Large tetraalkyl ammonium cations produce a reduced conductance state in the sheep cardiac sarcoplasmic reticulum Ca(2+)-release channel

Blocking effect of ferritin on the ryanodine receptor-isoform 2

Iron, an essential element for most living organism, participates in a wide variety of physiological processes. Disturbance in iron homeostasis has been associated with numerous pathologies, particularly in the heart and brain, which are the most susceptible organs. Under iron-overload conditions, the generation of reactive oxygen species leads to impairment in Ca2+ signaling, fundamentally implicated in cardiac and neuronal physiology. Since iron excess is accompanied by increased expression of iron-storage protein, ferritin, we examined whether ferritin has an effect on the ryanodine receptor - isoform 2 (RYR2), which is one of the major components of Ca2+ signaling. Using the method of planar lipid membranes, we show that ferritin induced an abrupt, permanent blockage of the RYR2 channel. The ferritin effect was strongly voltage dependent and competitively antagonized by cytosolic TEA+, an impermeant RYR2 blocker. Our results collectively indicate that monomeric ferritin highly likely blocks the RYR2 channel by a direct electrostatic interaction within the wider region of the channel permeation pathway.

Electrical polarity-dependent gating and a unique subconductance of RyR2 induced by S-adenosyl methionine via the ATP binding site

S-Adenosyl-l-methionine (SAM) was used to probe the functional effects exerted via the RyR2 adenine nucleotide binding site. Single channel experiments revealed that SAM applied to the cytoplasmic face of RyR2 had complex voltage dependent effects on channel gating and conductance. At positive transmembrane holding potentials, SAM caused a striking reduction in channel openings and a reduced channel conductance. In contrast, at negative potentials SAM promoted a clearly resolved subconductance state. At membrane potentials between -75 and -25 mV the open probability of the subconductance state was independent of voltage. ATP, but not the non-adenosine based RyR activator 4-chloro-m-cresol interfered with the effects of SAM at both negative and positive potentials. This suggests that ATP and SAM interact with a common binding site. Molecular docking showed SAM bound to the adenine nucleotide-binding site and formed a hydrogen bond to Glu4886 in the C-terminal end of the S6 alpha helix. In this configuration SAM may alter the conformation of the RyR2 ion conduction pathway. This work provides novel insight into potential functional outcomes of ligand binding to the RyR adenine nucleotide binding site.

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.

Unambiguous observation of blocked states reveals altered, blocker-induced, cardiac ryanodine receptor gating

The flow of ions through membrane channels is precisely regulated by gates. The architecture and function of these elements have been studied extensively, shedding light on the mechanisms underlying gating. Recent investigations have focused on ion occupancy of the channel’s selectivity filter and its ability to alter gating, with most studies involving prokaryotic K⁺ channels. Some studies used large quaternary ammonium blocker molecules to examine the effects of altered ionic flux on gating. However, the absence of blocking events that are visibly distinct from closing events in K⁺ channels makes unambiguous interpretation of data from single channel recordings difficult. In this study, the large K⁺ conductance of the RyR2 channel permits direct observation of blocking events as distinct subconductance states and for the first time demonstrates the differential effects of blocker molecules on channel gating. This experimental platform provides valuable insights into mechanisms of blocker-induced modulation of ion channel gating.

Effect of flecainide derivatives on sarcoplasmic reticulum calcium release suggests a lack of direct action on the cardiac ryanodine receptor

BACKGROUND AND PURPOSE

Flecainide is a use-dependent blocker of cardiac Na(+) channels. Mechanistic analysis of this block showed that the cationic form of flecainide enters the cytosolic vestibule of the open Na(+) channel. Flecainide is also effective in the treatment of catecholaminergic polymorphic ventricular tachycardia but, in this condition, its mechanism of action is contentious. We investigated how flecainide derivatives influence Ca(2) (+) -release from the sarcoplasmic reticulum through the ryanodine receptor channel (RyR2) and whether this correlates with their effectiveness as blockers of Na(+) and/or RyR2 channels.

EXPERIMENTAL APPROACH

We compared the ability of fully charged (QX-FL) and neutral (NU-FL) derivatives of flecainide to block individual recombinant human RyR2 channels incorporated into planar phospholipid bilayers, and their effects on the properties of Ca(2) (+) sparks in intact adult rat cardiac myocytes.

KEY RESULTS

Both QX-FL and NU-FL were partial blockers of the non-physiological cytosolic to luminal flux of cations through RyR2 channels but were significantly less effective than flecainide. None of the compounds influenced the physiologically relevant luminal to cytosol cation flux through RyR2 channels. Intracellular flecainide or QX-FL, but not NU-FL, reduced Ca(2) (+) spark frequency.

CONCLUSIONS AND IMPLICATIONS

Given its inability to block physiologically relevant cation flux through RyR2 channels, and its lack of efficacy in blocking the cytosolic-to-luminal current, the effect of QX-FL on Ca(2) (+) sparks is likely, by analogy with flecainide, to result from Na(+) channel block. Our data reveal important differences in the interaction of flecainide with sites in the cytosolic vestibules of Na(+) and RyR2 channels.

The mechanism of flecainide action in CPVT does not involve a direct effect on RyR2

RATIONALE

Flecainide, a class 1c antiarrhythmic, has emerged as an effective therapy in preventing arrhythmias in patients with catecholaminergic polymorphic ventricular tachycardia (CPVT) refractory to β-adrenergic receptor blockade. It has been proposed that the clinical efficacy of flecainide in CPVT is because of the combined actions of direct blockade of ryanodine receptors (RyR2) and Na(+) channel inhibition. However, there is presently no direct evidence to support the notion that flecainide blocks RyR2 Ca(2+) flux in the physiologically relevant (luminal-to-cytoplasmic) direction. The mechanism of flecainide action remains controversial.

OBJECTIVE

To examine, in detail, the effect of flecainide on the human RyR2 channel and to establish whether the direct blockade of physiologically relevant RyR2 ion flow by the drug contributes to its therapeutic efficacy in the clinical management of CPVT.

METHODS AND RESULTS

Using single-channel analysis, we show that, even at supraphysiological concentrations, flecainide did not inhibit the physiologically relevant, luminal-to-cytosolic flux of cations through the channel. Moreover, flecainide did not alter RyR2 channel gating and had negligible effect on the mechanisms responsible for the sarcoplasmic reticulum charge-compensating counter current. Using permeabilized cardiac myocytes to eliminate any contribution of plasmalemmal Na(+) channels to the observed actions of the drug at the cellular level, flecainide did not inhibit RyR2-dependent sarcoplasmic reticulum Ca(2+) release.

CONCLUSIONS

The principal action of flecainide in CPVT is not via a direct interaction with RyR2. Our data support a model of flecainide action in which Na(+)-dependent modulation of intracellular Ca(2+) handling attenuates RyR2 dysfunction in CPVT.

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.

Patch-clamp electrophysiology of intracellular Ca2+ channels

The modulation of cytoplasmic free Ca(2+) concentration ([Ca(2+)]i) is a universal intracellular signaling pathway that regulates numerous cellular physiological processes. Ubiquitous intracellular Ca(2+)-release channels localized to the endoplasmic/sarcoplasmic reticulum-inositol 1,4,5-trisphosphate receptor (InsP3R) and ryanodine receptor (RyR) channels-play a central role in [Ca(2+)]i signaling in all animal cells. Despite their intracellular localization, electrophysiological studies of the single-channel permeation and gating properties of these Ca(2+)-release channels using the powerful patch-clamp approach have been possible by application of this technique to isolated nuclei because the channels are present in membranes of the nuclear envelope. Here we provide a concise description of how nuclear patch-clamp experiments have been used to study single-channel properties of different InsP3R channels in the outer nuclear membrane. We compare this with other methods for studying intracellular Ca(2+) release. We also briefly describe application of the technique to InsP3R channels in the inner nuclear membrane and to channels in the outer nuclear membrane of HEK293 cells expressing recombinant RyR.

The interaction of an impermeant cation with the sheep cardiac RyR channel alters ryanoid association

In previous studies, we have demonstrated that the interaction of ryanoids with the sarcoplasmic reticulum Ca(2+)-release channel [ryanodine receptor (RyR)] incorporated into planar lipid bilayers reduced the effectiveness of tetraethylammonium (TEA(+)) as a blocker of K(+) translocation (J Gen Physiol 117: 385-393, 2001). In the current study, we investigated both the effect of TEA(+) on [(3)H]ryanodine binding and the actions of this impermeant cation on the interaction of the reversible ryanoid 21-amino-9alpha-hydroxyryanodine with individual, voltage-clamped RyR channels. A dose-dependent inhibition of [(3)H]ryanodine binding was observed in the presence of TEA(+), suggesting that the cation and alkaloid compete for access to a common site of interaction. Single channel studies gave further insights into the mechanism of the competition between the two classes of ligands. TEA(+) decreases the association rate of 21-amino-9alpha-hydroxyryanodine with its receptor, whereas the dissociation rate of the ryanoid from the channel was unaffected. Our results demonstrate that TEA(+) inhibits both K(+) translocation through RyR, and ryanoid interaction at the high affinity ryanodine site on the channel. These actions involve binding of TEA(+) to different, but weakly interacting, sites in the RyR channel.

Permeation and inhibition of polycystin-L channel by monovalent organic cations

Polycystin-L (PCL), homologous to polycystin-2 (71% similarity in protein sequence), is the third member of the polycystin family of proteins. Polycystin-1 and -2 are mutated in autosomal dominant polycystic kidney disease, but the physiological role of PCL has not been determined. PCL acts as a Ca-regulated non-selective cation channel permeable to mono- and divalent cations. To further understand the biophysical and pharmacological properties of PCL, we examined a series of organic cations for permeation and inhibition, using single-channel patch clamp and whole-cell two-microelectrode voltage clamp techniques in conjunction with Xenopus oocyte expression. We found that PCL is permeable to organic amines, methlyamine (MA, 3.8 A), dimethylamine (DMA, 4.6 A) and triethylamine (TriEA, 6 A), and to tetra-alkylammonium cation (TAA) tetra-methylammonium (TMA, 5.5-6.4 A). TAA compounds tetra-ethylammonium (TEA, 6.1-8.2 A) and tetra-propylammonium (TPA, 9.8 A) were impermeable through PCL and exhibited weak inhibition on PCL (IC50 values>13 mM). Larger TAA cations tetra-butylammonium (TBA, 11.6 A) and tetra-pentylammonium (TPeA, 13.2 A) were impermeable through PCL as well and showed strong inhibition (IC50 values of 2.7 mM and 1.3 microM, respectively). Inhibition by TBA was on decreasing the single-channel current amplitude and exhibited no effect on open probability (NPo) or mean open time (MOT), suggesting that it blocks the PCL permeation pathway. In contract, TEA, TPA and TPeA reduced NPo and MOT values but had no effect on the amplitude, suggesting their binding to a different site in PCL, which affects the channel gating. Taken together, our studies revealed that PCL is permeable to organic amines and TAA cation TMA, and that inhibition of PCL by large TAA cations exhibits two different mechanisms, presumably through binding either to the pore pathway to reduce permeant flux or to another site to regulate the channel gating. These data allow to estimate a channel pore size of approximately 7 A for PCL.

Organic cation permeation through the channel formed by polycystin-2

Polycystin-2 (PC2), a member of the transient receptor potential family of ion channels (TRPP2), forms a calcium-permeable cation channel. Mutations in PC2 lead to polycystic kidney disease. From the primary sequence and by analogy with other channels in this family, PC2 is modeled to have six transmembrane domains. However, most of the structural features of PC2, such as how large the channel is and how many subunits make up the pore of the channel, are unknown. In this study, we estimated the pore size of PC2 from the permeation properties of the channel. Organic cations of increasing size were used as current carriers through the PC2 channel after PC2 was incorporated into lipid bilayers. We found that dimethylamine, triethylamine, tetraethylammonium, tetrabutylammonium, tetrapropylammonium, and tetrapentylammonium were permeable through the PC2 channel. The slope conductance of the PC2 channel decreased as the ionic diameter of the organic cation increased. For each organic cation tested, the currents were inhibited by gadolinium and anti-PC2 antibody. Using the dimensions of the largest permeant cation, the minimum pore diameter of the PC2 channel was estimated to be at least 11 A. The large pore size suggests that the primary state of this channel found in vivo is closed to avoid rundown of cation gradients across the plasma membrane and excessive calcium leak from endoplasmic reticulum stores.

A model of the putative pore region of the cardiac ryanodine receptor channel

Using the bacterial K+ channel KcsA as a template, we constructed models of the pore region of the cardiac ryanodine receptor channel (RyR2) monomer and tetramer. Physicochemical characteristics of the RyR2 model monomer were compared with the template, including homology, predicted secondary structure, surface area, hydrophobicity, and electrostatic potential. Values were comparable with those of KcsA. Monomers of the RyR2 model were minimized and assembled into a tetramer that was, in turn, minimized. The assembled tetramer adopts a structure equivalent to that of KcsA with a central pore. Characteristics of the RyR2 model tetramer were compared with the KcsA template, including average empirical energy, strain energy, solvation free energy, solvent accessibility, and hydrophobic, polar, acid, and base moments. Again, values for the model and template were comparable. The pores of KcsA and RyR2 have a common motif with a hydrophobic channel that becomes polar at both entrances. Quantitative comparisons indicate that the assembled structure provides a plausible model for the pore of RyR2. Movement of Ca2+, K+, and tetraethylammonium (TEA+) through the model RyR2 pore were simulated with explicit solvation. These simulations suggest that the model RyR2 pore is permeable to Ca2+ and K+ with rates of translocation greater for K+. In contrast, simulations indicate that tetraethylammonium blocks movement of metal cations.

Role of the proposed pore-forming segment of the Ca2+ release channel (ryanodine receptor) in ryanodine interaction

In earlier studies we showed that point mutations introduced into the proposed pore-forming segment, GVRAGGGIGD (amino acids 4820-4829), of the mouse cardiac ryanodine receptor reduced or abolished high affinity [3H]ryanodine binding. Here we investigate the effects of these mutations on the affinity and dissociation properties of [3H]ryanodine binding and on ryanodine modification of the ryanodine receptor channel at the single channel and whole cell levels. Scatchard analysis and dissociation studies reveal that mutation G4824A decreases the equilibrium dissociation constant (K(d)) and the dissociation rate constant (k(off)), whereas mutations G4828A and D4829A increase the K(d) and k(off) values. The effect of ryanodine on single G4828A and D4829A mutant channels is reversible on the time scale of single channel experiments, in contrast to the irreversible effect of ryanodine on single wild-type channels. Ryanodine alone is able to induce a large and sustained Ca2+ release in HEK293 cells transfected with the R4822A or G4825A mutant cDNA at the resting cytoplasmic Ca2+ but causes little or no Ca2+ release in cells transfected with the wild-type cDNA. Mutation G4826C diminishes the functional effect of ryanodine on Ca2+ release but spares caffeine-induced Ca2+ release in HEK293 cells. Co-expression of the wild-type and G4826C mutant proteins produces single channels that interact with ryanodine reversibly and display altered conductance and ryanodine response. These results are consistent with the view that the proposed pore-forming segment is a critical determinant of ryanodine interaction. A putative model of ryanodine-ryanodine receptor interaction is proposed.

Subconductance activity induced by quinidine and quinidinium in purified cardiac sarcoplasmic reticulum calcium release channels

This study examined the effects of quinidine, quinine, and the quaternary quinidine derivative, quinidinium, on the conductance and activity of purified cardiac sarcoplasmic reticulum calcium release channels/ryanodine receptors (RyR) incorporated into planar lipid bilayers. Quinidine (50-500 microM) reduced the single-channel open probability in a voltage- and concentration-dependent manner. Reduction of channel activity was evident only at positive holding potentials where current flow is from the cytoplasmic to luminal side of the channel and when the drug was present only on the cytoplasmic face of the channel. A more pronounced effect was the appearance of a subconductance state at positive potentials. Single channel recordings and dose-response experiments revealed that at least two quinidine molecules were involved in reduction of the RyR activity. The permanently charged quinidinium compound produced nearly identical effects as quinidine when present only on cytoplasmic side of the channel, suggesting the positive-charged form of quinidine is responsible for the effects on the channel. There was no stereospecificity in the effects of quinidine because the levoisomer, 100 microM quinine, produced a similar subconductance activity of the channel. Ryanodine modification of the channel prevented subconductance activity. These findings suggest that the quinidine-induced subconductance activity may be the result of a partial occlusion of the channel pore interfering with ion conduction. Modification of the channel by ryanodine alters quinidine binding to the channel through a conformational change in protein structure.

Excess noise in modified conductance states following the interaction of ryanoids with cardiac ryanodine receptor channels

The interaction of ryanodine with the ryanodine receptor (RyR) produces profound changes in channel function. Open probability increases dramatically and conductance is reduced. In this report we describe differences in the properties of reduced conductance states produced by the interaction of ryanodine derivatives with RyR channels. Some reduced conductance states are considerably noisier than the normal open state of the RyR channel. Inspection and analysis of these events reveals that the excess noise arises from transitions between two conductance states. Following the interaction of certain ryanodine derivatives, RyR channels undergo transitions between two conformations with slightly different ion-handling properties.

Block of the ryanodine receptor channel by neomycin is relieved at high holding potentials

In this study we have investigated the actions of the aminoglycoside antibiotic neomycin on K+ conductance in the purified sheep cardiac sarcoplasmic reticulum (SR) calcium-release channel (RyR). Neomycin induces a concentration- and voltage-dependent partial block from both the cytosolic and luminal faces of the channel. Blocking parameters for cytosolic and luminal block are markedly different. Neomycin has a greater affinity for the luminal site of interaction than the cytosolic site: zero-voltage dissociation constants (Kb(0)) are respectively 210.20 +/- 22.80 and 589.70 +/- 184.00 nM for luminal and cytosolic block. However, neomycin also exhibits voltage-dependent relief of block at holding potentials >+60 mV when applied to the cytosolic face and a similar phenomenon may occur with luminal neomycin at high negative holding potentials. These observations indicate that, under appropriate conditions, neomycin is capable of passing through the RyR channel.

Ryanodine-induced structural alterations in the RyR channel suggested by neomycin block

In Mead and Williams, (Biophys. J. 82:1953-1963, 2002) we have reported that neomycin is a potent partial blocker of single purified sheep cardiac SR calcium release channels. Neomycin is unusual in that it is capable of blocking when applied to either the cytosolic or the luminal face of the channel. Block at either aspect of the channel is both concentration- and voltage-dependent, but exhibits different blocking parameters. In this study we have investigated the actions of neomycin on ion handling in the ryanodine-modified channel. Neomycin is more effective at the cytosolic face, having a Kb(0) value of 534.9 +/- 35.17 nM compared with a Kb(0) value of 971.5 +/- 66.62 nM for the luminal face. The voltage dependence also differs at the two sites. Values of zdelta for cytosolic and luminal neomycin are 1.09 +/- 0.04 and -0.57 +/- 0.03, respectively. The interaction of neomycin with the ryanodine-modified channel differs notably from that in the unmodified channel. Voltage-dependent relief of block is not observed after ryanodine modification, and the luminal blocking characteristics are altered. This suggests that ryanodine induces changes at the luminal mouth of the channel and may confer increased rigidity to the channel protein.

Ionic blockade of the rat connexin40 gap junction channel by large tetraalkylammonium ions

The rat connexin40 gap junction channel is permeable to monovalent cations including tetramethylammonium and tetraethylammonium ions. Larger tetraalkyammonium (TAA(+)) ions beginning with tetrabutylammonium (TBA(+)) reduced KCl junctional currents disproportionately. Ionic blockade by tetrapentylammonium (TPeA(+)) and tetrahexylammonium (THxA(+)) ions were concentration- and voltage-dependent and occurred only when TAA(+) ions were on the same side as net K(+) efflux across the junction, indicative of block of the ionic permeation pathway. The voltage-dependent dissociation constants (K(m)(V(j))) were lower for THxA(+) than TPeA(+), consistent with steric effects within the pore. The K(m)-V(j) relationships for TPeA(+) and THxA(+) were fit with different reaction rate models for a symmetrical (homotypic) connexin gap junction channel and were described by either a one- or two-site model that assumed each ion traversed the entire V(j) field. Bilateral addition of TPeA(+) ions confirmed a common site of interaction within the pore that possessed identical K(m)(V(j)) values for cis-trans concentrations of TPeA(+) ions as indicated by the modeled I-V relations and rapid channel block that precluded unitary current measurements. The TAA(+) block of K(+) currents and bilateral TPeA(+) interactions did not alter V(j)-gating of Cx40 gap junctions. N-octyl-tributylammonium and -triethylammonium also blocked rCx40 channels with higher affinity and faster kinetics than TBA(+) or TPeA(+), indicative of a hydrophobic site within the pore near the site of block.

Two domains in dihydropyridine receptor activate the skeletal muscle Ca(2+) release channel

The II-III cytoplasmic loop of the skeletal muscle dihydropyridine receptor (DHPR) alpha(1)-subunit is essential for skeletal-type excitation-contraction coupling. Single channel and [(3)H]ryanodine binding studies with a full-length recombinant peptide (p(666-791)) confirmed that this region specifically activates skeletal muscle Ca2+ release channels (CRCs). However, attempts to identify shorter domains of the II-III loop specific for skeletal CRC activation have yielded contradictory results. We assessed the specificity of the interaction of five truncated II-III loop peptides by comparing their effects on skeletal and cardiac CRCs in lipid bilayer experiments; p(671-680) and p(720-765) specifically activated the submaximally Ca2+-activated skeletal CRC in experiments using both mono and divalent ions as current carriers. A third peptide, p(671-690), showed a bimodal activation/inactivation behavior indicating a high-affinity activating and low-affinity inactivating binding site. Two other peptides (p(681-690) and p(681-685)) that contained an RKRRK-motif and have previously been suggested in in vitro studies to be important for skeletal-type E-C coupling, failed to specifically stimulate skeletal CRCs. Noteworthy, p(671-690), p(681-690), and p(681-685) induced similar subconductances and long-lasting channel closings in skeletal and cardiac CRCs, indicating that these peptides interact in an isoform-independent manner with the CRCs.

Ruthenium red modifies the cardiac and skeletal muscle Ca(2+) release channels (ryanodine receptors) by multiple mechanisms

The effects of ruthenium red (RR) on the skeletal and cardiac muscle ryanodine receptors (RyRs) were studied in vesicle-Ca(2+) flux, [(3)H]ryanodine binding, and single channel measurements. In vesicle-Ca(2+) flux measurements, RR was more effective in inhibiting RyRs at 0.2 microM than 20 microM free Ca(2+). [(3)H]Ryanodine binding measurements suggested noncompetitive interactions between RR inhibition and Ca(2+) regulatory sites of RyRs. In symmetric 0.25 M KCl with 10-20 microM cytosolic Ca(2+), cytosolic RR decreased single channel activities at positive and negative holding potentials. In close to fully activated skeletal (20 microM Ca(2+) + 2 mM ATP) and cardiac (200 microM Ca(2+)) RyRs, cytosolic RR induced a predominant subconductance at a positive but not negative holding potential. Lumenal RR induced a major subconductance in cardiac RyR at negative but not positive holding potentials and several subconductances in skeletal RyR. The RR-related subconductances of cardiac RyR showed a nonlinear voltage dependence, and more than one RR molecule appeared to be involved in their formation. Cytosolic and lumenal RR also induced subconductances in Ca(2+)-conducting skeletal and cardiac RyRs recorded at 0 mV holding potential. These results suggest that RR inhibits RyRs and induces subconductances by binding to cytosolic and lumenal sites of skeletal and cardiac RyRs.

Imperatoxin A induces subconductance states in Ca2+ release channels (ryanodine receptors) of cardiac and skeletal muscle

Single-channel and [3H]ryanodine binding experiments were carried out to examine the effects of imperatoxin activator (IpTxa), a 33 amino acid peptide isolated from the venom of the African scorpion Pandinus imperator, on rabbit skeletal and canine cardiac muscle Ca2+ release channels (CRCs). Single channel currents from purified CRCs incorporated into planar lipid bilayers were recorded in 250 mM KCl media. Addition of IpTxa in nanomolar concentration to the cytosolic (cis) side, but not to the lumenal (trans) side, induced substates in both ryanodine receptor isoforms. The substates displayed a slightly rectifying current-voltage relationship. The chord conductance at -40 mV was approximately 43% of the full conductance, whereas it was approximately 28% at a holding potential of +40 mV. The substate formation by IpTxa was voltage and concentration dependent. Analysis of voltage and concentration dependence and kinetics of substate formation suggested that IpTxa reversibly binds to the CRC at a single site in the voltage drop across the channel. The rate constant for IpTxa binding to the skeletal muscle CRC increased e-fold per +53 mV and the rate constant of dissociation decreased e-fold per +25 mV applied holding potential. The effective valence of the reaction leading to the substate was approximately 1.5. The IpTxa binding site was calculated to be located at approximately 23% of the voltage drop from the cytosolic side. IpTxa induced substates in the ryanodine-modified skeletal CRC and increased or reduced [3H]ryanodine binding to sarcoplasmic reticulum vesicles depending on the level of channel activation. These results suggest that IpTxa induces subconductance states in skeletal and cardiac muscle Ca2+ release channels by binding to a single, cytosolically accessible site different from the ryanodine binding site.

Electrophysiological effects of ryanodine derivatives on the sheep cardiac sarcoplasmic reticulum calcium-release channel

We have examined the effects of a number of derivatives of ryanodine on K+ conduction in the Ca2+ release channel purified from sheep cardiac sarcoplasmic reticulum (SR). In a fashion comparable to that of ryanodine, the addition of nanomolar to micromolar quantities to the cytoplasmic face (the exact amount depending on the derivative) causes the channel to enter a state of reduced conductance that has a high open probability. However, the amplitude of that reduced conductance state varies between the different derivatives. In symmetrical 210 mM K+, ryanodine leads to a conductance state with an amplitude of 56.8 +/- 0.5% of control, ryanodol leads to a level of 69.4 +/- 0.6%, ester A ryanodine modifies to one of 61.5 +/- 1.4%, 9,21-dehydroryanodine to one of 58.3 +/- 0.3%, 9 beta,21beta-epoxyryanodine to one of 56.8 +/- 0.8%, 9-hydroxy-21-azidoryanodine to one of 56.3 +/- 0.4%, 10-pyrroleryanodol to one of 52.2 +/- 1.0%, 3-epiryanodine to one of 42.9 +/- 0.7%, CBZ glycyl ryanodine to one of 29.4 +/- 1.0%, 21-p-nitrobenzoyl-amino-9-hydroxyryanodine to one of 26.1 +/- 0.5%, beta-alanyl ryanodine to one of 14.3 +/- 0.5%, and guanidino-propionyl ryanodine to one of 5.8 +/- 0.1% (chord conductance at +60 mV, +/- SEM). For the majority of the derivatives the effect is irreversible within the lifetime of a single-channel experiment (up to 1 h). However, for four of the derivatives, typified by ryanodol, the effect is reversible, with dwell times in the substate lasting tens of seconds to minutes. The effect caused by ryanodol is dependent on transmembrane voltage, with modification more likely to occur and lasting longer at +60 than at -60 mV holding potential. The addition of concentrations of ryanodol insufficient to cause modification does not lead to an increase in single-channel open probability, such as has been reported for ryanodine. At concentrations of > or = 500 mu M, ryanodine after initial rapid modification of the channel leads to irreversible closure, generally within a minute. In contrast, comparable concentrations of beta-alanyl ryanodine do not cause such a phenomenon after modification, even after prolonged periods of recording (>5 min). The implications of these results for the site(s) of interaction with the channel protein and mechanism of the action of ryanodine are discussed. Changes in the structure of ryanodine can lead to specific changes in the electrophysiological consequences of the interaction of the alkaloid with the sheep cardiac SR Ca2+ release channel.

Characteristics of cocaine block of purified cardiac sarcoplasmic reticulum calcium release channels

We have examined the effects of cocaine on the SR Ca2+ release channel purified from canine cardiac muscle. Cocaine induced a flicker block of the channel from the cytoplasmic side, which resulted in an apparent reduction in the single-channel current amplitude without a marked reduction in the single-channel open probability. This block was evident only at positive holding potentials. Analysis of the block revealed that cocaine binds to a single site with an effective valence of 0.93 and an apparent dissociation constant at 0 mV (Kd(0)) of 38 mM. The kinetics of cocaine block were analyzed by amplitude distribution analysis and showed that the voltage and concentration dependence lay exclusively in the blocking reaction, whereas the unblocking reaction was independent of both voltage and concentration. Modification of the channel by ryanodine dramatically attenuated the voltage and concentration dependence of the on rates of cocaine block while diminishing the off rates to a lesser extent. In addition, ryanodine modification changed the effective valence of cocaine block to 0.52 and the Kd(0) to 110 mM, suggesting that modification of the channel results in an alteration in the binding site and its affinity for cocaine. These results suggest that cocaine block of the SR Ca2+ release channel is due to the binding at a single site within the channel pore and that modification of the channel by ryanodine leads to profound changes in the kinetics of cocaine block.

Measuring the length of the pore of the sheep cardiac sarcoplasmic reticulum calcium-release channel using related trimethylammonium ions as molecular calipers

After incorporation of purified sheep cardiac Ca(2+)-release channels into planar phospholipid bilayers, we have investigated the blocking effects of a series of monovalent (CH3-(CH2)n-1-N+(CH3)3) and divalent ((CH3)3N(+)-(CH2)n-N+(CH3)3) trimethylammonium derivatives under voltage clamp conditions. All the compounds tested produce voltage-dependent block from the cytoplasmic face of the channel. With divalent (Qn) derivatives the effective valence of block decreases with increasing chain length, reaching a plateau with a chain length of n > or = 7. No decline in effective valence is observed with the monovalent (Un) derivatives. A plausible interpretation of this phenomena suggests that for the 90% of the voltage drop measured, the increase in length following the addition of a CH2 in the chain spans 12.7% of the electrical field. Extrapolating this distance to include the remaining 10% suggests that the applied holding potential falls over a total distance of 10.4 A. In addition, at high positive holding potentials there is evidence for permeation of the trimethylammonium ions and a valency specific relief of block.

The effects of tetrahexyl ammonium cations (THA+) on inositol 1,4,5-trisphosphate-induced calcium release from porcine cerebellar microsomes: THA+ can induce calcium release selectively from the InsP3-sensitive calcium stores

In this study we show that the potassium-channel blocker tetrahexyl ammonium chloride (THA+) is able to inhibit inositol 1,4,5-trisphosphate (InsP3)-induced calcium release in an apparently biphasic fashion with a IC50 of 3 microM. This inhibition was not alleviated by valinomycin and, therefore, is not consistent with the blocking of K+ counter-ion movement, an observation initially made by Palade et al. (Palade, P., Dettbarn, C., Volpe, P., Alderson, B. and Otero, A.S (1989) Mol. Pharmacol. 36, 664-672). THA+ affected quantal calcium release by reducing the amount of calcium released by InsP3, but did not greatly affect the concentration of InsP3 required to cause half-maximal calcium release. THA+ did not affect the metabolism of InsP3 or its binding to porcine cerebellar microsomes. THA+ could also itself induce calcium release. At concentrations below 100 microM, THA+ appears to release Ca2+ selectively from the InsP3-sensitive calcium stores, since prior depletion of these stores with supramaximal doses of InsP3 abolishes this response. At higher THA+ concentrations (above 100 microM) Ca2+ is released non-selectively from all stores. THA+ has no effect on the Ca(2+)-ATPase activity at concentrations below 100 microM, indicating that selective THA(+)-induced Ca2+ release is not due to non-specific inhibition of the microsomal Ca2+ pumps and does not affect Ca2+ leakage. A number of pharmacological modulators of intracellular calcium channels were also tested on THA(+)-induced calcium release with little effect, except for spermidine which reduced this release by up to 50%. Our observations are consistent with the view that THA+, at concentrations below 100 microM, selectively releases calcium from the InsP3-sensitive calcium stores.

Using large organic cations to probe the nature of ryanodine modification in the sheep cardiac sarcoplasmic reticulum calcium release channel

We have reported that the large impermeant organic cations tetrabutyl ammonium (TBA+), tetrapentyl ammonium, and the charged local anesthetic QX314 produce unique reduced conductance states in the purified sheep cardiac sarcoplasmic reticulum Ca2+ release channel when present at the cytoplasmic face of the channel. We have interpreted this as a form of partial occlusion by the blocking cation in wide vestibules of the conduction pathway. Following modification with ryanodine, which causes the channel to enter a reduced conductance state with long open dwell time, these cations block the receptor channel to a level that is indistinguishable from the closed state. The voltage dependence of TBA+'s interaction with the Ca2+ release channel is the same before and after ryanodine modification. The concentration dependence is different, in that the ryanodine-modified channel has one-third the affinity for TBA+, which is accounted for predominantly by changes in the TBA+ on rate. The data are compatible with a structural change in the vestibule of the conduction pathway consequent upon ryanodine binding that reduces the capture radius for blocking ion entry.

Charged local anesthetics block ionic conduction in the sheep cardiac sarcoplasmic reticulum calcium release channel

We have examined the effect of the charged local anesthetics QX314, QX222, and Procaine on monovalent cation conduction in the Ca2+ release channel of the sheep cardiac sarcoplasmic reticulum. All three blockers only affect cation conductance when present at the cytoplasmic face of the channel. QX222 and Procaine act as voltage-dependent blockers. With 500 Hz filtering, this is manifest as a relatively smooth reduction in single-channel current amplitude most prominent at positive holding potentials. Quantitative analysis gives an effective valence of approximately 0.9 for both ions and Kb(0)s of 9.2 and 15.8 mM for QX222 and Procaine, respectively. Analysis of the concentration dependence of block suggests that QX222 is binding to a single site with a Km of 491 microM at a holding potential of 60 mV. The use of amplitude distribution analysis, with the data filtered at 1 to 2 kHz, reveals that the voltage and concentration dependence of QX222 block occurs largely because of changes in the blocker on rate. The addition of QX314 has a different effect, leading to the production of a substate with an amplitude of approximately one-third that of the control. The substate's occurrence is dependent on holding potential and QX314 concentration. Quantitative analysis reveals that the effect is highly voltage dependent, with a valence of approximately 1.5 caused by approximately equal changes in the on and off rates. Kinetic analysis of the concentration dependence of the substate occurrence reveals positive cooperativity with at least two QX314s binding to the conduction pathway, and this is largely accounted for by changes in the on rate. A paradoxical increase in the off rate at high positive holding potentials and with increasing QX314 concentration at 80 mV suggests the existence of a further QX314-dependent reaction that is both voltage and concentration dependent. The substate block is interpreted physically as a form of partial occlusion in the vestibule of the conduction pathway giving a reduction in single-channel current by electrostatic means.

Block of the sheep cardiac sarcoplasmic reticulum Ca(2+)-release channel by tetra-alkyl ammonium cations

The purified ryanodine receptor channel of the sheep cardiac muscle sarcoplasmic reticulum (SR) membrane functions as a calcium-activated cation-selective channel under voltage-clamp conditions following reconstitution into planar phospholipid bilayers. We have investigated the effects of the tetra-alkyl ammonium (TAA) cations, (CnH2n+1)4N+ and the trimethyl ammonium cations, ethyltrimethyl ammonium and propyltrimethyl ammonium, on potassium conductance through the receptor channel. Small TAA cations (n = 1-3) and the trimethyl ammonium derivatives act as asymmetric, voltage-dependent blockers of potassium current. Quantitative analysis of the voltage dependence of block indicates that the conduction pathway of the sheep cardiac SR ryanodine receptor channel contains two distinct sites for the interaction of these small organic cations. Sites are located at approximately 50% for tetramethyl ammonium (TMA+) and 90% for tetraethyl ammonium (TEA+) and tetrapropyl ammonium (TPrA+) of the voltage drop across the channel from the cytosolic face of the protein. The chemical substitution of an ethyl or propyl group for one of the methyl groups in TMA+ increases the voltage dependence of block to a level similar to that of TEA+ and TPrA+. The zero-voltage dissociation constant (Kb(0)) falls with the increasing number of methyl and methylene groups for those blockers acting 90% of the way across the voltage drop. This is interpreted as suggesting a hydrophobic binding site at this point in the conduction pathway. The degree of block increases as the concentration of small TAA cations is raised. The concentration dependence of tetraethyl ammonium block indicates that the cation interacts with a single site within the conduction pathway with a Km of 9.8 +/- 1.7 mM (mean +/- SD) at 40 mV. Larger TAA cations (n = 4-5) do not induce voltage-dependent block of potassium current of the form seen with the smaller TAA cations. These data support the contention that the sheep cardiac SR ryanodine receptor channel may be occupied by at most one ion at a time and suggest that a large proportion of the voltage drop falls over a relatively wide region of the conduction pathway.