Eu3+ detects two functionally distinct luminal Ca2+ binding sites in ryanodine receptors

Ryanodine receptors (RyRs) are Ca2+ release channels, gated by Ca2+ in the cytosol and the sarcoplasmic reticulum lumen. Their regulation is impaired in certain cardiac and muscle diseases. Although a lot of data is available on the luminal Ca2+ regulation of RyR, its interpretation is complicated by the possibility that the divalent ions used to probe the luminal binding sites may contaminate the cytoplasmic sites by crossing the channel pore. In this study, we used Eu3+, an impermeable agonist of Ca2+ binding sites, as a probe to avoid this complication and to gain more specific information about the function of the luminal Ca2+ sensor. Single-channel currents were measured from skeletal muscle and cardiac RyRs (RyR1 and RyR2) using the lipid bilayer technique. We show that RyR2 is activated by the luminal addition of Ca2+, whereas RyR1 is inhibited. These results were qualitatively reproducible using Eu3+. The luminal regulation of RyR1 carrying a mutation associated with malignant hyperthermia was not different from that of the wild-type. RyR1 inhibition by Eu3+ was extremely voltage dependent, whereas RyR2 activation did not depend on the membrane potential. These results suggest that the RyR1 inhibition site is in the membrane's electric field (channel pore), whereas the RyR2 activation site is outside. Using in silico analysis and previous results, we predicted putative Ca2+ binding site sequences. We propose that RyR2 bears an activation site, which is missing in RyR1, but both isoforms share the same inhibitory Ca2+ binding site near the channel gate.

4-chloro-orto-cresol activates ryanodine receptor more selectively and potently than 4-chloro-meta-cresol

In this study we performed the comprehensive pharmacological analysis of two stereoisomers of 4-chloro-meta-cresol (4CMC), a popular ryanodine receptor (RyR) agonist used in muscle research. Experiments investigating the Ca²⁺-releasing action of the isomers demonstrated that the most potent isomer was 4-chloro-orto-cresol (4COC) (EC₅₀ = 55 ± 14 μM), although 3-chloro-para-cresol (3CPC) was more effective, as it was able to induce higher magnitude of Ca²⁺ flux from isolated terminal cisterna vesicles. Nevertheless, 3CPC stimulated the hydrolytic activity of the sarcoplasmic reticulum ATP-ase (SERCA) with an EC₅₀ of 91 ± 17 μM, while 4COC affected SERCA only in the millimolar range (IC₅₀ = 1370 ± 88 μM). IC₅₀ of 4CMC for SERCA pump was 167 ± 8 μM, indicating that 4CMC is not a specific RyR agonist either, as it activated RyR in a similar concentration (EC₅₀ = 121 ± 20 μM). Our data suggest that the use of 4COC might be more beneficial than 4CMC in experiments, when Ca²⁺ release should be triggered through RyRs without influencing SERCA activity.

Dantrolene Requires Mg2+ and ATP To Inhibit the Ryanodine Receptor

Dantrolene is a ryanodine receptor (RyR) inhibitor, which is used to relax muscles in malignant hyperthermia syndrome. Although dantrolene binds to the RyR protein, its mechanism of action is unknown, mainly because of the controversial results showing that dantrolene inhibited Ca²⁺ release from intact fibers and sarcoplasmic reticulum (SR) vesicles, but failed to inhibit single RyR channel currents in bilayers. Accordingly, it was concluded that an important factor for dantrolene's action was lost during the purification procedure of RyR. Recently, Mg²⁺ was demonstrated to be the essential factor for dantrolene to inhibit Ca²⁺ release in skinned muscle fibers. The aim of the present study was to confirm these results in Ca²⁺ release and bilayer experiments, using SR vesicles and solubilized channels, respectively. Our Ca²⁺ release experiments demonstrated that the effect of dantrolene and Mg²⁺ was cooperative and that ATP enhanced the inhibiting effect of dantrolene. Namely, 10 µM dantrolene reduced RyR channel open probability by ∼50% in the presence of 3 mM free Mg²⁺ and 1 mM ATP, whereas channel activity further decreased to ∼20% of control when [ATP] was increased to 2 mM. Our data provide important complementary information that supports the direct, Mg²⁺-dependent mechanism of dantrolene's action and suggests that dantrolene also requires ATP to inhibit RyR.

Laser induced calcium oscillations in fluorescent calcium imaging

Phototoxicity is the most common problem investigators may encounter when performing live cell imaging. It develops due to excess laser exposure of cells loaded with fluorophores and can lead to often overlooked but significant artifacts, such as massive increase of intracellular Ca2+ concentration, which would make data interpretation problematic. Because information about laser- and dye-related changes in cytoplasmic calcium concentration is very limited, we aimed to describe this phenomenon to help investigators using laser scanning confocal microscopy in a non-invasive way. Therefore, in the present study we evaluated fluorescent fluctuations, which evolved in Fluo-3/4/8 loaded mouse pancreatic acinar cells during very low intensity laser excitation. We demonstrate that after standard loading procedure (2 µM Fluo-3/4/8-AM, 30 min at room temperature), applying 488 nm laser at as low as ca. 10 µW incident laser power (0.18 µW/µm2) at 1 Hz caused repetitive, 2-3 fold elevations of the resting intracellular fluorescence. The first latency and the pattern of the fluorescence fluctuations were laser power dependent and were related to Ca2+-release from intracellular stores, as they were abolished by BAPTA-AM treatment in Ca2+-free medium, but were not diminished by the reactive oxygen species (ROS) scavenger DMPO. Worryingly enough, the qualitative and quantitative features of the Ca2+-waves were practically indistinguishable from the responses evoked by secretagogue stimulation. Since using similar imaging conditions, a number of other cell types were reported to display spontaneous Ca2+ oscillations, we propose strategies to distinguish the real signals from artifacts.

Lanthanides Report Calcium Sensor in the Vestibule of Ryanodine Receptor

Ca²⁺ regulates ryanodine receptor's (RyR) activity through an activating and an inhibiting Ca²⁺-binding site located on the cytoplasmic side of the RyR channel. Their altered sensitivity plays an important role in the pathology of malignant hyperthermia and heart failure. We used lanthanide ions (Ln³⁺) as probes to investigate the Ca²⁺ sensors of RyR, because they specifically bind to Ca²⁺-binding proteins and they are impermeable to the channel. Eu³⁺'s and Sm³⁺'s action was tested on single RyR1 channels reconstituted into planar lipid bilayers. When the activating binding site was saturated by 50 μM Ca²⁺, Ln³⁺ potently inhibited RyR's open probability (K_d Eu³⁺ = 167 ± 5 nM and K_d Sm³⁺ = 63 ± 3 nM), but in nominally 0 [Ca²⁺], low [Eu³⁺] activated the channel. These results suggest that Ln³⁺ acts as an agonist of both Ca²⁺-binding sites. More importantly, the voltage-dependent characteristics of Ln³⁺'s action led to the conclusion that the activating Ca²⁺ binding site is located within the electrical field of the channel (in the vestibule). This idea was tested by applying the pore blocker toxin maurocalcine on the cytoplasmic side of RyR. These experiments showed that RyR lost reactivity to changing cytosolic [Ca²⁺] from 50 μM to 100 nM when the toxin occupied the vestibule. These results suggest that maurocalcine mechanically prevented Ca²⁺ from dissociating from its binding site and support our vestibular Ca²⁺ sensor-model further.

Effects of fluvastatin and coenzyme Q10 on skeletal muscle in normo- and hypercholesterolaemic rats

Myalgia and muscle weakness may appreciably contribute to the poor adherence to statin therapy. Although the pathomechanism of statin-induced myopathy is not completely understood, changes in calcium homeostasis and reduced coenzyme Q10 levels are hypothesized to play important roles. In our experiments, fluvastatin and/or coenzyme Q10 was administered chronically to normocholesterolaemic or hypercholaestherolaemic rats, and the modifications of the calcium homeostasis and the strength of their muscles were investigated. While hypercholesterolaemia did not change the frequency of sparks, fluvastatin increased it on muscles both from normocholesterolaemic and from hypercholesterolaemic rats. This effect, however, was not mediated by a chronic modification of the ryanodine receptor as shown by the unchanged ryanodine binding in the latter group. While coenzyme Q10 supplementation significantly reduced the frequency of the spontaneous calcium release events, it did not affect their amplitude and spatial spread in muscles from fluvastatin-treated rats. This indicates that coenzyme Q10 supplementation prevented the spark frequency increasing effect of fluvastatin without having a major effect on the amount of calcium released during individual sparks. In conclusion, we have found that fluvastatin, independently of the cholesterol level in the blood, consistently and specifically increased the frequency of calcium sparks in skeletal muscle cells, an effect which could be prevented by the addition of coenzyme Q10 to the diet. These results support theories favouring the role of calcium handling in the pathophysiology of statin-induced myopathy and provide a possible pathway for the protective effect of coenzyme Q10 in statin treated patients symptomatic of this condition.

Synthesis of fluorescent ristocetin aglycon derivatives with remarkable antibacterial and antiviral activities

Isoindole and benzoisoindole derivatives of ristocetin aglycon have been prepared by reaction with o-phthalaldehyde or naphthalene-2,3-dialdehyde and various thiols. The new compounds exhibited potent antibacterial and anti-influenza virus activity. The cluster forming and fluorescent properties of the aglycon derivatives were also studied.

The Na+/Ca2+ exchange blocker SEA0400 fails to enhance cytosolic Ca2+ transient and contractility in canine ventricular cardiomyocytes

AIMS

This study was designed to evaluate the effects of the Na(+)/Ca(2+) exchange (NCX) inhibitor SEA0400 on Ca(2+) handling in isolated canine ventricular myocytes.

METHODS AND RESULTS

Intracellular Ca(2+) ([Ca(2+)](i)) transients, induced by either field stimulation or caffeine flush, were monitored using Ca(2+) indicator dyes. [Ca(2+)](i)-dependent modulation of the inhibitory effect of SEA0400 on NCX was characterized by the changes in Ni(2+)-sensitive current in voltage-clamped myocytes. Sarcoplasmic reticulum (SR) Ca(2+) release and uptake were studied in SR membrane vesicles. Gating properties of single-ryanodine receptors were analysed in lipid bilayers. Ca(2+) sensitivity of the contractile machinery was evaluated in chemically skinned myocytes. In myocytes paced at 1 Hz, neither diastolic [Ca(2+)](i) nor the amplitude of [Ca(2+)](i) transients was significantly altered by SEA0400 up to the concentration of 1 microM, which was shown to inhibit the exchange current. The blocking effect of SEA0400 on NCX decreased with increasing [Ca(2+)](i), and it was more pronounced in reverse than in forward mode operation at every [Ca(2+)](i) examined. The rate of decay of the caffeine-induced [Ca(2+)](i) transients was decreased significantly by 1 microM SEA0400; however, this effect was only a fraction of that observed with 10 mM NiCl(2). Neither SR Ca(2+) release and uptake nor cell shortening and Ca(2+) sensitivity of the contractile proteins were influenced by SEA0400.

CONCLUSION

The lack of any major SEA0400-induced shift in Ca(2+) transients or contractility of myocytes can well be explained by its limited inhibitory effect on NCX (further attenuated by elevated [Ca(2+)](i) levels) and a concomitant reduction in Ca(2+) influx due to the predominantly reverse mode blockade of NCX and suppression of L-type Ca(2+) current.

Effect of natural phenol derivatives on skeletal type sarcoplasmic reticulum Ca2+-ATPase and ryanodine receptor

The effect of natural phenol derivatives was studied on skeletal type sarcoplasmic reticulum Ca(2+)-ATPase and ryanodine receptor. The majority of the tested derivatives exerted inhibitory effect on the Ca(2+)-ATPase with an ascending sequence in regard to their effectiveness (IC(50)): cineole (3.33 mM) < ortho-vanillin (IC(50 )=1.13 mM) < 4-methyl-2-nitrophenol (1104 microM) < vanillin (525 microM) < thymol (224 microM) < carvacrol (162 microM). In two cases biphasic characteristic was observed: trans-anethole and meta-anisaldehyde first caused activation followed by inhibition (with IC(50)-s of 141 and 1903 microM respectively) as their concentration was increased. In some cases (cineole, ortho-vanillin, meta-anisaldehyde) total inhibition of Ca(2+)-ATPase could not be reached as the result of the limited solubility of these drugs. Para-anisaldehyde and 6-amino-meta-cresol did not show any effect up to 3 mM. In Ca(2+) release experiments drugs were applied on heavy sarcoplasmic reticulum vesicles isolated from skeletal muscle and actively loaded with calcium. Only thymol and carvacrol were able to evoke Ca(2+) release with EC(50) values of 158 +/- 16 and 211 +/- 55 microM respectively. Furthermore the effect of thymol and carvacrol was tested on the isolated ryanodine receptor incorporated into artificial lipid bilayer. Both drugs activated the RyR when applied in concentrations identical to their EC(50) values. These observations show that small differences in the structure of phenol derivatives sometimes have little impact on their effect on the sarcoplasmic reticulum Ca(2+)-ATPase or ryanodine receptor (thymol and carvacrol) whereas in certain cases they can completely abolish a particular effect (para- and meta-anisaldehyde).

Alterations in the calcium homeostasis of skeletal muscle from postmyocardial infarcted rats

In chronic heart failure, skeletal muscles develop a weakness that is not associated to an impaired circulatory function but rather to alterations in the skeletal muscle fibers themselves. To understand these changes, the steps in excitation-contraction coupling of rats that underwent a left anterior coronary artery occlusion were studied. About 24 weeks after the myocardial infarction, neither the total amount nor the voltage dependence of intramembrane charge were altered. In contrast, calcium release from the sarcoplasmic reticulum was considerably suppressed, and its voltage dependence shifted toward more positive voltages. Elementary calcium-release events showed altered morphology as the relative proportion of embers increased. Calcium sparks were smaller in amplitude and had larger time-to-peak values. Isolated ryanodine receptors (RyR) displayed an unusual rectification with increased single-channel conductance at positive (cis vs trans) voltages. In addition, the bell-shaped calcium dependence of channel activity was broader, with a slight shift of activation to lower and a larger shift in inactivation to higher calcium concentrations. These data indicate that the number of channels that open during a calcium-release event is decreased and that RyR function is altered; thus, calcium-release is suppressed after a myocardial infarction. These observations give an explanation for the impaired skeletal muscle function in these animals.

Effect of gadolinium on the ryanodine receptor/sarcoplasmic reticulum calcium release channel of skeletal muscle

The effect of gadolinium ions on the sarcoplasmic reticulum (SR) calcium release channel/ryanodine receptor (RyR1) was studied using heavy SR (HSR) vesicles and RyR1 isolated from rabbit fast twitch muscle. In the [(3)H]ryanodine binding assay, 5 microM Gd(3+) increased the K(d) of the [(3)H]ryanodine binding of the vesicles from 33.8 nM to 45.6 nM while B(max), referring to the binding capacity, was not affected significantly. In the presence of 18 nM[(3)H]ryanodine and 100 microM free Ca(2+), Gd(3+) inhibited the binding of the radiolabeled ryanodine with an apparent K(d) value of 14.7 microM and a Hill coefficient of 3.17. In (45)Ca(2+) experiments the time constant of (45)Ca(2+) efflux from HSR vesicles increased from 90.9 (+/- 11.1) ms to 187.7 (+/- 24.9) ms in the presence of 20 microM gadolinium. In single channel experiments gadolinium inhibited the channel activity from both the cytoplasmic (cis) (IC(50) = 5.65 +/- 0.33 microM, n(Hill) = 4.71) and the luminal (trans) side (IC(50) = 5.47 +/- 0.24 microM, n(Hill) = 4.31). The degree of inhibition on the cis side didn't show calcium dependency in the 100 microM to 1 mM Ca(2+) concentration range which indicates no competition with calcium on its regulatory binding sites. When Gd(3+) was applied at the trans side, EGTA was present at the cis side to prevent the binding of Gd(+3) to the cytoplasmic calcium binding regulatory sites of the RyR1 if Gd(3+) accidentally passed through the channel. The inhibition of the channel did not show any voltage dependence, which would be the case if Gd(3+) exerted its effect after getting to the cis side. Our results suggest the presence of inhibitory binding sites for Gd(3+) on both sides of the RyR1 with similar Hill coefficients and IC(50) values.

Effect of thymol on calcium handling in mammalian ventricular myocardium

Concentration-dependent effects of thymol on calcium handling were studied in canine and guinea pig cardiac preparations (Langendorff-perfused guinea pig hearts, canine ventricular trabeculae, canine sarcoplasmic reticular vesicles and single ryanodine receptors). Thymol induced a concentration-dependent negative inotropic action in both canine and guinea pig preparations (EC(50) = 297 +/- 12 microM in dog). However, low concentrations of thymol reduced intracellular calcium transients in guinea pig hearts without decreasing contractility. At higher concentrations both calcium transients and contractions were suppressed. In canine sarcoplasmic reticular vesicles thymol induced rapid release of calcium (V(max) = 0.47 +/- 0.04 nmol s(-1), EC(50) = 258 +/- 21 microM, Hill coefficient = 3.0 +/- 0.54), and decreased the activity of the calcium pump (EC(50) = 253 +/- 4.7 microM, Hill coefficient = 1.62 +/- 0.05). Due to the less sharp concentration-dependence of the ATPase inhibition, this effect was significant from 50 microM, whereas the thymol-induced calcium release only from 100 microM. In single ryanodine receptors incorporated into artificial lipid bilayer thymol induced long lasting openings, having mean open times increased with 3 orders of magnitude, however, the specific conductance of the channel remained unaltered. This effect of thymol was not voltage-dependent and failed to prevent the binding of ryanodine. In conclusion, the negative inotropic action of thymol can be explained by reduction in calcium content of the sarcoplasmic reticulum due to the combination of the thymol-induced calcium release and inhibition of the calcium pump. The calcium-sensitizer effect, observed at lower thymol concentrations, indicates that thymol is likely to interact with the contractile machinery also.

Troponin I converts the skeletal muscle ryanodine receptor into a rectifying calcium release channel

The goal of our present studies has been to find novel ryanodine receptor (RyR1) interacting polypeptides that modulate the channel activity from the luminal side of RyR1. Using K(+) as charge carrier for recording of single channel events here we demonstrate a very unexpected observation that troponin I substantially alters RyR's gating behavior, and that RyR1 in association with troponin I becomes a rectifying Ca(2+) release channel. Troponin I rapidly locks the RyR1 in a non-conducting state only at a negative holding potential, and only when applied to the luminal side; switching to a positive holding potential results in the channel returning to its original activity, immediately. A hypothesis is proposed to account for how an intraluminally located, positively charged molecule might function as a RyR1 regulator under physiological conditions.

Altered inhibition of the rat skeletal ryanodine receptor/calcium release channel by magnesium in the presence of ATP

Magnesium-induced inhibition of the skeletal ryanodine receptor/calcium-release channel (RyR) was studied in the presence and absence of ATP under isolated conditions and in situ, by examining the RyR incorporated into a planar lipid bilayer and the calcium release flux (Rrel) in isolated single fibres mounted in the double Vaseline gap system. When the incorporated RyR had been activated by calcium (50 microM) in the absence of ATP, the magnesium-induced inhibition showed co-operativity with a Hill coefficient (N) of 1.83 and a half-inhibitory concentration (IC50) of 635 microM. When the open probability was measured in the presence of 5 mM ATP and at a low calcium concentration, the magnesium-induced inhibition was non-cooperative (N=1.1, IC50= 860 microM). In isolated muscle fibres, in the presence of ATP, lowering the intracellular magnesium concentration ([Mg2+]i) increased the maximal Rrel and shifted its voltage dependence to more negative membrane potentials. Increasing [Mg2+]i had the opposite effect. The concentration dependence was described with an IC50 of 174 microM, N=1, under depolarized conditions and showed a tenfold increase in affinity in polarized fibres. At the concentration required for the measurements from isolated fibres, ATP had a full activatory effect on the isolated channel. At a low calcium concentration, the RyR had two ATP-binding sites with half-activatory concentrations of 19 and 350 microM, respectively.

Biphasic effect of bimoclomol on calcium handling in mammalian ventricular myocardium

1. Concentration-dependent effects of bimoclomol, the novel heat shock protein coinducer, on intracellular calcium transients and contractility were studied in Langendorff-perfused guinea-pig hearts loaded with the fluorescent calcium indicator dye Fura-2. Bimoclomol had a biphasic effect on contractility: both peak left ventricular pressure and the rate of force development significantly increased at a concentration of 10 nM or higher. The maximal effect was observed between 0.1 and 1 microM, and the positive inotropic action disappeared by further increasing the concentration of bimoclomol. The drug increased systolic calcium concentration with a similar concentration-dependence. In contrast, diastolic calcium concentration increased monotonically in the presence of bimoclomol. Thus low concentrations of the drug (10 - 100 nM) increased, whereas high concentrations (10 microM) decreased the amplitude of intracellular calcium transients. 2. Effects of bimoclomol on action potential configuration was studied in isolated canine ventricular myocytes. Action potential duration was increased at low (10 nM), unaffected at intermediate (0.1 - 1 microM) and decreased at high (10 - 100 microM) concentrations of the drug. 3. In single canine sarcoplasmic calcium release channels (ryanodine receptor), incorporated into artificial lipid bilayer, bimoclomol significantly increased the open probability of the channel in the concentration range of 1 - 10 microM. The increased open probability was associated with increased mean open time. The effect of bimoclomol was again biphasic: the open probability decreased below the control level in the presence of 1 mM bimoclomol. 4. Bimoclomol (10 microM - 1 mM) had no significant effect on the rate of calcium uptake into sarcoplasmic reticulum vesicles of the dog, indicating that in vivo calcium reuptake might not substantially be affected by the drug. 5. In conclusion, the positive inotropic action of bimoclomol is likely due to the activation of the sarcoplasmic reticulum calcium release channel in mammalian ventricular myocardium.

Regulation of the rat sarcoplasmic reticulum calcium release channel by calcium

The regulation by calcium of the ryanodine receptor/SR calcium release channel (RyR) from rat skeletal muscle was studied under isolated conditions and in situ. RyRs were either solubilized and incorporated into lipid bilayers or single fibres were mounted into a Vaseline gap voltage clamp. Single channel data were compared to parameters determined from the calculated calcium release flux. With K+ (250 mM) being the charge carrier the single channel conductance was 529 pS at 50 microM Ca2+ cis and trans, and decreased with increasing cis [Ca2+]. Open probability showed a bell shaped calcium dependence revealing an activatory and an inhibitory Ca2+ binding site (Hill coefficients of 1.18 and 1.28, respectively) with half activatory and inhibitory concentrations of 9.4 and 298 microM. The parameters of the inhibitory site agreed with the calcium dependence of channel inactivation deduced from the decline in SR calcium release in isolated fibres. Mean open time showed slight [Ca2+] dependence following a single exponential at every Ca2+ concentration tested. Closed time histograms, at high [Ca2+], were fitted with three exponentials, from which the longest was calcium independent, and resembled the recovery time constant of SR inactivation (115+/-15 ms) obtained in isolated fibres. The data are in agreement with a model where calcium binding to the inhibitory site on RyR would be responsible for the calcium dependent inactivation in situ.

Calsequestrin: more than 'only' a luminal Ca2+ buffer inside the sarcoplasmic reticulum

In striated muscle, the sarcoplasmic reticulum (SR) Ca2+ release/ryanodine receptor (RyR) channel provides the pathway through which stored Ca2+ is released into the myoplasm during excitation-contraction coupling. Various luminal Ca2+-binding proteins are responsible for maintaining the free [Ca2+] at 10(-3)-10(-4) M in the SR lumen; in skeletal-muscle SR, it is mainly calsequestrin. Here we show that, depending on its phosphorylation state, calsequestrin selectively controls the RyR channel activity at 1 mM free luminal [Ca2+]. Calsequestrin exclusively in the dephosphorylated state enhanced the open probability by approx. 5-fold with a Hill coefficient (h) of 3.3, and increased the mean open time by about 2-fold, i.e. solely dephosphorylated calsequestrin regulates Ca2+ release from the SR. Because calsequestrin has been found to occur mainly in the phosphorylated state in the skeletal-muscle SR for the regulation of RyR channel activity, the dephosphorylation of calsequestrin would appear to be a quintessential physiological event.