Mechanism of chloride-dependent release of Ca2+ in the sarcoplasmic reticulum of rabbit skeletal muscle

Sasanquasaponin induces increase of Cl‑/HCO3‑ exchange of anion exchanger 3 and promotes intracellular Cl‑ efflux in hypoxia/reoxygenation cardiomyocytes

Anion exchanger 3 (AE3) is known to serve crucial roles in maintaining intracellular chloride homeostasis by facilitating the reversible electroneutral exchange of Cl‑ for HCO3‑ across the plasma membrane. Our previous studies reported that sasanquasaponin (SQS) can inhibit hypoxia/reoxygenation (H/R)‑induced elevation of intracellular Cl‑ concentration ([Cl‑]i) and elicit cardioprotection by favoring Cl‑/HCO3‑ exchange of AE3. However, the molecular basis for SQS‑induced increase of Cl‑/HCO3‑ exchange of AE3 remains unclear. The present study demonstrated that SQS activates protein kinase Cε (PKCε) and stimulates the phosphorylation of AE3 in H9c2 cells. Notably, SQS‑induced AE3 phosphorylation was blocked by the PKCε selective inhibitor εV1‑2, and a S67A mutation of AE3, indicating that SQS could promote phosphorylation of Ser67 of AE3 via a PKCε‑dependent regulatory signaling pathway. Additionally, both inhibition of PKCε by εV1‑2 and S67A mutation of AE3 eradicated the SQS‑induced increase of AE3 activity, reversed the inhibitory effect of SQS on H/R‑induced elevation of [Cl‑]i, Ca2+ overload and generation of reactive oxygen species, and eliminated SQS‑induced cardioprotection. In conclusion, PKCε‑dependent phosphorylation of serine 67 on AE3 may be responsible for the increase of Cl‑/HCO3‑ exchange of AE3 and intracellular chloride efflux by SQS, and contributes to the cardioprotection of SQS against H/R in H9c2 cells.

Extracellular Cl--free-induced cardioprotection against hypoxia/reoxygenation is associated with attenuation of mitochondrial permeability transition pore

The isotonic substitution of extracellular chloride by gluconate (extracellular Cl^--free) has been demonstrated to elicit cardioprotection by attenuating ischaemia/reperfusion-induced elevation of intracellular chloride ion concentration ([Cl^-]_i). However, the downstream mechanism underlying the cardioprotective effect of extracellular Cl^--free is not fully established. Here, it was investigated whether extracellular Cl^--free attenuates mitochondrial dysfunction after hypoxia/reoxygenation (H/R) and whether mitochondrial permeability transition pore (mPTP) plays a key role in the extracellular Cl^--free cardioprotection. H9c2 cells were incubated with or without Cl^--free solution, in which Cl^- was replaced with equimolar gluconate, during H/R. The involvement of mPTP was determined with atractyloside (Atr), a specific mPTP opener. The results showed that extracellular Cl^--free attenuated H/R-induced the elevation of [Cl^-]_i, accompanied by increase of cell viability and reduction of lactate dehydrogenase release. Moreover, extracellular Cl^--free inhibited mPTP opening, and improved mitochondria function, as indicated by preserved mitochondrial membrane potential and respiratory chain complex activities, decreased mitochondrial reactive oxygen species generation, and increased ATP content. Intriguingly, pharmacologically opening of the mPTP with Atr attenuated all the protective effects caused by extracellular Cl^--free, including suppression of mPTP opening, maintenance of mitochondrial membrane potential, and subsequent improvement of mitochondrial function. These results indicated that extracellular Cl^--free protects mitochondria from H/R injury in H9c2 cells and inhibition of mPTP opening is a crucial step in mediating the cardioprotection of extracellular Cl^--free.

Sasanquasaponin-induced cardioprotection involves inhibition of mPTP opening via attenuating intracellular chloride accumulation

Sasanquasaponin (SQS) has been reported to elicit cardioprotection by suppressing hypoxia/reoxygenation (H/R)-induced elevation of intracellular chloride ion concentration ([Cl^-]_i). Given that the increased [Cl^-]_i is involved to modulate the mitochondrial permeability transition pore (mPTP), we herein sought to further investigate the role of mPTP in the cardioprotective effect of SQS on H/R injury. H9c2 cells were incubated for 24h with or without 10μM SQS followed by H/R. The involvement of mPTP was determined with a specific mPTP agonist atractyloside (ATR). The results showed that SQS attenuated H/R-induced the elevation of [Cl^-]_i, accompanied by reduction of lactate dehydrogenase release and increase of cell viability. Moreover, SQS suppressed mPTP opening, and protected mitochondria, as indicated by preserved mitochondrial membrane potential and respiratory chain complex activities, decreased mitochondrial reactive oxygen species generation, and increased ATP content. Interestingly, extracellular Cl^--free condition created by replacing Cl^- with equimolar gluconate resulted in a decrease in [Cl^-]_i and induced protective effects similar to SQS preconditioning, whereas pharmacologically opening of the mPTP with ATR abolished all the protective effects induced by SQS or Cl^--free, including suppression of mPTP opening, maintenance of mitochondrial membrane potential, and subsequent improvement of mitochondrial function. The above results allow us to conclude that SQS-induced cardioprotection may be mediated by preserving the mitochondrial function through preventing mPTP opening via inhibition of H/R-induced elevation of [Cl^-]_i.

Electrophysiological remodeling in heart failure

Heart failure affects nearly 6 million Americans, with a half-million new cases emerging each year. Whereas up to 50% of heart failure patients die of arrhythmia, the diverse mechanisms underlying heart failure-associated arrhythmia are poorly understood. As a consequence, effectiveness of antiarrhythmic pharmacotherapy remains elusive. Here, we review recent advances in our understanding of heart failure-associated molecular events impacting the electrical function of the myocardium. We approach this from an anatomical standpoint, summarizing recent insights gleaned from pre-clinical models and discussing their relevance to human heart failure.

Calcium-Induced Calcium Release in Skeletal Muscle

Calcium-induced calcium release (CICR) was first discovered in skeletal muscle. CICR is defined as Ca2+ release by the action of Ca2+ alone without the simultaneous action of other activating processes. CICR is biphasically dependent on Ca2+ concentration; is inhibited by Mg2+, procaine, and tetracaine; and is potentiated by ATP, other adenine compounds, and caffeine. With depolarization of the sarcoplasmic reticulum (SR), a potential change of the SR membrane in which the luminal side becomes more negative, CICR is activated for several seconds and is then inactivated. All three types of ryanodine receptors (RyRs) show CICR activity. At least one RyR, RyR1, also shows non-CICR Ca2+ release, such as that triggered by the t-tubule voltage sensor, by clofibric acid, and by SR depolarization. Maximum rates of CICR, at the optimal Ca2+ concentration in the presence of physiological levels of ATP and Mg2+ determined in skinned fibers and fragmented SR, are much lower than the rate of physiological Ca2+ release. The primary event of physiological Ca2+ release, the Ca2+ spark, is the simultaneous opening of multiple channels, the coordinating mechanism of which does not appear to be CICR because of the low probability of CICR opening under physiological conditions. The coordination may require Ca2+, but in that case, some other stimulus or stimuli must be provided simultaneously, which is not CICR by definition. Thus CICR does not appear to contribute significantly to physiological Ca2+ release. On the other hand, CICR appears to play a key role in caffeine contracture and malignant hyperthermia. The potentiation of voltage-activated Ca2+ release by caffeine, however, does not seem to occur through secondary CICR, although the site where caffeine potentiates voltage-activated Ca2+ release might be the same site where caffeine potentiates CICR.

Lack of CFTR in skeletal muscle predisposes to muscle wasting and diaphragm muscle pump failure in cystic fibrosis mice

Cystic fibrosis (CF) patients often have reduced mass and strength of skeletal muscles, including the diaphragm, the primary muscle of respiration. Here we show that lack of the CF transmembrane conductance regulator (CFTR) plays an intrinsic role in skeletal muscle atrophy and dysfunction. In normal murine and human skeletal muscle, CFTR is expressed and co-localized with sarcoplasmic reticulum-associated proteins. CFTR-deficient myotubes exhibit augmented levels of intracellular calcium after KCl-induced depolarization, and exposure to an inflammatory milieu induces excessive NF-kB translocation and cytokine/chemokine gene upregulation. To determine the effects of an inflammatory environment in vivo, sustained pulmonary infection with Pseudomonas aeruginosa was produced, and under these conditions diaphragmatic force-generating capacity is selectively reduced in Cftr(-/-) mice. This is associated with exaggerated pro-inflammatory cytokine expression as well as upregulation of the E3 ubiquitin ligases (MuRF1 and atrogin-1) involved in muscle atrophy. We conclude that an intrinsic alteration of function is linked to the absence of CFTR from skeletal muscle, leading to dysregulated calcium homeostasis, augmented inflammatory/atrophic gene expression signatures, and increased diaphragmatic weakness during pulmonary infection. These findings reveal a previously unrecognized role for CFTR in skeletal muscle function that may have major implications for the pathogenesis of cachexia and respiratory muscle pump failure in CF patients.

Calcium and arrhythmogenesis

Triggered activity in cardiac muscle and intracellular Ca2+ have been linked in the past. However, today not only are there a number of cellular proteins that show clear Ca2+ dependence but also there are a number of arrhythmias whose mechanism appears to be linked to Ca2+-dependent processes. Thus we present a systematic review of the mechanisms of Ca2+ transport (forward excitation-contraction coupling) in the ventricular cell as well as what is known for other cardiac cell types. Second, we review the molecular nature of the proteins that are involved in this process as well as the functional consequences of both normal and abnormal Ca2+ cycling (e.g., Ca2+ waves). Finally, we review what we understand to be the role of Ca2+ cycling in various forms of arrhythmias, that is, those associated with inherited mutations and those that are acquired and resulting from reentrant excitation and/or abnormal impulse generation (e.g., triggered activity). Further solving the nature of these intricate and dynamic interactions promises to be an important area of research for a better recognition and understanding of the nature of Ca2+ and arrhythmias. Our solutions will provide a more complete understanding of the molecular basis for the targeted control of cellular calcium in the treatment and prevention of such.

Effects of Sasanquasaponin on Ischemia and Reperfusion Injury in Mouse Hearts

We investigated effects of sasanquasaponin (SQS), a traditional Chinese herb's effective component, on ischemia and reperfusion injury in mouse hearts and the possible role of intracellular Cl- homeostasis on SQS's protective effects during ischemia and reperfusion. An in vivo experimental ischemia model was made in mice (weight 27-45 g) using ligation of left anterior descending coronary artery, and in vitro models were made in perfused hearts by stopping flow or in isolated ventricular myocytes by hypoxia. The in vivo results showed that SQS inhibited cardiac arrhythmias during ischemia and reperfusion. Incidence of arrhythmias during ischemia and reperfusion, including ventricular premature beats and ventricular fibrillation, was significantly decreased in the SQS-pretreated group (P<0.05). Results in perfused hearts showed that SQS suppressed the arrhythmias, prevented against ischemia-induced decrease in contract force and promoted the force recovery from reperfusion. Furthermore, intracellular Cl- concentrations ([Cl-]i) were measured using a MQAE fluorescence method in isolated ventricular myocytes in vitro. SQS slightly decreased [Cl-]i in non-hypoxic myocytes and delayed the hypoxia/reoxygenation-induced increase in [Cl-]i during ischemia and reperfusion (P<0.05). Our results showed that SQS protected against ischemia/reperfusion-induced cardiac injury in mouse hearts and that modulation of intracellular Cl- homeostasis by SQS would play a role in its anti-arrhythmia effects during ischemia and reperfusion.

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.

Properties of Ca(2+) release induced by clofibric acid from the sarcoplasmic reticulum of mouse skeletal muscle fibres

1. To characterize the effect of clofibric acid (Clof) on the Ca(2+) release mechanism in the sarcoplasmic reticulum (SR) of skeletal muscle, we analysed the properties of Clof-induced Ca(2+) release under various conditions using chemically skinned skeletal muscle fibres of the mouse. 2. Clof (>0.5 mM) released Ca(2+) from the SR under Ca(2+)-free conditions buffered with 10 mM EGTA (pCa >8). 3. Co-application of ryanodine and Clof at pCa >8 but not ryanodine alone reduced the Ca(2+) uptake capacity of the SR. Thus, Ca(2+) release induced by Clof at pCa >8 must be a result of the activation of the ryanodine receptor (RyR). 4. At pCa >8, (i) Clof-induced Ca(2+) release was inhibited by adenosine monophosphate (AMP), (ii) the inhibitory effect of Mg(2+) on the Clof-induced Ca(2+) release was saturated at about 1 mM, and (iii) Clof-induced Ca(2+) release was not inhibited by procaine (10 mM). These results indicate that Clof may activate the RyR-Ca(2+) release channels in a manner different from Ca(2+)-induced Ca(2+) release (CICR). 5. In addition to this unique mode of opening, Clof also enhanced the CICR mode of opening of RyR-Ca(2+) release channels. 6. Apart from CICR, a high concentration of Ca(2+) might also enhance the unique mode of opening by Clof. 7. These results suggest that some features of Ca(2+) release activated by Clof are similar to those of physiological Ca(2+) release (PCR) in living muscle cells and raise the possibility that Clof may be useful in elucidating the mechanism of PCR in skeletal muscle.

Iodide and bromide inhibit Ca(2+) uptake by cardiac sarcoplasmic reticulum

Recent studies indicate that the Ca(2+) permeability of the sarcoplasmic reticulum (SR) can be affected by its anionic environment. Additionally, anions could directly modulate the SR Ca(2+) pump or the movement of compensatory charge across the SR membrane during Ca(2+) uptake or release. To examine the effect of anion substitution on cardiac SR Ca(2+) uptake, fluorometric Ca(2+) measurements and spectrophotometric ATPase assays were used. Ca(2+) uptake into SR vesicles was inhibited in a concentration-dependent manner when Br(-) or I(-) replaced extravesicular Cl(-) (when Br(-) completely replaced Cl(-), uptake velocity was approximately 70% of control; when I(-) completely replaced Cl(-), uptake velocity was approximately 39% of control). Replacement of Cl(-) with SO(2)(-4) had no effect on SR uptake. Although both I(-) and Br(-) inhibited net Ca(2+) uptake, neither anion directly inhibited the SR Ca(2+) pump nor did they increase the permeability of the SR membrane to Ca(2+). Our results support the hypothesis that an anionic current that occurs during SR Ca(2+) uptake is reduced by the substitution of Br(-) or I(-) for Cl(-).

Calcium release in frog cut twitch fibers exposed to different ionic environments under voltage clamp

Calcium release was measured in highly stretched frog cut twitch fibers mounted in a double Vaseline-gap voltage clamp chamber, with the internal solution containing 20 mM EGTA plus 0.4 or 1.8 mM added calcium. Rise in myoplasmic [Ca(2+)] was monitored with antipyrylazo III as the indicator at a temperature of 13 to 14 degrees C. The waveform of calcium release rate (Rel) computed from the absorbance change showed an early peak (Rel(p)) followed by a maintained phase (Rel(m)). Each Rel(p)-versus-V plot was fitted with a Boltzmann distribution function. The maximum value of Rel(p) (Rel(p,max)) was compared in various calcium-containing external solutions. The average value in a Cl(-) solution was about one-third larger than those in a CH(3)SO(3)(-) or gluconate solution, whereas the values in the CH(3)SO(3)(-) and gluconate solutions had no statistically significant difference. In external solutions containing CH(3)SO(3)(-) or gluconate, a replacement of the Ca(2+) with Mg(2+) reduced Rel(p,max) by 30 to 50%, on average. The values of Rel(p, max) also had no statistically significant difference among calcium-free external solutions containing different impermeant anions. An increase of the nominal free [Ca(2+)] in the end-pool solution from a reduced to the normal physiological level increased the value of Rel(p,max), and also slowed the decay of the maintained phase of the Rel waveform. The Rel waveforms in the Cl(-) and CH(3)SO(3)(-) solutions were compared in the same fiber at a fixed potential. CH(3)SO(3)(-) increased the time to peak, reduced Rel(p), and increased Rel(m), and the effects were partially reversible. Under the hypothesis that the decay of the peak was due to calcium inactivation of calcium release, the inactivation was larger in Cl(-) than in CH(3)SO(3)(-), in qualitative agreement with the ratio of Rel(p) in the two solutions. Under the alternative hypothesis that the peak and the maintained phase were separately gated by calcium and depolarization, respectively, then CH(3)SO(3)(-) appeared to decrease the calcium-gated component and increase the voltage-gated component.

Cooperative block of the plant endomembrane ion channel by ruthenium red

Effects of ruthenium red (RR) on the slow Ca(2+)-activated Ca(2+)-permeable vacuolar channel have been studied by patch-clamp technique. Applied to the cytosolic side of isolated membrane patches, RR at concentrations of 0.1-5 microM produced two distinct effects on single channel kinetics, long lasting closures and a flickering block of the open state. The first effect was largely irreversible, whereas the second one could be washed out. The extent of flickering block steeply increased (zdelta = approximately 1.35) with the increase of cytosol-positive voltage, dragging RR into the channel pore. At least two RR ions are involved in the block according to Hill coefficient n = approximately 1.30 for the dose response curves. The on-rate rate of the drug binding linearly depended on the RR concentration, implying that one RR ion already plugged the pore. The blocked state was further stabilized by binding of the second RR. This stabilization was in excess of that predicted by independent binding as the dependence of unblocking rate on RR concentration revealed. A cooperative model was therefore employed to describe the kinetic behavior of RR binding. At zero voltage the half-blocking RR concentration of 36 microM and the bimolecular on-rate constant of 1.8 x 10(8) M(-1) s(-1) were estimated.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

The aim of this review is to provide basic information on the electrophysiological changes during acute ischemia and reperfusion from the level of ion channels up to the level of multicellular preparations. After an introduction, section II provides a general description of the ion channels and electrogenic transporters present in the heart, more specifically in the plasma membrane, in intracellular organelles of the sarcoplasmic reticulum and mitochondria, and in the gap junctions. The description is restricted to activation and permeation characterisitics, while modulation is incorporated in section III. This section (ischemic syndromes) describes the biochemical (lipids, radicals, hormones, neurotransmitters, metabolites) and ion concentration changes, the mechanisms involved, and the effect on channels and cells. Section IV (electrical changes and arrhythmias) is subdivided in two parts, with first a description of the electrical changes at the cellular and multicellular level, followed by an analysis of arrhythmias during ischemia and reperfusion. The last short section suggests possible developments in the study of ischemia-related phenomena.

The structure, function, and cellular regulation of ryanodine-sensitive Ca2+ release channels

The fundamental biological process of Ca2+ signaling is known to be important in most eukaryotic cells, and inositol 1,2,5-trisphosphate and ryanodine receptors, intracellular Ca2+ release channels encoded by two distantly related gene families, are central to this phenomenon. Ryanodine receptors in the sarcoplasmic reticulum of skeletal and cardiac muscle have a predominant role in excitation-contraction coupling, but the channels are also present in the endoplasmic reticulum of noncontractile tissues including the central nervous system and the immune system. In all, three highly homologous ryanodine receptor isoforms have been identified, all very large proteins which assemble as (homo)tetramers of approximately 2 MDa. They contain large cytoplasmically disposed regulatory domains and are always associated with other structural or regulatory proteins, including calmodulin and immunophilins, which can have marked effects on channel function. The type 1 isoform in skeletal muscle is electromechanically coupled to surface membrane voltage sensors, whereas the remaining isoforms appear to be activated solely by endogenous cytoplasmic second messengers or other ligands, including Ca2+ itself ("Ca(2+)-induced Ca2+ release"). This review concentrates on ryanodine receptor structure-function relationships as probed by a variety of methods and on the molecular mechanisms of channel modulation at the cellular level (including evidence for the regulation of gene expression and transcription). It also touches on the relevance of ryanodine receptors to complex cellular functions and disease.

Calcium pool size modulates the sensitivity of the ryanodine receptor channel and calcium-dependent ATPase of heavy sarcoplasmic reticulum to extravesicular free calcium concentration

We have examined calcium cycling and associated ATP consumption by isolated heavy sarcoplasmic reticulum (HSR) vesicles incubated in conditions believed to exist in resting muscle. Our goals were to estimate the magnitude of calcium cycling under those conditions and identify the main mechanisms involved in its regulation. The integrity of the HSR vesicles was documented by the retention of [14C]-sucrose and electron microscopy. HSR actively exchanged Ca2+ with the medium through a partially open ryanodine-binding channel (RyR), as evidenced by the rapid attainment of a steady-state gradient between HSR and medium, which was promptly increased by the closure of the channel with ruthenium red (RR) or collapsed by its opening with caffeine. The ATP dependency was evidenced by the sustained ATP consumption after the steady state was attained and by the abrogation of the gradient following inhibition of the pump with thapsigargin (Tg) or the omission of ATP. When HSR vesicles were incubated in a comparatively large pool of calcium (approximately 1 micromol/mg HSR protein), ATP consumption was 1-1.5 micromol x [min x mg protein](-1) at 0.1 microM free Ca2+. Under such conditions, the main regulator of the sarcoplasmic Ca2+-dependent ATPase (SERCA) was extravesicular-free Ca2+ concentration, with a four- to fivefold increase between 0.1 and 2 microM Ca2+, whereas RyR channel activity and the replenishment of the HSR vesicles had only a modest effect on ATP consumption. When calcium pool size was reduced to 0.1 micromol/mg HSR protein, a steady state was established at a lower level of HSR calcium. In spite of a slightly lower free extravesicular Ca2+ at equilibrium (approximately 0.07 microM following an initial concentration of 0.1 microM), both ATP consumption and the open probability of the RyR channel were increased by a factor of three to five. Compared to the large calcium pool, the sensitivity of both RyR channel and SERCA to extravesicular free Ca2+ concentration as well as to caffeine and RR was markedly enhanced.

CONCLUSIONS

1) In conditions present in resting muscle, HSR calcium is in dynamic equilibrium with the medium through a partially open RyR channel, which requires continuous ATP hydrolysis. 2) The availability of calcium is a major determinant of the sensitivity of both RyR channel and SERCA to free extravesicular Ca2+ and possibly other stimuli. 3) These observations are consistent with the concept that calcium cycling in resting muscle may account for a significant fraction of muscle energy demands and further suggest that restricting calcium availability may enhance the energetic demands of this process.

ATP inhibition and rectification of a Ca2+-activated anion channel in sarcoplasmic reticulum of skeletal muscle

We describe ATP-dependent inhibition of the 75-105-pS (in 250 mM Cl-) anion channel (SCl) from the sarcoplasmic reticulum (SR) of rabbit skeletal muscle. In addition to activation by Ca2+ and voltage, inhibition by ATP provides a further mechanism for regulating SCl channel activity in vivo. Inhibition by the nonhydrolyzable ATP analog 5'-adenylylimidodiphosphate (AMP-PNP) ruled out a phosphorylation mechanism. Cytoplasmic ATP (approximately 1 mM) inhibited only when Cl- flowed from cytoplasm to lumen, regardless of membrane voltage. Flux in the opposite direction was not inhibited by 9 mM ATP. Thus ATP causes true, current rectification in SCl channels. Inhibition by cytoplasmic ATP was also voltage dependent, having a K(I) of 0.4-1 mM at -40 mV (Hill coefficient approximately 2), which increased at more negative potentials. Luminal ATP inhibited with a K(I) of approximately 2 mM at +40 mV, and showed no block at negative voltages. Hidden Markov model analysis revealed that ATP inhibition 1) reduced mean open times without altering the maximum channel amplitude, 2) was mediated by a novel, single, voltage-independent closed state (approximately 1 ms), and 3) was much less potent on lower conductance substates than the higher conductance states. Therefore, the SCl channel is unlikely to pass Cl- from cytoplasm to SR lumen in vivo, and balance electrogenic Ca2+ uptake as previously suggested. Possible roles for the SCl channel in the transport of other anions are discussed.

Rat brain p64H1, expression of a new member of the p64 chloride channel protein family in endoplasmic reticulum

Many plasma membrane Cl- channels have been cloned, including the cystic fibrosis transmembrane conductance regulator and several members of the voltage-gated ClC family. In contrast, very little is known about the molecular identity of intracellular Cl- channels. We used a polymerase chain reaction-based approach to identify candidate genes in mammalian brain and cloned the cDNA corresponding to rat brain p64H1. This encoded a microsomal membrane protein of predicted Mr 28,635 homologous to the putative intracellular bovine kidney Cl- channel p64. In situ mRNA hybridization histochemistry showed marked expression in hippocampus and cerebellum, and in vitro expression revealed a large cytoplasmic domain, one membrane-spanning segment, and a small nonglycosylated N-terminal luminal domain. The predicted protein contained consensus phosphorylation sites for protein kinase C and protein kinase A, and protein kinase C-mediated phosphorylation increased the Mr of p64H1 to approximately 43,000, characteristic of the native protein in Western blots. Recombinant p64H1 was immunolocalized to the endoplasmic reticulum of human embryonic kidney 293 and HT-4 cells, and incorporation of human embryonic kidney 293 endoplasmic reticulum vesicles into planar lipid bilayers gave rise to intermediate conductance, outwardly rectifying anion channels. Although p64H1 is the first intracellular Cl- channel component or regulator to be identified in brain, Northern blotting revealed transcripts in many other rat tissues. This suggests that p64H1 may contribute widely to intracellular Cl- transport.

Inositol 1,4,5-trisphosphate and basal Ca2+ release is affected by the cytoplasmic concentration of Cl- in endothelial cells

The effects of varying Cl- concentration in the intracellular bathing medium, on IP3-induced 45Ca2+ release from internal stores, were examined in saponin-permeabilised bovine aortic endothelial (BAE) cells. Results from this study show that the release of Ca2+ from the internal stores is affected by the cytoplasmic concentration of Cl- ions. Complete replacement of Cl- with gluconate augmented IP3 (3 microM)-induced 45Ca2+ release by 33 +/- 8%. Replacement of both Cl- and K+ with gluconate and NMG, respectively, had no significant effect on 45Ca2+ release. However, resting levels of internal 45Ca2+ were found to be affected by Cl- removal. These data suggest that in BAE cells, IP3 and also basal 45Ca2+ release may be regulated by the physiological intracellular Cl- concentration.

Single-channel properties of a rat brain endoplasmic reticulum anion channel

Many intracellular membranes contain ion channels, although their physiological roles are often poorly understood. In this study we incorporated single anion channels colocalized with rat brain endoplasmic reticulum (ER) ryanodine-sensitive Ca(2+)-release channels into planar lipid bilayers. The channels opened in bursts, with more activity at negative (cytoplasm-ER lumen) membrane potentials, and they occupied four open conductance levels with frequencies well described by the binomial equation. The probability of a protomer being open decreased from approximately 0.7 at -40 mV to approximately 0.2 at +40 mV, and the channels selected between different anions in the order PSCN > PNO3 > PBr > PCl > PF. They were also permeant to cations, including the large cation Tris+ (PTris/PCl = 0.16). Their conductance saturated at 170 pS in choline Cl. The channels were inactivated by 15 microM 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) and blocked with low affinity (KD of 1-100 microM) by anthracene-9-carboxylic acid, ethacrynic acid, frusemide (furosemide), HEPES, the indanyloxyacetic acid derivative IAA-94, 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB), and Zn2+. Unlike protein translocation pores, the channels were unaffected by high salt concentrations or puromycin. They may regulate ER Ca2+ release, or be channel components en route to their final cellular destinations. Alternatively, they may contribute to the fusion machinery involved in intracellular membrane trafficking.

Sphingosylphosphocholine modulates the ryanodine receptor/calcium-release channel of cardiac sarcoplasmic reticulum membranes

Sphingosylphosphocholine (SPC) modulates Ca2+ release from isolated cardiac sarcoplasmic reticulum membranes; 50 microM SPC induces the release of 70 80% of the accumulated calcium. SPC release calcium from cardiac sarcoplasmic reticulum through the ryanodine receptor, since the release is inhibited by the ryanodine receptor channel antagonists ryanodine. Ruthenium Red and sphingosine. In intact cardiac myocytes, even in the absence of extracellular calcium. SPC causes a rise in diastolic Ca2+, which is greatly reduced when the sarcoplasmic reticulum is depleted of Ca2+ by prior thapsigargin treatment. SPC action on the ryanodine receptor is Ca(2+)-dependent. SPC shifts to the left the Ca(2+)-dependence of [3H]ryanodine binding, but only at high pCa values, suggesting that SPC might increase the sensitivity to calcium of the Ca(2+)-induced Ca(2+)-release mechanism. At high calcium concentrations (pCa 4.0 or lower), where [3H]ryanodine binding is maximally stimulated, no effect of SPC is observed. We conclude that SPC releases calcium from cardiac sarcoplasmic reticulum membranes by activating the ryanodine receptor and possibly another intracellular Ca(2+)-release channel, the sphingolipid Ca(2+)-release-mediating protein of endoplasmic reticulum (SCaMPER) [Mao, Kim, Almenoff, Rudner, Kearney and Kindman (1996) Proc.Natl.Acad.Sci. U.S.A 93, 1993-1996], which we have identified for the first time in cardiac tissue.

Bastadins relate ryanodine-sensitive and -insensitive Ca2+ efflux pathways in skeletal SR and BC3H1 cells

Bastadins potently interact with the FK-506-binding protein of 12 kDa (FKBP12)-ryanodine receptor (Ry1R) complex in skeletal muscle to enhance a high-affinity ryanodine binding conformation (M. M. Mack, T. F. Molinski, E. D. Buck, and I. N. Pessah. J. Biol. Chem. 269: 23236-23249, 1994). Bastadins are used to examine the relationship between ryanodine-sensitive and ryanodine-insensitive Ca2+ efflux pathways that coexist in junctional sarcoplasmic reticulum (SR) vesicles from rabbit skeletal muscle and differentiated BC3H1 cells. Complete block of caffeine-sensitive Ca2+ channels with micromolar ryanodine or ruthenium red does not alter the steady-state loading capacity of SR. Inhibition of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) pumps with thapsigargin unmasks a ryanodine- and ruthenium red-insensitive Ca2+ efflux pathway. Bastadin 5 alone does not inhibit Ca2+ efflux unmasked by inhibition of SERCA pumps, but, in combination with blocking concentrations of ryanodine or ruthenium red, it eliminates the ryanodine-insensitive Ca2+ "leak" and enhances steady-state loading capacity of SR vesicles approximately 2.5-fold. These actions of bastadins occur in the same concentration range that enhances the number of high-affinity binding sites for [3H]ryanodine (50% effective concentration of approximately 2 microM). Similar effects on SR Ca2+ transport are found with FK-506 and ryanodine in combination. Block of Ry1R in intact BC3H1 cells with ryanodine does not eliminate the prominent Ca2+ leak unmasked by thapsigargin. A membrane-permeant mixture of bastadins in combination with ryanodine nearly eliminates the Ca2+ leak unmasked by thapsigargin, even though the Ca2+ stores are replete. The requirement of both a known Ry1R blocker and bastadins in combination provides a pharmacological link between ryanodine-sensitive Ca2+ channels and ryanodine-insensitive leak pathways in isolated junctional SR and BC3H1 cells. Together, these results strongly suggest that bastadins, through their modulatory actions on the FKBP12-Ry1R complex, convert ryanodine-insensitive leak states into ryanodine-sensitive channels that recognize [3H]ryanodine with high affinity.

Regulation of skeletal muscle Ca2+ release channel (ryanodine receptor) by Ca2+ and monovalent cations and anions

The effects of ionic composition and strength on rabbit skeletal muscle Ca2+ release channel (ryanodine receptor) activity were investigated in vesicle-45Ca2+ flux, single channel and [3H]ryanodine binding measurements. In <0.01 microM Ca2+ media, the highest 45Ca2+ efflux rate was measured in 0.25 M choline-Cl medium followed by 0.25 M KCl, choline 4-morpholineethanesulfonic acid (Mes), potassium 1,4-piperazinediethanesulfonic acid (Pipes), and K-Mes medium. In all five media, the 45Ca2+ efflux rates were increased when the free [Ca2+] was raised from <0.01 microM to 20 microM and decreased as the free [Ca2+] was further increased to 1 mM. An increase in [KCl] augmented Ca2+-gated single channel activity and [3H]ryanodine binding. In [3H]ryanodine binding measurements, bell-shaped Ca2+ activation/inactivation curves were obtained in media containing different monovalent cations (Li+, Na+, K+, Cs+, and choline+) and anions (Cl-, Mes-, and Pipes-). In choline-Cl medium, substantial levels of [3H]ryanodine binding were observed at [Ca2+] <0.01 microM. Replacement of Cl- by Mes- or Pipes- reduced [3H]ryanodine binding levels at all [Ca2+]. In all media, the Ca2+-dependence of [3H]ryanodine binding could be well described assuming that the skeletal muscle ryanodine receptor possesses cooperatively interacting high-affinity Ca2+ activation and low-affinity Ca2+ inactivation sites. AMP primarily affected [3H]ryanodine binding by decreasing the apparent affinity of the Ca2+ inactivation site(s) for Ca2+, while caffeine increased the apparent affinity of the Ca2+ activation site for Ca2+. Competition studies indicated that ionic composition affected Ca2+-dependent receptor activity by at least three different mechanisms: (i) competitive binding of Mg2+ and monovalent cations to the Ca2+ activation sites, (ii) binding of divalent cations to the Ca2+ inactivation sites, and (iii) binding of anions to specific anion regulatory sites.

Chloride-dependent sarcoplasmic reticulum Ca2+ release correlates with increased Ca2+ activation of ryanodine receptors

The mechanism by which chloride increases sarcoplasmic reticulum (SR) Ca2+ permeability was investigated. In the presence of 3 microM Ca2+, Ca2+ release from 45Ca(2+)-loaded SR vesicles prepared from procine skeletal muscle was increased approximately 4-fold when the media contained 150 mM chloride versus 150 mM propionate, whereas in the presence of 30 nM Ca2+, Ca2+ release was similar in the chloride- and the propionate-containing media. Ca(2+)-activated [3H]ryanodine binding to skeletal muscle SR was also increased (2- to 10-fold) in media in which propionate or other organic anions were replaced with chloride; however, chloride had little or no effect on cardiac muscle SR 45Ca2+ release or [3H]ryanodine binding. Ca(2+)-activated [3H]ryanodine binding was increased approximately 4.5-fold after reconstitution of skeletal muscle RYR protein into liposomes, and [3H]ryanodine binding to reconstituted RYR protein was similar in chloride- and propionate-containing media, suggesting that the sensitivity of the RYR protein to changes in the anionic composition of the media may be diminished upon reconstitution. Together, our results demonstrate a close correlation between chloride-dependent increases in SR Ca2+ permeability and increased Ca2+ activation of skeletal muscle RYR channels. We postulate that media containing supraphysiological concentrations of chloride or other inorganic anions may enhance skeletal muscle RYR activity by favoring a conformational state of the channel that exhibits increased activation by Ca2+ in comparison to the Ca2+ activation exhibited by this channel in native membranes in the presence of physiological chloride (< or = 10 mM). Transitions to this putative Ca(2+)-activatable state may thus provide a mechanism for controlling the activation of RYR channels in skeletal muscle.

Purification and characterization of ryanotoxin, a peptide with actions similar to those of ryanodine

We purified and characterized ryanotoxin, an approximately 11.4-kDa peptide from the venom of the scorpion Buthotus judiacus that induces changes in ryanodine receptors of rabbit skeletal muscle sarcoplasmic reticulum analogous to those induced by the alkaloid ryanodine. Ryanotoxin stimulated Ca2+ release from sarcoplasmic reticulum vesicles and induced a state of reduce unit conductance with a mean duration longer than that of unmodified ryanodine receptor channels. With Cs+ as the current carrier, the slope conductance of the state induced by 1 microM ryanotoxin was 163 +/- 12 pS, that of the state induced by 1 microM ryanodine was 173 +/- 26 pS, and that of control channels was 2.3-fold larger (396 +/- 25 pS). The distribution of substate events induced by 1 microM RyTx was biexponential and was fitted with time constants approximately 10 times shorter than those fitted to the distribution of substates induced by 1 microM ryanodine. Bath-applied 5 microM ryanotoxin had no effect on the excitability of mouse myotubes in culture. When 5 microM ryanotoxin was dialyzed into the cell through the patch pipette in the whole-cell configuration, there was a voltage-dependent increase in the amplitude of intracellular Ca2+ transients elicited by depolarizing potentials in the range of -30 to +50 mV. Ryanotoxin increased the binding affinity of [3H]ryanodine in a reversible manner with a 50% effective dose (ED50) of 0.16 microM without altering the maximum number (Bmax) of [3H]ryanodine-binding sites. This result suggested that binding sites for ryanotoxin and ryanodine were different. Ryanotoxin should prove useful in identifying domains coupling the ryanodine receptor to the voltage sensor, or domains affecting the gating and conductance of the ryanodine receptor channel.

Characteristics of two types of chloride channel in sarcoplasmic reticulum vesicles from rabbit skeletal muscle

A comparison is made of two types of chloride-selective channel in skeletal muscle sarcoplasmic reticulum (SR) vesicles incorporated into lipid bilayers. The I/V relationships of both channels, in 250/50 mM Cl- (cis/trans), were linear between -20 and +60 mV (cis potential,) reversed near Ecl and had slope conductances of approximately 250 pS for the big chloride (BCl) channel and approximately 70 pS for the novel, small chloride (SCl) channel. The protein composition of vesicles indicated that both channels originated from longitudinal SR and terminal cisternae. BCl and SCl channels responded differently to cis SO4(2-) (30-70 mM), 4,4'-diisothiocyanatostilbene 2,2'-disulfonic acid (8-80 microM) and to bilayer potential. The BCl channel open probability was high at all potentials, whereas SCl channels exhibited time-dependent activation and inactivation at negative potentials and deactivation at positive potentials. The duration and frequency of SCl channel openings were minimal at positive potentials and maximal at -40 mV, and were stationary during periods of activity. A substate analysis was performed using the Hidden Markov Model (S. H. Chung, J. B. Moore, L. Xia, L. S. Premkumar, and P. W. Gage, 1990, Phil. Trans. R. Soc. Lond. B., 329:265-285) and the algorithm EVPROC (evaluated here). SCl channels exhibited transitions between 5 and 7 conductance levels. BCl channels had 7-13 predominant levels plus many more short-lived substates. SCl channels have not been described in previous reports of Cl- channels in skeletal muscle SR.

Chloride channels of intracellular organelles

Chloride channels are present in a variety of intracellular organelles (Golgi, endosomes, endoplasmic reticulum, and sarcoplasmic reticulum) where they serve largely to shunt the membrane potential created by other ion-translocating processes. Electrophysiological studies have shown that the Cl- channels of the endoplasmic and sarcoplasmic reticula facilitate the efflux of Ca2+. In the Golgi and some endosomes, the open Cl- channels (probably the cystic fibrosis transmembrane conductance regulator) favor accumulation of H+.

Postischemic changes in cardiac sarcoplasmic reticulum Ca2+ channels. A possible mechanism of ischemic preconditioning

We investigated the modifications of cardiac ryanodine receptors/sarcoplasmic reticulum Ca2+ release channels occurring in ischemic preconditioning. In an isolated rat heart model, the injury produced by 30 minutes of global ischemia was reduced by preexposure to three 3-minute periods of global ischemia (preconditioning ischemia). The protection was still present 120 minutes after preconditioning ischemia but disappeared after 240 minutes. Three 1-minute periods of global ischemia did not provide any protection. In the crude homogenate obtained from ventricular myocardium, the density of [3H]ryanodine binding sites averaged 372 +/- 18 fmol/mg of protein in the control condition, decreased 5 minutes after preconditioning ischemia (290 +/- 15 fmol/mg, P < .01), was still significantly reduced after 120 minutes (298 +/- 17 fmol/mg, P < .05), and recovered after 240 minutes (341 +/- 21 fmol/mg). Three 1-minute periods of ischemia did not produce any change in ryanodine binding. The Kd for ryanodine (1.5 +/- 0.3 nmol/L) was unchanged in all cases. In parallel experiments, the crude homogenate or a microsomal fraction was passively loaded with 45Ca, and Ca(2+)-induced Ca2+ release was studied by the quick filtration technique. In both preparations, the rate constant of Ca(2+)-induced Ca2+ release decreased 5 and 120 minutes after preconditioning ischemia (homogenate values: 19.7 +/- 1.4 and 18.9 +/- 0.9 s-1 vs a control value of 25.4 +/- 1.7 s-1, P < .05 in both cases) and recovered after 240 minutes (23.0 +/- 1.9 s-1). The Ca2+ dependence of Ca(2+)-induced Ca2+ release was not affected by preconditioning ischemia.(ABSTRACT TRUNCATED AT 250 WORDS)

Removal of Mg2+ inhibition of cardiac ryanodine receptor by palmitoyl coenzyme A

45Ca2+ fluxes and planar bilayer recordings indicated that the fatty acid metabolite palmitoyl coenzyme A, but not free coenzyme A or palmitic acid, stimulated the cardiac ryanodine receptor channel of pig heart sarcoplasmic reticulum. Palmitoyl CoA reactivated channels inhibited by concentrations of cytoplasmic free Mg2+ in the physiological range. Reactivation by palmitoyl CoA in the presence of Mg2+ was stimulated by myoplasmic free Ca2+ in the micromolar range. Acyl coenzyme A derivatives may be utilized by cardiac muscle cells to compensate for the severe Mg2+ inhibition of ryanodine receptors which would otherwise leave Ca2+ stores unresponsive to Ca2+ and to other cytosolic ligands involved in signal transduction.