Disulfide linkage of biotin identifies a 106-kDa Ca2+ release channel in sarcoplasmic reticulum

Relaxin abrogates genomic remodeling of the aged heart

"Healthy" aging drives structural and functional changes in the heart including maladaptive electrical remodeling, fibrosis and inflammation, which lower the threshold for cardiovascular diseases such as heart failure (HF) and atrial fibrillation (AF). Despite mixed results in clinical trials, Relaxin-therapy for 2-days reduced mortality by 37% at 180-days post-treatment, in patients with acute decompensated HF. Relaxin's short lifespan (2-3h) but long-lasting protective actions suggested that relaxin acts at a genomic level to reverse maladaptive remodeling in AF, HF and aging. Our recent studies showed that a 2-week treatment with Relaxin (0.4mg/kg/day) of aged (24months old F-344 rats) increases the expression of voltage-gated Na⁺ channels (mRNA, Nav1.5 and I_Na), connexin-43, abrogates inflammatory and immune responses and reverses myocardial fibrosis and cellular hypertrophy of the aged hearts. Relaxin acts directly at a wide range of cell types in the cardiovascular system that express its cognate GPCR receptor, RXFP1. RNA-seq analysis of young and aged hearts with and without Relaxin treatment revealed that "normal" aging altered the expression of ~10% of genes expressed in the ventricles, including: ion channels, components of fibrosis, hemodynamic biomarkers, immune and inflammatory responses which were reversed by Relaxin. The extensive cardiovascular remodeling caused by Relaxin was mediated through the activation of the Wnt/β-catenin signaling pathway which was otherwise suppressed by in adult cardiomyocytes intracellular by cytosolic Dickkopf1 (Dkk1). Wnt/β-catenin signaling is a mechanism that can explain the pleiotropic actions of Relaxin and the marked reversal of genomic changes that occur in aged hearts.

Thioltransferases

A family of small molecular weight proteins with thiol-disulfide exchange activity have been discovered, widely distributed from E. coli to mammalian systems, called thioltransferases or glutaredoxins. There are no substantiated reports of thioltransferases-glutaredoxins in plants; however, partially purified dehydroascorbate reductase from peas had thiol-disulfide exchange catalytic activity using glutathione as reductant and S-sulfocysteine as thiosulfate cosubstrate (unpublished data). Thus, this class of proteins is universally distributed. Based on mutagenesis studies, a sequence of Cys-Pro-Tyr(Phe)-Cys- followed by Arg-Lys- or Lys alone is critical for both the thiol-disulfide exchange reaction and the dehydroascorbate reductase activity. The dithiol-disulfide loop represented by this structure is unique since the cystine closer to the N-terminus has a highly acidic thiol pKa (3.8 as determined for the pig liver enzyme) that contributes to the protein's high S- nucleophilicity. Compared with the microbial enzyme, the mammalian thioltransferases (glutaredoxins) are extended at both N and C termini by 10-12 amino acid residues, including a second pair of cysteines toward the C-terminus with no known special function. Yeast thioltransferase is more like mammalian enzymes in length (106 amino acids) but more like E. coli glutaredoxin in being unblocked at the N-terminus and having only one set of cysteines; that is, at the active center. The three mammalian enzymes, for which sequences are available, are blocked at the N-terminus by an acetyl group linked to alanine with no known special function other than possibly to impart greater cellular turnover stability. A report of carbohydrate (8.6%) content in rat liver thioltransferase has not been verified by more sensitive methods of carbohydrate analysis, nor has carbohydrate been identified in samples of purified glutaredoxin from any source. Thiol transferase and glutaredoxin are two names for the same protein based on similarity of amino acid sequence, immunochemical cross-reactivity, and other enzyme properties. The inability of thioltransferase from some mammalian sources to act as an electron carrier in ribonucleotide reductase systems, whether homologous or heterologous in origin, remains to be explained in future studies.

Molecular interaction between nitric oxide and ryanodine receptors of skeletal and cardiac sarcoplasmic reticulum

In striated muscle, the sarcoplasmic reticulum (SR) is the major storage compartment of intracellular Ca2+ that controls cytosolic free Ca2+ (Cai) and developed force by sequestering and releasing Ca2+ during each contraction. Ca2+ release from the SR occurs through high-conductance Ca2+ release channels or ryanodine receptors (RyR), which are regulated by various signaling processes. Over the last 15 years, there has been a growing consensus that critical sulfhydryl sites on RyRs can be oxidized and reduced, respectively, to open and close the release channels. The pharmacological actions of various classes of sulfhydryl reagents have demonstrated the existence of hyperreactive thiols on RyRs, which could play a role in the regulation of normal contractile function and explain contractile dysfunctions in pathological conditions. More recent studies show that redox regulation of release channels may occur by nitric oxide (NO), a physiological signaling mechanism. This article is intended to review current concepts in thiol regulation of RyRs and present new data on the possible identification of the primary cysteine residues, which may be the site of oxidation and S-nitrosylation involved in channel opening.

Reversible thiol-dependent activation of ryanodine-sensitive Ca2+ release channel by etoposide (VP-16) phenoxyl radical

Many phenolic compounds can act as antioxidants by donating a proton to peroxyl radicals and quenching lipid peroxidation. Phenoxyl radicals produced this way or from metabolism by peroxidases, tyrosinase, or mixed-function oxidases, however, may react with sulfhydryl groups of proteins and other endogenous thiols. In this regard, phenolic compounds may have cytotoxic potential instead of antioxidant effects. We employed the anticancer drug, etoposide (VP-16), as a model phenolic compound to study the sensitivity of ryanodine-sensitive Ca2+ channel (RyR) to VP-16 phenoxyl radicals. The combination of VP-16 and tyrosinase, used to generate the etoposide phenoxyl radical, produced marked Ca2+ release from Ca2+-loaded RyR-rich vesicles prepared from terminal cisternae fraction of sarcoplasmic reticulum (SR). This effect was reversed by the SH-reagent, dithiothreitol (DTT), suggesting that cysteines within the RyR-protein complex were targets for modification by VP-16 phenoxyl radicals. VP-16/tyrosinase-induced release of Ca2+ was attenuated in vesicles prepared from longitudinal SR, which contain relatively little RyR. The effects of the VP-16 phenoxyl radical on Ca2+-ATPase in SR vesicles resembled those observed with caffeine or 4,4'-dithiodipyridine, both of which activate RyR Ca2+ release and lead to activation of Ca2+-ATPase via prolonged Ca2+ cycling. The addition of ruthenium red returned Ca2+-ATPase to its original level. Thus, under these conditions Ca2+-ATPase was not directly affected by VP-16 phenoxyl radical. The hypersensitive SH-groups on RyR are shown to be targets for oxidation of VP-16 phenoxyl radical, and suggest that other phenolic compounds could similarly disrupt Ca2+ homeostasis.

Calcium release and subsequent development induced by modification of sulfhydryl groups in porcine oocytes

The mechanism of Ca2+ release induced by modification of sulfhydryl groups and the subsequent activation of porcine oocytes were investigated. Thimerosal, a sulfhydryl-oxidizing compound, induced Ca2+ oscillation in matured oocytes. In thimerosal-preincubated oocytes, the amount of Ca2+ released after microinjection of inositol 1,4,5-trisphosphate (InsP3) or ryanodine increased strikingly, indicating that thimerosal potentiated both InsP3- and ryanodine-sensitive Ca2+ release pathways. Thimerosal also enhanced the sensitivity of oocytes to microinjected Ca2+ so that in pretreated oocytes a Ca2+ injection triggered a larger transient. Heparin at concentrations that normally block the InsP3-induced Ca2+ release were without effect; higher doses significantly increased the time leading up to the first spike. The thimerosal-induced Ca2+ release could not be blocked by procaine, and it did not require the formation of InsP3 since preinjection with neomycin did not prevent the oscillation. Immunocytochemistry revealed that thimerosal treatment destroyed the meiotic spindle, preventing further development, an effect that could be reversed by dithiothreitol. The combined thimerosal/dithiothreitol treatment triggered second polar body extrusion in 50% of the oocytes, and as a result of this activation scheme approximately 15% of the in vitro- and approximately 60% of the in vivo-matured oocytes developed to blastocyst during a 7-day culture in vitro.

Toxoplasma gondii: dithiol-induced Ca2+ flux causes egress of parasites from the parasitophorous vacuole

Ca2+ is an essential activator of motility in the obligate intracellular parasite Toxoplasma gondii. Ca2+ ionophore A23187 and intracellular microinjection of Ca2+ initiate motility of parasites residing in parasitophorous vacuoles (PV). The source of Ca2+ and the mechanism by which it activates motility in vivo remain uncertain. Exposure of the parasites to dithiothreitol (DTT) can activate egress of previously nonmotile intravacuolar parasites within 60 sec. DTT is also known to activate both isoforms of the highly concentrated nucleoside triphosphate hydrolase (NTPase) produced by T. gondii. Using an adherent cell analysis system (ACAS) for Ca2+ imaging, a brief 15-50% increase in intra-PV fluorescence ratio was observed after exposure of infected fibroblasts to 5 mM DTT. Chelation of intracellular Ca2+ with BAPTA-AM and extracellular Ca2+ with EGTA blocked the DTT effect; however, this chelation did not prevent the activation of parasites nor the Ca2+ response to the Ca2+ ionophore ionomycin, suggesting that the Ca2+ that activates motility may reside near or within the parasite itself. This result demonstrates that an increase in Ca2+ within the vacuole precedes the onset of motility and the correlation of the DTT effect on motility and tachyzoite NTPase suggests that NTPase activation may be involved in the Ca2+ flux.

Cross-linking of the ryanodine receptor/Ca2+ release channel from skeletal muscle

The relationship between the tetrameric organization of the ryanodine receptor (RyR) and its activity in binding of ryanodine was approached through cross-linking studies using several bifunctional reagents, differing in their linear dimensions and flexibility, as well as in the reactivity of the active groups. Cross-linking with: 1,5-difluoro-2,4-dinitrobenzene (DFDNB); di(fluoro-3-nitrophenyl)sulfone (DFNPS), 1-ethyl-3-(3-dimethylamino)propyl)carbodiimide (EDC); dimethyl suberimidate (DMS); ethylene glycol bis(succinimidylsuccinate) (EGS); and glutaraldehyde resulted in the disappearance of the, 470 kDa, RyR monomer protein band with concomitant appearance of additional bands of molecular masses higher than the monomer. At the relatively low concentrations of the reagents and the conditions used, RyR is the only cross-linked protein of SR membranes. The 'new' protein bands cross-react with antibodies against the RyR and correspond to dimers and tetramers of the RyR subunits while trimers were not detectable. DFDNB and DFNPS produced also a 560 kDa protein band which probably represents an intramolecular cross-linked monomer. The SDS-electrophoretic patterns of the cross-linked purified RyR resemble those of the membrane-bound receptor. Ryanodine binding to the high-affinity site was inhibited by modification of SR membranes with DFDNB and DFNPS, but not with DMS, EDC, EGS and glutaraldehyde, although RyR was completely cross-linked. The inhibition by DFDNB and DFNPS is due to modification of a specific lysyl residue which is also involved in the control of Ca2+ release. On the other hand, cross linking of the RyR with glutaraldehyde or EGS resulted in inhibition of ryanodine binding to the low-affinity, but not to the high-affinity binding sites. Thus, the cross-linking of two or more sites in each monomer (which lead to fixation of dimers or tetramers) did not prevent the conformational changes involved in the binding and occlusion of ryanodine at the high-affinity site, but inhibited its binding to the low-affinity sites.

Oxygen free radicals and calcium homeostasis in the heart

Many experiments have been done to clarify the effects of oxygen free radicals on Ca2+ homeostasis in the hearts. A burst of oxygen free radicals occurs immediately after reperfusion, but we have to be reminded that the exact levels of oxygen free radicals in the hearts are yet unknown in both physiological and pathophysiological conditions. Therefore, we should give careful consideration to this point when we perform the experiments and analyze the results. It is, however, evident that Ca2+ overload occurs when the hearts are exposed to an excess amount of oxygen free radicals. Through ATP-independent Ca2+ binding is increased, Ca2+ influx through Ca2+ channel does not increase in the presence of oxygen free radicals. Another possible pathway through which Ca2+ can enter the myocytes is Na(+)-Ca2+ exchanger. Although, the activities of Na(+)-K+ ATPase and Na(+)-H(+) exchange are inhibited by oxygen free radicals, it is not known whether intracellular Na(+) level increases under oxidative stress or not. The question has to be solved for the understanding of the importance of Na(+)-Ca2+ exchange in Ca2+ influx process from extracellular space. Another question is 'which way does Na(+)-Ca2+ exchange work under oxidative stress? Net influx or efflux of Ca2+?' Membrane permeability for Ca2+ may be maintained in a relatively early phase of free radical injury. Since sarcolemmal Ca(2+)-pump ATPase activity is depressed by oxygen free radicals, Ca2+ extrusion from cytosol to extracellular space is considered to be reduced. It has also been shown that oxygen free radicals promote Ca2+ release from sarcoplasmic reticulum and inhibit Ca2+ sequestration to sarcoplasmic reticulum. Thus, these changes in Ca2+ handling systems could cause the Ca2+ overload due to oxygen free radicals.

Oxygen free radicals and calcium homeostasis in the heart

Many experiments have been done to clarify the effects of oxygen free radicals on Ca2+ homeostasis in the hearts. A burst of oxygen free radicals occurs immediately after reperfusion, but we have to be reminded that the exact levels of oxygen free radicals in the hearts are yet unknown in both physiological and pathophysiological conditions. Therefore, we should give careful consideration to this point when we perform the experiments and analayze the results. It is, however, evident that Ca2+ overload occurs when the hearts are exposed to an excess amount of oxygen free radicals. Though ATP-independent Ca2+ binding is increased, Ca2+ influx through Ca2+ channel does not increase in the presence of oxygen free radicals. Another possible pathway through which Ca2+ can enter the myocytes is Na(+)-Ca2+ exchanger. Although, the activities of Na(+)-K+ ATPase and Na(+)-Ca2+ exchanger. Although, the activities of Na(+)-H+ exchange are inhibited by oxygen free radicals, it is not known whether intracellular Na+ level increases under oxidative stress or not. The question has to be solved for the understanding of the importance of Na(+)-Ca2+ exchange in Ca2+ influx process from extracellular space. Another question is 'which way does Na(+)-Ca2+ exchange work under oxidative stress? Net influx or efflux of Ca2+?' Membrane permeability for Ca2+ may be maintained in a relatively early phase of free radical injury. Since sarcolemmal Ca(2+)-pump ATPase activity is depressed by oxygen free radicals, Ca2+ extrusion from cytosol to extracellular space is considered to be reduced. It has also been shown that oxygen free radicals promote Ca2+ release from sarcoplasmic reticulum and inhibit Ca2+ sequestration to sarcoplasmic reticulum. Thus, these changes in Ca2+ handling systems could cause the Ca2+ overload due to oxygen free radicals.

Sulfhydryl oxidation induces calcium release from fragmented sarcoplasmic reticulum even in the presence of glutathione

Alcian blue and plumbagin induced transient Ca2+ release from fragmented sarcoplasmic reticulum. Dithiothreitol (DTT) and glutathione (GSH) partially blocked Ca2+ release induced by these oxidizing compounds. Pretreatment of alcian blue and plumbagin with DTT or GSH for more than 1 min was required to abolish the ability of the oxidizing compounds to release Ca2+. Mg2+ and ruthenium red completely blocked alcian blue-and plumbagin-induced Ca2+ release. These results suggest that oxidation of sulfhydryls on Ca2+ release channels induces Ca2+ release even in the presence of GSH in situ.

Ryanodine-affinity chromatography purifies 106 kD Ca2+ release channels from skeletal and cardiac sarcoplasmic reticulum

A 106 kD protein was isolated from skeletal sarcoplasmic reticulum (SR) vesicles and shown to have the properties of SR Ca2+ release channels, including blockade by 5 nM ryanodine. In view of extensive reports that the ryanodine-receptor complex consists of four 565 kD junctional feet proteins (JFPs) and is the 'physiological' Ca2+ release channel, we prepared ryanodine-affinity columns to isolate its receptor site(s). Conditions known to maximize the association and dissociation of ryanodine to SR proteins were respectively used to link, then elute, the receptor(s) from ryanodine-affinity columns. The method purified a protein at about 100 kD from both rabbit skeletal and canine cardiac SR vesicles. The skeletal and cardiac proteins isolated by ryanodine-affinity chromatography were identified as the low molecular weight Ca2+ release channel through their antigenic reaction with an anti-106 kD monoclonal antibody. Upon reconstitution in planar bilayers, both skeletal and cardiac proteins revealed the presence of functional SR Ca2+ release channels. Surprisingly, ryanodine-affinity columns did not retain JFPs but purified 106 kD Ca2+ release channels which are a minor component (0.1-0.3%) of SR proteins.

Antibody to the inositol trisphosphate receptor blocks thimerosal-enhanced Ca(2+)-induced Ca2+ release and Ca2+ oscillations in hamster eggs

The sulfhydryl reagent thimerosal enhanced the sensitivity of hamster eggs to injected inositol 1,4,5-trisphosphate (InsP3) or Ca2+ to generate regenerative Ca2+ release from intracellular pools. A monoclonal antibody (mAb) to the InsP3 receptor blocked both the InsP3-induced Ca2+ release (IICR) and Ca(2+)-induced Ca2+ release (CICR). The mAb also blocked Ca2+ oscillations induced by thimerosal. The results indicate that thimerosal enhances IICR sensitized by cytosolic Ca2+, but not CICR from InsP3-insensitive pools, and causes repetitive Ca2+ releases from InsP3-sensitive pools.

Block of Ca2+ wave and Ca2+ oscillation by antibody to the inositol 1,4,5-trisphosphate receptor in fertilized hamster eggs

The concentration of cytoplasmic free calcium (Ca2+) increases in various stimulated cells in a wave (Ca2+ wave) and in periodic transients (Ca2+ oscillations). These phenomena are explained by inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ release (IICR) and Ca(2+)-induced Ca2+ release (CICR) from separate intracellular stores, but decisive evidence is lacking. A monoclonal antibody to the IP3 receptor inhibited both IICR and CICR upon injection of IP3 and Ca2+ into hamster eggs, respectively. The antibody completely blocked sperm-induced Ca2+ waves and Ca2+ oscillations. The results indicate that Ca2+ release in fertilized hamster eggs is mediated solely by the IP3 receptor, and Ca(2+)-sensitized IICR, but not CICR, generates Ca2+ waves and Ca2+ oscillations.

The Ca2+ release channel in junctional sarcoplasmic reticulum: gating and blockade by cations

1. By using a sarcoplasmic reticulum preparation containing feet structures and the 45Ca2+/filtration technique, the opening and closing response of the Ca(2+)-channel was studied. 2. Extravesicular Sr2+ can activate the channel even though this cation is less efficient than Ca2+ in stimulating the Ca2+ release. Higher Sr2+ concentrations display inhibitory action. 3. By studying the closing response high- and low-affinity cations can be distinguished, according to the concentration range required to exert their effect. 4. The synergistic behavior observed by combining high- and low-affinity blocking cations suggest that they interact through the same binding site. 5. The high-and low-affinity cations are noncompetitive blockers of the activating Ca2+ suggesting the existence of an inhibitory site which is different to the activating site.

Rose bengal activates the Ca2+ release channel from skeletal muscle sarcoplasmic reticulum

The photooxidizing xanthene dye rose bengal (10 nM to 1 microM) stimulates rapid Ca2+ release from skeletal muscle sarcoplasmic reticulum vesicles. Following fusion of sarcoplasmic reticulum (SR) vesicles to an artificial bilayer, reconstituted Ca2+ channel activity is stimulated by nanomolar concentrations of rose bengal in the presence of a broad-spectrum light source. Rose bengal does not appear to affect K+ channels present in the SR. Following reconstitution of the sulfhydryl-activated 106-kDa Ca2+ channel protein into a bilayer, rose bengal activates the isolated protein in a light-dependent manner. Ryanodine at a concentration of 10 nM is shown to lock the 106-kDa channel protein in a subconductance state which can be reversed by subsequent addition of 500 nM rose bengal. This apparent displacement of bound ryanodine by nanomolar concentrations of rose bengal is also directly observed upon measurement of [3H]ryanodine binding to JSR vesicles. These observations indicate that photooxidation of rose bengal causes a stimulation of the Ca2+ release protein from skeletal muscle sarcoplasmic reticulum by interacting with the ryanodine binding site. Furthermore, similar effects of rose bengal on isolated SR vesicles, on single channel measurements following fusion of SR vesicles, and following incorporation of the isolated 106-kDa protein strongly implicates the 106-kDa sulfhydryl-activated Ca2+ channel protein in the Ca2+ release process.

Photooxidation of skeletal muscle sarcoplasmic reticulum induces rapid calcium release

The photooxidizing xanthene dye rose bengal is shown to induce rapid Ca2+ release from skeletal muscle sarcoplasmic reticulum (SR) vesicles. In the presence of light, nanomolar concentrations of rose bengal increase the Ca2+ permeability of the SR and stimulate the production of singlet oxygen (1O2). In the absence of light, no 1O2 production is measured. Under these conditions, higher concentrations of rose bengal (micromolar) are required to stimulate Ca2+ release. Furthermore, removal of oxygen from the release medium results in marked inhibition of the light-dependent reaction rate. Rose bengal-induced Ca2+ release is relatively insensitive to Mg2+. At nanomolar concentrations, rose bengal inhibits [3H]ryanodine binding to its receptor. beta,gamma-Methyleneadenosine 5'-triphosphate, a nonhydrolyzable analog of ATP, inhibits rose bengal-induced Ca2+ release and prevents rose bengal inhibition of [3H]ryanodine binding. Ethoxyformic anhydride, a histidine modifying reagent, at millimolar concentrations induces Ca2+ release from SR vesicles in a manner similar to that of rose bengal. The molecular mechanism underlying rose bengal modification of the Ca2+ release system of the SR appears to involve a modification of a histidyl residue associated with the Ca2+ release protein from SR. The light-dependent reaction appears to be mediated by singlet oxygen.

Different localization of inositol 1,4,5-trisphosphate and ryanodine binding sites in rat liver

The distribution of inositol 1,4,5-trisphosphate and ryanodine binding sites between plasma membrane, microsomal, and mitochondrial fractions of rat liver were compared. IP3 bound mostly to the plasma membrane fraction (Kd = 6 nM; Bmax = 802 fmol/mg protein). Some IP3 binding sites were also present in the microsomal and mitochondrial fractions (Kd = 2.5 and 2.9 nM; Bmax = 35 and 23 fmol/mg protein respectively). The possibility that these binding sites are due to contamination of the fractions with plasma membrane cannot be excluded. Binding of IP3 to the plasma membrane was inhibited by heparin but not by either caffeine or tetracaine. High-affinity ryanodine binding sites were present mostly in the microsomal fraction (Kd = 13 nM; Bmax = 301 fmol/mg protein). Lower affinity binding sites were also found to be present in the mitochondrial and plasma membrane fractions. Binding of ryanodine to the microsomal fraction was inhibited by both caffeine and tetracaine but not by heparin. These data demonstrate that IP3 and ryanodine binding sites are present in different cellular compartments in the liver. These differences in the localization of the binding sites might be indicative of their functional differences.

Properties of immunoaffinity purified 106-kDa Ca2+ release channels from the skeletal sarcoplasmic reticulum

The sulfhydryl-gated 106-kDa Ca(2+)-release channel (SG-106) was purified by biotin-avidin chromatography from skeletal sarcoplasmic reticulum (SR) vesicles and used as an antigen to raise polyclonal antibodies. Western blots showed that the antisera crossreacted with the antigenic SG-106 and not with SR Ca2+, Mg(2+)-ATPase or with junctional foot proteins (JFPs) (Zaidi et al., 1989, J. Biol. Chem. 264(36), 21, 725-21, 736; 21, 737-21, 747). Polyclonal antibody-affinity columns were used to selectively purify SG-106-kDa proteins which, upon incorporation in planar bilayers, revealed the presence of a cationic channels with properties similar to "native" Ca(2+)-release channels obtained through the fusion of SR vesicles with planar bilayers. In agreement with measurements of Ca2+ release from SR vesicles, sulfhydryl oxidizing and reducing agents (i.e., 2,2'-dithiodipyridine and dithiothreitol) respectively increased and decreased the open-time probability of 106-kDa Ca(2+)-release channels. In contrast with reports on JFPs, ryanodine at 0.5-1 nM increased the open-time probability and at 2-10 nM locked 106-kDa Ca(2+)-release channels in a closed state rather than an open subconductance state. The SG-106 was activated by millimolar ATP, inhibited by millimolar Mg2+, and blocked by micromolar ruthenium red. Adriamycin (2-10 microM) caused a transient activation of SG-106 Ca(2+)-release channels, followed by closure in about 5 min, and intermittent activation to a subconductance state. Polyclonal antibodies used to purify the SG-106 also activated the channel when added to the cis side but not the trans side of the bilayer. Thus, SG-106 channels possess features that are similar to "native" SR Ca(2+)-release channels, are immunologically distinct from JFPs, and interact in seconds with nanomolar ryanodine in planar bilayers.

Stunning: a radical re-view

The recovery from trauma, whether ischemia or some other form of tissue injury, is never instantaneous; time is always required for repair and the return of normal metabolism and function. To what extent the delay in recovery of contractile activity (stunning) after a brief period of ischemia represents convalescence from ischemia-induced injury, as opposed to the expression of reperfusion-induced injury, is perhaps not as clear as the proponents of stunning would hope. Definitive evidence for a distinct reperfusion-induced pathology, which compromises the recovery of contractile function from the depressed state induced by ischemia, is elusive. If reperfusion-induced injury accounts for a significant proportion of stunning, then the molecular mechanisms responsible for initiating the event and those responsible for orchestrating the event at the level of the contractile protein are far from clear. Perturbations of calcium homeostasis are frequently cited as responsible for the depressed contractile state, however, some metabolic derangement must precede any pathologically induced ionic disturbance. In this connection, evidence indicates that free-radical-induced oxidant stress, during the early moments of reperfusion, may modify the activity of a number of thiol-regulated proteins that are directly, or indirectly, responsible for controlling the movement of calcium. Sarcolemmal sodium-calcium exchange and the calcium release channel of the sarcoplasmic reticulum may be activated, whereas the sarcolemmal calcium pump and sodium-potassium ATPase, together with the calcium pump of the sarcoplasmic reticulum, may be inhibited. Under the conditions prevailing during ischemia and reperfusion, this would be expected to promote an early intracellular calcium overload. It is difficult to reconcile such a change with the decreased inotropic state that characterizes stunning; however, it seems likely that the calcium overload is transient and that the stunned myocardium rapidly reestablishes normal levels of intracellular calcium. It is still difficult to explain adequately the reduced inotropic state; clearly, the mechanism of stunning is not quite as simple as its definition.

Purification of a K(+)-channel protein of sarcoplasmic reticulum by assaying the channel activity in the planar lipid bilayer system

A K(+)-channel protein of the sarcoplasmic reticulum (SR) was purified by assaying the channel activity in a planar lipid bilayer system. The light fraction of SR vesicles was solubilized in 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS) and fractionated by an anion-exchange chromatography and followed by gel filtration chromatography and affinity chromatography with concanavalin A. All fractions in each steps were mixed with asolectin solubilized in CHAPS and reconstituted into vesicles by dialysis. The channel activity of each fraction was assayed after the reconstituted vesicles had been fused into a planar lipid bilayer. The final fraction which showed the K(+)-channel activity contained only 100 kDa protein in a silver-stained gel after SDS-PAGE and an anti-Ca(2+)-ATPase antibody did not recognize the protein. The characteristics of the K(+)-channel were identical to those observed in native SR vesicles when using the same method. The channel showed a single-channel conductance of 120 pS in 0.1 M KCl and marked voltage dependence. The channel did not permeate Ca2+ and Cl- and was blocked by neomycin B.

Localization and partial characterization of the oligomeric disulfide-linked molecular weight 95,000 protein (triadin) which binds the ryanodine and dihydropyridine receptors in skeletal muscle triadic vesicles

A monoclonal antibody, GE 4.90, has been produced following immunization of mice with the 95-kDa protein (triadin) of terminal cisternae of rabbit fast skeletal muscle isolated in nondenaturing detergent. The antibody binds to a protein of Mr95K in Western blots of microsomal vesicles electrophoresed in the presence of mercaptoethanol. The greatest intensity of the immunoblot reaction is to enriched terminal cisternae vesicles while little binding is seen to longitudinal reticulum and transverse tubules. The content of antigen in different microsomal subfractions has been estimated by immunoassay: terminal cisternae/triads contain 5.6 micrograms/mg of protein while heavy terminal cisternae contain 32 micrograms/mg. The molar content of triadin in vesicles is approximately the same as that of the ryanodine receptor. When Western blots of gels of terminal cisternae are run in nonreducing conditions, little protein of Mr95K is visible. A number of bands, however, forming a ladder of higher molecular weight are discerned, indicating that the 95-kDa protein forms a disulfide-linked homopolymer. A biotinylated aromatic disulfide reagent (biotin-HPDP) labels the 95-kDa protein, the junctional foot protein, and the Mr 106K protein described by others as a Ca(2+)-release channel (SG 106). This latter protein migrates in gel electrophoresis under nonreducing conditions at a molecular weight different from that of the 95-kDa protein. We did not detect any alteration of binding of the 95-kDa protein to the dihydropyridine receptor or junctional foot protein dependent on the state of oxidation of cysteine residues of either triadin or receptor protein used as the overlay probe.

Thimerosal causes calcium oscillations and sensitizes calcium-induced calcium release in unfertilized hamster eggs

Calcium-induced-calcium-release (CICR) was assayed in unfertilized golden hamster eggs by injecting Ca2+ and monitoring Ca2(+)-dependent hyperpolarizing responses (HRs) and Ca2(+)-sensitive fluo-3 fluorescence. Incubating eggs in the sulfhydryl reagent thimerosal caused [Ca2+]i oscillations as monitored by Ca2(+)-dependent HRs and decreased approximately 10-fold the Ca2+ injection current required to generate an HR and cause a large intracellular Ca2+ increase. Thimerosal also enhanced the sensitivity of eggs to Ca2+ injection in a calcium-free medium. The effects of thimerosal on CICR were prevented by dithiothreitol and were not mimicked by injecting inositol 1,4,5-trisphosphate. The data suggest that thimerosal may be an alternative agent for studying CICR in caffeine-insensitive cells.

Reactive disulfide compounds induce Ca2+ release from cardiac sarcoplasmic reticulum

Reactive disulfide compounds (RDSs) with a pyridyl ring adjacent to a disulfide bond, 2,2'dithiodipyridine (2,2' DTDP) and 4,4' dithiodipyridine (4,4' DTDP), induce Ca2+ release from isolated canine cardiac sarcoplasmic reticulum (SR) vesicles. RDSs are absolutely specific to free sulfhydryl (SH) groups and oxidize SH sites of low pKa via a thiol-disulfide exchange reaction, with the stoichiometric production of thiopyridone in the medium. As in skeletal SR, this reaction caused large increases in the Ca2+ permeability of cardiac SR and the number of SH sites oxidized by RDSs was kinetically and quantitatively measured through the absorption of thiopyridone. RDS-induced Ca2+ release from cardiac SR was characterized and compared to the action of RDSs on skeletal SR and to Ca2(+)-induced Ca2+ release. (i) RDS-induced Ca2+ release from cardiac SR was dependent on ionized Mg2+, with maximum rates of release occurring at 0.5 and 1 mM Mg2+free for 2,2' DTDP and 4,4' DTDP, respectively. (ii) In the presence of adenine nucleotides (0.1-1 mM), the oxidation of SH sites in cardiac SR by exogenously added RDS was inhibited, which, in turn, inhibited Ca2+ release induced by RDSs. (iii) Conversely, when the oxidation reaction between RDSs and cardiac SR was completed and Ca2+ release pathways were opened, subsequent additions of adenine nucleotides stimulated Ca2+ efflux induced by RDSs. (iv) Sulfhydryl reducing agents (e.g., dithiothreitol, DTT, 1-5 mM) inhibited RDS-induced Ca2+ efflux in a concentration-dependent manner. (v) RDSs elicited Ca2+ efflux from passively loaded cardiac SR vesicles (i.e., with nonfunctional Ca2+ pumps in the absence of Mg-ATP) and stimulated Ca2(+)-dependent ATPase activity, which indicated that RDS uncoupled Ca2+ uptake and did not act at the Ca2+, Mg2(+)-ATPase. These results indicate that RDSs selectively oxidize critical sulfhydryl site(s) on or adjacent to a Ca2+ release channel protein channel and thereby trigger Ca2+ release. Conversely, reduction of these sites reverses the effects of RDSs by closing Ca2+ release channels, which results in active Ca2+ reuptake by Ca2+, Mg2(+)-ATPase. These compounds can thus provide a method to covalently label and identify the protein involved in Ca2+ release from cardiac SR.

Isolation of a terminal cisterna protein which may link the dihydropyridine receptor to the junctional foot protein in skeletal muscle

The isolated dihydropyridine receptor and junctional foot protein were employed as protein ligands in overlay experiments to investigate the mode of interaction of these two proteins. As previously demonstrated by Brandt et al. [Brandt et al. (1990) J. Membr. Biol. 113, 237-251], the DHP receptor directly binds to an intrinsic terminal cisterna protein of Mr 95,000 (95-kDa protein). The junctional foot protein also binds to an Mr 95,000 protein showing similar organelle distribution to the 95-kDa protein which binds to the dihydropyridine receptor. The 95-kDa protein which binds to the dihydropyridine receptor was isolated to over 85% purity employing sequential column chromatography. Junctional foot protein and dihydropyridine receptor overlays of the column fractions at successive stages of isolation show an identical pattern of distribution, indicating that both probes bind to the same protein. When CHAPS-solubilized terminal cisterna/triads were passed through Sepharose with attached 95-kDa protein, the junctional foot protein was specifically retained, as evidenced by ryanodine binding. The junctional foot protein was incompletely released by 1 M NaCl. The alpha 1 subunit but not the beta subunit of the dihydropyridine receptor was also specifically retained, as evidenced by immunoblotting employing dihydropyridine receptor subunit-specific antibodies. A 170-kDa Stains-all blue staining protein, which appears to be bound to the luminal side of the terminal cisterna, was also retained on the 95-kDa protein column. From these findings, a model for the triad junction is proposed.

The heavy metal ions Ag+ and Hg2+ trigger calcium release from cardiac sarcoplasmic reticulum

Heavy metal ions have been shown to induce Ca2+ release from skeletal sarcoplasmic reticulum (SR) by binding to free sulfhydryl groups on a Ca2+ channel protein and are now examined in cardiac SR. Ag+ and Hg2+ (at 10-25 microM) induced Ca2+ release from isolated canine cardiac SR vesicles whereas Ni2+, Cd2+, and Cu2+ had no effect at up to 200 microM. Ag(+)-induced Ca2+ release was measured in the presence of modulators of SR Ca2+ release was compared to Ca2(+)-induced Ca2+ release and was found to have the following characteristics. (i) Ag(+)-induced Ca2+ release was dependent on free [Mg2+], such that rates of efflux from actively loaded SR vesicles increased by 40% in 0.2 to 1.0 mM Mg2+ and decreased by 50% from 1.0 to 10.0 mM Mg2+. (ii) Ruthenium red (2-20 microM) and tetracaine (0.2-1.0 mM), known inhibitors of SR Ca2+ release, inhibited Ag(+)-induced Ca2+ release. (iii) Adenine nucleotides such as cAMP (0.25-2.0 mM) enhanced Ca2(+)-induced Ca2+ release, and stimulated Ag(+)-induced Ca2+ release. (iv) Low Ag+ to SR protein ratios (5-50 nmol Ag+/mg protein) stimulated Ca2(+)-dependent ATPase activity in Triton X-100-uncoupled SR vesicles. (v) At higher ratios of Ag+ to SR proteins (50-250 nmol Ag+/mg protein), the rate of Ca2+ efflux declined and Ca2(+)-dependent ATPase activity decreased gradually, up to a maximum of 50% inhibition. (vi) Ag+ stimulated Ca2+ efflux from passively loaded SR vesicles (i.e., in the absence of ATP and functional Ca2+ pumps), indicating a site of action distinct from the SR Ca2+ pump. Thus, at low Ag+ to SR protein ratios, Ag+ is very selective for the Ca2+ release channel. At higher ratios, this selectivity declines as Ag+ also inhibits the activity of Ca2+,Mg2(+)-ATPase pumps. Ag+ most likely binds to one or more sulfhydryl sites "on" or "adjacent" to the physiological Ca2+ release channel in cardiac SR to induce Ca2+ release.