Planar bilayer recording of ryanodine receptors of sarcoplasmic reticulum

Ryanodine receptors/calcium release channels in heart failure and sudden cardiac death

Calcium (Ca2+) ions are second messengers in signaling pathways in all types of cells. They regulate muscle contraction, electrical signals which determine the cardiac rhythm and cell growth pathways in the heart. In the past decade cDNA cloning has provided clues as to the molecular structure of the intracellular Ca2+ release channels (ryanodine receptors, RyR, and inositol 1,4,5-trisphosphate receptors, IP3R) on the sarcoplasmic and endoplasmic reticulum (SR/ER) and an understanding of how these molecules regulate Ca2+ homeostasis in the heart is beginning to emerge. The intracellular Ca2+ release channels form a distinct class of ion channels distinguished by their structure, size, and function. Both RyRs and IP3Rs have gigantic cytoplasmic domains that serve as scaffolds for modulatory proteins that regulate the channel pore located in the carboxy terminal 10% of the channel sequence. The channels are tetramers comprised of four RyR or IP3R subunits. RyR2 is required for excitation-contraction (EC) coupling in the heart. Using co-sedimentation and co-immunoprecipitation we have defined a macromolecular complex comprised of RyR2, FKBP12.6, PKA, the protein phosphatases PP1 and PP2A, and an anchoring protein mAKAP. We have shown that protein kinase A (PKA) phosphorylation of RyR2 dissociates FKBP12.6 and regulates the channel open probability (P(o)). In failing human hearts RyR2 is PKA hyperphosphorylated resulting in defective channel function due to increased sensitivity to Ca2+-induced activation.

Activation and inhibition of purified skeletal muscle calcium release channel by NO donors in single channel current recordings

The actions of the nitric oxide (NO) donors 1-hydroxy-2-oxo-3-(N-methyl-3-aminopropyl)-3 methyl-1-triazine (NOC-7), S-nitrosoacetylcysteine (CySNO) and S-nitrosoglutathione (GSNO) on the purified calcium release channel (ryanodine receptor) of rabbit skeletal muscle were determined by single channel current recordings. In addition, the activation of the NO donor modulated calcium release channel by the sulfhydryl oxidizing organic mercurial compound 4-(chloromercuri)phenylsulfonic acid (4-CMPS) was investigated. NOC-7 (0.1 and 0.3 mM) and CySNO (0.4 and 0.8 mM) increased the open probability (P(o)) of the calcium release channel at activating calcium concentrations (20-100 microM Ca(2+)) by 60-100%, with no effect on the current amplitude; this activation was abolished by the specific sulfhydryl reducing agent DTT. High concentrations of CySNO (1.6-2 mM) decreased P(o). Activation by GSNO (1 mM) was observed in two thirds of the experiments, but 2 mM and 4 mM GSNO markedly reduced P(o) at activating Ca(2+) (20-100 microM). In contrast to 4-CMPS, NOC-7 or GSNO had no effect at subactivating free Ca(2+) (0.6 microM). 4-CMPS further increased the open probability of NOC-7- or CySNO-stimulated channels and reversed transiently the reduced open probability of CySNO or GSNO inhibited channels at activating free Ca(2+). High concentrations of GSNO did not prevent channel activation of 4-CMPS at subactivating free Ca(2+). The NOC-7-, CySNO- or GSNO-modified channels were completely blocked by ruthenium red. It is suggested that nitrosylation/oxidation of sulfhydryls by NO donors and oxidation of sulfhydryls by 4-CMPS affect different cysteine residues essential in the gating of the calcium release channel.

FKBP12 modulates gating of the ryanodine receptor/calcium release channel

Excitation-contraction (EC) coupling in muscle requires the activation of intracellular calcium release channels (CRC). Four type 1 ryanodine receptor (RyR1) molecules form each tetrameric CRC. Each RyR1 contains a binding site for the FK506 binding protein (FKBP12), a cis-trans peptidyl-prolyl isomerase that is required for coordinated gating of the four RyR1 subunits comprising the channel. When FKBP12 is bound to RyR1, it stabilizes the four subunits that form each CRC. We propose that binding of one FKBP12 to each RyR1 lowers the energy of twisted-amide peptidyl-prolyl bonds and stabilizes RyR1 in a conformation that permits coordinated gating of the four RyR1 subunits.

Modification of sulfhydryls of the skeletal muscle calcium release channel by organic mercurial compounds alters Ca2+ affinity of regulatory Ca2+ sites in single channel recordings and [3H]ryanodine binding

The actions of two organic mercurial compounds, 4-(chloromercuri)phenyl-sulfonic acid (4-CMPS) and p-chloromercuribenzoic acid (p-CMB) on the calcium release channel (ryanodine receptor) from rabbit skeletal muscle were determined by single channel recordings with the purified calcium release channel, radioligand binding to sarcoplasmic reticulum vesicles (HSR) and calcium release from HSR. p-CMB or 4-CMPS (20-100 microM) increased the mean open probability (Po) of the calcium channel at subactivating (20 nM), maximally activating (20-100 microM and inhibitory (1-4 mM) Ca2+ concentrations, with no effect on unitary conductance. This activation was partly reversed by 2 mM DTT. Both compounds affected the channels only from the cytosolic side, but not from the trans side. 100 microM 4-CMPS caused a transient increase in Po, followed by a low activity state within 1 min. At inhibitory Ca2+ concentrations Po was increased to values observed with maximally activating Ca2+ or lower, inhibitory Ca2+ concentrations. The p-CMB/4-CMPS modified channels were ryanodine sensitive and blocked by ruthenium red. [3H]Ryanodine binding was increased up to four-fold with 3-15 microM 4-CMPS/p-CMB (Hill coefficient 1.7-2.0) at 4 microM Ca2+ and reduced at high concentrations (50-200 microM). The increase in [3H]ryanodine binding by 10 microM 4-CMPS was completely inhibited by 2 mM DTT. 4-CMPS significantly increased the affinity for the high affinity calcium activation sites and decreased the affinity of low affinity calcium inhibitory sites of specific [3H]ryanodine binding. 4-CMPS increased the affinity of the ryanodine receptor for high affinity ryanodine binding without a change in receptor density. 4-CMPS induced a rapid, concentration-dependent, biphasic calcium release from passively calcium-loaded HSR vesicles at subactivating Ca2+ concentrations (20 nM), which was partly inhibited by 4 mM DTT and completely blocked by 20 microM ruthenium red. It is suggested that the 4-CMPS-induced modulation of essential sulfhydryls involved in the gating of the calcium release channel results in a modulation of the apparent calcium affinity of the activating high affinity and inhibitory low affinity calcium binding sites of the calcium release channel.

Coupled gating between individual skeletal muscle Ca2+ release channels (ryanodine receptors)

Excitation-contraction coupling in skeletal muscle requires the release of intracellular calcium ions (Ca2+) through ryanodine receptor (RyR1) channels in the sarcoplasmic reticulum. Half of the RyR1 channels are activated by voltage-dependent Ca2+ channels in the plasma membrane. In planar lipid bilayers, RyR1 channels exhibited simultaneous openings and closings, termed "coupled gating." Addition of the channel accessory protein FKBP12 induced coupled gating, and removal of FKBP12 uncoupled channels. Coupled gating provides a mechanism by which RyR1 channels that are not associated with voltage-dependent Ca2+ channels can be regulated.

Intercellular calcium signaling via gap junction in connexin-43-transfected cells

In excitable cells, intracellular Ca2+ is released via the ryanodine receptor from the intracellular Ca2+ storing structure, the sarcoplasmic reticulum. To determine whether this released Ca2+ propagates through gap junctions to neighboring cells and thereby constitutes a long range signaling network, we developed a cell system in which cells expressing both connexin-43 and ryanodine receptor are surrounded by cells expressing only connexin-43. When the ryanodine receptor in cells was activated by caffeine, propagation of Ca2+ from these caffeine-responsive cells to neighboring cells was observed with a Ca2+ imaging system using fura-2/AM. Inhibitors of gap junctional communication rapidly and reversibly abolished this propagation of Ca2+. Together with the electrophysiological analysis of transfected cells, the observed intercellular Ca2+ wave was revealed to be due to the reconstituted gap junction of transfected cells. We next evaluated the functional roles of cysteine residues in the extracellular loops of connexin-43 in gap junctional communication. Mutations of Cys54, Cys187, Cys192, and Cys198 to Ser showed the failure of Ca2+ propagation to neighboring cells in accordance with the electrical uncoupling between transfected cells, whereas mutations of Cys61 and Cys68 to Ser showed the same pattern as the wild type. [14C]Iodoacetamide labeling of free thiols of cysteine residues in mutant connexin-43s showed that two pairs of intramolecular disulfide bonds are formed between Cys54 and Cys192 and between Cys187 and Cys198. These results suggest that intercellular Ca2+ signaling takes place in cultured cells expressing connexin-43, leading to their own synchronization and that the extracellular disulfide bonds of connexin-43 are crucial for this process.

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

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

Ketamine, at clinical concentrations, does not alter the function of cardiac sarcoplasmic reticulum calcium release channels

In the absence of sympathetically mediated stimulation, ketamine depresses myocardial contractility. This results from a decrease in the availability of intracellular Ca2+ for excitation-contraction coupling. Although sites of action other than the Ca2+ release channel of sarcoplasmic reticulum have been implicated, ketamine-induced alterations in Ca2+ efflux from the sarcoplasmic reticulum remain contentious. The purpose of the present study was to identify interactions of ketamine with the calcium release channel using sarcoplasmic reticulum enriched vesicles from porcine left ventricle. Ketamine did not alter [3H]ryanodine binding at concentrations of 1 mM or less, while binding was almost completely inhibited at 10 mM. Gating and conductance of SR Ca2+ channels studied in planar bilayers was not altered by clinical concentrations of ketamine over the range of physiologic cytoplasmic free Ca2+ concentrations. Channel inactivation was observed at 10 mM ketamine, well in excess of clinical concentrations. These findings indicate that clinical concentrations of ketamine do not alter the function of the Ca2+ release channel. Alterations in intracellular Ca2+ homeostasis that result in depression of myocardial contractility must therefore result from effects at other sites along the excitation-contraction coupling pathway.

Primary structure and properties of helothermine, a peptide toxin that blocks ryanodine receptors

Helothermine, a protein from the venom of the Mexican beaded lizard (Heloderma horridum horridum), was found to inhibit [3H]ryanodine binding to cardiac and skeletal sarcoplasmic reticulum, to block cardiac and skeletal ryanodine receptor channels incorporated into planar bilayers, and to block Ca(2+)-induced Ca2+ release triggered by photolysis of nitr-5 in saponin-permeabilized trabeculae from rat ventricle. Cloning of the helothermine cDNA revealed that the protein is composed of 223 amino acids with a molecular mass of 25,376 daltons, and apparently is stabilized by eight disulfide bridges. The peptide sequence showed significant homology with a family of cysteine-rich secretory proteins found in the male genital tract and in salivary glands. The interaction of helothermine and ryanodine receptors should serve to define functional domains within the channel structure involved in the control of Ca2+ release from sarcoplasmic reticulum.

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

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

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

We investigated the effect of Cl- on the Ca2+ permeability of rabbit skeletal muscle junctional sarcoplasmic reticulum (SR) using 45Ca2+ fluxes and single channel recordings. In 45Ca2+ efflux experiments, the lumen of the SR was passively loaded with solutions of 150 mM univalent salt containing 5 mM 45Ca2+. Release of 45Ca2+ was measured by rapid filtration in the presence of extravesicular 0.4-0.8 microM free Ca2+ and 150 mM of the same univalent salt loaded into the SR lumen. The rate of release was 5-10 times higher when the univalent salt equilibrated across the SR-contained Cl- (Tris-Cl, choline-Cl, KCl) instead of an organic anion or other halides (gluconate-, methanesulfonate-, acetate-, HEPES-, Br-, I-). Cations (K+, Tris+) could be interchanged without a significant effect on the release rate. To determine whether Cl- stimulated ryanodine receptors, we measured the stimulation of release by ATP (5 mM total) and caffeine (20 mM total) and the inhibition by Mg2+ (0.8 mM estimated free) in Cl(-)-free and Cl(-)-containing solutions. The effects of ATP, caffeine, and Mg2+ were the largest in K-gluconate and Tris-gluconate, intermediate in KCl, and notably poor or absent in choline-Cl and Tris-Cl. Procaine (10 mM) inhibited the caffeine-stimulated release measured in K-gluconate, whereas the Cl- channel blocker clofibric acid (10 mM) but not procaine inhibited the caffeine-insensitive release measured in choline-Cl. Ruthenium red (20 microM) inhibited release in all solutions. In SR fused to planar bilayers we identified a nonselective Cl- channel (PCl: PTris: PCa = 1:0.5:0.3) blocked by ruthenium red and clofibric acid but not by procaine. These conductive and pharmacological properties suggested the channel was likely to mediate Cl(-)-dependent SR Ca2+ release. The absence of a contribution of ryanodine receptors to the Cl(-)-dependent release were indicated by the lack of an effect of Cl- on the open probability of this channel, a complete block by procaine, and a stimulation rather than inhibition by clofibric acid. A plug model of Cl(-)-dependent release, whereby Cl- removed the inhibition of the nonselective channel by large anions, was formulated under the assumption that nonselective channels and ryanodine receptor channels operated separately from each other in the terminal cisternae. The remarkably large contribution of Cl- to the SR Ca2+ permeability suggested that nonselective Cl- channels may control the Ca2+ permeability of the SR in the resting muscle cell.

Structure and function of ryanodine receptors

Membrane depolarization, neurotransmitters, and hormones evoke a release of Ca2+ from intracellular Ca(2+)-storing organelles like the endoplasmic reticulum and, in muscle, the sarcoplasmic reticulum (SR). In turn, the released Ca2+ serves to trigger a variety of cellular responses. The presence of Ca2+ pumps to replenish intracellular stores was described more than 20 years ago. The presence of Ca2+ channels, like the ryanodine receptor, which suddenly release the organelle-stored Ca2+, is a more recent finding. This review describes the progress made in the last five years on the structure, function, and regulation of the ryanodine receptor. Numerous reports have described the response of ryanodine receptors to cellular ions and metabolites, kinases and other proteins, and pharmacological agents. In many cases, comparative measurements have been made using Ca2+ fluxes in SR vesicles, single-channel recordings in planar bilayers, and radioligand binding assays using [3H]ryanodine. These techniques have helped to relate the activity of single ryanodine receptors to global changes in the SR Ca2+ permeability. Molecular information on functional domains within the primary structure of the ryanodine receptor is also available. There are at least three ryanodine receptor isoforms in various tissues. Some cells, such as amphibian muscle cells, express more than a single isoform. The diversity of ligands known to modulate gating and the diversity of tissues known to express the protein suggest that the ryanodine receptor has the potential to participate in many types of cell stimulus-Ca(2+)-release coupling mechanisms.

Stabilization of calcium release channel (ryanodine receptor) function by FK506-binding protein

FK506-binding protein (FKBP12) was originally identified as the cytosolic receptor for the immunosuppressant drugs FK506 and rapamycin. The cellular function of FKBP12, a ubiquitously expressed 12,000-dalton proline isomerase, has been unknown. FKBP12 copurifies with the 565,000-dalton ryanodine receptor (RyR), four of which form intracellular Ca2+ release channels of the sarcoplasmic and endoplasmic reticula. By coexpressing the RyR and FKBP12 in insect cells, we have demonstrated that FKBP12 modulates channel gating by increasing channels with full conductance levels (by > 400%), decreasing open probability after caffeine activation (from 0.63 +/- 0.09 to 0.04 +/- 0.02), and increasing mean open time (from 4.4 +/- 0.6 ms to 75 +/- 41 ms). FK506 or rapamycin, inhibitors of FKBP12 isomerase activity, reverse these stabilizing effects. These results provide the first natural cellular function for FKBP12, and establish that the functional Ca2+ release channel complex includes FKBP12.

Comparison of large-conductance Ca(2+)-activated K+ channels in artificial bilayer and patch-clamp experiments

We compared the gating, ion conduction, and pharmacology of large-conductance Ca(2+)-activated K+ channels (BK channels) from canine colon in artificial lipid bilayers and in excised patches. Both protocols identified 270-pS K(+)-selective channels activated by depolarization and Ca2+ (approximately 130-mV shift of half-activation voltage per 10-fold change in Ca2+) that were inhibited by extracellular tetraethylammonium (TEA) and charybdotoxin. These similarities suggest that the same BK channels are studied in the two techniques. However, we found three quantitative differences between channels in artificial bilayers and patches. 1) Channels in artificial bilayers required fivefold higher free Ca2+ or 80-mV stronger depolarization for activation. 2) The voltage dependence of TEA block was smaller for channels in artificial bilayers. The apparent distance across the membrane field for the TEA binding site was 0.031 for channels in artificial bilayers and 0.23 for channels in patches. 3) ATP (2 mM) decreased open probability (Po) of channels in artificial bilayers, whereas channels in patches were unaffected. Neither GTP nor UTP reduced Po of channels in artificial bilayers. It is possible that these differences may be due to a lack of molecular identity between the channels studied in the two protocols. Alternatively, they may be attributed to alterations in channel properties during reconstitution or to influences of the artificial lipid environment.

Cyclic ADP-ribose induced Ca2+ release in rabbit skeletal muscle sarcoplasmic reticulum

The Ca(2+)-mobilizing metabolite cyclic ADP-ribose (cADPR) has been shown to release Ca2+ from ryanodine-sensitive stores in many cells. We show that this metabolite at a concentration of 17 microM, but not its precursor beta-NAD+ nor non-cyclic ADPR at the same concentration, is active in releasing Ca2+ from rabbit skeletal muscle sarcoplasmic reticulum. The release was not sensitive to Ruthenium red (1 microM) nor to the ryanodine receptor-specific scorpion toxin Buthotus1-1 (10 microM). In planar bilayer single channel recordings, concentrations up to 50 microM cADPR did not increase the open probability of Ruthenium red and toxin-sensitive Ca2+ release channels. Thus Ca2+ release induced by cADPR in skeletal muscle sarcoplasmic reticulum may not involve opening of ryanodine receptors.

Activation of the Ca2+ release channel of skeletal muscle sarcoplasmic reticulum by palmitoyl carnitine

Studies of [3H]ryanodine binding, 45Ca2+ efflux, and single channel recordings in planar bilayers indicated that the fatty acid metabolite palmitoyl carnitine produced a direct stimulation of the Ca2+ release channel (ryanodine receptor) of rabbit and pig skeletal muscle junctional sarcoplasmic reticulum. At a concentration of 50 microM, palmitoyl carnitine (a) stimulated [3H]ryanodine binding 1.6-fold in a competitive manner at all pCa in the range 6 to 3; (b) released approximately 65% (30 nmol) of passively loaded 45Ca2+/mg protein; and (c) increased 7-fold the open probability of Ca2+ release channels incorporated into planar bilayers. Neither carnitine nor palmitic acid could reproduce the effect of palmitoyl carnitine on [3H]ryanodine binding, 45Ca2+ release, or channel open probability. 45Ca2+ release was induced by several long-chain acyl carnitines (C14, C16, C18) and acyl coenzyme A derivatives (C12, C14, C16), but not by the short-chain derivative C8 or by free saturated fatty acids of chain length C8 to C18, at room temperature or 36 degrees C. This newly identified interaction of esterified fatty acids and ryanodine receptors may represent a pathway by which metabolism of skeletal muscle could influence intracellular Ca2+ and may be responsible for the pathophysiology of disorders of beta-oxidation such as carnitine palmitoyl transferase II deficiency.