Electrically silent divalent cation entries in resting and active voltage-controlled muscle fibers

Ryanodine receptor activity and store-operated Ca2+ entry: Critical regulators of Ca2+ content and function in skeletal muscle

Store-operated Ca2+ entry (SOCE) is critical to cell function. In skeletal muscle, SOCE has evolved alongside excitation-contraction coupling (EC coupling); as a result, it displays unique properties compared to SOCE in other cells. The plasma membrane of skeletal muscle is mostly internalized as the tubular system, with the tubules meeting the sarcoplasmic reticulum (SR) terminal cisternae, forming junctions where the proteins that regulate EC coupling and SOCE are positioned. In this review, we describe the properties and roles of SOCE based on direct measurements of Ca2+ influx during SR Ca2+ release and leak. SOCE is activated immediately and locally as the [Ca2+ ] of the junctional SR terminal cisternae ([Ca2+ ]jSR ) depletes. [Ca2+ ]jSR changes rapidly and steeply with increasing activity of the SR ryanodine receptor isoform 1 (RyR1). The high fidelity of [Ca2+ ]jSR with RyR1 activity probably depends on the SR Ca2+ -buffer calsequestrin that is located immediately behind RyR1 inside the SR. This arrangement provides in-phase activation and deactivation of SOCE with a large dynamic range, allowing precise grading of SOCE flux. The in-phase activation of SOCE as the SR partially depletes traps Ca2+ in the cytoplasm, preventing net Ca2+ loss. Mild presentation of RyR1 leak can occur under physiological conditions, providing fibre Ca2+ redistribution without changing fibre Ca2+ content. This condition preserves normal contractile function at the same time as increasing basal metabolic rate. However, higher RyR1 leak drives excess cytoplasmic and mitochondrial Ca2+ load, setting a deleterious intracellular environment that compromises the function of the skeletal muscle.

Stability of Drinking Water Distribution Systems and Control of Disinfection By-Products

The stability of drinking water distribution systems and the management of disinfection by-products are critical to ensuring public health safety. In this paper, the interrelationships between corrosion products in the network, microbes, and drinking water quality are elucidated. This review also discusses the mechanisms through which corrosive by-products from the piping network influence the decay of disinfectants and the formation of harmful disinfection by-products. Factors such as copper corrosion by-products, CuO, Cu₂O, and Cu²⁺ play a significant role in accelerating disinfectant decay and catalyzing the production of by-products. Biofilms on pipe walls react with residual chlorine, leading to the formation of disinfection by-products (DBPs) that also amplify health risks. Finally, this paper finally highlights the potential of peroxymonosulfate (PMS), an industrial oxidant, as a disinfectant that can reduce DBP formation, while acknowledging the risks associated with its corrosive nature. Overall, the impact of the corrosive by-products of pipe scale and microbial communities on water quality in pipe networks is discussed, and recommendations for removing DBPs are presented.

Store-Operated Calcium Entry in Skeletal Muscle: What Makes It Different?

Current knowledge on store-operated Ca²⁺ entry (SOCE) regarding its localization, kinetics, and regulation is mostly derived from studies performed in non-excitable cells. After a long time of relative disinterest in skeletal muscle SOCE, this mechanism is now recognized as an essential contributor to muscle physiology, as highlighted by the muscle pathologies that are associated with mutations in the SOCE molecules STIM1 and Orai1. This review mainly focuses on the peculiar aspects of skeletal muscle SOCE that differentiate it from its counterpart found in non-excitable cells. This includes questions about SOCE localization and the movement of respective proteins in the highly organized skeletal muscle fibers, as well as the diversity of expressed STIM isoforms and their differential expression between muscle fiber types. The emerging evidence of a phasic SOCE, which is activated during EC coupling, and its physiological implication is described as well. The specific issues related to the use of SOCE modulators in skeletal muscles are discussed. This review highlights the complexity of SOCE activation and its regulation in skeletal muscle, with an emphasis on the most recent findings and the aim to reach a current picture of this mesmerizing phenomenon.

Divalent cations permeation in a Ca2+ non-conducting skeletal muscle dihydropyridine receptor mouse model

In response to excitation of skeletal muscle fibers, trains of action potentials induce changes in the configuration of the dihydropyridine receptor (DHPR) anchored in the tubular membrane which opens the Ca²⁺ release channel in the sarcoplasmic reticulum membrane. The DHPR also functions as a voltage-gated Ca²⁺ channel that conducts L-type Ca²⁺ currents routinely recorded in mammalian muscle fibers, which role was debated for more than four decades. Recently, to allow a closer look into the role of DHPR Ca²⁺ influx in mammalian muscle, a knock-in (ki) mouse model (ncDHPR) carrying mutation N617D (adjacent to domain II selectivity filter E) in the DHPRα_1S subunit abolishing Ca²⁺ permeation through the channel was generated [Dayal et al., 2017]. In the present study, the Mn²⁺ quenching technique was initially intended to be used on voltage-clamped muscle fibers from this mouse to determine whether Ca²⁺ influx through a pathway distinct from DHPR may occur to compensate for the absence of DHPR Ca²⁺ influx. Surprisingly, while N617D DHPR muscle fibers of the ki mouse do not conduct Ca²⁺, Mn²⁺ entry and subsequent quenching did occur because Mn²⁺ was able to permeate and produce L-type currents through N617D DHPR. N617D DHPR was also found to conduct Ba²⁺ and Ba²⁺ currents were strongly blocked by external Ca²⁺. Ba²⁺ permeation was smaller, current kinetics slower and Ca²⁺ block more potent than in wild-type DHPR. These results indicate that residue N617 when replaced by the negatively charged residue D is suitably located at entrance of the pore to trap external Ca²⁺ impeding in this way permeation. Because Ba²⁺ binds with lower affinity to D, Ba²⁺ currents occur, but with reduced amplitudes as compared to Ba²⁺ currents through wild-type channels. We conclude that mutations located outside the selectivity filter influence channel permeation and possibly channel gating in a fully differentiated skeletal muscle environment.

Voltage-gated Ca(2+) influx through L-type channels contributes to sarcoplasmic reticulum Ca(2+) loading in skeletal muscle

Muscle contraction is triggered by Ca(2+) ions released from the sarcoplasmic reticulum (SR) in response to depolarization of skeletal muscle fibres. Muscle activation is also associated with a voltage-activated trans-sarcolemmal Ca(2+) influx early identified as a current flowing through L-type Ca(2+) channels. Because removal of external Ca(2+) does not impede fibres from contracting, a negligible role was given to this voltage-activated Ca(2+) entry, although the decline of Ca(2+) release is more pronounced in the absence of Ca(2+) during long-lasting activation. Furthermore, it is not clearly established whether Ca(2+) exclusively flows through L-type channels or in addition through a parallel voltage-activated pathway distinct from L-type channels. Here, by monitoring the quenching of fura-2 fluorescence resulting from Mn(2+) influx in voltage-controlled mouse and zebrafish isolated muscle fibres, we show that the L-type current is the only contributor to Ca(2+) influx during long-lasting depolarizations in skeletal muscle. Calibration of the Mn(2+) quenching signal allowed us to estimate a mean Mn(2+) current of 0.31 ± 0.06 A F(-1) flowing through L-type channels during a train of action potentials. Measurements of SR Ca(2+) changes with fluo-5N in response to depolarization revealed that an elevated voltage-activated Ca(2+) current potentiated SR Ca(2+) loading and addition of external Mn(2+) produced quenching of fluo-5N in the SR, indicating that voltage-activated Ca(2+) /Mn(2+) influx contributes to SR Ca(2+) /Mn(2+) loading.

Modulation effects of cordycepin on the skeletal muscle contraction of toad gastrocnemius muscle

Isolated toad gastrocnemius muscle is a typical skeletal muscle tissue that is frequently used to study the motor system because it is an important component of the motor system. This study investigates the effects of cordycepin on the skeletal muscle contractile function of isolated toad gastrocnemius muscles by electrical field stimulation. Results showed that cordycepin (20 mg/l to 100 mg/l) significantly decreased the contractile responses in a concentration-dependent manner. Cordycepin (50 mg/l) also produced a rightward shift of the contractile amplitude-stimulation intensity relationship, as indicated by the increases in the threshold stimulation intensity and the saturation stimulation intensity. However, the most notable result was that the maximum amplitude of the muscle contractile force was significantly increased under cordycepin application (122±3.4% of control). This result suggests that the skeletal muscle contractile function and muscle physical fitness to the external stimulation were improved by the decreased response sensitivity in the presence of cordycepin. Moreover, cordycepin also prevented the repetitive stimulation-induced decrease in muscle contractile force and increased the recovery amplitude and recovery ratio of muscle contraction. However, these anti-fatigue effects of cordycepin on muscle contraction during long-lasting muscle activity were absent in Ca2+-free medium or in the presence of all Ca2+ channels blocker (0.4 mM CdCl2). These results suggest that cordycepin can positively affect muscle performance and provide ergogenic and prophylactic benefits in decreasing skeletal muscle fatigue. The mechanisms involving excitation-coupled Ca2+ influxes are strongly recommended.

Major contribution of sarcoplasmic reticulum Ca(2+) depletion during long-lasting activation of skeletal muscle

Depolarization of skeletal muscle fibers induces sarcoplasmic reticulum (SR) Ca²⁺ release and contraction that progressively decline while depolarization is maintained. Voltage-dependent inactivation of SR Ca²⁺ release channels and SR Ca²⁺ depletion are the two processes proposed to explain the decline of SR Ca²⁺ release during long-lasting depolarizations. However, the relative contribution of these processes, especially under physiological conditions of activation, is not clearly established. Using Fura-2 and Fluo-5N to monitor cytosolic and SR Ca²⁺ changes, respectively, in voltage-controlled mouse muscle fibers, we show that 2-min conditioning depolarizations reduce voltage-activated cytosolic Ca²⁺ signals with a V_1/2 of −53 mV but also induce SR Ca²⁺ depletion that decreased the releasable pool of Ca²⁺ with the same voltage sensitivity. In contrast, measurement of SR Ca²⁺ changes indicated that SR Ca²⁺ release channels were inactivated after SR had been depleted and in response to much higher depolarizations with a V_1/2 of −13 mV. In response to trains of action potentials, cytosolic Ca²⁺ signals decayed with time, whereas SR Ca²⁺ changes remained stable over 1-min stimulation, demonstrating that SR Ca²⁺ depletion is exclusively responsible for the decline of SR Ca²⁺ release under physiological conditions of excitation. These results suggest that previous studies using steady-state inactivation protocols to investigate the voltage dependence of Ca²⁺ release inactivation in fact probed the voltage dependence of SR Ca²⁺ depletion, and that SR Ca²⁺ depletion is the only process that leads to Ca²⁺ release decline during continuous stimulation of skeletal muscle.

Store-operated Ca²⁺ entry is not required for store refilling in skeletal muscle

The present review describes store-operated Ca²⁺ entry (SOCE) in skeletal muscle. Fundamental discoveries in the field of skeletal muscle SOCE are described and the techniques that were used to make these. The advantages and limitations in these techniques are discussed to provide a means of questioning and determining the physiological role(s) of SOCE in skeletal muscle. It is concluded that SOCE has little or no role in the filling of the sarcoplasmic reticulum with Ca²⁺ at rest or during a single contracture. It is likely that SOCE is activated during fatigue, although direct measurements of SOCE are lacking and the physiological significance remains uncertain.

Dihydropyridine receptors actively control gating of ryanodine receptors in resting mouse skeletal muscle fibres

Contraction of skeletal muscle is triggered by the release of Ca(2+) from the sarcoplasmic reticulum (SR) in response to depolarization of the muscle membrane. Depolarization is known to elicit a conformational change of the dihydropyridine receptor (DHPR) in the tubular membrane that controls in a time- and voltage-dependent manner the opening of the ryanodine receptor (RyR), the SR Ca(2+) release channel. At rest, it is assumed that RyRs are kept in a closed state imposed by the repressive action of DHPRs; however, a direct control of the RyR gating by the DHPR has up to now never been demonstrated in resting adult muscle. In this study, we monitored slow changes in SR Ca(2+) content using the Ca(2+) indicator fluo-5N loaded in the SR of voltage-clamped mouse muscle fibres. We first show that external Ca(2+) removal induced a reversible SR Ca(2+) efflux at -80 mV and prevented SR Ca(2+) refilling following depolarization-evoked SR Ca(2+) depletion. The dihydropyridine compound nifedipine induced similar effects. The rate of SR Ca(2+) efflux was also shown to be controlled in a time- and voltage-dependent manner within a membrane potential range more negative than -50 mV. Finally, intracellular addition of ryanodine produced an irreversible SR Ca(2+) efflux and kept the SR in a highly depleted state following depolarization-evoked SR Ca(2+) depletion. The fact that resting SR Ca(2+) efflux is modulated by conformational changes of DHPRs induced by external Ca(2+), nifedipine and voltage demonstrates that DHPRs exert an active control on gating of RyRs in resting skeletal muscle.

Sarcoplasmic reticulum Ca2+ permeation explored from the lumen side in mdx muscle fibers under voltage control

Under resting conditions, external Ca²⁺ is known to enter skeletal muscle cells, whereas Ca²⁺ stored in the sarcoplasmic reticulum (SR) leaks into the cytosol. The nature of the pathways involved in the sarcolemmal Ca²⁺ entry and in the SR Ca²⁺ leak is still a matter of debate, but several lines of evidence suggest that these Ca²⁺ fluxes are up-regulated in Duchenne muscular dystrophy. We investigated here SR calcium permeation at resting potential and in response to depolarization in voltage-controlled skeletal muscle fibers from control and mdx mice, the mouse model of Duchenne muscular dystrophy. Using the cytosolic Ca²⁺ dye Fura2, we first demonstrated that the rate of Ca²⁺ increase in response to cyclopiazonic acid (CPA)–induced inhibition of SR Ca²⁺-ATPases at resting potential was significantly higher in mdx fibers, which suggests an elevated SR Ca²⁺ leak. However, removal of external Ca²⁺ reduced the rate of CPA-induced Ca²⁺ increase in mdx and increased it in control fibers, which indicates an up-regulation of sarcolemmal Ca²⁺ influx in mdx fibers. Fibers were then loaded with the low-affinity Ca²⁺ dye Fluo5N-AM to measure intraluminal SR Ca²⁺ changes. Trains of action potentials, chloro-m-cresol, and depolarization pulses evoked transient Fluo5N fluorescence decreases, and recovery of voltage-induced Fluo5N fluorescence changes were inhibited by CPA, demonstrating that Fluo5N actually reports intraluminal SR Ca²⁺ changes. Voltage dependence and magnitude of depolarization-induced SR Ca²⁺ depletion were found to be unchanged in mdx fibers, but the rate of the recovery phase that followed depletion was found to be faster, indicating a higher SR Ca²⁺ reuptake activity in mdx fibers. Overall, CPA-induced SR Ca²⁺ leak at −80 mV was found to be significantly higher in mdx fibers and was potentiated by removal of external Ca²⁺ in control fibers. The elevated passive SR Ca²⁺ leak may contribute to alteration of Ca²⁺ homeostasis in mdx muscle.

Store-operated calcium entry remains fully functional in aged mouse skeletal muscle despite a decline in STIM1 protein expression

Store-operated Ca(2+) entry (SOCE) is a robust mechanism in skeletal muscle, supported by abundant STIM1 and Orai1 in the junctional membranes. The precise role of SOCE in skeletal muscle Ca(2+) homeostasis and excitation-contraction coupling remains to be defined. Regardless, it remains important to determine whether the function and capacity of SOCE changes in aged skeletal muscle. We identified an approximate 40% decline in the expression of the integral SOCE protein, stromal interacting molecule 1 (STIM1), but no such decline in its coupling partner, Orai1, in muscle fibers from aged mice. To determine whether this changed aspects of SOCE functionality in skeletal muscle in aged mice, Ca(2+) in the cytoplasm and t-system were continuously and simultaneously imaged on a confocal microscope during sarcoplasmic reticulum Ca(2+) release and compared to experiments under identical conditions using muscle fibers from young mice. Normal activation, deactivation, Ca(2+) influx, and spatiotemporal characteristics of SOCE were found to persist in skeletal muscle from aged mice. Thus, SOCE remains a robust mechanism in aged skeletal muscle despite the decline in STIM1 protein expression, suggesting STIM1 is in excess in young skeletal muscle.

Mechanisms of carbacholine and GABA action on resting membrane potential and Na+/K+-ATPase of Lumbricus terrestris body wall muscles

This work was aimed to identify the action of several ion channel and pump inhibitors as well as nicotinic, GABAergic, purinergic and serotoninergic drugs on the resting membrane potential (RMP) and assess the role of cholinergic and GABAergic sensitivity in earthworm muscle electrogenesis. The nicotinic agonists acetylcholine (ACh), carbacholine (CCh) and nicotine depolarize the RMP at concentrations of 5 μM and higher. The nicotinic antagonists (+)tubocurarine, α-bungarotoxin, muscarinic antagonists atropine and hexamethonium do not remove or prevent the CCh-induced depolarization. Verapamil, tetrodotoxin, removal of Cl(-) and Ca(2+) from the solution also cannot prevent the depolarization by CCh. In a Na(+)-free medium, however, CCh lost this depolarization ability and this indicates that the drug opens the sodium permeable pathway. Serotonin, glutamate, glycine, adenosine triphosphate (ATP) and cis-4-aminocrotonic acid (GABA(C) receptor antagonist) had no effect on the RMP. On the other hand, isoguvacin, γ-aminobutyric acid (GABA) and baclofen (GABA(B) receptor agonist) hyperpolarized the RMP. Ouabain, bicucullin (GABA(A) antagonist) and phaclofen (GABA(B) antagonist), as well as the removal of Cl(-), suppressed the effect of GABA and baclofen. CCh did not enhance the depolarization generated by ouabain but, on the other hand, hindered the hyperpolarizing activity of baclofen both in the absence and presence of atropine and (+)tubocurarine. The long-term application of CCh depolarizes the RMP primarily by inhibiting the Na(+)/K(+)-ATPase. The muscle membrane also contains A and B type GABA binding sites, the activation of which increases the RMP at the expense of increasing the action of ouabain- and Cl(-) -sensitive electrogenic pumps.

Ultra-rapid activation and deactivation of store-operated Ca(2+) entry in skeletal muscle

Skeletal muscle is highly specialized for the rapid delivery of Ca(2+) to the contractile apparatus during excitation-contraction coupling (EC coupling). Previous studies have shown the presence of a relatively fast-activated store-operated Ca(2+) entry (SOCE) mechanism (<1s) to be present in skeletal muscle, unlike the situation occurring in non-excitable cells. We simultaneously imaged [Ca(2+)] in the t-system and cytoplasm in mechanically skinned fibers during SR Ca(2+) release and observed both cell-wide Ca(2+) release and Ca(2+) waves. SOCE activation followed cell-wide Ca(2+) release from high sarcoplasmic reticulum (SR) [Ca(2+)] ([Ca(2+)](SR)) by seconds, consistent with depletion of [Ca(2+)](SR) to an absolute threshold for SOCE and an unformed SOCE complex at high [Ca(2+)](SR). Ca(2+) waves occurred at low [Ca(2+)](SR), close to the threshold for SOCE, minimizing the time between Ca(2+) release and Ca(2+) influx. Local activation of SOCE during Ca(2+) waves occurred in approximately 27ms following local initiation of SR depletion indicating a steep relationship between [Ca(2+)](SR) and SOCE activation. Most of this delay was due to slow release of Ca(2+) from SR, leaving only milliseconds at most for the activation of Ca(2+) entry following store depletion. SOCE was also observed to deactivate effectively instantly during store refilling at low [Ca(2+)](SR). These rapid kinetics of SOCE persisted as subsequent Ca(2+) waves propagated along the fiber. Thus we show for the first time millisecond activation and deactivation of SOCE during low amplitude [Ca(2+)](SR) oscillations at low [Ca(2+)](SR). To account for the observed Ca(2+) movements we propose the SOCE complex forms during the progressive depletion of [Ca(2+)](SR) prior to reaching the activation threshold of SOCE and this complex remains stable at low [Ca(2+)](SR).

Toward the roles of store-operated Ca2+ entry in skeletal muscle

Store-operated Ca(2+) entry (SOCE) has been found to be a rapidly activated robust mechanism in skeletal muscle fibres. It is conducted across the junctional membranes by stromal interacting molecule 1 (STIM1) and Orai1, which are housed in the sarcoplasmic reticulum (SR) and tubular (t-) system, respectively. These molecules that conduct SOCE appear evenly distributed throughout the SR and t-system of skeletal muscle, allowing for rapid and local control in response to depletions of Ca(2+) from SR. The significant depletion of SR Ca(2+) required to reach the activation threshold for SOCE could only be achieved during prolonged bouts of excitation-contraction coupling (EC coupling) in a healthy skeletal muscle fibre, meaning that this mechanism is not responsible for refilling the SR with Ca(2+) during periods of fibre quiescence. While Ca(2+) in SR remains below the activation threshold for SOCE, a low-amplitude persistent Ca(2+) influx is provided to the junctional cleft. This article reviews the properties of SOCE in skeletal muscle and the proposed molecular mechanism, assesses its potential physiological roles during EC coupling, namely refilling the SR with Ca(2+) and simple balancing of Ca(2+) within the cell, and also proposes the possibility of SOCE as a potential regulator of t-system and SR membrane protein function.

Upregulation of store-operated Ca2+ entry in dystrophic mdx mouse muscle

Store-operated Ca(2+) entry (SOCE) is an important mechanism in virtually all cells. In adult skeletal muscle, this mechanism is highly specialized for the rapid delivery of Ca(2+) from the transverse tubule into the junctional cleft during periods of depleting Ca(2+) release. In dystrophic muscle fibers, SOCE may be a source of Ca(2+) overload, leading to cell necrosis. However, this possibility is yet to be examined in an adult fiber during Ca(2+) release. To examine this, Ca(2+) in the tubular system and cytoplasm were simultaneously imaged during direct release of Ca(2+) from sarcoplasmic reticulum (SR) in skeletal muscle fibers from healthy (wild-type, WT) and dystrophic mdx mouse. The mdx fibers were found to have normal activation and deactivation properties of SOCE. However, a depression of the cytoplasmic Ca(2+) transient in mdx compared with WT fibers was observed, as was a shift in the SOCE activation and deactivation thresholds to higher SR Ca(2+) concentrations ([Ca(2+)](SR)). The shift in SOCE activation and deactivation thresholds was accompanied by an approximately threefold increase in STIM1 and Orai1 proteins in dystrophic muscle. While the mdx fibers can introduce more Ca(2+) into the fiber for an equivalent depletion of [Ca(2+)](SR) via SOCE, it remains unclear whether this is deleterious.

New factors contributing to dynamic calcium regulation in the skeletal muscle triad-a crowded place

Skeletal muscle is a highly organized tissue that has to be optimized for fast signalling events conveying electrical excitation to contractile response. The site of electro-chemico-mechanical coupling is the skeletal muscle triad where two membrane systems, the extracellular t-tubules and the intracellular sarcoplasmic reticulum, come into very close contact. Structure fits function here and the signalling proteins DHPR and RyR1 were the first to be discovered to bridge this gap in a conformational coupling arrangement. Since then, however, new proteins and more signalling cascades have been identified just in the last decade, adding more diversity and fine tuning to the regulation of excitation-contraction coupling (ECC) and control over Ca2+ store content. The concept of Ca2+ entry into working skeletal muscle has become attractive again with the experimental evidence summarized in this review. Store-operated Ca2+ entry (SOCE), excitation-coupled Ca2+ entry (ECCE), action-potential-activated Ca2+ current (APACC), and retrograde EC-coupling (ECC) are new concepts additional to the conventional orthograde ECC; they have provided fascinating new insights into muscle physiology. In this review, we discuss the discovery of these pathways, their potential roles, and the signalling proteins involved that show that the triad may become a crowded place in time.

Role of TRPC1 channel in skeletal muscle function

Skeletal muscle contraction is reputed not to depend on extracellular Ca2+. Indeed, stricto sensu, excitation-contraction coupling does not necessitate entry of Ca2+. However, we previously observed that, during sustained activity (repeated contractions), entry of Ca2+ is needed to maintain force production. In the present study, we evaluated the possible involvement of the canonical transient receptor potential (TRPC)1 ion channel in this entry of Ca2+ and investigated its possible role in muscle function. Patch-clamp experiments reveal the presence of a small-conductance channel (13 pS) that is completely lost in adult fibers from TRPC1(-/-) mice. The influx of Ca2+ through TRPC1 channels represents a minor part of the entry of Ca(2+) into muscle fibers at rest, and the activity of the channel is not store dependent. The lack of TRPC1 does not affect intracellular Ca2+ concentration ([Ca2+](i)) transients reached during a single isometric contraction. However, the involvement of TRPC1-related Ca2+ entry is clearly emphasized in muscle fatigue. Indeed, muscles from TRPC1(-/-) mice stimulated repeatedly progressively display lower [Ca2+](i) transients than those observed in TRPC1(+/+) fibers, and they also present an accentuated progressive loss of force. Interestingly, muscles from TRPC1(-/-) mice display a smaller fiber cross-sectional area, generate less force per cross-sectional area, and contain less myofibrillar proteins than their controls. They do not present other signs of myopathy. In agreement with in vitro experiments, TRPC1(-/-) mice present an important decrease of endurance of physical activity. We conclude that TRPC1 ion channels modulate the entry of Ca(2+) during repeated contractions and help muscles to maintain their force during sustained repeated contractions.

Sildenafil inhibits chronically hypoxic upregulation of canonical transient receptor potential expression in rat pulmonary arterial smooth muscle

In pulmonary arterial smooth muscle cells (PASMCs), Ca2+ influx through store-operated Ca2+ channels thought to be composed of canonical transient receptor potential (TRPC) proteins is an important determinant of intracellular free calcium concentration ([Ca2+](i)) and pulmonary vascular tone. Sildenafil, a type V phosphodiesterase inhibitor that increases cellular cGMP, is recently identified as a promising agent for treatment of pulmonary hypertension. We previously demonstrated that chronic hypoxia elevated basal [Ca2+](i) in PASMCs due in large part to enhanced store-operated Ca2+ entry (SOCE); moreover, ex vivo exposure to prolonged hypoxia (4% O2 for 60 h) upregulated TRPC1 and TRPC6 expression in PASMCs. We examined the effect of sildenafil on basal [Ca2+](i), SOCE, and the expression of TRPC in PASMCs under prolonged hypoxia exposure. We also examined the effect of sildenafil on TRPC1 and TRPC6 expression in pulmonary arterial smooth muscle (PA) from rats that developed chronically hypoxic pulmonary hypertension (CHPH). Compared with vehicle control, treatment with sildenafil (300 nM) inhibited prolonged hypoxia induced increases of 1) basal [Ca2+](i), 2) SOCE, and 3) mRNA and protein expression of TRPC in PASMCs. Moreover, sildenafil (50 mg . kg(-1) . day(-1)) inhibited mRNA and protein expression of TRPC1 and TRPC6 in PA from chronically hypoxic (10% O2 for 21 days) rats, which was associated with decreased right ventricular pressure and right ventricular hypertrophy. Furthermore, we found, in PASMCs exposed to prolonged hypoxia, that knockdown of TRPC1 or TRPC6 by their specific small interference RNA attenuated the hypoxic increases of SOCE and basal [Ca2+]i, suggesting a cause and effect link between increases of TRPC1 and TRPC6 expression and the hypoxic increases of SOCE and basal [Ca2+]i. These results suggest that sildenafil may alter basal [Ca2+](i) in PASMCs by decreasing SOCE through downregulation of TRPC1 and TRPC6 expression, thereby contributing to decreased vascular tone of pulmonary arteries during the development of CHPH.

Transient receptor potential canonical type 1 (TRPC1) operates as a sarcoplasmic reticulum calcium leak channel in skeletal muscle

Extensive studies performed in nonexcitable cells and expression systems have shown that type 1 transient receptor potential canonical (TRPC1) channels operate mainly in plasma membranes and open through phospholipase C-dependent processes, membrane stretch, or depletion of Ca(2+) stores. In skeletal muscle, it is proposed that TRPC1 channels are involved in plasmalemmal Ca(2+) influx and stimulated by store depletion or membrane stretch, but direct evidence for TRPC1 sarcolemmal channel activity is not available. We investigated here the functional role of TRPC1 using an overexpressing strategy in adult mouse muscle fibers. Immunostaining for endogenous TRPC1 revealed a striated expression pattern that matched sarcoplasmic reticulum (SR) Ca(2+) pump immunolabeling. In cells expressing TRPC1-yellow fluorescent protein (YFP), the same pattern of expression was observed, compatible with a longitudinal SR localization. Resting electric properties, action potentials, and resting divalent cation influx were not altered in TRPC1-YFP-positive cells. Poisoning with the SR Ca(2+) pump blocker cyclopiazonic acid elicited a contracture of the fiber at the level of the overexpression site in presence and absence of external Ca(2+) which was not observed in control cells. Ca(2+) measurements indicated that resting Ca(2+) and the rate of Ca(2+) increase induced by cyclopiazonic acid were higher in the TRPC1-YFP-positive zone than in the TRPC1-YFP-negative zone and control cells. Ca(2+) transients evoked by 200-ms voltage clamp pulses decayed slower in TRPC1-YFP-positive cells. In contrast to previous hypotheses, these data demonstrate that TRPC1 operates as a SR Ca(2+) leak channel in skeletal muscle.

The cardiac alpha(1C) subunit can support excitation-triggered Ca2+ entry in dysgenic and dyspedic myotubes

Depolarization-induced entry of divalent ions into skeletal muscle has been attributed to a process termed Excitation-Coupled Ca(2+) Entry (ECCE), which is hypothesized to require the interaction of the ryanodine receptor (RyR1), the L-type Ca(2+) channel (DHPR) and another unidentified cation channel. Thus, ECCE is absent in myotubes lacking either the DHPR (dysgenic) or RyR1 (dyspedic). Furthermore, ECCE, as measured by Mn(2+) quench of Fura-2, is reconstituted by expression of a mutant DHPR alpha(1S) subunit (SkEIIIK) thought to be impermeable to divalent cations. Previously, we showed that the bulk of depolarization-induced Ca(2+) entry could be explained by the skeletal L-type current. Accordingly, one would predict that any Ca(2+) current similar to the endogenous current would restore such entry and that this entry would not require coupling to either the DHPR or RyR1. Here, we show that expression of the cardiac alpha(1C) subunit in either dysgenic or dyspedic myotubes does result in Ca(2+) entry similar to that ascribed to ECCE. We also demonstrate that, when potentiated by strong depolarization and Bay K 8644, SkEIIIK supports entry of Mn(2+). These results strongly support the idea that the L-type channel is the major route of Ca(2+) entry in response to repetitive or prolonged depolarization of skeletal muscle.