Regulation of the calcium release channel from skeletal muscle by suramin and the disulfonated stilbene derivatives DIDS, DBDS, and DNDS

ent-Verticilide B1 Inhibits Type 2 Ryanodine Receptor Channels and is Antiarrhythmic in Casq2 -/- Mice

Intracellular Ca2+ leak from cardiac ryanodine receptor (RyR2) is an established mechanism of sudden cardiac death (SCD), whereby dysregulated Ca2+ handling causes ventricular arrhythmias. We previously discovered the RyR2-selective inhibitor ent-(+)-verticilide (ent-1), a 24-membered cyclooligomeric depsipeptide that is the enantiomeric form of a natural product (nat-(-)-verticilide). Here, we examined its 18-membered ring-size oligomer (ent-verticilide B1; "ent-B1") in RyR2 single channel and [3H]ryanodine binding assays, and in Casq2 -/- cardiomyocytes and mice, a gene-targeted model of SCD. ent-B1 inhibited RyR2 single channels and RyR2-mediated spontaneous Ca2+ release in Casq2 -/- cardiomyocytes with sub-micromolar potency. ent-B1 was a partial RyR2 inhibitor, with maximal inhibitory efficacy of less than 50%. ent-B1 was stable in plasma, with a peak plasma concentration of 1460 ng/ml at 10 minutes and half-life of 45 minutes after intraperitoneal administration of 3 mg/kg in mice. In vivo, ent-B1 significantly reduced catecholamine-induced ventricular arrhythmias in Casq2 -/- mice in a dose-dependent manner. Hence, we have identified a novel chemical entity - ent-B1 - that preserves the mechanism of action of a hit compound and shows therapeutic efficacy. These findings strengthen RyR2 as an antiarrhythmic drug target and highlight the potential of investigating the mirror-image isomers of natural products to discover new therapeutics. SIGNIFICANCE STATEMENT: The cardiac ryanodine receptor (RyR2) is an untapped target in the stagnant field of antiarrhythmic drug development. We have confirmed RyR2 as an antiarrhythmic target in a mouse model of sudden cardiac death and shown the therapeutic efficacy of a second enantiomeric natural product.

ent -Verticilide B1 inhibits type 2 ryanodine receptor channels and is antiarrhythmic in Casq2-/- mice

Ca 2+ leak from cardiac ryanodine receptor (RyR2) is an established mechanism of sudden cardiac death (SCD), whereby dysregulated Ca 2+ handling causes ventricular arrhythmias. We previously discovered the RyR2-selective inhibitor ent- (+)-verticilide ( ent -1), a 24-membered cyclooligomeric depsipeptide that is the enantiomeric form of a natural product ( nat -(-)-verticilide). Here, we examined its 18-membered ring-size oligomer ( ent -verticilide B1; " ent -B1") in single RyR2 channel assays, [ 3 H]ryanodine binding assays, and in Casq2 -/- cardiomyocytes and mice, a gene-targeted model of SCD. ent -B1 inhibited RyR2 single-channels and [ 3 H]ryanodine binding with low micromolar potency, and RyR2-mediated spontaneous Ca 2+ release in Casq2-/- cardiomyocytes with sub-micromolar potency. ent -B1 was a partial RyR2 inhibitor, with maximal inhibitory efficacy of less than 50%. ent -B1 was stable in plasma, with a peak plasma concentration of 1460 ng/ml at 10 min and half-life of 45 min after intraperitoneal administration of 3 mg/kg in mice. Both 3 mg/kg and 30 mg/kg ent -B1 significantly reduced catecholamine-induced ventricular arrhythmia in Casq2-/- mice. Hence, we have identified a novel chemical entity - ent -B1 - that preserves the mechanism of action of a hit compound and shows therapeutic efficacy. These findings strengthen RyR2 as an antiarrhythmic drug target and highlight the potential of investigating the mirror-image isomers of natural products to discover new therapeutics.

Significance statement

The cardiac ryanodine receptor (RyR2) is an untapped target in the stagnant field of antiarrhythmic drug development. We have confirmed RyR2 as an antiarrhythmic target in a mouse model of sudden cardiac death and shown the therapeutic efficacy of a second enantiomeric natural product.

Ion currents through Kir potassium channels are gated by anionic lipids

Ion currents through potassium channels are gated. Constriction of the ion conduction pathway at the inner helix bundle, the textbook gate of Kir potassium channels, has been shown to be an ineffective permeation control, creating a rift in our understanding of how these channels are gated. Here we present evidence that anionic lipids act as interactive response elements sufficient to gate potassium conduction. We demonstrate the limiting barrier to K+ permeation lies within the ion conduction pathway and show that this gate is operated by the fatty acyl tails of lipids that infiltrate the conduction pathway via fenestrations in the walls of the pore. Acyl tails occupying a surface groove extending from the cytosolic interface to the conduction pathway provide a potential means of relaying cellular signals, mediated by anionic lipid head groups bound at the canonical lipid binding site, to the internal gate.

RYR2 Channel Inhibition Is the Principal Mechanism of Flecainide Action in CPVT

RATIONALE

The class Ic antiarrhythmic drug flecainide prevents ventricular tachyarrhythmia in patients with catecholaminergic polymorphic ventricular tachycardia (CPVT), a disease caused by hyperactive RyR2 (cardiac ryanodine receptor) mediated calcium (Ca) release. Although flecainide inhibits single RyR2 channels in vitro, reports have claimed that RyR2 inhibition by flecainide is not relevant for its mechanism of antiarrhythmic action and concluded that sodium channel block alone is responsible for flecainide's efficacy in CPVT.

OBJECTIVE

To determine whether RyR2 block independently contributes to flecainide's efficacy for suppressing spontaneous sarcoplasmic reticulum Ca release and for preventing ventricular tachycardia in vivo.

METHODS AND RESULTS

We synthesized N-methylated flecainide analogues (QX-flecainide and N-methyl flecainide) and showed that N-methylation reduces flecainide's inhibitory potency on RyR2 channels incorporated into artificial lipid bilayers. N-methylation did not alter flecainide's inhibitory activity on human cardiac sodium channels expressed in HEK293T cells. Antiarrhythmic efficacy was tested utilizing a Casq2 (cardiac calsequestrin) knockout (Casq2-/-) CPVT mouse model. In membrane-permeabilized Casq2-/- cardiomyocytes-lacking intact sarcolemma and devoid of sodium channel contribution-flecainide, but not its analogues, suppressed RyR2-mediated Ca release at clinically relevant concentrations. In voltage-clamped, intact Casq2-/- cardiomyocytes pretreated with tetrodotoxin to inhibit sodium channels and isolate the effect of flecainide on RyR2, flecainide significantly reduced the frequency of spontaneous sarcoplasmic reticulum Ca release, while QX-flecainide and N-methyl flecainide did not. In vivo, flecainide effectively suppressed catecholamine-induced ventricular tachyarrhythmias in Casq2-/- mice, whereas N-methyl flecainide had no significant effect on arrhythmia burden, despite comparable sodium channel block.

CONCLUSIONS

Flecainide remains an effective inhibitor of RyR2-mediated arrhythmogenic Ca release even when cardiac sodium channels are blocked. In mice with CPVT, sodium channel block alone did not prevent ventricular tachycardia. Hence, RyR2 channel inhibition likely constitutes the principal mechanism of antiarrhythmic action of flecainide in CPVT.

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

KEY POINTS

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

ABSTRACT

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

Comparative Analyses of the 12 Most Abundant PCB Congeners Detected in Human Maternal Serum for Activity at the Thyroid Hormone Receptor and Ryanodine Receptor

Polychlorinated biphenyls (PCBs) pose significant risk to the developing human brain; however, mechanisms of PCB developmental neurotoxicity (DNT) remain controversial. Two widely posited mechanisms are tested here using PCBs identified in pregnant women in the MARBLES cohort who are at increased risk for having a child with a neurodevelopmental disorder (NDD). As determined by gas chromatography-triple quadruple mass spectrometry, the mean PCB level in maternal serum was 2.22 ng/mL. The 12 most abundant PCBs were tested singly and as a mixture mimicking the congener profile in maternal serum for activity at the thyroid hormone receptor (THR) and ryanodine receptor (RyR). Neither the mixture nor the individual congeners (2 fM to 2 μM) exhibited agonistic or antagonistic activity in a THR reporter cell line. However, as determined by equilibrium binding of [3H]ryanodine to RyR1-enriched microsomes, the mixture and the individual congeners (50 nM to 50 μM) increased RyR activity by 2.4-19.2-fold. 4-Hydroxy (OH) and 4-sulfate metabolites of PCBs 11 and 52 had no TH activity; but 4-OH PCB 52 had higher potency than the parent congener toward RyR. These data support evidence implicating RyRs as targets in environmentally triggered NDDs and suggest that PCB effects on the THR are not a predominant mechanism driving PCB DNT. These findings provide scientific rationale regarding a point of departure for quantitative risk assessment of PCB DNT, and identify in vitro assays for screening other environmental pollutants for DNT potential.

Cardiac Calcium Release Channel (Ryanodine Receptor 2) Regulation by Halogenated Anesthetics

Background

Halogenated anesthetics activate cardiac ryanodine receptor 2–mediated sarcoplasmic reticulum Ca2+ release, leading to sarcoplasmic reticulum Ca2+ depletion, reduced cardiac function, and providing cell protection against ischemia-reperfusion injury. Anesthetic activation of ryanodine receptor 2 is poorly defined, leaving aspects of the protective mechanism uncertain.

Methods

Ryanodine receptor 2 from the sheep heart was incorporated into artificial lipid bilayers, and their gating properties were measured in response to five halogenated anesthetics.

Results

Each anesthetic rapidly and reversibly activated ryanodine receptor 2, but only from the cytoplasmic side. Relative activation levels were as follows: halothane (approximately 4-fold; n = 8), desflurane and enflurane (approximately 3-fold,n = 9), and isoflurane and sevoflurane (approximately 1.5-fold, n = 7, 10). Half-activating concentrations (Ka) were in the range 1.3 to 2.1 mM (1.4 to 2.6 minimum alveolar concentration [MAC]) with the exception of isoflurane (5.3 mM, 6.6 minimum alveolar concentration). Dantrolene (10 μM with 100 nM calmodulin) inhibited ryanodine receptor 2 by 40% but did not alter the Ka for halothane activation. Halothane potentiated luminal and cytoplasmic Ca2+ activation of ryanodine receptor 2 but had no effect on Mg2+ inhibition. Halothane activated ryanodine receptor 2 in the absence and presence (2 mM) of adenosine triphosphate (ATP). Adenosine, a competitive antagonist to ATP activation of ryanodine receptor 2, did not antagonize halothane activation in the absence of ATP.

Conclusions

At clinical concentrations (1 MAC), halothane desflurane and enflurane activated ryanodine receptor 2, whereas isoflurane and sevoflurane were ineffective. Dantrolene inhibition of ryanodine receptor 2 substantially negated the activating effects of anesthetics. Halothane acted independently of the adenine nucleotide–binding site on ryanodine receptor 2. The previously observed adenosine antagonism of halothane activation of sarcoplasmic reticulum Ca2+ release was due to competition between adenosine and ATP, rather than between halothane and ATP.

P2Y2 purinergic receptors are highly expressed in cardiac and diaphragm muscles of mdx mice, and their expression is decreased by suramin

INTRODUCTION

In Duchenne muscular dystrophy (DMD) and in the mdx mouse model of DMD, the lack of dystrophin leads to increased calcium influx and muscle necrosis. Patients suffer progressive muscle loss, and cardiomyopathy is an important determinant of morbidity. P2 purinergic receptors participate in the increased calcium levels in dystrophic skeletal muscles.

METHODS

In this study, we evaluated whether P2 receptors are involved in cardiomyopathy in mdx mice at later stages of the disease.

RESULTS

Western blotting revealed that P2Y₂ receptor levels were upregulated (54%) in dystrophic heart compared with a normal heart. Suramin reduced the levels of P2Y₂ to almost normal values. Suramin also decreased heart necrosis (reduced CK-MB) and the expression of the stretch-activated calcium channel TRPC1.

CONCLUSIONS

This study suggests that P2Y₂ may participate in cardiomyopathy in mdx mice. P2-selective drugs with specific actions in the dystrophic heart may ameliorate cardiomyopathy in dystrophinopathies. Muscle Nerve 55: 116-121, 2017.

An Extended Structure-Activity Relationship of Nondioxin-Like PCBs Evaluates and Supports Modeling Predictions and Identifies Picomolar Potency of PCB 202 Towards Ryanodine Receptors

Nondioxin-like polychlorinated biphenyls (NDL PCBs) activate ryanodine-sensitive Ca²⁺channels (RyRs) and this activation has been associated with neurotoxicity in exposed animals. RyR-active congeners follow a distinct structure-activity relationship and a quantitative structure-activity relationship (QSAR) predicts that a large number of PCBs likely activate the receptor, which requires validation. Additionally, previous structural based conclusions have been established using receptor ligand binding assays but the impact of varying PCB structures on ion channel gating behavior is not understood. We used [³H]Ryanodine ([³H]Ry) binding to assess the RyR-activity of 14 previously untested PCB congeners evaluating the predictability of the QSAR. Congeners determined to display widely varying potency were then assayed with single channel voltage clamp analysis to assess direct influences on channel gating kinetics. The RyR-activity of individual PCBs assessed in in vitro assays followed the general pattern predicted by the QSAR but binding and lipid bilayer experiments demonstrated higher potency than predicted. Of the 49 congeners tested to date, tetra-ortho PCB 202 was found to be the most potent RyR-active congener increasing channel open probability at 200 pM. Shifting meta-substitutions to the para-position resulted in a > 100-fold reduction in potency as seen with PCB 197. Non-ortho PCB 11 was found to lack activity at the receptor supporting a minimum mono-ortho substitution for PCB RyR activity. These findings expand and support previous SAR assessments; where out of the 49 congeners tested to date 42 activate the receptor demonstrating that the RyR is a sensitive and common target of PCBs.

Essential Role of Calmodulin in RyR Inhibition by Dantrolene

Dantrolene is the first line therapy of malignant hyperthermia. Animal studies suggest that dantrolene also protects against heart failure and arrhythmias caused by spontaneous Ca(2+) release. Although dantrolene inhibits Ca(2+) release from the sarcoplasmic reticulum of skeletal and cardiac muscle preparations, its mechanism of action has remained controversial, because dantrolene does not inhibit single ryanodine receptor (RyR) Ca(2+) release channels in lipid bilayers. Here we test the hypothesis that calmodulin (CaM), a physiologic RyR binding partner that is lost during incorporation into lipid bilayers, is required for dantrolene inhibition of RyR channels. In single channel recordings (100 nM cytoplasmic [Ca(2+)] + 2 mM ATP), dantrolene caused inhibition of RyR1 (rabbit skeletal muscle) and RyR2 (sheep) with a maximal inhibition of Po (Emax) to 52 ± 4% of control only after adding physiologic [CaM] = 100 nM. Dantrolene inhibited RyR2 with an IC50 of 0.16 ± 0.03 µM. Mutant N98S-CaM facilitated dantrolene inhibition with an IC50 = 5.9 ± 0.3 nM. In mouse cardiomyocytes, dantrolene had no effect on cardiac Ca(2+) release in the absence of CaM, but reduced Ca(2+) wave frequency (IC50 = 0.42 ± 0.18 µM, Emax = 47 ± 4%) and amplitude (IC50 = 0.19 ± 0.04 µM, Emax = 66 ± 4%) in the presence of 100 nM CaM. We conclude that CaM is essential for dantrolene inhibition of RyR1 and RyR2. Its absence explains why dantrolene inhibition of single RyR channels has not been previously observed.

Suppression of cell membrane permeability by suramin: involvement of its inhibitory actions on connexin 43 hemichannels

BACKGROUND AND PURPOSE

Suramin is a clinically prescribed drug for treatment of human African trypanosomiasis, cancer and infection. It is also a well-known pharmacological antagonist of P2 purinoceptors. Despite its clinical use and use in research, the biological actions of this molecule are still incompletely understood. Here, we investigated the effects of suramin on membrane channels, as exemplified by its actions on non-junctional connexin43 (Cx43) hemichannels, pore-forming α-haemolysin and channels involved in ATP release under hypotonic conditions.

EXPERIMENTAL APPROACH

Hemichannels were activated by removing extracellular Ca(2+) . The influences of suramin on hemichannel activities were evaluated by its effects on influx of fluorescent dyes and efflux of ATP. The membrane permeability and integrity were assessed through cellular retention of preloaded calcein and LDH release.

KEY RESULTS

Suramin blocked Cx43 hemichannel permeability induced by removal of extracellular Ca(2+) without much effect on Cx43 expression and gap junctional intercellular communication. This action of suramin was mimicked by its analogue NF023 and NF449 but not by another P2 purinoceptor antagonist PPADS. Besides hemichannels, suramin also significantly blocked intracellular and extracellular exchanges of small molecules caused by α-haemolysin from Staphylococcus aureus and by exposure of cells to hypotonic solution. Furthermore, it prevented α-haemolysin- and hypotonic stress-elicited cell injury.

CONCLUSION AND IMPLICATIONS

Suramin blocked membrane channels and protected cells against toxin- and hypotonic stress-elicited injury. Our finding provides novel mechanistic insights into the pharmacological actions of suramin. Suramin might be therapeutically exploited to protect membrane integrity under certain pathological situations.

Multiple modes of ryanodine receptor 2 inhibition by flecainide

Catecholaminergic polymorphic ventricular tachycardia (CPVT) causes sudden cardiac death due to mutations in cardiac ryanodine receptors (RyR2), calsequestrin, or calmodulin. Flecainide, a class I antiarrhythmic drug, inhibits Na(+) and RyR2 channels and prevents CPVT. The purpose of this study is to identify inhibitory mechanisms of flecainide on RyR2. RyR2 were isolated from sheep heart, incorporated into lipid bilayers, and investigated by single-channel recording under various activating conditions, including the presence of cytoplasmic ATP (2 mM) and a range of cytoplasmic [Ca(2+)], [Mg(2+)], pH, and [caffeine]. Flecainide applied to either the cytoplasmic or luminal sides of the membrane inhibited RyR2 by two distinct modes: 1) a fast block consisting of brief substate and closed events with a mean duration of ∼1 ms, and 2) a slow block consisting of closed events with a mean duration of ∼1 second. Both inhibition modes were alleviated by increasing cytoplasmic pH from 7.4 to 9.5 but were unaffected by luminal pH. The slow block was potentiated in RyR2 channels that had relatively low open probability, whereas the fast block was unaffected by RyR2 activation. These results show that these two modes are independent mechanisms for RyR2 inhibition, both having a cytoplasmic site of action. The slow mode is a closed-channel block, whereas the fast mode blocks RyR2 in the open state. At diastolic cytoplasmic [Ca(2+)] (100 nM), flecainide possesses an additional inhibitory mechanism that reduces RyR2 burst duration. Hence, multiple modes of action underlie RyR2 inhibition by flecainide.

Divergent regulation of ryanodine receptor 2 calcium release channels by arrhythmogenic human calmodulin missense mutants

RATIONALE

Calmodulin (CaM) mutations are associated with an autosomal dominant syndrome of ventricular arrhythmia and sudden death that can present with divergent clinical features of catecholaminergic polymorphic ventricular tachycardia (CPVT) or long QT syndrome (LQTS). CaM binds to and inhibits ryanodine receptor (RyR2) Ca release channels in the heart, but whether arrhythmogenic CaM mutants alter RyR2 function is not known.

OBJECTIVE

To gain mechanistic insight into how human CaM mutations affect RyR2 Ca channels.

METHODS AND RESULTS

We studied recombinant CaM mutants associated with CPVT (N54I and N98S) or LQTS (D96V, D130G, and F142L). As a group, all LQTS-associated CaM mutants (LQTS-CaMs) exhibited reduced Ca affinity, whereas CPVT-associated CaM mutants (CPVT-CaMs) had either normal or modestly lower Ca affinity. In permeabilized ventricular myocytes, CPVT-CaMs at a physiological intracellular concentration (100 nmol/L) promoted significantly higher spontaneous Ca wave and spark activity, a typical cellular phenotype of CPVT. Compared with wild-type CaM, CPVT-CaMs caused greater RyR2 single-channel open probability and showed enhanced binding affinity to RyR2. Even a 1:8 mixture of CPVT-CaM:wild-type-CaM activated Ca waves, demonstrating functional dominance. In contrast, LQTS-CaMs did not promote Ca waves and exhibited either normal regulation of RyR2 single channels (D96V) or lower RyR2-binding affinity (D130G and F142L). None of the CaM mutants altered Ca/CaM binding to CaM-kinase II.

CONCLUSIONS

A small proportion of CPVT-CaM is sufficient to evoke arrhythmogenic Ca disturbances, whereas LQTS-CaMs do not. Our findings explain the clinical presentation and autosomal dominant inheritance of CPVT-CaM mutations and suggest that RyR2 interactions are unlikely to explain arrhythmogenicity of LQTS-CaM mutations.

ß-Adrenergic stimulation increases RyR2 activity via intracellular Ca2+ and Mg2+ regulation

Here we investigate how ß-adrenergic stimulation of the heart alters regulation of ryanodine receptors (RyRs) by intracellular Ca²⁺ and Mg²⁺ and the role of these changes in SR Ca²⁺ release. RyRs were isolated from rat hearts, perfused in a Langendorff apparatus for 5 min and subject to 1 min perfusion with 1 µM isoproterenol or without (control) and snap frozen in liquid N₂ to capture their phosphorylation state. Western Blots show that RyR2 phosphorylation was increased by isoproterenol, confirming that RyR2 were subject to normal ß-adrenergic signaling. Under basal conditions, S2808 and S2814 had phosphorylation levels of 69% and 15%, respectively. These levels were increased to 83% and 60%, respectively, after 60 s of ß-adrenergic stimulation consistent with other reports that ß-adrenergic stimulation of the heart can phosphorylate RyRs at specific residues including S2808 and S2814 causing an increase in RyR activity. At cytoplasmic [Ca²⁺] <1 µM, ß-adrenergic stimulation increased luminal Ca²⁺ activation of single RyR channels, decreased luminal Mg²⁺ inhibition and decreased inhibition of RyRs by mM cytoplasmic Mg²⁺. At cytoplasmic [Ca²⁺] >1 µM, ß-adrenergic stimulation only decreased cytoplasmic Mg²⁺ and Ca²⁺ inhibition of RyRs. The K_a and maximum levels of cytoplasmic Ca²⁺ activation site were not affected by ß-adrenergic stimulation.

Our RyR2 gating model was fitted to the single channel data. It predicted that in diastole, ß-adrenergic stimulation is mediated by 1) increasing the activating potency of Ca²⁺ binding to the luminal Ca²⁺ site and decreasing its affinity for luminal Mg²⁺ and 2) decreasing affinity of the low-affinity Ca²⁺/Mg²⁺ cytoplasmic inhibition site. However in systole, ß-adrenergic stimulation is mediated mainly by the latter.

Does the lipid environment impact the open-state conductance of an engineered β-barrel protein nanopore?

Using rational membrane protein design, we were recently able to obtain a β-barrel protein nanopore that was robust under an unusually broad range of experimental circumstances. This protein nanopore was based upon the native scaffold of the bacterial ferric hydroxamate uptake component A (FhuA) of Escherichia coli. In this work, we expanded the examinations of the open-state current of this engineered protein nanopore, also called FhuA ΔC/Δ4L, employing an array of lipid bilayer systems that contained charged and uncharged as well as conical and cylindrical lipids. Remarkably, systematical single-channel analysis of FhuA ΔC/Δ4L indicated that most of its biophysical features, such as the unitary conductance and the stability of the open-state current, were not altered under the conditions tested in this work. However, electrical recordings at high transmembrane potentials revealed that the presence of conical phospholipids within the bilayer catalyzes the first, stepwise current transition of the FhuA ΔC/Δ4L protein nanopore to a lower-conductance open state. This study reinforces the stability of the open-state current of the engineered FhuA ΔC/Δ4L protein nanopore under various experimental conditions, paving the way for further critical developments in biosensing and molecular biomedical diagnosis.

The circular bacteriocin, carnocyclin A, forms anion-selective channels in lipid bilayers

Bacterial resistance to conventional antibiotics is a major challenge in controlling infectious diseases and has necessitated the development of novel approaches in antimicrobial therapy. One such approach is the use of antimicrobial peptides, such as the bacterially produced bacteriocins. Carnocyclin A (CclA) is a 60-amino acid circular bacteriocin produced by Carnobacterium maltaromaticum UAL307 that exhibits potent activity against many Gram-positive bacteria. Lipid bilayer and single channel recording techniques were applied to study the molecular mechanisms by which CclA interacts with the lipid membrane and exerts its antimicrobial effects. Here we show that CclA can form ion channels with a conductance of 35 pS in 150 mM NaCl solution. This channel displays a linear current-voltage relationship, is anion-selective, and its activation is strongly voltage-dependent. The formation of ion channels by CclA is driven by the presence of a negative membrane potential and may result in dissipation of membrane potential. Carnocyclin A's unique functional activities as well as its circular structure make it a potential candidate for developing novel antimicrobial drugs.

Amyloid-β protein impairs Ca2+ release and contractility in skeletal muscle

Inclusion body myositis (IBM), the most common muscle disorder in the elderly, is partly characterized by dysregulation of β-amyloid precursor protein (βAPP) expression and abnormal, intracellular accumulation of full-length βAPP and β-amyloid epitopes. The present study examined the effects of β-amyloid accumulation on force generation and Ca(2+) release in skeletal muscle from transgenic mice harboring human βAPP and assessed the consequence of Aβ(1-42) modulation of the ryanodine receptor Ca(2+) release channels (RyRs). β-Amyloid laden muscle produced less peak force and exhibited Ca(2+) transients with smaller amplitude. To determine whether modification of RyRs by β-amyloid underlie the effects observed in muscle, in vitro Ca(2+) release assays and RyR reconstituted in planar lipid bilayer experiments were conducted in the presence of Aβ(1-42). Application of Aβ(1-42) to RyRs in bilayers resulted in an increased channel open probability and changes in gating kinetics, while addition of Aβ(1-42) to the rabbit SR vesicles resulted in RyR-mediated Ca(2+) release. These data may relate altered βAPP metabolism in IBM to reductions in RyR-mediated Ca(2+) release and muscle contractility.

Luminal Mg2+, a key factor controlling RYR2-mediated Ca2+ release: cytoplasmic and luminal regulation modeled in a tetrameric channel

In cardiac muscle, intracellular Ca(2+) and Mg(2+) are potent regulators of calcium release from the sarcoplasmic reticulum (SR). It is well known that the free [Ca(2+)] in the SR ([Ca(2+)](L)) stimulates the Ca(2+) release channels (ryanodine receptor [RYR]2). However, little is known about the action of luminal Mg(2+), which has not been regarded as an important regulator of Ca(2+) release. The effects of luminal Ca(2+) and Mg(2+) on sheep RYR2 were measured in lipid bilayers. Cytoplasmic and luminal Ca(2+) produced a synergistic increase in the opening rate of RYRs. A novel, high affinity inhibition of RYR2 by luminal Mg(2+) was observed, pointing to an important physiological role for luminal Mg(2+) in cardiac muscle. At diastolic [Ca(2+)](C), luminal Mg(2+) inhibition was voltage independent, with K(i) = 45 microM at luminal [Ca(2+)] ([Ca(2+)](L)) = 100 microM. Luminal and cytoplasmic Mg(2+) inhibition was alleviated by increasing [Ca(2+)](L) or [Ca(2+)](C). Ca(2+) and Mg(2+) on opposite sides of the bilayer exhibited competitive effects on RYRs, indicating that they can compete via the pore for common sites. The data were accurately fitted by a model based on a tetrameric RYR structure with four Ca(2+)-sensing mechanisms on each subunit: activating luminal L-site (40-microM affinity for Mg(2+) and Ca(2+)), cytoplasmic A-site (1.2 microM for Ca(2+) and 60 microM for Mg(2+)), inactivating cytoplasmic I(1)-site (approximately 10 mM for Ca(2+) and Mg(2+)), and I(2)-site (1.2 microM for Ca(2+)). Activation of three or more subunits will cause channel opening. Mg(2+) inhibition occurs primarily by Mg(2+) displacing Ca(2+) from the L- and A-sites, and Mg(2+) fails to open the channel. The model predicts that under physiological conditions, SR load-dependent Ca(2+) release (1) is mainly determined by Ca(2+) displacement of Mg(2+) from the L-site as SR loading increases, and (2) depends on the properties of both luminal and cytoplasmic activation mechanisms.

Discovery of potent CLC chloride channel inhibitors

Anion-transport proteins are central to all of physiology, for processes ranging from regulating bone-density, muscle excitability, and blood pressure, to facilitating extreme-acid survival of pathogenic bacteria. 4,4-Diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) has been used as an anion-transport inhibitor for decades. In this study, we demonstrate that polythiourea products derived from DIDS hydrolysis inhibit three different CLC chloride-transport proteins, ClC-ec1, ClC-0, and ClC-Ka, more effectively than DIDS itself. The structures of the five major products were determined by NMR spectroscopy, mass spectrometry, and chemical synthesis. These compounds bind directly to the CLC proteins, as evidenced by the fact that inhibition of ClC-0 occurs only from the intracellular side and inhibition of ClC-Ka is prevented by the point mutation N68D. These polythioureas are the highest affinity inhibitors known for the CLCs and provide a new class of chemical probes for dissecting the molecular mechanisms of chloride transport.

Calcium-activated chloride current contributes to action potential alternations in left ventricular hypertrophy rabbit

T-wave alternans, characterized by a beat-to-beat change in T-wave morphology, amplitude, and/or polarity on the ECG, often heralds the development of lethal ventricular arrhythmias in patients with left ventricular hypertrophy (LVH). The aim of our study was to examine the ionic basis for a beat-to-beat change in ventricular repolarization in the setting of LVH. Transmembrane action potentials (APs) from epicardium and endocardium were recorded simultaneously, together with transmural ECG and contraction force, in arterially perfused rabbit left ventricular wedge preparation. APs and Ca(2+)-activated chloride current (I(Cl,Ca)) were recorded from left ventricular myocytes isolated from normal rabbits and those with renovascular LVH using the standard microelectrode and whole cell patch-clamping techniques, respectively. In the LVH rabbits, a significant beat-to-beat change in endocardial AP duration (APD) created beat-to-beat alteration in transmural voltage gradient that manifested as T-wave alternans on the ECG. Interestingly, contraction force alternated in an opposite phase ("out of phase") with APD. In the single myocytes of LVH rabbits, a significant beat-to-beat change in APD was also observed in both left ventricular endocardial and epicardial myocytes at various pacing rates. APD alternans was suppressed by adding 1 microM ryanodine, 100 microM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), and 100 microM 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS). The density of the Ca(2+)-activated chloride currents (I(Cl,Ca)) in left ventricular myocytes was significantly greater in the LVH rabbits than in the normal group. Our data indicate that abnormal intracellular Ca(2+) fluctuation may exert a strong feedback on the membrane I(Cl,Ca), leading to a beat-to-beat change in the net repolarizing current that manifests as T-wave alternans on the ECG.

A domain peptide of the cardiac ryanodine receptor regulates channel sensitivity to luminal Ca2+ via cytoplasmic Ca2+ sites

The clustering of cardiac RyR mutations, linked to sudden cardiac death (SCD), into several regions in the amino acid sequence underlies the hypothesis that these mutations interfere with stabilising interactions between different domains of the RyR2. SCD mutations cause increased channel sensitivity to cytoplasmic and luminal Ca(2+). A synthetic peptide corresponding to part of the central domain (DPc10:(2460)G-P(2495)) was designed to destabilise the interaction of the N-terminal and central domains of wild-type RyR2 and mimic the effects of SCD mutations. With Ca(2+) as the sole regulating ion, DPc10 caused increased channel activity which could be reversed by removal of the peptide whereas in the presence of ATP DPc10 caused no activation. In support of the domain destablising hypothesis, the corresponding peptide (DPc10-mut) containing the CPVT mutation R2474S did not affect channel activity under any circumstances. DPc10-induced activation was due to a small increase in RyR2 sensitivity to cytoplasmic Ca(2+) and a large increase in the magnitude of luminal Ca(2+) activation. The increase in the luminal Ca(2+) response appeared reliant on the luminal-to-cytoplasmic Ca(2+) flux in the channel, indicating that luminal Ca(2+) was activating the RyR2 via its cytoplasmic Ca(2+) sites. DPc10 had no significant effect on the RyR2 gating associated with luminal Ca(2+) sensing sites. The results were fitted by the luminal-triggered Ca(2+) feed-through model and the effects of DPc10 were explained entirely by perturbations in cytoplasmic Ca(2+)-activation mechanism.

The ryanodine receptor pore blocker neomycin also inhibits channel activity via a previously undescribed high-affinity Ca(2+) binding site

In this study, we present evidence for the mechanism of neomycin inhibition of skeletal ryanodine receptors (RyRs). In single-channel recordings, neomycin produced monophasic inhibition of RyR open probability and biphasic inhibition of [(3)H]ryanodine binding. The half-maximal inhibitory concentration (IC(50)) for channel blockade by neomycin was dependent on membrane potential and cytoplasmic [Ca(2+)], suggesting that neomycin acts both as a pore plug and as a competitive antagonist at a cytoplasmic Ca(2+) binding site that causes allosteric inhibition. This novel Ca(2+)/neomycin binding site had a neomycin affinity of 100 nM: and a Ca(2+) affinity of 35 nM,: which is 30-fold higher than that of the well-described cytoplasmic Ca(2+) activation site. Therefore, a new high-affinity class of Ca(2+) binding site(s) on the RyR exists that mediates neomycin inhibition. Neomycin plugging of the channel pore induced brief (1-2 ms) conductance substates at 30% of the fully open conductance, whereas allosteric inhibition caused complete channel closure with durations that depended on the neomycin concentration. We quantitatively account for these results using a dual inhibition model for neomycin that incorporates voltage-dependent pore plugging and Ca(2+)-dependent allosteric inhibition.

Ca2+ stores regulate ryanodine receptor Ca2+ release channels via luminal and cytosolic Ca2+ sites

The free [Ca2+] in endoplasmic/sarcoplasmic reticulum Ca2+ stores regulates excitability of Ca2+ release by stimulating the Ca2+ release channels. Just how the stored Ca2+ regulates activation of these channels is still disputed. One proposal attributes luminal Ca2+-activation to luminal facing regulatory sites, whereas another envisages Ca2+ permeation to cytoplasmic sites. This study develops a unified model for luminal Ca2+ activation for single cardiac ryanodine receptors (RyR2) and RyRs in coupled clusters in artificial lipid bilayers. It is shown that luminal regulation of RyR2 involves three modes of action associated with Ca2+ sensors in different parts of the molecule; a luminal activation site (L-site, 60 microM affinity), a cytoplasmic activation site (A-site, 0.9 microM affinity), and a novel cytoplasmic inactivation site (I2-site, 1.2 microM affinity). RyR activation by luminal Ca2+ is demonstrated to occur by a multistep process dubbed luminal-triggered Ca2+ feedthrough. Ca2+ binding to the L-site initiates brief openings (1 ms duration at 1-10 s(-1)) allowing luminal Ca2+ to access the A-site, producing up to 30-fold prolongation of openings. The model explains a broad data set, reconciles previous conflicting observations and provides a foundation for understanding the action of pharmacological agents, RyR-associated proteins, and RyR2 mutations on a range of Ca2+-mediated physiological and pathological processes.

Aldolase potentiates DIDS activation of the ryanodine receptor in rabbit skeletal sarcoplasmic reticulum

DIDS (4,4'-di-isothiocyanostilbene-2,2'-disulfonate), an anion channel blocker, triggers Ca2+ release from skeletal muscle SR (sarcoplasmic reticulum). The present study characterized the effects of DIDS on rabbit skeletal single Ca2+-release channel/RyR1 (ryanodine receptor type 1) incorporated into a planar lipid bilayer. When junctional SR vesicles were used for channel incorporation (native RyR1), DIDS increased the mean P(o) (open probability) of RyR1 without affecting unitary conductance when Cs+ was used as the charge carrier. Lifetime analysis of single RyR1 activities showed that 10 microM DIDS induced reversible long-lived open events (P(o)=0.451+/-0.038) in the presence of 10 microM Ca2+, due mainly to a new third component for both open and closed time constants. However, when purified RyR1 was examined in the same condition, 10 microM DIDS became considerably less potent (P(o)=0.206+/-0.025), although the caffeine response was similar between native and purified RyR1. Hence we postulated that a DIDS-binding protein, essential for the DIDS sensitivity of RyR1, was lost during RyR1 purification. DIDS-affinity column chromatography of solubilized junctional SR, and MALDI-TOF (matrix-assisted laser-desorption ionization-time-of-flight) MS analysis of the affinity-column-associated proteins, identified four major DIDS-binding proteins in the SR fraction. Among them, aldolase was the only protein that greatly potentiated DIDS sensitivity. The association between RyR1 and aldolase was further confirmed by co-immunoprecipitation and aldolase-affinity batch-column chromatography. Taken together, we conclude that aldolase is physically associated with RyR1 and could confer a considerable potentiation of the DIDS effect on RyR1.

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

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

Regulation of skeletal ryanodine receptors by dihydropyridine receptor II-III loop C-region peptides: relief of Mg2+ inhibition

The aim of the present study was to explore interactions between surface-membrane DHPR (dihydropyridine receptor) Ca2+ channels and RyR (ryanodine receptor) Ca2+ channels in skeletal-muscle sarcoplasmic reticulum. The C region (725Phe-Pro742) of the linker between the 2nd and 3rd repeats (II-III loop) of the a1 subunit of skeletal DHPRs is essential for skeletal excitation-contraction coupling, which requires a physical interaction between the DHPR and RyR and is independent of external Ca2+. Little is known about the regulatory processes that might take place when the two Ca2+ channels interact. Indeed, interactions between C fragments of the DHPR (C peptides) and RyR have different reported effects on Ca2+ release from the sarcoplasmic reticulum and on RyR channels in lipid bilayers. To gain insight into functional interactions between the proteins and to explore different reported effects, we examined the actions of C peptides on RyR1 channels in lipid bilayers with three key RyR regulators, Ca2+, Mg2+ and ATP. We identified four discrete actions: two novel, low-affinity (>10 microM), rapidly reversible effects (fast inhibition and decreased sensitivity to Mg2+ inhibition) and two slowly reversible effects (high-affinity activation and a slow-onset, low-affinity inhibition). Fast inhibition and high-affinity activation were decreased by ATP. Therefore peptide activation in the presence of ATP and Mg2+, used with Ca2+ release assays, depends on a mechanism different from that seen when Ca2+ is the sole agonist. The relief of Mg2+ inhibition was particularly important since RyR activation during excitation-contraction coupling depends on a similar decrease in Mg2+ inhibition.

Coupled calcium release channels and their regulation by luminal and cytosolic ions

Contraction in skeletal and cardiac muscle occurs when Ca(2+) is released from the sarcoplasmic reticulum (SR) through ryanodine receptor (RyR) Ca(2+) release channels. Several isoforms of the RyR exist throughout the animal kingdom, which are modulated by ATP, Ca(2+) and Mg(2+) in the cytoplasm and by Ca(2+) in the lumen of the SR. This review brings to light recent findings on their mechanisms of action in the mammalian isoforms RyR-1 and RyR-2 with an emphasis on RyR-1 from skeletal muscle. Cytoplasmic Mg(2+) is a potent RyR antagonist that binds to two classes of cytoplasmic site, identified as low-affinity, non-specific inhibition sites and high-affinity Ca(2+) activation sites (A-sites). Mg(2+) inhibition at the A-sites is very sensitive to the cytoplasmic and luminal milieu. Cytoplasmic Ca(2+), Mg(2+) and monovalent cations compete for the A-sites. In isolated RyRs, luminal Ca(2+) alters the Mg(2+) affinity of the A-site by an allosteric mechanism mediated by luminal sites. However, in close-packed RyR arrays luminal Ca(2+) can also compete with cytoplasmic ions for the A-site. Activation of RyRs by luminal Ca(2+) has been attributed to either Ca(2+) feedthrough to A-sites or to Ca(2+) regulatory sites on the luminal side of the RyR. As yet there is no consensus on just how luminal Ca(2+) alters RyR activation. Recent evidence indicates that both mechanisms operate and are likely to be important. Allosteric regulation of A-site Mg(2+) affinity could trigger Ca(2+) release, which is reinforced by Ca(2+) feedthrough.

Suramin and disulfonated stilbene derivatives stimulate the Ca2+-induced Ca2+ -release mechanism in A7r5 cells

We have described previously a novel Ca2+-induced Ca2+-release (CICR) mechanism in permeabilized A7r5 cells (embryonic rat aorta) and 16HBE14o-cells (human bronchial mucosa) cells (J Biol Chem 278:27548-27555, 2003). This CICR mechanism was activated upon the elevation of the free cytosolic calcium concentration [Ca2+]c and was not inhibited by pharmacological inhibitors of the inositol-1,4,5-trisphosphate (IP3) receptor nor of the ryanodine receptor. This CICR mechanism was inhibited by calmodulin (CaM)1234, a Ca2+-insensitive CaM mutant, and by different members of the superfamily of CaM-like Ca2+-binding proteins. Here, we present evidence that the CICR mechanism that is expressed in A7r5 and 16HBE14o-cells is strongly activated by suramin and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS). We found several indications that both activation mechanisms are indeed two different modes of the same release system. Suramin/DIDS-induced Ca2+ release was only detected in cells that displayed the CICR mechanism, and cell types that do not express this type of CICR mechanism did not exhibit suramin/DIDS-induced Ca2+ release. Furthermore, we show that the suramin-stimulated Ca2+ release is regulated by Ca2+ and CaM in a similar way as the previously described CICR mechanism. The pharmacological characterization of the suramin/DIDS-induced Ca2+ release further confirms its properties as a novel CaM-regulated Ca2+-release mechanism. We also investigated the effects of disulfonated stilbene derivatives on IP3-induced Ca2+ release and found, in contrast to the effect on CICR, a strong inhibition by DIDS and 4'-acetoamido-4'-isothiocyanostilbene-2',2'-disulfonic acid.

Luminal Ca2+-regulated Mg2+ inhibition of skeletal RyRs reconstituted as isolated channels or coupled clusters

In resting muscle, cytoplasmic Mg(2+) is a potent inhibitor of Ca(2+) release from the sarcoplasmic reticulum (SR). It is thought to inhibit calcium release channels (RyRs) by binding both to low affinity, low specificity sites (I-sites) and to high affinity Ca(2+) sites (A-sites) thus preventing Ca(2+) activation. We investigate the effects of luminal and cytoplasmic Ca(2+) on Mg(2+) inhibition at the A-sites of skeletal RyRs (RyR1) in lipid bilayers, in the presence of ATP or modified by ryanodine or DIDS. Mg(2+) inhibits RyRs at the A-site in the absence of Ca(2+), indicating that Mg(2+) is an antagonist and does not simply prevent Ca(2+) activation. Cytoplasmic Ca(2+) and Cs(+) decreased Mg(2+) affinity by a competitive mechanism. We describe a novel mechanism for luminal Ca(2+) regulation of Ca(2+) release whereby increasing luminal [Ca(2+)] decreases the A-site affinity for cytoplasmic Mg(2+) by a noncompetitive, allosteric mechanism that is independent of Ca(2+) flow. Ryanodine increases the Ca(2+) sensitivity of the A-sites by 10-fold, which is insufficient to explain the level of activation seen in ryanodine-modified RyRs at nM Ca(2+), indicating that ryanodine activates independently of Ca(2+). We describe a model for ion binding at the A-sites that predicts that modulation of Mg(2+) inhibition by luminal Ca(2+) is a significant regulator of Ca(2+) release from the SR. We detected coupled gating of RyRs due to luminal Ca(2+) permeating one channel and activating neighboring channels. This indicated that the RyRs existed in stable close-packed rafts within the bilayer. We found that luminal Ca(2+) and cytoplasmic Mg(2+) did not compete at the A-sites of single open RyRs but did compete during multiple channel openings in rafts. Also, luminal Ca(2+) was a stronger activator of multiple openings than single openings. Thus it appears that RyRs are effectively "immune" to Ca(2+) emanating from their own pore but sensitive to Ca(2+) from neighboring channels.

Multiple actions of imperatoxin A on ryanodine receptors: interactions with the II-III loop "A" fragment

Imperatoxin A is a high affinity activator of ryanodine receptors. The toxin contains a positively charged surface structure similar to that of the A fragment of skeletal dihydropyridine receptors (peptide A), suggesting that the toxin and peptide could bind to a common site on the ryanodine receptor. However, the question of a common binding site has not been resolved, and the concentration dependence of the actions of the toxin has not been fully explored. We characterize two novel high affinity actions of the toxin on the transient gating of cardiac and skeletal channels, in addition to the well documented lower affinity induction of prolonged substates. Transient activity was (a) enhanced with 0.2-10 nm toxin and (b) depressed by >50 nm toxin. The toxin at >/=1 nm enhanced Ca(2+) release from SR in a manner consistent with two independent activation processes. The effects of the toxin on transient activity, as well as the toxin-induced substate, were independent of cytoplasmic Ca(2+) or Mg(2+) concentrations or the presence of adenine nucleotide and were seen in diisothiocyanostilbene-2',2'-disulfonic acid-modified channels. Peptide A activated skeletal and cardiac channels with 100 nm cytoplasmic Ca(2+) and competed with Imperatoxin A in the high affinity enhancement of transient channel activity and Ca(2+) release from SR. In contrast to transient activity, prolonged substate openings induced by the toxin were not altered in the presence of peptide A. The results suggest that Imperatoxin A has three independent actions on ryanodine receptor channels and competes with peptide A for at least one action.