A non-specific Ca2+ (or Mg2+)-stimulated ATPase in rat heart sarcoplasmic reticulum

ATPase activity in rat heart sarcoplasmic reticulum was stimulated in a concentration-dependent manner by both Ca²⁺ and Mg²⁺ in the complete absence of the other cation. Increasing concentrations of Mg²⁺ produced an apparent inhibition of the Ca²⁺-dependent ATP hydrolysis. CDTA (trans-1,2-diaminocyclohexane-N,N,N,N-tetraacetate) had no effect on these responses. The results indicate the presence of a low affinity non-specific divalent cation-stimulated ATPase in rat heart sarcoplasmic reticulum. However, sarcoplasmic reticulum vesicles transported Ca²⁺ with a high affinity (K_0.5 Ca²⁺ = 0.41 M) suggesting the presence of a high affinity Ca²⁺-transporting ATPase. Calmodulin did not stimulate rat heart sarcoplasmic reticulum ATPase activity over a range of Ca²⁺ and Mg²⁺ concentrations and failed to stimulate membrane phosphorylation and Ca²⁺ transport into sarcoplasmic reticulum vesicles. Calmodulin antagonists trifluoperazine and compound 48180 did not affect the ATPase activity. Catalytic subunit of cAMP-dependent protein kinase was also ineffective in stimulating the ATPase activity. These results suggest the presence of an ATPase activity in rat heart sarcoplasmic reticulum with different properties from the high affinity Ca²⁺-pumping ATPase previously characterized in dog heart and other species.Abbreviations cAMP adenosine 3,5-monophosphate - CaM calmodulin - CDTA trans-1,2-diaminocyclohexane-N,N,N,N-tetraacetate - EDTA ethylene-diaminetetraacetate - EGTA ethylene glycol bis(-aminoethyl ether)-N,N,N,N-tetraacetate - PLB phospholamban - SR sarcoplasmic reticulum - TFP trifluoperazine     

Ca2+ binding and translocation by the sarcoplasmic reticulum ATPase: Functional and structural considerations

Three experimental systems are described including sarcoplasmic reticulum (SR) vesicles, reconstituted proteoliposomes, and recombinant protein obtained by gene transfer and expression in foreign cells. It is shown that the Ca²⁺ ATPase of sarcoplasmic reticulum (SR) includes an extramembranous globular head which is connected through a stalk to a membrane bound region. Cooperative binding of two calcium ions occurs sequentially, within a channel formed by four clustered helices within the membrane bound region. Destabilization of the helical cluster is produced following enzyme phosphorylation by ATP at the catalytic site in the extramembranous region. The affinity and orientation of the Ca²⁺ binding site are thereby changed, permitting vectorial dissociation of bound Ca²⁺ against a concentration gradient. A long range linkage between phosphorylation and Ca²⁺ binding sites is provided by an intervening peptide segment that retains high homology in cation transport ATPases, and whose function is highly sensitive to mutational perturbations.     

Trypsin-induced calcium efflux from sarcoplasmic reticulum: Evidence for the involvement of the (Ca2++Mg2+)-ATPase

Summary Trypsin digestion of the sarcoplasmic reticulum membrane at 35 to 43°C leads to an increased calcium permeability, the temperature dependence of which suggests tryptic exposure or creation of a channel rather than tryptic release of a mobile carrier (K.C. Toogood et al.,Membr. Biochem. 5:49–75, 1983). Here we show that: (1) the digested vesicles both pump and leak calcium, demonstrating that the vesicles remain intact; (2) an increased rate of efflux is not observed for membranes digested and kept at 15°C, but a temperature shift to 35°C following arrested digestion leads to the development of increased calcium permeability, indicating that a digestion step at the lower temperature potentiates increased permeability which develops rapidly as a result of a trypsin-facilitated protein conformational change at the higher temperature; (3) two inhibitors of the ATPase, adenyl-5-yl imidodiphosphate and dicyclohexyl-carbodiimide, both measurably retard the development of increased permeability at the higher temperature following arrested digestion, suggesting that these inhibitors bind to the target protein and prevent the conformational change responsible for the permeability increase, and further suggesting that the ATPase is the target for the trypsin; (4) digestion of the ATPase at 15°C follows the same initial cleavage pattern as at 35°C, but the cleavage stops or drastically slows down after the second digestion step at the lower temperature, whereas the digestion continues beyond the second step at the higher temperature, showing that an early digestion step may be responsible for potentiating increased permeability; (5) the permeability increase following digestion at 15°C and incubation at 35°C correlates (r>0.98) with the second tryptic cleavage step of the calcium ATPase, providing more support for the ATPase as the trypsin-sensitive efflux site; and (6) the rate of efflux depends on the concentration of the doubly cleaved ATPase molecules to the first power; the null hypothesis that the efflux actually depends on the cleaved ATPase concentration to the second or higher power was examined using the F test and can be rejected (confidence>0.90 to 0.98), suggesting that the efflux pathway is through a single ATPase molecule. We speculate that the pathway for increased calcium permeability is the one employed during calcium uptake and that there is a functional separation of the ATPase and calcium channel activities by trypsin digestion at 15°C followed by incubation at 35°C.     

Regulation of cardiac sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase

Summary The two high affinity calcium binding sites of the cardiac (Ca²⁺ + Mg²⁺)-ATPase have been identified with the use of Eu³⁺. Eu³⁺ competes for the two high affinity calcium sites on the enzyme. With the use of laser-pulsed fluorescent spectroscopy, the environment of the two sites appear to be heterogeneous and contain different numbers of H₂O molecules coordinated to the ion. The ion appears to be occluded even further in the presence of ATP. Using non-radiative energy transfer studies, we were able to estimate the distance between the two Ca²⁺ sites to be between 9.4 to 10.2 A in the presence of ATP. Finally, from the assumption that the calcium site must contain four carboxylic side chains to provide the 6–8 ligands needed to coordinate calcium, and based on our recently published data, we predict the peptidic backbone of the two sites.     

Heterologous expression of sarcoplasmic reticulum Ca2+-ATPase

In recent years, expression of rabbit sarcoplasmic reticulum (SR) Ca²⁺-ATPase in heterologous systems has been a widely used strategy to study altered enzymes generated by site-directed mutagenesis. Various eukaryotic expression systems have been tested, all of them yielding comparable amounts of recombinant protein. However, the relatively low yield of recombinant protein obtained so far suggests that novel purification techniques will be required to allow further characterization of this enzyme based on direct ligand-binding measurements.     

Estimation of inter-binding-site distances in sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase using Eu(III) luminescence energy transfer

We have used several trivalent lanthanides as probes for the high-affinity Ca(II)-binding site of the Ca(II) + Mg(II)-ATPase of skeletal muscle sarcoplasmic reticulum. The luminescent probes Eu(III) and Tb(III) were excited directly with pulsed laser light and the energy transfer efficiencies to several lanthanide acceptors were measured, under conditions in which most donor-acceptor pair occupied high-affinity Ca(II) sites. We obtain an inter-ionic site distance of about 0.8-0.9 nm. Energy transfer measurements were also done with Eu(III) in at least one Ca(II) site and bidentate Cr-ATP complex at the ATP hydrolytic site. Quenching of Eu(III) luminescence by Cr-ATP was total under these conditions. We calculate an upper limit of 1.0 nm for the distance from the Ca(II) site(s) to the complexed Cr(III) ion at the hydrolytic site.     

How do Ca2+ ions pass through the sarcoplasmic reticulum membrane

We propose an overview of the mechanism of Ca²⁺ transport through the sarcoplasmic reticulum membrane via the Ca²⁺-ATPase. We describe cytoplasmic calcium binding, calcium occlusion in the membrane and lumenal calcium dissociation. A channel-like structure is discussed and related to structural data on the membranous domain of the Ca²⁺-ATPase.Abbreviations SR Sarcoplasmic Reticulum - AMPPNP adenylyl-imidodiphosphate - AMPPCP adenylyl (,-methylene)-diphosphonate - FITC fluorescein 5-isothiocyanate - NBD 4-nitrobenzo-2-oxa-1,3-diazole - DCCD dicyclohexylcarbodiimide     

Uncoupling of Ca2+ transport from ATP hydrolysis activity of sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase

Summary In reconstituted rabbit skeletal muscle (Ca²⁺ + Mg²⁺)-ATPase proteoliposomes, Ca²⁺-uptake is decreased by more than 90% with T₂ cleavage (Arg-198). However, no difference in the ATP dependence of hydrolysis activity is seen between SR and trypsin-treated SR. A large decrease in E-P formation and hydrolysis activity of the enzyme appear only at T₃ cleavage, which represents the cleavage of A₁ fragment to A_1a + A_1b forms. The disappearance of hydrolysis activity due to digestion is prior to the disappearance of E-P formation. No significant difference is found in the passive Ca²⁺ efflux between control SR and tryptically digested SR in the absence of Mg⁺ ruthenium red or in the presence of ATP. However, the passive Ca²⁺ efflux rate for tryptically digested SR is much larger than control SR in the presence of Mg²⁺ + ruthenium red. These results show that the Ca²⁺ channel cannot be closed after trypsin digestion of SR membranes by the presence of the Ca²⁺ channel inhibitors, Mg²⁺ and ruthenium red. In the reconstituted ATPase proteoliposomes, the Ca²⁺ efflux rates are the same regardless of digestion (T₂); also, efflux is not affected by the presence or absence of Mg²⁺ + ruthenium red. These results indicate that T₂ cleavage causes uncoupling of the Ca²⁺-pump from ATP hydrolytic activity.A theoretical model is developed in order to fit the extent of tryptic digestion of the A fragment of the (Ca²⁺ + Mg²⁺)-ATPase polypeptide with the loss of Ca²⁺-transport. Fits of the theoretical equations to the data are consistent with that Ca²⁺-transport system appears to require a dimer of the polypeptide (Ca²⁺ + Mg²⁺)-ATPase.     

Vanadate inhibition of sarcoplasmic reticulum Ca2+-ATPase and other ATPases.

Vanadate is a potent inhibitor of the Ca²⁺-ATPase activity of sarcoplasmic reticulum in the presence of A-23187. The purified enzyme is sensitive to vanadate even in the absence of the ionophore. Ca²⁺ and norepinephrine protect the enzyme against inhibition of vanadate. The nonspecificity of vanadate is emphasized by the finding of inhibition of several other ATPases including the Ca²⁺Mg²⁺-ATPases of the ascites and human red cell plasma membranes, Mg²⁺-ATPase of the ascites plasma membrane, and the K⁺-ATPases of $\text{E.}\text{coli}$ and hog gastric mucosal cell membranes. The ascites plasma membrane Ca²⁺-ATPase (an ecto ATPase) and mitochondrial ATPase are not inhibited by vanadate.     

Calcium binding and allosteric signaling mechanisms for the sarcoplasmic reticulum Ca(2+) ATPase

The sarcoplasmic reticulum Ca(2+) ATPase (SERCA) is a membrane-bound pump that utilizes ATP to drive calcium ions from the myocyte cytosol against the higher calcium concentration in the sarcoplasmic reticulum. Conformational transitions associated with Ca(2+) -binding are important to its catalytic function. We have identified collective motions that partition SERCA crystallographic structures into multiple catalytically-distinct states using principal component analysis. Using Brownian dynamics simulations, we demonstrate the important contribution of surface-exposed, polar residues in the diffusional encounter of Ca(2+) . Molecular dynamics simulations indicate the role of Glu309 gating in binding Ca(2+) , as well as subsequent changes in the dynamics of SERCA's cytosolic domains. Together these data provide structural and dynamical insights into a multistep process involving Ca(2+) binding and catalytic transitions.     

Ligand-gated channel of the sarcoplasmic reticulum Ca2+ transport ATPase

In resting muscle, cytoplasmic Ca²⁺ concentration is maintained at a low level by active Ca²⁺ transport mediated by the Ca²⁺ ATPase from sarcoplasmic reticulum. The region of the protein that contains the catalytic site faces the cytoplasmic side of the membrane, while the transmembrane helices form a channel-like structure that allows Ca²⁺ translocation across the membrane. When the coupling between the catalytic and transport domains is lost, the ATPase mediates Ca²⁺ efflux as a Ca²⁺ channel. The Ca²⁺ efflux through the ATPase channel is activated by different hydrophobic drugs and is arrested by ligands and substrates of the ATPase at physiological pH. At acid pH, the inhibitory effect of cations is no longer observed. It is concluded that the Ca²⁺ efflux through the ATPase may be sufficiently fast to support physiological Ca²⁺ oscillations in skeletal muscle, that occur mainly in conditions of intracellular acidosis.     

质粒DNA增强大鼠缺血骨骼肌肌浆网钙转运功能

在大鼠肢体缺轿模型上观察质粒ｐｃＤＮＡ３对缺血骨骼肌肌浆网（ＳＲ）Ｃａ＾２＋转运的影响。结果显示,骨骼肌缺血时ＳＲＣａ＾２＋转运（Ｃａ＾２＋摄入与释放）较非缺血肌肉增强,而质粒ｐｃＤＮＡ３与ＳＲ上ＤＮＡ结合蛋白结合之后,可进一步增强缺备骨骼肌ＳＲＣａ＾２＋摄入（Ｐ＜０．０１）及释放速率（Ｐ＜０．０５）。提示质粒ＤＮＡ对正常及缺血大鼠骨骼肌的ＳＲＣａ＾２＋转运能力均有影响,其临床病理生理意义值得进一     

Fluorometric study on conformational changes in the catalytic cycle of sarcoplasmic reticulum Ca2+-ATPase

Changes in the fluoresence ofN-acetyl-N-(5-sulfo-1-naphthyl)ethylenediamine (EDANS), being attached to Cys-674 of sarcoplasmic reticulum Ca²⁺-ATPase without affecting the catalytic activity, as well as changes in the intrinsic tryptophan fluorescence were followed throughout the catalytic cycle by the steady-state measurements and the stopped-flow spectrofluorometry. EDANS-fluorescence changes reflect conformational changes near the ATP binding site in the cytoplasmic domain, while tryptophan-fluorescence changes most probably reflect conformational changes in or near the transmembrane domain in which the Ca²⁺ binding sites are located. Formation of the phosphoenzyme intermediates (EP) was also followed by the continuous flow-rapid quenching method. The kinetic analysis of EDANS-fluorescence changes andEP formation revealed that, when ATP is added to the calcium-activated enzyme, conformational changes in the ATP binding site occur in three successive reaction steps; conformational change in the calcium enzyme substrate complex, formation of ADP-sensitiveEP, and transition of ADP-sensitiveEP to ADP-insensitiveEP. In contrast, the ATP-induced tryptophan-fluorescence changes occur only in the latter two steps. Thus, we conclude that conformational changes in the ATP binding site in the cytoplasmic domain are transmitted to the Ca²⁺-binding sites in the transmembrane domain in these latter two steps.Abbreviations SR sarcoplasmic reticulum - EP phosphoenzyme - EDANS N-acetyl-N-(5-sulfo-1-naphthyl)ethylenediamine - AMP-PCP adenosine 5-(, -methylene)triphosphate - NEM N-ethylmaleimide     

Spectroscopic determination of sarcoplasmic reticulum Ca2+ uptake and Ca2+ release

In this report we describe the application of spectroscopic methods to the study of Ca2+ release by isolated native sarcoplasmic reticulum (SR) membranes from rabbit skeletal muscle. To date, dual-wavelength spectroscopy of arsenazo III and antipyrylazo III difference absorbance have been the most common spectroscopic methods for the assay of SR Ca2+ transport. The utility of these methods is the ability to manipulate intraluminal Ca2+ loading of SR vesicles. These methods have also been useful for studying the effect of both agonists and antagonists upon SR Ca2+ release and Ca2+ uptake. In this study, we have developed the application of Calcium Green-2, a long-wavelength excitable fluorescent indicator, for the study of SR Ca2+ uptake and release. With this method we demonstrate how ryanodine receptor Ca2+ channel opening and closing is regulated in a complex manner by the relative distribution of Ca2+ between extraluminal and intraluminal Ca2+ compartments. Intraluminal Ca2+ is shown to be a key regulator of Ca2+ channel opening. However, these methods also reveal that the intraluminal Ca2+ threshold for Ca2+-induced Ca2+ release varies as a function of extraluminal Ca2+ concentration. The ability to study how the relative distribution of a finite pool of Ca2+ across the SR membrane influences Ca2+ uptake and Ca2+ release may be useful for understanding how the ryanodine receptor is regulated, in vivo.     

白细胞介素-2对大鼠心肌Ca2+ATPase和Na+ /K+ATPase的影响

为了探讨IL－2对心肌细胞内钙影响的可能机制,用光学法检测心肌肌浆网Ca^2 ATPase的活性,以及细胞膜Ca^2 ATPase和Na^ /K^ ATPase的活性。结果：（1）用IL－2（10、40、200、800U/ml）灌流心脏后,其肌浆网Ca^2 ATPase的活性随IL－2浓度的升高而增强;（2）在ATP浓度为0.1-4mmol/L时,Ca^2 ATPase的活性随ATP浓度的升庙则增强,由IL－2（200U／ml）灌流后的心脏获得肌浆网（SR）,其Ca^2 ATPase的活性对ATP的反应强于对照组;（3）在[Ca^2 ]为1－40μmol/L时,心脏SR Ca^2 ATPase的活性随[Ca^2 ]增加而增强,而IL－2灌流心脏后分离的SR,其Ca^2 ATPase活性在[Ca^2 ]升高时没有明显改变;（4）用nor-BNI(10nmol/L）预处理5min后,IL－2（200U／ml）灌流后不再使SR Ca^2 ATPase的活性增强;（5）用PTX（5mg/L）预处理后,IL－2对SR Ca^2 ATPase的影响减弱;（6）用磷脂酶C（PLC）抑制剂U73122（5μmol/L）处理后,IL－2不再使SR Ca^2 ATPase活性增高;（7）用IL－2直接处理从正常大鼠分离的SR后,对SR Ca^2 ATPase活性无明显影响;（8）IL－2灌流后,对心肌细胞膜Ca^2 ATPase和Na^ /K^ ATPase活性没有显著。上述结果表明,IL－2灌流心脏后使心肌肌浆网Ca^2 ATPase的活性增加,心肌细胞膜上的κ－阿片受体及其下游的G蛋白和PLC介导了IL－2的作用。尽管IL－2提高SR Ca^2 ATPase对ATP的反应性,但却抑制SR Ca^2 ATPase对钙离子的敏感性。IL－2对心肌细胞膜Ca^2 ATPase和Na^ /K^ ATPase的活性无明显影响。     

Catalytic and structural modifications of sarcoplasmic reticulum and plasma membrane (Ca2+ + Mg2+)ATPases induced by organic solutes that accumulate in living systems

Organic solutes such as urea, methylamines, polyols and amino acid can accumulate in the cytoplasm of cells to compensate for hyperosmotic conditions in the external medium. Whereas urea is considered to be typical of solutes that destabilize structure and function of proteins, methylamines, polyols and some amino acids appear to have the opposite effect, and can also compensate for the perturbing effects of urea. These effects have been extensively analyzed for a variety of proteins in terms of global changes in enzyme structure and acceleration or inhibition of overall reaction rates. Here the influence of these solutes on sarcoplasmic reticulum and plasma membrane (Ca²⁺ + Mg²⁺)ATPases is reviewed. The focus is on the changes induced by perturbing and stabilizing solutes at specific steps of the catalytic cycles of these enzymes, which can run forward (leading to ATP hydrolysis) and backward (leading to ATP synthesis). Structural changes promoted by osmolytes are correlated with functional changes, especially those that are related to energy coupling.This review is dedicated to Prof. Carlos Chagas Filho, founder of the Institute of Biophysics, on the occasion of its 50th anniversary.     

Partial ischemia reduces the efficiency of sarcoplasmic reticulum Ca2+ transport in rat EDL

To investigate the hypothesis that prolonged partial ischemia would result in a depression in homogenate sarcoplasmic reticulum (SR) Ca²⁺-sequestering and mechanical properties in muscle, a cuff was placed around the hindlimb of 8 adult Sprague–Dawley rats (267 ± 5.8 g; × ± S.E.) and partially inflated (315 mm Hg) for 2 h. Following occlusion, the EDL was sampled both from the ischemic (I) and contralateral control (C) leg and SR properties compared with the EDL muscles extracted from rats (n = 8) immediately following anaesthetization (CC). Ischemia was indicated by a lower (p < 0.05) concentration (mmol.kg dry wt^–1) of ATP (19.0 ± 0.7 vs. 16.7 ± 0.7) and phosphocreatine (58.1 ± 5.7 vs. 35.0 ± 4.6) in I compared to C. Although Ca²⁺-ATPase activity (mol·g protein^–1.sec^–1 ), both maximal and submaximal, was not different between C and I (19.7 ± 0.4 vs. 18.5 ± 1.3), reductions (p < 0.05) in Ca²⁺-uptake (mmol·g protein^–1.sec^–1 ) of between 18.2 and 24.7% across a range of submaximal free Ca²⁺-levels were observed in I compared to C. Lower submaximal Ca²⁺-ATPase activity and Ca²⁺-uptake were also observed in the EDL in C compared to CC animals. Time dependent reductions (p < 0.05) were found in peak twitch and maximal tetanic tension in EDL from I but not C. It is concluded that partial ischemia, resulting in modest reductions in energy state in EDL, induces a reduction in Ca²⁺-uptake independent of changes in Ca²⁺-ATPase activity. These changes reduce the coupling ratio and the efficiency of Ca²⁺-transport by SR.     

Structural and functional degradation of Ca2+:Mg2+-ATPase rich sarcoplasmic reticulum vesicles photosensitized by erythrosin B

Erythrosin B (Red Dye No. 3) and Rose Bengal photosensitize the destruction of the Ca2+:Mg2+-ATPase pump protein in sarcoplasmic reticulum (SR) vesicles with respective quantum efficiencies of (1.53 +/- 0.19) X 10(-3) and (1.25 +/- 0.18) X 10(-3). Damage to vesicle function was assayed by measurements of increases in passive Ca2+ permeability. Rates of passive Ca2+ movement into the SR lumen were increased by dye photosensitization in proportion to radiation absorbed. Active Ca2+ transport into SR vesicles was blocked independent of radiation absorbed by Erythrosin B and Rose Bengal at free concentrations of 0.69 microM and 1.16 microM, respectively. The photochemical lability of the Ca2+ pump protein and alterations in passive and active Ca2+ transport may be dependent on the concentration of the dye in the membrane. The photosensitization results may have implications with respect to the suitability of Erythrosin B usage in vivo, since the brightness of our irradiation source is comparable to that of sunlight at 480 nm.     

Calcium binding proteins in the sarcoplasmic/endoplasmic reticulum of muscle and nonmuscle cells

In this paper we review some of the large quantities of information currently available concerning the identification, structure and function of Ca²⁺-binding proteins of endoplasmic and sarcoplasmic reticulum membranes. The review places particular emphasis on identification and discussion of Ca²⁺ storage proteins in these membranes. We believe that the evidence reviewed here supports the contention that the Ca²⁺-binding capacity of both calsequestrin and calreticulin favor their contribution as the major Ca²⁺-binding proteins of muscle and nonmuscle cells, respectively. Other Ca²⁺-binding proteins discovered in both endoplasmic reticulum and sarcoplasmic reticulum membranes probably contribute to the overall Ca²⁺ storage capacity of these membrane organelles, and they also play other important functional role such as posttranslational modification of newly synthesized proteins, a cytoskeletal (structural) function, or movement of Ca²⁺ within the lumen of the sarcoplasmic/endoplasmic reticulum towards the storage sites.Abbreviations SR Sarcoplasmic Reticulum - ER Endoplasmic Reticulum - InsP₃ Inositol 1,4,5-trisphosphate - SDS-PAGE Sodium Dodecyl Sulfate-Polyacrylamide Gel Electrophoresis - PDI Protein Disulphide Isomerase - T₃BP Thyroid Hormone Binding Protein - Grp Glucose regulated proteins - HCP Histidine-rich Ca²⁺ binding Protein - LDL Low Density Lipoprotein     

Sulfhydryl oxidation and Ca2+ release from sarcoplasmic reticulum

Summary Our interest in the role of sulfhydryl groups (SH) in regulating or altering transport across biological membranes has focused on the significance of a critical SH group associated with the Ca²⁺-release protein from skeletal muscle sarcoplasmic reticulum (SR). We have shown that binding of heavy metals to this group or oxidation of this sulfhydryl to a disulfide induces rapid Ca²⁺ release from SR vesicles [1, 2] and induces contraction in skinned muscle fibers [3]. Several models are described in which oxidation and reduction might control the state of the Ca²⁺-release channel from SR.Abbreviations DTT Dithiothreitol, redox. - oxidation-reduction - SDS Sodium Dodecyl Sulfate - SH Sulfhydryl - SR Sarcoplasmic Reticulum - T-tubule Transverse tubule