscle (0.28 ± 0.04 g/kg) was significantly lower than in that of mouse (2.88 ± 0.38 g/kg) or rat (0.96 ± 0.25 g/kg) gastrocnemius muscle. The isometric twitch of the gastrocnemius muscle was somewhat faster than the twitch of the diaphragm in all three species. The isometric contractions of shrew and mouse skeletal muscles were generally very similar in duration, with the exception of the relaxation time of the gastrocnemius muscle, which was shorter in the mouse. Diaphragm and gastrocnemius muscle of the rat were clearly slower than the respective muscles in the mouse or shrew with regard to both the contraction and relaxation phases. The half-relaxation time of isometric contractions correlated relatively weakly with parvalbumin content of the muscles (r = 0.40) but more strongly with their fiber IIB content (r = 0.81). The unexpectedly low parvalbumin content and relatively slow rate of contraction in shrew skeletal muscles are attributed to the exceptional fiber type composition, i.e., a high proportion of type IID fibers.

Expression of sarcoplasmic/endoplasmic reticulum Ca2+ ATPases in the rat extensor digitorum longus (EDL) muscle regenerating from notexin-induced necrosis

The level of sarcoplasmic/endoplasmic reticulum Ca2+ ATPase (SERCA) mRNAs and proteins have been assessed by RT-PCR, immunoblotting and immunocytochemistry in the rat extensor digitorum longus (EDL) muscles during regeneration from notexin-induced necrosis. As a result of the necrosis, SERCA1 and SERCA2 declined on days 1 and 3 after administration of the toxin. Thereupon the mRNA of the fast isoform SERCA1 rapidly increased between days 5 and 10 to the normal level. The mRNA level of the "housekeeping" SERCA2b isoform increased markedly during the actual necrosis at days 1 and 5, probably due to invading cells. Then the mRNA level of the neonatal SERCA1b splice variant increased first, and exceeded the level of the adult SERCA1a transcript on day 5. At later stages of regeneration the neonatal form was gradually replaced by the adult SERCA1a form, thus recapitulating similar changes known to occur during normal ontogenesis. Along with SERCA1, the levels of the slow isoform (SERCA2a) mRNA and protein increased on day 5, but the SERCA2a mRNA levels never rose above 10% of SERCA1 and after 10 days gradually declined again. In the normal and regenerated muscles, SERCA1 was expressed in 97% of the fibres which accounted for the population of fast-twitch fibres (type IIa, type IIb and probably type IIx/d). SERCA2a was present in 6% of the fibres of normal muscle (mostly in the slow-twitch type I fibres). At the end of regeneration the number of fibres expressing SERCA2a was twice as high and were found in small groups, unlike in normal EDL, but about 50% of these clustered fibres also expressed SERCA1.

Inhibition and kinetic properties of membrane-bound carbonic anhydrases in rabbit skeletal muscles

It was the aim of this study to investigate whether the carbonic anhydrases associated with the sarcoplasmic reticulum (SR) and sarcolemmal membranes differ in their kinetic and inhibitory properties. To this end, sarcolemmal and SR membrane vesicle fractions were prepared from rabbit white and red skeletal muscles, the white muscle sarcolemmal fraction (WSL), the red muscle sarcolemmal fraction (RSL), the white muscle SR fraction (WSR), and the red muscle SR fraction (RSR). WSL displayed a specific carbonic anhydrase activity of 22.1 U . ml/mg and RSL of 7.5 U . ml/mg, whereas the SR fractions showed a much lower activity of 0.5 U . ml/mg for WSR and of 2.4 U . ml/mg for RSR. In both SR fractions phase separation experiments with Triton X-114 demonstrated that the carbonic anhydrase activity is due to a membrane-bound enzyme and not due to a cytosolic isozyme. The kinetic properties of carbonic anhydrase from the four distinct membane fractions were evaluated by determination of the Michaelis constant, Km, and of the catalytic centre activity kcat. Km appears to be somewhat lower for SR than for SL. Inhibition constants of SR and SL carbonic anhydrases were determined applying six carbonic anhydrase inhibitors: chlorzolamide, ethoxzolamide, methazolamide, benzolamide, and acetazolamide, and also cyanate. The inhibition constants of the SR fractions were significantly different from those of the corresponding sarcolemmal fractions, indicating that the carbonic anhydrase measured in the SR fractions does not originate from contaminating sarcolemmal membrane vesicles, but appears to represent a distinct carbonic anhydrase associated with the SR membrane.

Comparison of sarcoplasmic reticulum capabilities in toadfish (Opsanus tau) sonic muscle and rat fast twitch muscle

The sonic muscle of the oyster toadfish, Opsanus tau, can produce unfused contractions at 300 Hz. Electron microscopy shows a great abundance of the Sarcoplasmic reticulum (SR) in this muscle, but no functional characterization of the capabilities of the SR has been reported. We measured the oxalate-supported Ca2+ uptake rate and capacities of homogenates of toadfish sonic muscle and rat extensor digitorum longus (EDL) muscle, and estimated the number of pump units by titration with thapsigargin, a high-affinity, specific inhibitor of the SR Ca-ATPase. The Ca2+ uptake rate averaged 70.9 +/- 9.5 mumol min -1 per g tissue for the toad fish sonic muscle, and 73.5 +/- 3.7 mumol min -1 g-1 for rat EDL. The capacity for Ca2+ -oxalate uptake was 161 +/- 20 mumol g -1 and 33 +/- 2 mumol g -1 for toadfish sonic muscle and rat EDL, respectively. Thus, the rates of Ca2+ uptake were similar in the two muscles, but the toadfish sonic muscle had about five times the capacity of the rat EDL. The number of pumps as estimated by thapsigargin titration was 68 +/- 4 nmol of Ca-ATPase per g tissue in the toadfish, and 42 +/- 5 nmol Ca-ATPase per g tissue in the rat EDL. The turnover number, defined as the Ca2+ uptake divided by the number of pumps, was 1065 +/- 150 min -1 for toadfish and 1786 +/- 230 min -1 for rat EDL (p < 0.05) at 37 degrees C. The Ca2+ uptake rate of toadfish sonic muscle at 22 degree C, a typical temperature for calling toadfish, averaged 42 +/- 1% of its rate at 37 degree C. At these operating temperatures, the toadfish SR is likely to be slower than the rat fast-twitch SR, yet the toadfish sonic muscle supports more rapid contractions. One explanation for this is that the voluminous SR provides activator Ca2+ for contraction, but the abundant parvalbumin plays a major role in relaxation.

Cation pumps in skeletal muscle: potential role in muscle fatigue

Two membrane bound pumps in skeletal muscle, the sarcolemma Na+-K+ adenosine triphosphatase (ATPase) and the sarcoplasmic reticulum Ca2+-ATPase, provide for the maintenance of transmembrane ionic gradients necessary for excitation and activation of the myofibrillar apparatus. The rate at which the pumps are capable of establishing ionic homeostasis depends on the maximal activity of the enzyme and the potential of the metabolic pathways for supplying adenosine triphosphate (ATP). The activity of the Ca2+-ATPase appears to be expressed in a fibre type specific manner with both the amount of the enzyme and the isoform type related to the speed of contraction. In contrast, only minimal differences exist between slow-twitch and fast-twitch fibres in Na+-K+ ATPase activity. Evidence is accumulating that both active transport of Na+ and K+ across the sarcolemma and Ca2+-uptake by the sarcoplasmic reticulum may be impaired in vivo in a task specific manner resulting in loss of contractile function. In contrast to the Ca2+-ATPase, the Na+-K+ ATPase can be rapidly upregulated soon after the onset of a sustained pattern of activity. Similar programmes of activity result in a downregulation of Ca2+-ATPase but at a much later time point. The manner in which the metabolic pathways reorganize following chronic activity to meet the changes in ATP demand by the cation pumps and the degree to which these adaptations are compartmentalized is uncertain.

Oligomerization is an intrinsic property of calsequestrin in normal and transformed skeletal muscle

In skeletal muscle fibers, the high-capacity medium-affinity Ca(2+)-binding protein calsequestrin functions as the major Ca(2+)-reservoir of the sarcoplasmic reticulum. To determine the oligomeric status of calsequestrin, immunoblotting of microsomal proteins following chemical crosslinking was performed. Diagonal non-reducing/reducing two-dimensional gel electrophoresis was employed to unequivocally differentiate between cross-linked species of 63 kDa calsequestrin and calsequestrin-like proteins of higher relative molecular mass. Since chronic low-frequency stimulation has a profound effect on the expression of many muscle-specific protein isoforms, we investigated normal and conditioned muscle fibers. Calsequestrin was found to exist in a wide range of high-molecular-mass clusters in normal and chronically stimulated skeletal muscle fibers. Hence, oligomerization is an intrinsic property of this important Ca(2+)-binding protein and does not appear to be influenced by the fast-to-slow transformation process. Although fiber-type specific differences exist in the physiology of the skeletal muscle Ca(2+)-regulatory system, oligomerization of calsequestrin seems to be essential for proper functioning.

Early functional and biochemical adaptations to low-frequency stimulation of rabbit fast-twitch muscle

To examine mechanisms underlying force reduction after the onset of chronic low-frequency (10 Hz) stimulation (CLFS), we exposed rabbit tibialis anterior muscles to various durations of CLFS. To follow changes in isometric contractile properties and electromyographic (EMG) activity, we studied stimulated and contralateral muscles during a terminal test at 10 Hz for 10 min. In addition, activities and protein amounts of the sarcoplasmic reticulum Ca(2+)-ATPase, content of Na(+)-K(+)-ATPase, and expression patterns of triad junction components were examined. Force output and EMG amplitude declined abruptly soon after the onset of stimulation, suggesting refractoriness of a large fiber population. Although twitch force and to a lesser extent EMG activity gradually recovered after stimulation for 6 days and longer, the muscles exhibited profoundly altered properties, i.e., enhanced fatigue resistance, absence of twitch potentiation, and prolonged contraction and relaxation times. These changes were associated with significant increases in Na(+)-K(+)-ATPase concentration and significant decreases in Ca(2+)-ATPase, ryanodine receptor, dihydropyridine receptor, and triadin concentrations over the course of the 20 days of stimulation. Alterations in excitability, Ca2+ handling, and excitation-contraction coupling prior to changes in myofibrillar protein isoforms may thus be responsible for early functional alterations.

Existence of parvalbumin and biochemical characterization in quail and pigeon skeletal muscles with different fiber type compositions

The localization of parvalbumin was determined in skeletal muscles with different fiber type compositions from quails (Coturnix japonica) and pigeons (Columba livia) by sandwich ELISA. The biochemical profiles of these muscles were evaluated by the assay of total lactate dehydrogenase (LDH) activity as well as the LDH isozymes for anaerobic metabolism, and by the analysis of myoglobin for aerobic metabolism. The quail fast posterior latissimus dorsi (PLD) with a lower myoglobin content and higher LDH activity or M-type isozyme pattern was assessed as containing primarily fast-twitch glycolytic and oxidative-glycolytic (FG/FOG) fibers, and the mixed sartorius (SA) in quails and pigeons was shown to be composed mostly of FOG fibers because of the intermediate myoglobin content and LDH activity or H/M-type isozyme pattern. Parvalbumin, which functions as a relaxing factor in the fast-twitch fibers, was present in PLD and SA in significant amounts, whereas it was undetectable in quail anterior latissimus dorsi (ALD) and pigeon latissimus dorsi (LD) which contain exclusively slow-tonic (ST) fibers with the lowest LDH activity or predominant H-type isozyme characteristics. The pectoralis superficialis (PS) and pectoralis profundus (PP) muscles from quails and pigeons seem to consist mostly of FG/FOG fibers because of the highest myoglobin contents and LDH activities or M-type and H/M-type isozyme patterns. Despite fast-twitch fiber compositions, parvalbumin was absent from these pectoralis muscles. The tetanic contraction induced in avian fast-twitch pectoralis fibers during flapping flight might be independent of a function of parvalbumin as a relaxing factor.

Characterization of the promoter of the rat sarcoplasmic endoplasmic reticulum Ca2+-ATPase 1 gene and analysis of thyroid hormone responsiveness

Relaxation of skeletal muscle requires the re-uptake of Ca2+, which is mediated by the sarcoplasmic reticulum Ca2+-ATPase (SERCA). Thyroid hormone (T3) stimulates the expression of the SERCA1 isoform, which is essential for fast skeletal muscle fiber phenotype. We have cloned and studied the first 962 base pairs of the 5'-flanking region of the rat SERCA1 gene. This sequence was tested for T3-regulated expression in transient transfection experiments using COS7 cells and for binding of thyroid hormone receptor (TR) alpha in mobility shift assays. A construct of the 5'-flanking region and a reporter gene was unresponsive to T3 in the absence of co-transfected thyroid hormone receptor. In the presence of TRalpha, a T3 induction ratio of almost 4.0 was found, and this induction ratio was doubled with co-transfection of an RXR expression plasmid. Analysis of progressive 5'-deletion fragments of the sequence indicated multiple regions involved in T3 responsiveness. Three regions, R1, R2, and R3, were identified that bound TR complexes in mobility shift assays and conferred T3 responsiveness to a heterologous promoter. The most potent of these thyroid hormone response elements, R3, increased the 2-fold background T3 stimulation of the thymidine kinase promoter to nearly 6-fold. Detailed analysis of this element showed that four TR-binding half-sites, comprising two independent thyroid hormone response elements, interact cooperatively to give the maximal T3 response. T3 regulation of SERCA1 expression is mediated by a complex thyroid hormone response element that may serve to provide a greater range of response in interaction with nuclear receptor partners or cell-specific transcription factors.