Polymorphism of sarcoplasmic-reticulum adenosine triphosphatase of rabbit skeletal muscle

Exciting perspectives for Translational Myology in the Abstracts of the 2018Spring PaduaMuscleDays: Giovanni Salviati Memorial - Chapter I - Foreword

Myologists working in Padua (Italy) were able to continue a half-century tradition of studies of skeletal muscles, that started with a research on fever, specifically if and how skeletal muscle contribute to it by burning bacterial toxin. Beside main publications in high-impact-factor journals by Padua myologists, I hope to convince readers (and myself) of the relevance of the editing Basic and Applied Myology (BAM), retitled from 2010 European Journal of Translational Myology (EJTM), of the institution of the Interdepartmental Research Center of Myology of the University of Padova (CIR-Myo), and of a long series of International Conferences organized in Euganei Hills and Padova, that is, the PaduaMuscleDays. The 2018Spring PaduaMuscleDays (2018SpPMD), were held in Euganei Hills and Padua (Italy), in March 14-17, and were dedicated to Giovanni Salviati. The main event of the "Giovanni Salviati Memorial", was held in the Aula Guariento, Accademia Galileiana di Scienze, Lettere ed Arti of Padua to honor a beloved friend and excellent scientist 20 years after his premature passing. Using the words of Prof. Nicola Rizzuto, we all share his believe that Giovanni "will be remembered not only for his talent and originality as a biochemist, but also for his unassuming and humanistic personality, a rare quality in highly successful people like Giovanni. The best way to remember such a person is to gather pupils and colleagues, who shared with him the same scientific interests and ask them to discuss recent advances in their own fields, just as Giovanni have liked to do". Since Giovanni's friends sent many abstracts still influenced by their previous collaboration with him, all the Sessions of the 2018SpPMD reflect both to the research aims of Giovanni Salviati and the traditional topics of the PaduaMuscleDays, that is, basics and applications of physical, molecular and cellular strategies to maintain or recover functions of skeletal muscles. The translational researches summarized in the 2018SpPMD Abstracts are at the appropriate high level to attract approval of Ethical Committees, the interest of International Granting Agencies and approval for publication in top quality, international journals. This was true in the past, continues to be true in the present and will be true in the future. All 2018SpPMD Abstracts are indexed at the end of the Chapter IV.

Tubular aggregates: sarcoplasmic reticulum origin, calcium storage ability, and functional implications

Muscle biopsy specimens from three patients with an autosomal dominant myopathy and tubular aggregates in both type 1 and type 2 fibers were investigated for immunofluorescent staining with antibodies to sarcoplasmic reticulum (SR) Ca-pump protein and calsequestrin and for Ca2+ loading ability. The results show that type 1 and type 2 fibers are differentially reactive to anti-Ca-pump protein IgG and similarly reactive with affinity-purified antibody to calsequestrin, which is in agreement with earlier observations in rat skeletal muscle. Tubular aggregates, which are shown to be highly reactive with either kind of antibody, appear to be sites of calcium accumulation for oxalate-facilitated adenosine triphosphate (ATP) dependent Ca uptake by chemically skinned fibers and thereby increase markedly the Ca loading capacity of the affected fibers.

Glycogen- and PP1c-targeting Subunit GM Is Phosphorylated at Ser48 by Sarcoplasmic Reticulum-bound Ca2+-Calmodulin Protein Kinase in Rabbit Fast Twitch Skeletal Muscle

Multifunctional Ca(2+)-calmodulin-dependent protein kinase (CaMKII) is a Ser/Thr protein kinase uniformly distributed within the sarcoplasmic reticulum (SR) of skeletal muscle. In fast twitch muscle, no specific substrates of CaMKII have yet been identified in nonjunctional SR. Previous electron microscopy data showed that glycogen particles containing glycogen synthase (GS) associate with SR at the I band level. Furthermore, recent evidence implicates CaMKII in regulation of glucose and glycogen metabolism. Here, we demonstrate that the glycogen- and protein phosphatase 1-targeting subunit, also known as G(M), selectively localizes to the SR membranes of rabbit skeletal muscle and that G(M) and GS co-localize at the level of the I band. We further show that G(M), GS, and PP1c assemble in a structural complex that selectively localizes to nonjunctional SR and that G(M) is phosphorylated by SR-bound CaMKII and dephosphorylated by PP1c. On the other hand, no evidence for a structural interaction between G(M) and CaMKII was obtained. Using His-tagged G(M) recombinant fragments and site-directed mutagenesis, we demonstrate that the target of CaMKII is Ser(48). Taken together, these data suggest that SR-bound CaMKII participates in the regulation of GS activity through changes in the phosphorylation state of G(M). Based on these findings, we propose that SR-bound CaMKII participates in the regulation of glycogen metabolism, under physiological conditions involving repetitive raises elevations of [Ca(2+)](i).

A method for measuring sarcoplasmic reticulum calcium uptake in the skeletal muscle using Fura-2

We have presented an assay for measuring the rate of sarcoplasmic reticulum (SR) Ca2+ uptake and Ca2+ release in skeletal muscle homogenates using the fluorescent Ca2+ probe Fura-2. Using this assay, we investigated the effects of an elevated temperature (40 degrees C) and lowered pH (6.8), two factors proposed to be involved in skeletal muscle fatigue, on SR Ca2+ uptake. The EDL muscle was found to have a higher rate of Ca2+ uptake than the soleus (34%). Exposure of the muscles to a raised temperature, but not a reduced pH, resulted in a reduction in the rate of Ca2+ uptake in both the EDL and soleus homogenates. This uptake process was blocked by cyclopiazonic acid (CPA) a specific inhibitor of the major transport protein of the sarcoplasmic reticulum, the Ca(2+)-ATPase. Calcium release was induced using AgNO3 after loading of the vesicles during the uptake process. It was found that AgNO3 was only effective in producing Ca2+ release in the EDL muscles. The soleus muscles did not release Ca2+ under varying [Mg2+] or with Hg2+ substitution for Ag+, suggesting that fast- and slow-twitch muscle fibres require different conditions for maximum Ca2+ release, or that different isoforms of the Ca2+ release channels are present in the different fibres.

Age-related abnormalities in regulation of the ryanodine receptor in rat fast-twitch muscle

The tibialis anterior (TA) muscles of 6-month-old and 24-month-old male Wistar rats, after being characterized, at the fast motor unit level, for twitch properties, were dissected and processed by a procedure [Margreth A., Damiani E., Tobaldin G. Biochem Biophys Res Commun 1993; 197: 1303-1311] aimed at obtaining a representative total membrane fraction comprising 70-80% of the total muscle content of sarcoplasmic reticulum (SR) and transverse tubule (TT) membranes (about 20 mg protein/g). Skeletal muscle membranes were analyzed for protein composition, and the content and functional properties of specific components of the free and junctional subcompartments of the SR and of junctional TT. Our results, while confirming a twitch prolongation in TA of old rats, do not demonstrate any associated age-related change concerning: (a) the overall number and functional properties of Ca2+ pumps, as characterized by kinetic parameters, Ca(2+)-dependency, and the protein isoform specificity of SR Ca(2+)-ATPase; (b) the number of functional junctional SR Ca(2+)-release channels, on the basis of Bmax values for high-affinity binding of [3H]-ryanodine to skeletal muscle membranes at optimal Ca2+; (c) the overall muscle dihydropyridine receptor/ryanodine receptor (RyR) ratio. We conclude from these findings, and the additional negative evidence for changes in membrane density of specific components of junctional SR, including 60 kDa Ca(2+)-calmodulin protein kinase, that this membrane domain, like the Ca(2+)-pump domain of the SR, are in no way basically altered at early stages of the aging process, as investigated here. Because of that, we allege particular significance to the occurrence of age-related, specific abnormalities in regulation of RyR in rat TA. The main supportive evidence is as follows: (a) an increased sensitivity to Ca2+ of the RyR of old muscle, and, more importantly; (b) an increased sensitivity to caffeine of [3H]ryanodine binding to the RyR at optimal Ca2+ and also optimal for the activity of the Ca(2+)-release channel. The results reported here also demonstrate that there are two classes of caffeine sites in rat TA muscle, as defined by differences in EC50 values at resting (pCa 7) and at high Ca2+ (pCa 4-5), that sites involved in stimulation of [3H]-ryanodine binding to the RyR are distinguished by a higher affinity (caffeine below mM), and that only these sites undergo age-related changes. Thus, although the underlying age-related abnormality of the RyR remains to be elucidated, it appears to satisfy the requirement for being regarded as a specific change, which in itself might argue for its being fundamentally related to the twitch prolongation of the muscle.

Skeletal muscle sarcoplasmic reticulum phenotype in myotonic dystrophy

In this study we investigated the sarcoplasmic reticulum (SR), alongside myofibrillar phenotype, in muscle samples from five Myotonic Dystrophy (DM) patients and five control individuals. DM muscles exhibited as a common feature, a decrease in the slow isoform of myosin heavy chain (MHC) and of troponin C in myofibrils. We observed a match between myofibrillar changes and changes in SR membrane markers specific to fiber type, i.e. the fast (SERCA1) Ca(2+)-ATPase isoform increased concomitantly with a decrease of protein phospholamban (PLB), which in native SR membranes colocalizes with the slow (SERCA2a) SR Ca(2+)-ATPase, and regulates its activity depending on phosphorylation by protein kinases. Our results outline a cellular process selectively affecting slow-twitch fibers, and non-degenerative in nature, since neither the total number of Ca(2+)-pumps or of ryanodine receptor/Ca(2+)-release channels, or their ratio to the dihydropyridine receptor/voltage sensor in junctional transverse tubules, were found to be significantly changed in DM muscle. The only documented, apparently specific molecular changes associated with this process in the SR of DM muscle, are the defective expression of the slow/cardiac isoform of Ca(2+)-binding protein calsequestrin, together with an increased phosphorylation activity of membrane-bound 60 kDa Ca(2+)-calmodulin (CaM) dependent protein kinase. Enhanced phosphorylation of PLB by membrane-bound Ca(2+)-CaM protein kinase also appeared to be most pronounced in biopsy from a patient with a very high CTG expansion, as was the overall 'slow-to-fast' transformation of the same muscle biopsy. Animal studies showed that endogenous Ca(2+)-CaM protein kinase exerts a dual activatory role on SERCA2a SR Ca(2+)-ATPase, i.e. either by direct phosphorylation of the Ca(2+)-ATPase protein, or mediated by phosphorylation of PLB. Our results seem to be consistent with a maturational-related abnormality and/or with altered modulatory mechanisms of SR Ca(2+)-transport in DM slow-twitch muscle fibers.

Coexistence of two calsequestrin isoforms in rabbit slow-twitch skeletal muscle fibers

The cardiac and skeletal muscle isoforms of calsequestrin (CS), the low affinity, high capacity Ca2+ binding protein localized in the lumen of sarcoplasmic reticulum, are the products of two different genes (Fliegel, L., Leberer, E., Green, N.M. and MacLennan, D.H. (1982) FEBS Lett. 242, 297-300), and can be both purified from slow-twitch skeletal muscle of the rabbit (Damiani, E., Volpe, P. and Margreth, A. (1990) J. Muscle Res. Cell Motil. 11, 522-530). Here we show that both CS isoforms coexist in slow-twitch muscle fibers as indicated by indirect immunofluorescent staining of cryosections with affinity-purified antibodies specific for each CS isoform.

Calcium pump isoforms: diversity, selectivity and plasticity. Review article

Ca2+ pumps are essential for removing cytosolic Ca2+ either across the plasma membrane (PM) or into internal organelles such as the sarcoplasmic reticulum (SR). Four genes (PMCA1, PMCA2, PMCA3 and PMCA4) have been reported to encode the PM Ca2+ pumps and three (SERCA1, SERCA2 and SERCA3) to encode the SR Ca2+ pumps. The PM Ca2+ pumps are stimulated by calmodulin, the SR Ca2+ pumps encoded by SERCA1 and SERCA2 are stimulated by phospholamban while the product of SERCA3 may be regulated directly by cAMP-dependent protein kinase. Alternative splicing of the primary transcripts of several of these genes has been reported to occur in a tissue selective manner and for others to alter during ontogeny. For the PM Ca2+ pump, alternative RNA splicing may result in isoforms with altered cyclic nucleotide dependent protein kinase sensitivity. The diversity in distribution of Ca2+ pump isoforms and their regulatory factors when coupled with different Ca2+ entry mechanisms allows for tissue selectivity and plasticity in stimulus-response coupling. The roles of various Ca2+ pump isoforms, the rationale behind their tissue selective expression and the plasticity in this expression are among the new challenges to researchers in this field.

Coexpression of two isoforms of calsequestrin in rabbit slow-twitch muscle

The cardiac and fast-twitch skeletal muscle forms of the Ca2(+)-binding protein calsequestrin (CS) are the products of two different genes, both of which are transcribed in slow-twitch skeletal muscle, though at much different rates (Scott et al., 1988., Fliegel et al., 1989). We have investigated this problem more closely at the protein level, on isolated terminal cisternae (TC) of the sarcoplasmic reticulum (SR) of rabbit slow-twitch muscle, and following purification of two distinct forms of CS from whole tissue by DEAE-Cellulose chromatography and CA2(+)-dependent elution from phenyl-Sepharose. Two electrophoretically (apparent molecular mass of 64 kDa and 54 kDa, respectively), and antigenically distinct forms of CS, here shown to be related to the fast-twitch skeletal muscle and to cardiac-type isoform of CS, respectively, colocalize to junctional TC of slow-twitch muscle. The cardiac-type isoform that is expressed in slow-twitch muscle accounts for about 25% of total CS present in isolated TC, it binds Ca2+ as effectively as the major CS form, using a 45Ca-overlay technique, and it shares extensive similarities with dog cardiac CS, not only in size and antigenically, but also in pl, as well as in the DEAE-elution characteristics. No difference in behaviour with phenyl-Sepharose resin were observed between the two CS isoforms from slow-twitch muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural and functional aspects of calcium homeostasis in eukaryotic cells

The maintenance of a low cytosolic free-Ca2+ concentration, ([Ca2+]i) is a common feature of all eukaryotic cells. For this purpose a variety of mechanisms have developed during evolution to ensure the buffering of Ca2+ in the cytoplasm, its extrusion from the cell and/or its accumulation within organelles. Opening of plasma membrane channels or release of Ca2+ from intracellular pools leads to elevation of [Ca2+]i; as a result, Ca2+ binds to cytosolic proteins which translate the changes in [Ca2+]i into activation of a number of key cellular functions. The purpose of this review is to provide a comprehensive description of the structural and functional characteristics of the various components of [Ca2+]i homeostasis in eukaryotes.

Sarcolemmal carbonic anhydrase in red and white rabbit skeletal muscle

Sarcolemmal vesicles of white and red skeletal muscles of the rabbit were prepared by consecutive density gradient centrifugations in sucrose and dextran according to Seiler and Fleischer (1982, J. Biol. Chem. 257, 13,862-13,871). White and red muscle membrane fractions enriched in sarcolemma were characterized by high ouabain-sensitive Na+, K(+)-ATPase, by high Mg2(+)-ATPase activity, and by a high cholesterol content. Ca2(+)-ATPase activity, a marker enzyme for sarcoplasmic reticulum, was not detectable in the highly purified white and red muscle sarcolemmal fractions. White and red muscle sarcolemmal fractions exhibited no significant differences with regard to Na+, K(+)-ATPase, Mg2(+)-ATPase, and cholesterol. Specific activity of carbonic anhydrase in white muscle sarcolemmal fractions was 38 U.ml/mg and was 17.6 U.ml/mg in red muscle sarcolemma. Inhibition properties of sarcolemmal carbonic anhydrase were analyzed for acetazolamide, chlorzolamide, and cyanate. White muscle sarcolemmal carbonic anhydrase is characterized by inhibition constants, KI, toward acetazolamide of 4.6 X 10(-8) M, toward chlorzolamide of 0.75 X 10(-8) M, and toward cyanate of 1.3 X 10(-4) M. Red muscle sarcolemmal carbonic anhydrase is characterized by KI values toward acetazolamide of 8.1 X 10(-8) M, toward chlorzolamide of 6.3 X 10(-8) M, and toward cyanate of 0.81 X 10(-4) M. In contrast to the high specific carbonic anhydrase activities in sarcolemma, carbonic anhydrase activity in sarcoplasmic reticulum from white muscle varied between values of only 0.7 and 3.3 U.ml/mg. Carbonic anhydrase of red muscle sarcoplasmic reticulum ranged from 2.4 to 3.7 U.ml/mg.

The binding of monoclonal and polyclonal antibodies to the Ca2(+)-ATPase of sarcoplasmic reticulum: effects on interactions between ATPase molecules

We analyzed the interaction of 14 monoclonal and 5 polyclonal anti-ATPase antibodies with the Ca2(+)-ATPase of rabbit sarcoplasmic reticulum and correlated the location of their epitopes with their effects on ATPase-ATPase interactions and Ca2+ transport activity. All antibodies were found to bind with high affinity to the denatured Ca2(+)-ATPase, but the binding to the native enzyme showed significant differences, depending on the location of antigenic sites within the ATPase molecule. Of the seven monoclonal antibodies directed against epitopes on the B tryptic fragment of the Ca2(+)-ATPase, all except one (VIE8) reacted with the enzyme in native sarcoplasmic reticulum vesicles in both the E1 and E2V conformations. Therefore these regions of the Ca2(+)-ATPase molecule are freely accessible in the native enzyme. The monoclonal antibody VIE8 bound with high affinity to the Ca2(+)-ATPase only in the E1 conformation stabilized by 0.5 mM Ca2+ but not in the E2V conformation stabilized by 0.5 mM EGTA and 5 mM vanadate. Several antibodies that reacted with the B fragment interfered with the crystallization of Ca2(+)-ATPase in the presence of EGTA and vanadate and at least two of them destabilized preformed Ca2(+)-ATPase crystals, suggesting inhibition of interactions between ATPase molecules. Of five monoclonal antibodies with epitopes on the A1 tryptic fragment of the Ca2(+)-ATPase only one gave strong reaction with the native enzyme, and none interfered with ATPase-ATPase interactions as measured by the polarization of fluorescence of FITC-labeled Ca2(+)-ATPase. Therefore the regions of the molecule containing these epitopes are relatively inaccessible in the native structure. Partial tryptic cleavage of the Ca2(+)-ATPase into the A1, A2 and B fragments did not promote the reaction of anti-A1 antibodies with sarcoplasmic reticulum vesicles, but solubilization of the membrane with C12E8 rendered the antigenic site fully accessible to several of them, suggesting that their epitopes are located in areas of contacts between ATPase molecules. Two monoclonal anti-B antibodies that interfered with ATPase-ATPase interactions, produced close to 50% inhibition of the rate of ATP-dependent Ca2+ transport, with significant inhibition of ATPase; this may suggest a role for ATPase oligomers in the regulation of Ca2+ transport. The other antibodies that interact with the native Ca2(+)-ATPase produced no significant inhibition of ATPase activity even at saturating concentrations; therefore their antigenic sites do not undergo major movements during Ca2+ transport.

Function of the sarcoplasmic reticulum and expression of its Ca2(+)-ATPase gene in pressure overload-induced cardiac hypertrophy in the rat

The reduction in Ca2+ concentration during diastole and relaxation occurs differently in normal hearts and in hypertrophied hearts secondary to pressure overload. We have studied some possible molecular mechanisms underlying these differences by examining the function of the sarcoplasmic reticulum and the expression of the gene encoding its Ca2(+)-ATPase in rat hearts with mild and severe compensatory hypertrophy induced by abdominal aortic constriction. Twelve sham-operated rats and 31 operated rats were studied 1 month after surgery. Eighteen animals exhibited mild hypertrophy (left ventricular wt/body wt less than 2.6) and 13 animals severe hypertrophy (left ventricular wt/body wt greater than 2.6). During hypertrophy we observed a decline in the function of the sarcoplasmic reticulum as assessed by the oxalate-stimulated Ca2+ uptake of homogenates of the left ventricle. Values decreased from 12.1 +/- 1.2 nmol Ca2+/mg protein/min in sham-operated rats to 9.1 +/- 1.5 and 6.7 +/- 1.1 in rats with mild and severe hypertrophy, respectively (p less than 0.001 and p less than 0.001, respectively, vs. shams). This decrease was accompanied by a parallel reduction in the number of functionally active CA2(+)-ATPase molecules, as determined by the level of Ca2(+)-dependent phosphorylated intermediate: 58.8 +/- 7.4 and 48.1 +/- 13.5 pmol P/mg protein in mild and severe hypertrophy, respectively, compared with 69.7 +/- 8.2 in shams (p less than 0.05 and p less than 0.01, respectively, vs. shams). Using S1 nuclease mapping, we observed that the Ca2(+)-ATPase messenger RNA (mRNA) from sham-operated and hypertrophied hearts was identical. Finally, the relative level of expression of the Ca2(+)-ATPase gene was studied by dot blot analysis at both the mRNA and protein levels using complementary DNA clones and a monoclonal antibody specific to the sarcoplasmic reticulum Ca2(+)-ATPase. In mild hypertrophy, the concentrations of Ca2(+)-ATPase mRNA and protein in the left ventricle were unchanged when compared with shams (mRNA, 93.8 +/- 10.6% vs. sham, NS; protein, 105.5 +/- 14% vs. sham, NS). in severe hypertrophy, the concentration of Ca2(+)-ATPase mRNA decreased to 68.7 +/- 12.9% and that of protein to 80.1 +/- 15.5% (p less than 0.001 and p less than 0.05, respectively), whereas the total amount of mRNA and enzyme per left ventricle was either unchanged or slightly increased. The slow velocity of relaxation of severely hypertrophied heart can be at least partially explained by the absence of an increase in the expression of the Ca2(+)-ATPase gene and by the relative diminution in the density of the Ca2+ pumps.(ABSTRACT TRUNCATED AT 400 WORDS)

A monoclonal antibody against the slow isoform of skeletal muscle Ca2+-ATPase selectively stains a subpopulation of neurons in the central nervous system

A monoclonal antibody against the slow isoform of chicken skeletal muscle Ca2+-ATPase recognises, in nervous tissue, analogous membrane proteins which are most concentrated in the microsomal fraction. Histochemically, the immunoreactivity of the antibody is restricted to neurones where the staining is most intense in cell bodies and dendrites, weak in axons and absent from cell nuclei. The expression of the antigen varies greatly between different neuronal populations and is developmentally regulated. The antigen is also axonally transported.

Biochemical characteristics of free and junctional sarcoplasmic reticulum and of transverse tubules in human skeletal muscle

The microsomal fraction of normal human skeletal muscle was subfractionated by isopycnic sucrose-density centrifugation, using the procedure originally described by Saito et al. for rabbit fast muscle, and specific markers of the junctional face membrane of terminal cisternae (TC) (ryanodine receptor, high-molecular-weight feet proteins and membrane-associated calcium-binding protein calsequestrin), of the sarcoplasmic reticulum (SR) Ca-pump membrane (chicken antibody to rabbit Ca-ATPase), and of transverse tubules (TT) (dihydropiridine receptor, membrane cholesterol), respectively. The results show that isolated TC from human skeletal muscle share extensive morphological characteristics, protein composition, as well as Ca-release properties with rabbit TC, as tested with an inhibitor (Ruthenium red) and an activator (doxorubicin) of SR Ca-release. The Ca-pump membrane of human muscle SR, in distinction to rabbit fast muscle SR, showed a relatively low specific activity of the Ca-ATPase, as expected from the mixed fiber composition of human muscles, but shared the presence of minor protein components, such as a Con A binding protein of about 57 kDa and blue-staining peptides in the 170-120 kDa range of molecular weights. Human muscle TT, as isolated from the same sucrose gradient, demonstrated a high affinity (3H)-dihydropiridine binding activity in the range of previously reported values for purified TT from rabbit skeletal muscle.

The unraveling architecture of the junctional sarcoplasmic reticulum

The sarcoplasmic reticulum (SR) of skeletal muscle controls the contraction-relaxation cycle by raising and lowering the myoplasmic free-Ca2+ concentration. The coupling between excitation, i.e., depolarization of sarcolemma and transverse tubule (TT) and Ca2+ release from the terminal cisternae (TC) of SR takes place at the triad. The triad junction is formed by a specialized region of the TC, the junctional SR, and the TT. The molecular architecture and protein composition of the junctional SR are under active investigation. Since the junctional SR plays a central role in excitation-contraction coupling and Ca2+ release, some of its protein constituents are directly involved in these processes. The biochemical evidence supporting this contention is reviewed in this article.

Fibre typing using sarcoplasmic reticulum Ca2+-ATPase and myoglobin immunohistochemistry in rat gastrocnemius muscle

Skeletal muscle fibre types were identified by using immunohistochemical detection of sarcoplasmic reticulum Ca2+-ATPase and myoglobin content in rat gastrocnemius muscle. The strong Ca2+-ATPase-reactive fibres were identical with the fast-twitch population, while the fibres with weak reactivity represented the slow-twitch type. Strong myoglobin immunoreactivity reflected the fast oxidative glycolytic (FOG) and slow oxidative (SO) types. Slight to moderate myoglobin immunostaining was found in the fast glycolytic (FG) fibres. The staining intensity of the different fibre types differed as follows: for Ca2+-ATPase FG greater than FOG greater than SO, and for myoglobin FOG greater than SO greater than FG. The immunoreactivity of Ca2+-ATPase and myoglobin were well preserved after fixation of the muscles in Bouin's solution, or in formol/acetic acid fixative, and paraffin embedding. Detection of the primary antibodies was carried out by using the avidin-biotin-peroxidase complex, and the immunogold-silver-staining methods. The latter was found to be more sensitive and suitable for postembedding ultrastructural demonstration of the Ca2+-pump enzyme on Durcupan-embedded muscles. The method, using 5 nm immunogold conjugate with silver enhancement, offered the advantages of high sensitivity and excellent visualization of the reaction product. The postembedding detection of sarcoplasmic reticulum Ca2+-ATPase also proved to be useful in the retrospective identification of the main fibre classes in human muscle biopsies.

Smooth muscle expresses a cardiac/slow muscle isoform of the Ca2+-transport ATPase in its endoplasmic reticulum

Smooth muscle expresses in its endoplasmic reticulum an isoform of the Ca2+-transport ATPase that is very similar to or identical with that of the cardiac-muscle/slow-twitch skeletal-muscle form. However, this enzyme differs from that found in fast-twitch skeletal muscle. This conclusion is based on two independent sets of observations, namely immunological observations and phosphorylation experiments. Immunoblot experiments show that two different antibody preparations against the Ca2+-transport ATPase of cardiac-muscle sarcoplasmic reticulum also recognize the endoplasmic-reticulum/sarcoplasmic-reticulum enzyme of the smooth muscle and the slow-twitch skeletal muscle whereas they bind very weakly or not at all to the sarcoplasmic-reticulum Ca2+-transport ATPase of the fast-twitch skeletal muscle. Conversely antibodies directed against the fast-twitch skeletal-muscle isoform of the sarcoplasmic-reticulum Ca2+-transport ATPase do not bind to the cardiac-muscle, smooth-muscle or slow-twitch skeletal-muscle enzymes. The phosphorylated tryptic fragments A and A1 of the sarcoplasmic-reticulum Ca2+-transport ATPases have the same apparent Mr values in cardiac muscle, slow-twitch skeletal muscle and smooth muscle, whereas the corresponding fragments in fast-twitch skeletal muscle have lower apparent Mr values. This analytical procedure is a new and easy technique for discrimination between the isoforms of endoplasmic-reticulum/sarcoplasmic-reticulum Ca2+-transport ATPases.

Denervation-induced proliferative changes of triads in rabbit skeletal muscle

Protein compositional and functional differences exist between longitudinal and junctional sarcoplasmic reticulum (SR) in relation to Ca transport and to Ca release. In light of this knowledge, we have reinvestigated the effects of denervation on SR of rabbit gastrocnemius, a predominantly fast muscle. Electron microscopy of 2-weeks denervated muscle showed proliferation of transverse tubules (TT), forming junctional contacts with SR terminal cisternae (TC). At coincident periods, the yield of muscle microsomes was increased, and their fractionation by sucrose-density centrifugation demonstrated a relative increase of heavy vesicles. Thin-section electron microscopy of heavy SR from denervated muscle showed an increased number of vesicles containing calsequestrin (CS) as compared with control muscle. Electrophoretic analysis confirmed the relative decrease of Ca-ATPase protein and the striking increase of CS both in total microsomes and in heavy SR vesicles. Calcium loading and Ca-ATPase activity as well as the density of Ca-ATPase protein were decreased to a similar extent (20-30%) in denervated muscle microsomes. Stimulation of Ca-ATPase activity by Ca-ionophore A23187 showed that the vesicles were tightly sealed. When probed by competitive ELISA with antibody to SR Ca-ATPase from pure fast muscle, the Ca-ATPase of denervated microsomes was found to be highly cross reactive. Cleveland's peptide maps of the Ca-ATPase protein after partial digestion with S. aureus V8 protease also showed no significant change after denervation. Changes in cholesterol content and in the ratio of Mg-ATPase to Ca-ATPase activity of denervated muscle microsomes indicated a 4-fold increase of TT protein, i.e., from about 3% to not more than 12% of total protein, at 2 weeks after denervation. All these changes were totally reversed upon reinnervation of muscle fibers, and the consequent muscle recovery, as obtained by nerve crushing instead of nerve sectioning. From these results, we conclude that denervated adult fast muscle, similarly to immature fast muscle, contains more junctional SR. However, the molecular and catalytic properties of the Ca-ATPase are unaffected by denervation.

The rate and capacity of calcium uptake by sarcoplasmic reticulum in fast, slow, and cardiac muscle: effects of ryanodine and ruthenium red

The rate and capacity of oxalate-supported calcium uptake was measured in homogenates of rat fast, slow, and cardiac muscle. The contribution of the releasing fraction of the sarcoplasmic reticulum (SR) to the calcium uptake abilities was estimated using ruthenium red or ryanodine to block the release channel. A relatively small fraction (12-20%) of the calcium pumping activity was associated with the release channel in skeletal muscle compared to 50% or more in cardiac muscle. The total capacity of the SR in the muscle types was in the ratio 1:0.75:1.5 for cardiac, slow, and fast muscle, respectively, while the rates of uptake were in the ratio 1:3.8:14.4. The major difference in the muscle types appears to be the density of pumping activity in the SR rather than the volume of the SR. The difference in the density of pumping activity is due to intrinsic differences in the kinetics of the calcium pump units and in their surface density.

The Ca2+ ATPase content of slow and fast twitch fibers of guinea pig

The Ca ATPase content in the sarcoplasmic reticulum (SR) of fast and slow twitch skeletal fibers was estimated using two well-characterized muscles of the guinea pig: the white bundle of the vastus lateralis and the soleus. Ca ATPase surface density was determined by counting the projections of individual molecules revealed on the cytoplasmic surface of freeze-dried, rotary-shadowed microsomal vesicles isolated from the two muscles. The Ca ATPase densities were 32,000/micron 2 and 25,000/micron 2 for the vastus lateralis and soleus muscles, respectively. The percentage of membrane area occupied by Ca ATPase-free lipid patches was estimated using freeze-fractured, rotary-shadowed in situ SR. In soleus muscle the free SR of terminal cisternae and the longitudinal SR have 34.5 and 19.7% of their surface free of ATPase, respectively. In the white vastus less than 1% of the surface was not occupied by Ca ATPase. These values were combined with stereological data from the literature to give a ratio of total Ca ATPase content per unit fiber volume of 1:2, slow versus fast. This is considerably less than the approximately sixfold difference in the overall relaxation time and the half times to relaxation between the two fiber types. This suggests that other factor such as differences in enzyme kinetics or cytoplasmic Ca buffering proteins must also play a role in determining rate of relaxation.

Ca2+ release from sarcoplasmic reticulum of skinned fast- and slow-twitch muscle fibers

We have performed a comparative study of Ca2+ release from the sarcoplasmic reticulum (SR) of chemically skinned fibers from rabbit fast- and slow-twitch skeletal muscle. Ca2+ fluxes have been indirectly monitored by following either tension development or the inhibition of net Ca2+-loading rate by a light-scattering method. Several drugs (Ca2+-release modulators) have been used to either trigger or block Ca2+ release. Our results indicate that caffeine, doxorubicin, and ryanodine activate Ca2+ release, whereas ruthenium red blocks Ca2+ release from both fast- and slow-twitch skinned fibers. Caffeine has greater affinity for slow SR, whereas doxorubicin, ruthenium red, and ryanodine have greater affinity for fast SR. Our results indicate that Ca2+-release mechanisms in fast and slow SR are homologous but not identical and that differences in twitch-contraction time might be also related to the inherent properties of the Ca2+-release mechanism.

(Ca2+ + Mg2+)-dependent ATPase mRNA from smooth muscle sarcoplasmic reticulum differs from that in cardiac and fast skeletal muscles

We have investigated some characteristics of the sarcoplasmic reticulum (Ca2+ + Mg2+)-dependent ATPase (Ca2+-ATPase) mRNA from smooth muscle using specific cDNA probes isolated from a rat heart cDNA library. RNA blot analysis has shown that the Ca2+-ATPase mRNA expressed in smooth muscle is identical in size to the cardiac mRNA but differs from that of fast skeletal muscle. S1 nuclease mapping has moreover shown that the cardiac and smooth muscle isoforms possess different 3'-end sequences. These results indicate that a distinct sarcoplasmic reticulum Ca2+-ATPase mRNA is present in smooth muscle.

A monoclonal antibody to the Ca2+-ATPase of cardiac sarcoplasmic reticulum cross-reacts with slow type I but not with fast type II canine skeletal muscle fibers: an immunocytochemical and immunochemical study

Ca2+-ATPase of the sarcoplasmic reticulum was localized in cryostat sections from three different adult canine skeletal muscles (gracilis, extensor carpi radialis, and superficial digitalis flexor) by immunofluorescence labeling with monoclonal antibodies to the Ca2+-ATPase. Type I (slow) myofibers were strongly labeled for the Ca2+-ATPase with a monoclonal antibody (II D8) to the Ca2+-ATPase of canine cardiac sarcoplasmic reticulum; the type II (fast) myofibers were labeled at the level of the background with monoclonal antibody II D8. By contrast, type II (fast) myofibers were strongly labeled for Ca2+-ATPase of rabbit skeletal sarcoplasmic reticulum. The subcellular distribution of the immunolabeling in type I (slow) myofibers with monoclonal antibody II D8 corresponded to that of the sarcoplasmic reticulum as previously determined by electron microscopy. The structural similarity between the canine cardiac Ca2+-ATPase present in the sarcoplasmic reticulum of the canine slow skeletal muscle fibers was demonstrated by immunoblotting. Monoclonal antibody (II D8) to the cardiac Ca2+-ATPase binds to only one protein band present in the extract from either cardiac or type I (slow) skeletal muscle tissue. By contrast, monoclonal antibody (II H11) to the skeletal type II (fast) Ca2+-ATPase binds only one protein band in the extract from type II (fast) skeletal muscle tissue. These immunopositive proteins coelectrophoresed with the Ca2+-ATPase of the canine cardiac sarcoplasmic reticulum and showed an apparent Mr of 115,000. It is concluded that the Ca2+-ATPase of cardiac and type I (slow) skeletal sarcoplasmic reticulum have at least one epitope in common, which is not present on the Ca2+-ATPase of sarcoplasmic reticulum in type II (fast) skeletal myofibers. It is possible that this site is related to the assumed necessity of the Ca2+-ATPase of the sarcoplasmic reticulum in cardiac and type I (slow) skeletal myofibers to interact with phosphorylated phospholamban and thereby enhance the accumulation of Ca2+ in the lumen of the sarcoplasmic reticulum following beta-adrenergic stimulation.

Distribution of calcium ATPase in the sarcoplasmic reticulum of fast- and slow-twitch muscles determined with monoclonal antibodies

Four monoclonal antibodies against the calcium ATPase in sarcoplasmic reticulum (SR) of rabbit fast-twitch skeletal muscle were characterized using SDS-PAGE, Western blots and immunofluorescence. The ultrastructural distribution of the antigens was determined using post-embedding immunolabeling. The antibodies recognized the calcium ATPase in the SR but not in transverse (T-) tubule or plasma membranes. The antibody, D12, had the same binding affinity for the calcium ATPase from fast-twitch (rabbit sternomastoid) and slow-twitch (rabbit soleus) fibers and the affinity fell by 30% after fixation for electron microscopy in both types of muscle fiber. Ultrastructural studies revealed that the density of D12 antibody binding to the terminal cisternae membrane of extensor digitorum longus (edl) and sternomastoid fibers was on average seven times greater than in the slow-twitch soleus and semimembranosus fibers. Since the affinity of the ATPase for the antibody was the same in SR from fast- and slow-twitch muscles, the concentration of calcium ATPase in the terminal cisternae membrane of fast-twitch fibers was seven times greater than in slow-twitch fibers. This conclusion was supported by the fact that the concentration of calcium ATPase in light SR membranes was six times greater in SR from fast-twitch fibers than in SR from slow-twitch fibers. The results provide strong evidence that the different calcium accumulation rates in mammalian fast- and slow-twitch muscles are due to different concentrations of calcium ATPase molecules in the SR membrane.

Monoclonal antibodies to dog heart sarcoplasmic reticulum. Antibodies that inhibit Ca2+-pump systems of cardiac and skeletal muscles

Purified sarcoplasmic reticulum (SR) vesicles from dog heart were used as an antigen to produce monoclonal antibodies (mAbs) to the Ca2+-ATPase. Nine of twelve clones of hybridoma cells produce mAbs which cross-react with seven SR preparation isolated from cardiac and skeletal muscles of various species. Three mAbs of IgM type interact with the 45-kDa tryptic fragment of rabbit skeletal muscle Ca2+-ATPase and markedly inhibit Ca2+ uptake (by 95%) and ATPase activity (by 80%) and decrease (by 30-50%) the steady-state level of the Ca2+-ATPase phosphoenzyme. The ATPase activity could be completely blocked by one of these mAbs if the incubation medium was supplemented with 2 microM orthovanadate. On the other hand, when SR vesicles were treated with increasing concentrations of a nonionic detergent C12E8, the inhibiting effect of mAb 4B4 is diminished. It is concluded that the mAbs inhibit the Ca2+-ATPase only if the enzyme exists in an oligomeric form. The inhibition of the SR activities is due to an effect of the mAbs on the whole active center of the enzyme, rather than on a single partial reaction.

Common structural domains in the sarcoplasmic reticulum Ca-ATPase and the transverse tubule Mg-ATPase

Transverse tubule (TT) membranes isolated from chicken skeletal muscle possess a very active magnesium-stimulated ATPase (Mg-ATPase) activity. The Mg-ATPase has been tentatively identified as a 102-kD concanavalin A (Con A)-binding glycoprotein comprising 80% of the integral membrane protein (Okamoto, V.R., 1985, Arch. Biochem. Biophys., 237:43-54). To firmly identify the Mg-ATPase as the 102-kD TT component and to characterize the structural relationship between this protein and the closely related sarcoplasmic reticulum (SR) Ca-ATPase, polyclonal antibodies were raised against the purified SR Ca-ATPase and the TT 102-kD glycoprotein, and the immunological relationship between the two ATPases was studied by means of Western immunoblots and enzyme-linked immunosorbent assays (ELISA). Anti-chicken and anti-rabbit SR Ca-ATPase antibodies were not able to distinguish between the TT 102-kD glycoprotein and the SR Ca-ATPase. The SR Ca-ATPase and the putative 102-kD TT Mg-ATPase also possess common structural elements, as indicated by amino acid compositional and peptide mapping analyses. The two 102-kD proteins exhibit similar amino acid compositions, especially with regard to the population of charged amino acid residues. Furthermore, one-dimensional peptide maps of the two proteins, and immunoblots thereof, show striking similarities indicating that the two proteins share many common epitopes and peptide domains. Polyclonal antibodies raised against the purified TT 102-kD glycoprotein were localized by indirect immunofluorescence exclusively in the TT-rich I bands of the muscle cell. The antibodies substantially inhibit the Mg-ATPase activity of isolated TT vesicles, and Con A pretreatment could prevent antibody inhibition of TT Mg-ATPase activity. Further, the binding of antibodies to intact TT vesicles could be reduced by prior treatment with Con A. We conclude that the TT 102-kD glycoprotein is the TT Mg-ATPase and that a high degree of structural homology exists between this protein and the SR Ca-ATPase.

Reversible inhibition of sarcoplasmic reticulum Ca-ATPase by altered neuromuscular activity in rabbit fast-twitch muscle

A 50% decrease in both the initial rate and the total capacity of Ca2+ uptake by the sarcoplasmic reticulum (SR) occurred 2 days after the onset of chronic (10 Hz) nerve stimulation in rabbit fast-twitch muscle. Prolonged stimulation (up to 28 days) did not lead to further decreases. This reduction, which was detected in muscle homogenates using a Ca2+-sensitive electrode, was reversible after 6 days cessation of stimulation and was not accompanied by changes in the immunochemically (ELISA) determined tissue level or isozyme characteristics of the SR Ca2+-ATPase protein. However, as measured in isolated SR, it correlated with a reduced specific activity of the Ca2+-ATPase. Kinetic analyses demonstrated that affinities of the SR Ca2+-ATPase towards Ca2+ and ATP were unaltered. Positive cooperativity for Ca2+ binding (h = 1.5) was maintained. However, a 50% decrease in Ca2+-dependent phosphoprotein formation indicated the presence of inactive forms of Ca2+-ATPase in stimulated muscle. The reduced phosphorylation of the enzyme was accompanied by an approximately 50% lowered binding of fluorescein isothiocyanate, a competitor at the ATP-binding site. In view of the unaltered affinity for ATP, this finding suggests that active Ca2+-ATPase molecules coexist in stimulated muscle with inactive enzyme molecules, the latter displaying altered properties at the nucleotide-binding site.

Characteristics of skeletal muscle calsequestrin: comparison of mammalian, amphibian and avian muscles

Calsequestrin was identified in the isolated sarcoplasmic reticulum from skeletal muscle of three mammalian species (man, rat and rabbit) and from frog and chicken muscle, using electrophoretic and immunoblot techniques. It was further characterized in sarcoplasmic reticulum protein mixtures and at several stages of purification, following extraction with EDTA. We found extensive similarities in apparent molecular weight values, Stains All staining properties and in Cleveland's peptide maps, between mammalian calsequestrins, and no detectable difference within a species between fast and slow muscle. Human calsequestrin, with an apparent molecular weight of 60,000 when measured at alkaline pH and of 41,000 when measured at neutral pH, appears to be the smallest in size. Frog calsequestrin, although weakly cross-reactive with rabbit calsequestrin and having a relatively higher apparent molecular weight at alkaline pH (72,000), shares several significant properties with mammalian calsequestrins. It bound calcium with a high capacity (1300 nmol per mg protein), it contained about 32% acidic amino acid residues and focused at closely similar pI values. We observed the formation of a complex with Stains All absorbing maximally at 535 nm, rather than at 600 nm, and an even more marked shift in apparent molecular weight at neutral pH. We found distinct differences in the case of chicken calsequestrin, in addition to those previously reported. It is a highly acidic, calcium-precipitable protein, but its amino acid composition is contradistinguished by a higher ratio of glutamate to aspartate and its rate of electrophoretic mobility is minimally affected by changes in pH. It stained deep bluish with Stains All after gel electrophoresis and yielded a protein-dye complex in aqueous solution, absorbing maximally at 560 nm, and finally, it bound fluorescent Concanavalin A.

Immunochemical quantification of sarcoplasmic reticulum Ca-ATPase, of calsequestrin and of parvalbumin in rabbit skeletal muscles of defined fiber composition

Antibodies directed against purified Ca-ATPase from sarcoplasmic reticulum, calsequestrin and parvalbumin from rabbit fast-twitch muscle were raised in sheep. The specificity of the antibodies was shown by immunoblot analysis and by enzyme-linked immunoadsorbent assays (ELISAs). IgG against the sarcoplasmic reticulum Ca-ATPase inhibited the catalytic activities of Ca-ATPase from fast-twitch (psoas, tibialis anterior) and slow-twitch (soleus) muscles to the same degree. In non-equilibrium competitive ELISAs the anti(Ca-ATPase) IgG displayed a slightly higher affinity for the Ca-ATPase from fast-twitch muscle than for that from slow-twitch muscle. This suggests a fiber-type-specific polymorphism of the sarcoplasmic reticulum Ca-ATPase. Quantification of Ca-ATPase, calsequestrin and parvalbumin in various rabbit skeletal muscles of histochemically determined fiber composition was achieved by sandwich ELISA. Ca-ATPase was found to be 6-7 times higher in fast than in slow-twitch muscles. A slightly higher concentration was found in fast-twitch muscles with a higher percentage of IIb fibers when compared with fast-twitch muscles with a higher percentage of IIa fibers. Thus Ca-ATPase is distributed as follows, IIb greater than or equal to IIa much greater than I. Calsequestrin was uniformly distributed in fast-twitch muscles independently of their IIa/IIb fiber ratio and displayed 50% lower concentrations in slow than in fast-twitch muscles (IIb = IIa greater than I). Parvalbumin contents were 200-300-fold higher in fast than in slow-twitch muscles. Significantly lower parvalbumin concentrations were found in fast-twitch muscles with a higher percentage of IIa fibers than in fast-twitch muscles with a higher percentage of IIb fibers (IIb greater than IIa much greater than I).

Freeze-fractured sarcoplasmic reticulum in adult and embryonic fast and slow muscles

There is evidence to suggest that 8 nm calcium transport particles in the sarcoplasmic reticulum are involved in the regulation of twitch properties in adult muscles. We have studied ultrastructural characteristics of the sarcoplasmic reticulum in relation to previously defined physiological changes that take place in the normal course of development. The fast twitch posterior latissimus dorsi (PLD) and the slow tonic anterior latissimus dorsi (ALD) of the chicken were compared using the procedure of freeze-fracture. In the adult PLD, the sarcoplasmic reticulum was composed of longitudinal tubules which gave rise to fenestrated cisternae at the centre of the H band and to terminal cisternae that form triads regularly at each A-I junction. In most of the fibres (85%), 8 nm intramembrane particles were closely packed in the concave fracture face (P-face). In the ALD, a tubular network with an open circular pattern extended the entire length of the A band and usually throughout the I band as well. Dyads or triads, which were infrequent, were often oriented obliquely. The density of intramembrane particles was low in the majority of the fibres, but there was a significant minority population (30%) in which particle density was relatively high. At 10 days in ovo, when speed of contraction in both the ALD and PLD is slow, there was a circular configuration of sarcoplasmic reticulum components in both muscles, and particle density was low. Surprisingly, at 18 days in ovo, when the rate of tension development and relaxation have reached nearly adult values in the fast PLD, this muscle, like the ALD, continued to exhibit a circular arrangement of sarcoplasmic reticulum tubules. The density of P-face particles, although greater than at 10 days, was still low relative to the adult PLD. Estimated values for the 18-day PLD were similar to those calculated for the adult slow muscle. Our observations, along with those of other investigators, suggest that abundant intramembrane particles may be related to the fast twitch properties of the adult PLD. However, they indicate that neither the pattern of membranes typical of the adult fast muscle nor the high content of calcium transport particles is required for the differentiation of fast twitch characteristics.

Fast to slow transition induced by experimental myotonia in rat EDL muscle

Experimental myotonia was induced by feeding rats with 20,25-diazacholesterol for up to 8 months. Histochemical analysis of myotonic extensor digitorum longus (EDL) muscle showed a progressive decrease of type IIB fibres and a concomitant increase of type IIA and type I fibres. A transient hypertrophy of type IIA fibres was observed 6 months after beginning the treatment. Analysis of the pattern of myosin light chains of single fibres from EDL showed that myotonia caused a progressive decrease of fibres showing a pure fast myosin light chain pattern and an increase of fibres showing coexistence of fast and slow myosin light chains (intermediate fibres). Only a small percentage of intermediate fibres showed coexistence of fast and slow myosin heavy chains. Myotonic fibres presented an increased sensitivity to caffeine which approached that of normal soleus fibres. Furthermore, sarcoplasmic reticulum (SR) vesicles isolated from hind limb fast muscles of myotonic rats demonstrated a decrease of Ca2+-dependent ATPase and Ca2+-transport activities as well as a decrease of immunoreactivity with anti-rabbit SR fast Ca2+-ATPase antibody. These results suggest that the increased electrical activity brought about by 20,25-diazacholesterol-induced myotonia, caused a fast to slow transition in the phenotypic expression of myosin and sarcoplasmic reticulum proteins.

Distribution of sarcoplasmic reticulum Ca-ATPase and of calsequestrin in rabbit and rat skeletal muscle fibers

Muscle fibers in rabbit extensor digitorum longus (EDL), tibialis anterior (TA) and soleus, and rat soleus, were examined immunohistochemically for two proteins of the sarcoplasmic reticulum. Ca-ATPase and calsequestrin (CaS). Fibers were typed with the histochemical reaction for actomyosin ATPase. In the rabbit EDL and TA, type I fibers clearly reacted less for Ca-ATPase and CaS than type II fibers, but the difference was less with CaS than with Ca-ATPase. Although the differences were relatively small, IIB fibers consistently presented greater amounts of Ca-ATPase than IIA fibers. No type II subgroups could be recognized after incubation with anti-CaS. These findings confirm results from previous immunochemical measurements on whole muscles containing different proportions of IIA and IIB fibers (Leberer and Pette 1986). Type IIA and IIC in the rabbit and rat soleus reacted stronger for Ca-ATPase and for CaS than type I fibers. Small differences in Ca-ATPase, but not in CaS, were recognized within the type I fiber population. Therefore, type I fibers in the rabbit and rat soleus are not a homogeneous population.

Amino-acid sequence of a Ca2+ + Mg2+-dependent ATPase from rabbit muscle sarcoplasmic reticulum, deduced from its complementary DNA sequence

We have cloned and sequenced complementary DNA encoding a Ca2+-ATPase of rabbit muscle sarcoplasmic reticulum. We propose a model of the protein which has 3 cytoplasmic domains joined to a set of 10 transmembrane helices by a narrow, penta-helical stalk. In this model, ATP bound to one cytoplasmic domain would phosphorylate an aspartate in an adjoining cytoplasmic domain, inducing translocation of Ca2+ from binding sites on the stalk.

Effects of thyroid hormones on the biochemical specialization of human muscle fibers

The effects of thyrotoxicosis and of hypothyroidism on human muscle have been studied on single fiber preparations. In thyrotoxic muscle, the ratio between fibers showing the fast type of myofibrillar protein isoforms (fast fibers) and fibers showing the slow type (slow fibers) is increased, as is the percentage of fibers with incomplete segregation of fast and slow myosin (intermediate fibers). Furthermore, in fast fibers, the volume and, to a greater extent, the rate of Ca transport of sarcoplasmic reticulum (SR) are increased, without changes in the affinity for Ca2+ of the Ca-pump or in its sensitivity to the cyclic adenosine monophosphate (cAMP) dependent protein kinase system. These effects are completely reversed by the removal of thyroid hormones, as demonstrated by hypothyroid muscles. It is suggested that in human muscle cells thyroid hormones are critical for the expression of fast genes and for SR Ca transport.

Myofibrillar-protein isoforms and sarcoplasmic-reticulum Ca2+-transport activity of single human muscle fibres

In this study the polymorphism of myofibrillar proteins and the Ca2+-uptake activity of sarcoplasmic reticulum were analysed in single fibres from human skeletal muscles. Two populations of histochemically identified type-I fibres were found differing in the number of light-chain isoforms of the constituent myosin, whereas the pattern of light chains of fast myosin of type-IIA and type-IIB fibres was indistinguishable. Regulatory proteins, troponin and tropomyosin, and other myofibrillar proteins, such as M- and C-proteins, showed specific isoforms in type-I and type-II fibres. Furthermore, tropomyosin presented different stoichiometries of the alpha- and beta-subunits between the two types of fibres. Sarcoplasmic-reticulum volume, as indicated by the maximum capacity for calcium oxalate accumulation, was almost identical in type-I and type-II fibres, whereas the rate of Ca2+ transport was twice as high in type-II as compared with type-I fibres. It is concluded that, in normal human muscle fibres, there is a tight segregation of fast and slow isoforms of myofibrillar proteins that is very well co-ordinated with the relaxing activity of the sarcoplasmic reticulum. These findings may thus represent a molecular correlation with the differences of the twitch-contraction time between fast and slow human motor units. This tight segregation is partially lost in the muscle fibres of elderly individuals.

Restoration of fast muscle characteristics following cessation of chronic stimulation. The ultrastructure of slow-to-fast transformation

When fast-twitch skeletal muscles of the adult rabbit are subjected to continuous low-frequency activity by electrical stimulation of the corresponding motor nerves, the fibers undergo an ultrastructural transformation, so that after 6 weeks they have acquired an appearance typical of slow-twitch fibers. In the present study, stimulation was discontinued at this stage in order to follow the reverse transformation, in which the fibers recovered their original morphological characteristics under conditions of normal endogenous activity. Stereological techniques were used to assess the time course of this process over a period of 20 weeks in terms of fiber cross-sectional area, extent of T-system, thickness of the Z-band, and volume fraction of mitochondria in the fiber core. Fibers of transformed muscles were smaller than those of control muscles, but the differences were no longer evident after 9 weeks of recovery. After 2 weeks the T-system was still of limited extent, as is characteristic of slow-twitch fibers; it increased toward the amount typical of fast-twitch fibers between 2 and 4 weeks, and had reached its full extent by 12 weeks. The wide Z-bands characteristic of slow-twitch fibers were retained for 4 weeks, but the thickness had begun to decrease by 8 weeks and recovery was complete by 12 weeks. The mitochondrial volume did not increase during recovery, in contrast to the large increases which had been observed to take place between 2 and 6 weeks during the fast-to-slow transformation. Overall, the recovery of fast-twitch ultrastructural characteristics was complete, but followed a more extended time course, and involved less myofibrillar disruption at an intermediate stage, than the original fast-to-slow transformation.

Membrane crystals of Ca2+-ATPase in sarcoplasmic reticulum of fast and slow skeletal and cardiac muscles

Crystalline arrays of Ca2+ transport ATPase develop in sarcoplasmic reticulum membranes after treatment with Na3VO4 in a calcium-free medium [ Dux , L. and Martonosi , A. (1983) J. Biol. Chem. 258, 2599-2603]. The proportion of vesicles containing Ca2+-ATPase crystals in microsome preparations isolated from rat muscle of different fiber types (semimembranosus, levator ani, extensor digitorum longus, diaphragm, soleus, and heart) correlates well with the Ca2+-ATPase content and Ca2+-modulated ATPase activity. This implies that the concentration of Ca2+-ATPase in sarcoplasmic reticulum membranes of fast and slow skeletal or cardiac muscles differs only slightly, and the low Ca2+ transport activity of 'sarcoplasmic reticulum' preparations isolated from slow-twitch skeletal and cardiac muscles is due to the presence of large amount of non-sarcoplasmic-reticulum membrane elements. This is in accord with the relatively small differences in the density of 8.5-nm intramembranous particles seen by freeze-etch electron microscopy in sarcoplasmic reticulum of red and white muscles. The dimensions of the Ca2+-ATPase crystal lattice are similar in sarcoplasmic reticulum membranes of different fiber types; therefore if structural differences exist between 'isoenzymes' of Ca2+-ATPase, these are not reflected in the crystal-lattice.

Polymorphism of myosin light chains. An electrophoretic and immunological study of rabbit skeletal-muscle myosins

Antibodies specific for rabbit fast-twitch-muscle myosin LCIF light chain were purified by affinity chromatography and characterized by both non-competitive and competitive enzyme-linked immunosorbent assay (ELISA) and a gel-electrophoresis-derived assay (GEDELISA). The antibodies did not cross-react with myosin heavy chains, and were weakly cross-reactive with the LC2F [5,5'-dithio-(2-nitrobenzoic acid)-dissociated] light chain and with all classes of dissociated light chains (LC1Sa, LC1Sb and LC2S), as well as with the whole myosin, from hind-limb slow-twitch muscle. The immunoreactivity of myosins with a truly mixed light-chain pattern (e.g. vastus lateralis and gastrocnemius) correlated with percentage content of fast-twitch-muscle-type light chains. A more extensive immunoreactivity was observed with diaphragm and masseter myosins, which were also characterized, respectively, by a relative or absolute deficiency of LC1Sa light chain. Furthermore, it was found that the LC1Sb light chain of masseter myosin is antigenically different from its slow-twitch-muscle myosin analogue, and is immunologically related to the LC1F light chain. Rabbit masseter muscle from its metabolic and physiological properties and the content, activity and immunological properties of sarcoplasmic-reticulum adenosine triphosphatase, is classified as a red, predominantly fast-twitch, muscle. Therefore our results suggest that the two antigenically different iso-forms of LC1Sb light chain are associated with the myosins of fast-twitch red and slow-twitch red fibres respectively.

Transitions in membrane composition during postnatal development of rabbit fast muscle

Early postnatal changes (4-5 days to 15 days after birth) in the biochemical composition of microsomes were investigated in rabbit skeletal muscles destined to become fast-twitch muscles. During this period, a steady decrease in the microsomal content of cholesterol and of ouabain-sensitive Na + /K + -ATPase activity, as well as a decrease in protein electrophoretic components in the 80 000-70 000 molecular weight range, were observed. These changes are probably due to a diminishing yield of microsomal membranes derived from T-tubules, as the age of the animals increases, and are indicated from a knowledge of the mixed composition of muscle microsomes and previous biochemical data on isolated T-tubules. The content of cytochrome b5, which was found to be high in muscle microsomes of newborn animals, decreased strikingly as the amount of membrane-bound Ca2 + -ATPase protein increased, with a crossing-over point at about 7-10 days after birth. These changes, possibly corresponding to a transition from precursor sarcoplasmic reticulum (SR) to mature SR, were found to be temporally correlated with changes in [3H] alpha-tocopherol binding ability of the microsomes and in the mitochondrial content of glycerol phosphate dehydrogenase. At the same critical periods, coincident with the onset of motile activity, the immunological cross-reactivity of the Ca2 + -ATPase protein of microsomal vesicles, with antibody specific for the Ca2 + -ATPase of adult fast SR, was found to increase markedly, as tested by competitive enzyme-linked immunosorbent assay (ELISA). The immunological data are consistent with data in the literature demonstrating an increase in the concentration of Ca2 + -ATPase molecules in the SR membranes during ontogenic development. Both these data and catalytic data, however, suggest that the Ca2 + -ATPase protein is present in the same form in the SR of immature and of adult fast muscle and, in an antigenically different form, in slow muscle SR.

Biochemical heterogeneity of skeletal-muscle microsomal membranes. Membrane origin, membrane specificity and fibre types

1. Microsomes were isolated from rabbit fast-twitch and slow-twitch muscle and were separated into heavy and light fractions by centrifugation in a linear (0.3-2m) sucrose density gradient. The membrane origin of microsomal vesicles was investigated by studying biochemical markers of the sarcoplasmic-reticulum membranes and of surface and T-tubular membranes, as well as their freeze-fracture properties. 2. Polyacrylamide-gel electrophoresis showed differences in the Ca(2+)-dependent ATPase/calsequestrin ratio between heavy and light fractions, which were apparently consistent with their respective origin from cisternal and longitudinal sarcoplasmic reticulum, as well as unrelated differences, such as peptides specific to slow-muscle microsomes (mol.wts. 76000, 60000, 56000 and 45000). 3. Freeze-fracture electron microscopy of muscle microsomes demonstrated that vesicles truly derived from the sarcoplasmic reticulum, with an average density of 9nm particles on the concave face of about 3000/mum(2) for both fast and slow muscle, were admixed with vesicles with particle densities below 1000/mum(2). 4. As determined in the light fractions, the sarcoplasmic-reticulum vesicles accounted for 84% and 57% of the total number of microsomal vesicles, for fast and slow muscle respectively. These values agreed closely with the percentage values of Ca(2+)-dependent ATPase protein obtained by gel densitometry. 5. The T-tubular origin of vesicles with a smooth concave fracture face in slow-muscle microsomes is supported by their relative high content in total phospholipid and cholesterol, compared with the microsomes of fast muscle, and by other correlative data, such as the presence of (Na(+)+K(+))-dependent ATPase activity and of low amounts of Na(+)-dependent membrane phosphorylation. 6. Among intrinsic sarcoplasmic-reticulum membrane proteins, a proteolipid of mol.wt. 12000 is shown to be identical in the microsomes of both fast and slow muscle and the Ca(2+)-dependent ATPase to be antigenically and catalytically different, though electrophoretically homogeneous. 7. Basal Mg(2+)-activated ATPase activity was found to be high in light microsomes from slow muscle, but its identification with an enzyme different from the Ca(2+)-dependent ATPase is still not conclusive. 8. Enzyme proteins that are suggested to be specific to slow-muscle longitudinal sarcoplasmic reticulum are the flavoprotein NADH:cytochrome b(5) reductase (mol.wt. 32000), cytochrome b(5) (mol.wt. 17000) and the stearoyl-CoA desaturase, though essentially by criteria of plausibility.