Phosphate ion channels in sarcoplasmic reticulum of rabbit skeletal muscle

Toolkit for cellular studies of mammalian mitochondrial inorganic polyphosphate

Introduction: Inorganic polyphosphate (polyP) is an ancient polymer which is extremely well-conserved throughout evolution, and found in every studied organism. PolyP is composed of orthophosphates linked together by high-energy bonds, similar to those found in ATP. The metabolism and the functions of polyP in prokaryotes and simple eukaryotes are well understood. However, little is known about its physiological roles in mammalian cells, mostly due to its unknown metabolism and lack of systematic methods and effective models for the study of polyP in these organisms. Methods: Here, we present a comprehensive set of genetically modified cellular models to study mammalian polyP. Specifically, we focus our studies on mitochondrial polyP, as previous studies have shown the potent regulatory role of mammalian polyP in the organelle, including bioenergetics, via mechanisms that are not yet fully understood. Results: Using SH-SY5Y cells, our results show that the enzymatic depletion of mitochondrial polyP affects the expression of genes involved in the maintenance of mitochondrial physiology, as well as the structure of the organelle. Furthermore, this depletion has deleterious effects on mitochondrial respiration, an effect that is dependent on the length of polyP. Our results also show that the depletion of mammalian polyP in other subcellular locations induces significant changes in gene expression and bioenergetics; as well as that SH-SY5Y cells are not viable when the amount and/or the length of polyP are increased in mitochondria. Discussion: Our findings expand on the crucial role of polyP in mammalian mitochondrial physiology and place our cell lines as a valid model to increase our knowledge of both mammalian polyP and mitochondrial physiology.

Inorganic phosphate transporter in Giardia duodenalis and its possible role in ATP synthesis

Giardia duodenalis is a flagellated protozoan that inhabits vertebrate host intestines, causing the disease known as giardiasis. Similar to other parasites, G. duodenalis must take advantage of environmental resources to survive, such as inorganic phosphate (P_i) availability. P_i is an anionic molecule and an essential nutrient for all organisms because it participates in the biosynthesis of biomolecules, energy storage, and cellular structure formation. The first step in Pi metabolism is its uptake through specific transporters on the plasma membrane. We identified a symporter H⁺:P_i-type ORF sequence in the G. duodenalis genome (GenBank ID: GL50803_5164), named GdPho84, which is homologous to Saccharomyces cerevisiae PHO84. In trophozoites, P_i transport was linear for up to 15 min, and the cell density was 3 × 10⁷ cells/ml. Physiological variations in pH (6.4-8.0) did not influence P_i uptake. This P_i transporter had a high affinity, with K_0.5 = 67.7 ± 7.1 µM P_i. SCH28080 (inhibitor of H⁺, K⁺-ATPase), bafilomycin A₁ (inhibitor of vacuolar H⁺-ATPase), and FCCP (H⁺ ionophore) were able to inhibit P_i transport, indicating that an H⁺ gradient in the cell powered uphill P_i movement. PAA, an H⁺-dependent P_i transport inhibitor, reduced cell proliferation, P_i transport activity, and GdPHO48 mRNA levels. P_i starvation stimulated membrane potential-sensitive P_i uptake coupled to H⁺ fluxes, increased GdPho84 expression, and reduced intracellular ATP levels. These events indicate that these cells had an increased capacity to internalize P_i as a compensatory mechanism compared to cells maintained in control medium conditions. Internalized P_i can be used in glycolytic metabolism once iodoacetamide (GAPDH inhibitor) inhibits P_i influx. Together, these results reinforce the hypothesis that P_i is a crucial nutrient for G. duodenalis energy metabolism.

Monovalent: Divalent Anion Selectivity in the CFTR Channel Pore

The cystic fibrosis transmembrane conductance regulator (CFTR) Cl^- channel shows only weak selectivity between different small monovalent anions, however, little is known about its ability to discriminate between monovalent and divalent anions. The present study uses patch clamp recording to investigate the interaction between the small divalent anions S₂O₃^2- and SO₄^2- and wild-type and pore-mutant forms of human CFTR. Binding of these anions to wild-type CFTR appears weak; at 10 mM, intracellular S₂O₃^2- and SO₄^2- blocked <20 and <5% of macroscopic Cl^- current respectively, while these same concentrations had no discernible blocking effect when present in the extracellular solution. However, introduction of additional positive charge into the inner vestibule of the pore (in I344K and S1141K mutant channels) drastically strengthened block by intracellular (but not extracellular) S₂O₃^2- and SO₄^2-. Block of these mutant channels was highly voltage-dependent; at very negative membrane potentials, apparent binding affinities were ~100 µM for S₂O₃^2- and <1 mM for SO₄^2-. Permeability of S₂O₃^2- and SO₄^2- was too small to be quantified in wild-type CFTR, but was <1% of Cl^- permeability. Mutants that strengthened divalent binding (I344K, S1141K), as well as the selectivity-altering mutant F337A, also showed immeasurably low S₂O₃^2- and SO₄^2- permeabilities. Overall CFTR selects well for monovalent over divalent anions, both in terms of binding and permeability. The number or density of fixed positive charges in the pore appears well optimized to disfavour binding of divalent anions, which may be an important facet of the monovalent Cl^- permeation mechanism.

A chloride channel blocker prevents the suppression by inorganic phosphate of the cytosolic calcium signals that control muscle contraction

KEY POINTS

Accumulation of inorganic phosphate (P_i ) may contribute to muscle fatigue by precipitating calcium salts inside the sarcoplasmic reticulum (SR). Neither direct demonstration of this process nor definition of the entry pathway of P_i into SR are fully established.  We showed that P_i promoted Ca²⁺ release at concentrations below 10 mm and decreased it at higher concentrations. This decrease correlated well with that of [Ca²⁺ ]_SR .  Pre-treatment of permeabilized myofibres with 2 mm Cl^- channel blocker 9-anthracenecarboxylic acid (9AC) inhibited both effects of P_i .  The biphasic dependence of Ca²⁺ release on [P_i ] is explained by a direct effect of P_i acting on the SR Ca²⁺ release channel, combined with the intra-SR precipitation of Ca²⁺ salts. The effects of 9AC demonstrate that P_i enters the SR via a Cl^- pathway of an as-yet-undefined molecular nature.

ABSTRACT

Fatiguing exercise causes hydrolysis of phosphocreatine, increasing the intracellular concentration of inorganic phosphate (P_i ). P_i diffuses into the sarcoplasmic reticulum (SR) where it is believed to form insoluble Ca²⁺ salts, thus contributing to the impairment of Ca²⁺ release. Information on the P_i entrance pathway is still lacking. In amphibian muscles endowed with isoform 3 of the RyR channel, Ca²⁺ spark frequency is correlated with the Ca²⁺ load of the SR and can be used to monitor this variable. We studied the effects of P_i on Ca²⁺ sparks in permeabilized fibres of the frog. Relative event frequency (f/f_ref ) rose with increasing [P_i ], reaching 2.54 ± 1.6 at 5 mm, and then decreased monotonically, reaching 0.09 ± 0.03 at [P_i ] = 80 mm. Measurement of [Ca²⁺ ]_SR confirmed a decrease correlated with spark frequency at high [P_i ]. A large [Ca²⁺ ]_SR surge was observed upon P_i removal. Anion channels are a putative path for P_i into the SR. We tested the effect of the chloride channel blocker 9-anthracenecarboxylic acid (9AC) on P_i entrance. 9AC (400 µm) applied to the cytoplasm produced a non-significant increase in spark frequency and reduced the P_i effects on this parameter. Fibre treatment with 2 mm 9AC in the presence of high cytoplasmic Mg²⁺ suppressed the effects of P_i on [Ca²⁺ ]_SR and spark frequency up to 55 mm [P_i ]. These results suggest that chloride channels (or transporters) provide the main pathway of inorganic phosphate into the SR and confirm that P_i impairs Ca²⁺ release by accumulating and precipitating with Ca²⁺ inside the SR, thus contributing to myogenic fatigue.

Importance of Dietary Phosphorus for Bone Metabolism and Healthy Aging

Inorganic phosphate (Pi) plays a critical function in many tissues of the body: for example, as part of the hydroxyapatite in the skeleton and as a substrate for ATP synthesis. Pi is the main source of dietary phosphorus. Reduced bioavailability of Pi or excessive losses in the urine causes rickets and osteomalacia. While critical for health in normal amounts, dietary phosphorus is plentiful in the Western diet and is often added to foods as a preservative. This abundance of phosphorus may reduce longevity due to metabolic changes and tissue calcifications. In this review, we examine how dietary phosphorus is absorbed in the gut, current knowledge about Pi sensing, and endocrine regulation of Pi levels. Moreover, we also examine the roles of Pi in different tissues, the consequences of low and high dietary phosphorus in these tissues, and the implications for healthy aging.

Bioenergetic basis of skeletal muscle fatigue

Energetic demand from high-intensity exercise can easily exceed ATP synthesis rates of mitochondria leading to a reliance on anaerobic metabolism. The reliance on anaerobic metabolism results in the accumulation of intracellular metabolites, namely inorganic phosphate (P_i) and hydrogen (H⁺), that are closely associated with exercise-induced reductions in power. Cellular and molecular studies have revealed several steps where these metabolites impair contractile function demonstrating a causal role in fatigue. Elevated P_i or H⁺ directly inhibits force and power of the cross-bridge and decreases myofibrillar Ca²⁺ sensitivity, whereas P_i also inhibits Ca²⁺ release from the sarcoplasmic reticulum (SR). When both metabolites are elevated, they act synergistically to cause marked reductions in power, indicating that fatigue during high-intensity exercise has a bioenergetic basis.

Role of phosphate sensing in bone and mineral metabolism

Inorganic phosphate (P_i) is essential for signal transduction and cell metabolism, and is also an essential structural component of the extracellular matrix of the skeleton. P_i is sensed in bacteria and yeast at the plasma membrane, which activates intracellular signal transduction to control the expression of P_i transporters and other genes that control intracellular P_i levels. In multicellular organisms, P_i homeostasis must be maintained in the organism and at the cellular level, requiring an endocrine and metabolic P_i-sensing mechanism, about which little is currently known. This Review will discuss the metabolic effects of P_i, which are mediated by P_i transporters, inositol pyrophosphates and SYG1-Pho81-XPR1 (SPX)-domain proteins to maintain cellular phosphate homeostasis in the musculoskeletal system. In addition, we will discuss how P_i is sensed by the human body to regulate the production of fibroblast growth factor 23 (FGF23), parathyroid hormone and calcitriol to maintain serum levels of P_i in a narrow range. New findings on the crosstalk between iron and P_i homeostasis in the regulation of FGF23 expression will also be outlined. Mutations in components of these metabolic and endocrine phosphate sensors result in genetic disorders of phosphate homeostasis, cardiomyopathy and familial basal ganglial calcifications, highlighting the importance of this newly emerging area of research.

Molecular Basis for Exercise-Induced Fatigue: The Importance of Strictly Controlled Cellular Ca2+ Handling

The contractile function of skeletal muscle declines during intense or prolonged physical exercise, that is, fatigue develops. Skeletal muscle fibers fatigue acutely during highly intense exercise when they have to rely on anaerobic metabolism. Early stages of fatigue involve impaired myofibrillar function, whereas decreased Ca²⁺ release from the sarcoplasmic reticulum (SR) becomes more important in later stages. SR Ca²⁺ release can also become reduced with more prolonged, lower intensity exercise, and it is then related to glycogen depletion. Increased reactive oxygen/nitrogen species can cause long-lasting impairments in SR Ca²⁺ release resulting in a prolonged force depression after exercise. In this article, we discuss molecular and cellular mechanisms of the above fatigue-induced changes, with special focus on multiple mechanisms to decrease SR Ca²⁺ release to avoid energy depletion and preserve muscle fiber integrity. We also discuss fatigue-related effects of exercise-induced Ca²⁺ fluxes over the sarcolemma and between the cytoplasm and mitochondria.

Superhero physiology: the case for Captain America

Using pop icons in the science classroom represents a creative way to engage often-distracted students in a relevant and, perhaps more importantly, fun way. When the pop icon is as universally known as Captain America, the pedagogical stage is set. However, when the movies can also be employed to link dramatic references to the science concepts at hand, we may have a very powerful tool by which linkages between fiction and science can be forged. In this regard, Captain America's performances in several movies to date can be used to explain actual science. Granted, script writers and movie directors may or may not be interested in whether the physical performances they depict can be explained, but that is irrelevant. The point is to make a connection using science to explain how the superhero can run faster, jump higher, or lift more than is humanly possible. If a teachable moment has occurred and an important concept has been communicated, the educator has accomplished his or her job well.

Extracellular phosphates enhance activities of voltage-gated proton channels and production of reactive oxygen species in murine osteoclast-like cells

Osteoclasts are highly differentiated bone-resorbing cells and play a significant role in bone remodelling. In the resorption pit, inorganic phosphate (Pi) concentrations increase because of degradation of hydroxyapatite. We studied effects of extracellular Pi on voltage-gated H⁺ channels in osteoclast-like cells derived from a macrophage cell line (RAW264). Extracellular Pi (1.25-20 mM) increased the H⁺ channel currents dose dependently and reversibly. The Pi-induced increases were attenuated by removal of extracellular Na⁺ and by phosphonoformic acid, a blocker of Na⁺-dependent Pi transporters. Pi increased the maximal conductance, decreased activation time constant, increased deactivation time constant, and shifted the conductance-voltage relationship to more negative voltages. The most marked change was enhanced gating which was mainly caused by elevation of intracellular Pi levels. The Pi-induced enhanced gating was partially inhibited by protein kinase C (PKC) inhibitors, GF109203X and staurosporine, indicating that PKC-mediated phosphorylation was involved in part. The increase in the maximal conductance was mainly due to accompanying decrease in intracellular pH. These effects of Pi were not affected by intracellular Mg²⁺, bafilomycin A₁ (V-ATPase inhibitor) and removal of intracellular ATP. Extracellular Pi also upregulated reactive oxygen species (ROS). Diphenyleneiodonium chloride, an inhibitor of NADPH oxidases, decreased ROS production and partially attenuated the enhanced gating. In the cells during later passages where osteoclastogenesis declined, H⁺ channel activities and ROS production were both modest. These results suggest that, in osteoclasts, ambient Pi is a common enhancer for H⁺ channels and ROS production and that potentiation of H⁺ channels may help ROS production.

Exercise: Kinetic considerations for gas exchange

The activities of daily living typically occur at metabolic rates below the maximum rate of aerobic energy production. Such activity is characteristic of the nonsteady state, where energy demands, and consequential physiological responses, are in constant flux. The dynamics of the integrated physiological processes during these activities determine the degree to which exercise can be supported through rates of O₂ utilization and CO₂ clearance appropriate for their demands and, as such, provide a physiological framework for the notion of exercise intensity. The rate at which O₂ exchange responds to meet the changing energy demands of exercise--its kinetics--is dependent on the ability of the pulmonary, circulatory, and muscle bioenergetic systems to respond appropriately. Slow response kinetics in pulmonary O₂ uptake predispose toward a greater necessity for substrate-level energy supply, processes that are limited in their capacity, challenge system homeostasis and hence contribute to exercise intolerance. This review provides a physiological systems perspective of pulmonary gas exchange kinetics: from an integrative view on the control of muscle oxygen consumption kinetics to the dissociation of cellular respiration from its pulmonary expression by the circulatory dynamics and the gas capacitance of the lungs, blood, and tissues. The intensity dependence of gas exchange kinetics is discussed in relation to constant, intermittent, and ramped work rate changes. The influence of heterogeneity in the kinetic matching of O₂ delivery to utilization is presented in reference to exercise tolerance in endurance-trained athletes, the elderly, and patients with chronic heart or lung disease.

The multiple roles of phosphate in muscle fatigue

Muscle fatigue is the decline in performance of muscles observed during periods of intense activity. ATP consumption exceeds production during intense activity and there are multiple changes in intracellular metabolites which may contribute to the changes in crossbridge activity. It is also well-established that a reduction in activation, either through action potential changes or reduction in Ca(2+) release from the sarcoplasmic reticulum (SR), makes an additional contribution to fatigue. In this review we focus on the role of intracellular inorganic phosphate (P(i)) whose concentration can increase rapidly from around 5-30 mM during intense fatigue. Studies from skinned muscle fibers show that these changes substantially impair myofibrillar performance although the effects are strongly temperature dependent. Increased P(i) can also cause reduced Ca(2+) release from the SR and may therefore contribute to the reduced activation. In a recent study, we have measured both P(i) and Ca(2+) release in a blood-perfused mammalian preparation and the results from this preparation allows us to test the extent to which the combined effects of P(i) and Ca(2+) changes may contribute to fatigue.

ATP regulation of a large conductance voltage-gated cation channel in rough endoplasmic reticulum of rat hepatocytes

ATP-sensitive K+ channels play an important role in regulating membrane potential during metabolic stress. In this work we report the effect of ATP and ADP-Mg on a K+ channel present in the membrane of rough endoplasmic reticulum (RER) from rat hepatocytes incorporated into lipid bilayers. Channel activity was found to decrease in presence of ATP 100 microM on the cytoplasmic side and was totally inhibited at ATP concentrations greater than 0.25mM. The effect appeared voltage dependent, suggesting that the ATP binding site was becoming available upon channel opening. Channel activity was suppressed by the nonhydrolyzable ATP analog (ATPgammaS), ruling out a phosphorylation-based mechanism. Notably addition of 2.5mM ADP-Mg to the cytosolic side increased the channel open probability at negative potentials. We conclude that the large conductance voltage-gated cation channel in RER of rat hepatocytes is an ATP and ADP sensitive channel likely to be involved in cellular processes such as Ca(2+) signaling or control of membrane potential across the endoplasmic reticulum membrane.

Comparison of the effects of inorganic phosphate on caffeine-induced Ca2+ release in fast- and slow-twitch mammalian skeletal muscle

We compared the effects of 50 mM P(i) on caffeine-induced Ca(2+) release in mechanically skinned fast-twitch (FT) and slow-twitch (ST) skeletal muscle fibers of the rat. The time integral (area) of the caffeine response was reduced by approximately 57% (FT) and approximately 27% (ST) after 30 s of exposure to 50 mM P(i) in either the presence or absence of creatine phosphate (to buffer ADP). Differences in the sarcoplasmic reticulum (SR) Ca(2+) content between FT and ST fibers [ approximately 40% vs. 100% SR Ca(2+) content (pCa 6.7), respectively] did not contribute to the different effects of P(i) observed; underloading the SR of ST fibers so that the SR Ca(2+) content approximated that of FT fibers resulted in an even smaller ( approximately 21%), but not significant, reduction in caffeine-induced Ca(2+) release by P(i). These observed differences between FT and ST fibers could arise from fiber-type differences in the ability of the SR to accumulate Ca(2+)-P(i) precipitate. To test this, fibers were Ca(2+) loaded in the presence of 50 mM P(i). In FT fibers, the maximum SR Ca(2+) content (pCa 6.7) was subsequently increased by up to 13 times of that achieved when loading for 2 min in the absence of P(i). In ST fibers, the SR Ca(2+) content was only doubled. These data show that Ca(2+) release in ST fibers was less affected by P(i) than FT fibers, and this may be due to a reduced capacity of ST SR to accumulate Ca(2+)-P(i) precipitate. This may account, in part, for the fatigue-resistant nature of ST fibers.

Transport and transporters in the endoplasmic reticulum

Enzyme activities localized in the luminal compartment of the endoplasmic reticulum are integrated into the cellular metabolism by transmembrane fluxes of their substrates, products and/or cofactors. Most compounds involved are bulky, polar or even charged; hence, they cannot be expected to diffuse through lipid bilayers. Accordingly, transport processes investigated so far have been found protein-mediated. The selective and often rate-limiting transport processes greatly influence the activity, kinetic features and substrate specificity of the corresponding luminal enzymes. Therefore, the phenomenological characterization of endoplasmic reticulum transport contributes largely to the understanding of the metabolic functions of this organelle. Attempts to identify the transporter proteins have only been successful in a few cases, but recent development in molecular biology promises a better progress in this field.

Disulfonic stilbene permeation and block of the anion channel from the sarcoplasmic reticulum of rabbit skeletal muscle

Block of a sarcoplasmic reticulum anion channel (SCl channel) by disulfonic stilbene derivatives [DIDS, dibenzamidostilbene-2,2′-disulfonic acid (DBDS), and 4,4′-dinitrostilbene-2,2′-disulfonic acid (DNDS)] was investigated in planar bilayers using SO[Formula: see text] as the conducting ion. All molecules caused reversible voltage-dependent channel block when applied to either side of the membrane. DIDS also produced nonreversible channel block from both sides within 1–3 min. Reversible inhibition was associated with a decrease in channel open probability and mean open duration but not with any change in channel conductance. The half inhibitory concentration for cis- and trans-inhibition had voltage dependencies with minima of 190 nM and 33 μM for DBDS and 3.4 and 55 μM for DNDS. Our data supports a permeant blocker mechanism, in which stilbenes block SCl channels by lodging in the permeation pathway, where they may dissociate to either side of the membrane and thus permeate the channel. The stilbenes acted as open channel blockers where the binding of a single molecule occludes the channel. DBDS and DNDS, from opposite sides of the membrane, competed for common sites on the channel. Dissociation rates exhibited biphasic voltage dependence, indicative of two dissociation processes associated with ion movement in opposite directions within the trans-membrane electric field. The kinetics of DNDS and DBDS inhibition predict that there are two stilbene sites in the channel that are separated by 14–24 Å and that the pore constriction is ∼10 Å in diameter.

Anion channels transport ATP into the Golgi lumen

Anion channels provide a pathway for Cl(-) influx into the lumen of the Golgi cisternae. This influx permits luminal acidification by the organelle's H(+)-ATPase. Three different experimental approaches, electrophysiological, biochemical, and proteomic, demonstrated that two Golgi anion channels, GOLAC-1 and GOLAC-2, also mediate ATP anion transport into the Golgi lumen. First, GOLAC-1 and -2 were incorporated into planar lipid bilayers, and single-channel recordings were obtained. Low ionic activities of K(2)ATP added to the cis-chamber directly inhibited the Cl(-) subconductance levels of both channels, with K(m) values ranging from 16 to 115 microM. Substitution of either K(2)ATP or MgATP for Cl(-) on the cis, trans, or both sides indicated that ATP is conducted by the channels with a relative permeability sequence of Cl(-) > ATP(4-) > MgATP(2-). Single-channel currents were observed at physiological concentrations of Cl(-) and ATP, providing evidence for their importance in vivo. Second, transport of [alpha-(32)P]ATP into sealed Golgi vesicles that maintain in situ orientation was consistent with movement through the GOLACs because it exhibited little temperature dependence and was saturated with an apparent K(m) = 25 microM. Finally, after transport of [gamma-(32)P]ATP, a protease-protection assay demonstrated that proteins are phosphorylated within the Golgi lumen, and after SDS-PAGE, the proteins in the phosphorylated bands were identified by mass spectrometry. GOLAC conductances, [alpha-(32)P]ATP transport, and protein phosphorylation have identical pharmacological profiles. We conclude that the GOLACs play dual roles in the Golgi complex, providing pathways for Cl(-) and ATP influx into the Golgi lumen.

Calcium phosphate precipitation in the sarcoplasmic reticulum reduces action potential-mediated Ca2+release in mammalian skeletal muscle

During vigorous exercise, Piconcentration levels within the cytoplasm of fast-twitch muscle fibers may reach ≥30 mM. Cytoplasmic Pimay enter the sarcoplasmic reticulum (SR) and bind to Ca2+to form a precipitate (CaPi), thus reducing the amount of releasable Ca2+. Using mechanically skinned rat fast-twitch muscle fibers, which retain the normal action potential-mediated Ca2+release mechanism, we investigated the consequences of Piexposure on normal excitation-contraction coupling. The total amount of Ca2+released from the SR by a combined caffeine/low-Mg2+concentration stimulus was reduced by ∼20%, and the initial rate of force development slowed after 2-min exposure to 30 mM Pi(with or without the presence creatine phosphate). Peak (50 Hz) tetanic force was also reduced (by ∼25% and ∼45% after 10 and 30 mM Piexposure, respectively). Tetanic force responses produced after 30 mM Piexposure were nearly identical to those observed in the same fiber after depletion of total SR Ca2+by ∼35%. Ca2+content assays revealed that the total amount of Ca2+in the SR was not detectably changed by exposure to 30 mM Pi, indicating that Ca2+had not leaked from the SR but instead formed a precipitate with the Pi, reducing the amount of available Ca2+for rapid release. These results suggest that CaPiprecipitation that occurs within the SR could contribute to the failure of Ca2+release observed in the later stages of metabolic muscle fatigue. They also demonstrate that the total amount of Ca2+stored in the SR cannot drop substantially below the normal endogenous level without reducing tetanic force responses.

The Sarcoplasmic Reticulum in Muscle Fatigue and Disease: Role of the Sarco(endo)plasmic Reticulum Ca2+-ATPase

Skeletal muscles induced to contract repeatedly respond with a progressive loss in their ability to generate a target force or power. This condition is known simply as fatigue. Commonly, fatigue may persist for prolonged periods of time, particularly at low activation frequencies, which is called low-frequency fatigue. Failure to activate the contractile apparatus with the appropriate intracellular free calcium ([Ca2+]f) signal contributes to fatigue but the precise mechanisms involved are unknown. The sarcoplasmic reticulum (SR) is the major organelle in muscle that is responsible for the regulation of [Ca2+]f, and numerous studies have shown that SR function, both Ca2+ release and Ca2+ uptake, is impaired following fatiguing contractile activity. The major aim of this review is to provide insight into the various cellular mechanisms underlying the alterations in SR Ca2+ cycling and cytosolic [Ca2+]f that are associated both with the development of fatigue during repeated muscle contraction and with low-frequency or long-lasting fatigue. The primary focus will be on the role of the sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) in normal muscle function, fatigue, and disease. Key words: calcium release, calcium uptake, muscle relaxation, low-frequency fatigue, Brody disease

Metabolic factors contributing to altered Ca2+ regulation in skeletal muscle fatigue

AIM

Skeletal muscle fatigue is characterized by a failure to maintain force production or power output during intense exercise. Many recent studies on isolated fibres have used brief repetitive tetanic contractions to mimic fatigue resulting from intensive exercise and to investigate the underlying cellular mechanisms. Such studies have shown that characteristic changes in Ca2+ regulation occur during fatiguing stimulation. This includes prolongation of the 'Ca2+-tails' which follow each period of tetanic stimulation and a progressive rise in resting [Ca2+]. More importantly, the final stage of fatigue is associated with a rapid decrease in tetanic [Ca2+]i and force. These fatigue-induced changes in sarcoplasmic reticulum (SR) Ca2+ regulation are temporally associated with alterations in the intracellular levels of phosphate metabolites and a causal relationship has often been proposed. The aim of this review is to evaluate the evidence linking changes in the levels of phosphate metabolites and altered Ca2+ regulation during fatigue.

RESULTS

The following current hypotheses will be discussed: (1) the early changes in Ca2+ regulation reflect alterations in the intracellular levels of phosphate metabolites, (2) inhibition of the SR Ca2+ release mechanism (e.g. caused by ATP depletion and increased [Mg2+]) contributes to the decrease in tetanic [Ca2+]i during the final stages of fatigue and (iii) delayed entry of inorganic phosphate ions (Pi) into the SR, followed by precipitation of calcium phosphate (Ca-Pi), can explain the fatigue-induced decrease in tetanic [Ca2+]i.

CONCLUSION

There is strong evidence that changes in phosphate metabolite levels contribute to early changes in SR Ca2+ regulation during fatigue and that inhibition of the SR Ca2+ release mechanism can partially explain the rapid decrease in tetanic [Ca2+]i during the final stages of fatigue. While precipitation of Ca-Pi may occur within the SR during fatigue, there is currently insufficient evidence to establish whether this contributes to the late decline in tetanic [Ca2+]i.

Challenging accepted ion channel biology: p64 and the CLIC family of putative intracellular anion channel proteins (Review)

Parchorin, p64 and the related chloride intracellular channel (CLIC) proteins are widely expressed in multicellular organisms and have emerged as candidates for novel, auto-inserting, self-assembling intracellular anion channels involved in a wide variety of fundamental cellular events including regulated secretion, cell division and apoptosis. Although the mammalian phosphoproteins p64 and parchorin (49 and 65K, respectively) have only been indirectly implicated in anion channel activity, two CLIC proteins (CLIC1 and CLIC4, 27 and 29K, respectively) appear to be essential molecular components of anion channels, and CLIC1 can form anion channels in planar lipid bilayers in the absence of other cellular proteins. However, these putative ion channel proteins are controversial because they exist in both soluble and membrane forms, with at least one transmembrane domain. Even more surprisingly, soluble CLICs share the same glutaredoxin fold as soluble omega class glutathione-S-transferases. Working out how these ubiquitous, soluble proteins unfold, insert into membranes and then refold to form integral membrane proteins, and how cells control this potentially dangerous process and make use of the associated ion channels, are challenging prospects. Critical to this future work is the need for better characterization of membrane topology, careful functional analysis of reconstituted and native channels, including their conductances and selectivities, and detailed structure/function studies including targeted mutagenesis to investigate the structure of the putative pore, the role of protein phosphorylation and the role of conserved cysteine residues.

Role of phosphate and calcium stores in muscle fatigue

Intensive activity of muscles causes a decline in performance, known as fatigue, that is thought to be caused by the effects of metabolic changes on either the contractile machinery or the activation processes. The concentration of inorganic phosphate (P(i)) in the myoplasm ([P(i)](myo)) increases substantially during fatigue and affects both the myofibrillar proteins and the activation processes. It is known that a failure of sarcoplasmic reticulum (SR) Ca(2+) release contributes to fatigue and in this review we consider how raised [P(i)](myo) contributes to this process. Initial evidence came from the observation that increasing [P(i)](myo) causes reduced SR Ca(2+) release in both skinned and intact fibres. In fatigued muscles the store of releasable Ca(2+) in the SR declines mirroring the decline in SR Ca(2+) release. In muscle fibres with inoperative creatine kinase the rise of [P(i)](myo) is absent during fatigue and the failure of SR Ca(2+) release is delayed. These results can all be explained if inorganic phosphate can move from the myoplasm into the SR during fatigue and cause precipitation of CaP(i) within the SR. The relevance of this mechanism in different types of fatigue in humans is considered.