The regulatory proteins of the myofibril. Characterization and biological activity of the calcium-sensitizing factor (troponin A)

Historical perspective on heart function: the Frank-Starling Law

More than a century of research on the Frank-Starling Law has significantly advanced our knowledge about the working heart. The Frank-Starling Law mandates that the heart is able to match cardiac ejection to the dynamic changes occurring in ventricular filling and thereby regulates ventricular contraction and ejection. Significant efforts have been attempted to identify a common fundamental basis for the Frank-Starling heart and, although a unifying idea has still to come forth, there is mounting evidence of a direct relationship between length changes in individual constituents (cardiomyocytes) and their sensitivity to Ca²⁺ ions. As the Frank-Starling Law is a vital event for the healthy heart, it is of utmost importance to understand its mechanical basis in order to optimize and organize therapeutic strategies to rescue the failing human heart. The present review is a historic perspective on cardiac muscle function. We "revive" a century of scientific research on the heart's fundamental protein constituents (contractile proteins), to their assemblies in the muscle (the sarcomeres), culminating in a thorough overview of the several synergistically events that compose the Frank-Starling mechanism. It is the authors' personal beliefs that much can be gained by understanding the Frank-Starling relationship at the cellular and whole organ level, so that we can finally, in this century, tackle the pathophysiologic mechanisms underlying heart failure.

Studies on thermostable antigens, production of species-specific antiadrenal sera and comparison of immunological techniques in meat speciation

Heat-stable antigens (BE forms: resistant to heat and ethanol precipitation) of adrenal and muscle tissues of cattle, buffalo, sheep, goat and pig were prepared for use in detection of adulteration in meats. The physico-chemical characteristics of these antigens revealed that the antigens of adrenals had only one component corresponding to 'Troponin T'. Muscle antigens also contained a major troponin T component but were associated with low molecular weight fractions. Rabbit antiadrenal BE sera were developed and made species specific by immunoabsorption. The species-specific antisera were employed for identification of origin of fresh and cooked meats and their mixtures, using an immunodiffusion test-agar gel precipitation test (AGPT), counterimmunoelectrophoresis (CIEP), enzyme-linked immunosorbent assay (ELISA) and the unlabelled antibody peroxidase antiperoxidase (PAP) technique. The results indicated that absorbed antisera could successfully differentiate the fresh, cooked meats and the meat mixtures from the species under study. AGPT and CIEP were useful in identification of 5-10% addition, using water extracts of fresh meats and BE forms of cooked meats, whereas ELISA and PAP could detect adulteration down to the level of 1% when water extracts were used. Among the tests employed in the study, the PAP technique proved to be most sensitive. The antisera were also proved useful in identifying the species in canned meat products, milk, serum, plasma, semen, urine, organs, skin and spoilt flesh, employing AGPT and CIEP.

Isolation and characterization of a calcium-binding protein derived from mRNA termed p9Ka, pEL-98, 18A2, or 42A by the newly synthesized vasorelaxant W-66 affinity chromatography

W-66 (N-(2-aminoethyl)-N-[2-(4-chlorocinnamylamino) ethyl]-5-isoquinolinesulfonamide), a newly synthesized isoquinolinesulfonamide, was shown to have a potent vasodilatory action and calmodulin (CaM)-antagonizing action. Using the W-66 affinity chromatographic technique, we purified two Ca(2+)-binding proteins from the EGTA-soluble fraction of bovine aorta. One was CaM and the other was an acidic protein with a molecular mass of 11 kDa. It was tentatively named "calvasculin." Calvasculin was a dimeric protein. Equilibrium dialysis showed that 1 mol of calvasculin (dimer) bound to 1.98 +/- 0.30 mol of Ca2+ in the presence of 10(-3) M Ca2+. Calvasculin failed to activate Ca2+/CaM-dependent enzymes such as myosin light chain kinase, Ca2+/CaM-dependent phosphodiesterase, or Ca2+/CaM-dependent protein kinase II and to inhibit the CaM stimulation of these enzymes. The partial amino acid sequence of calvasculin revealed a high homology to the predicted protein derived from mRNA, named pEL-98, 18A2, 42A, or p9Ka. We also examined the physicochemical and biochemical properties of calvasculin. Using the antibody specific for calvasculin, we obtained evidence that calvasculin is present in abundance in bovine aorta but not in brain, lung, heart, or testis.

Thin filament proteins and thin filament-linked regulation of vertebrate muscle contraction

Recent developments in the field of myofibrillar proteins will be reviewed. Consideration will be given to the proteins that participate in the contractile process itself as well as to those involved in Ca-dependent regulation of striated (skeletal and cardiac) and smooth muscle. The relation of protein structure to function will be emphasized and the relation of various physiologically and histochemically defined fiber types to the proteins found in them will be discussed.

(32P) phosphate incorporation into ATP during ATP hydrolysis and its dependence on the interaction of actin and myosin

The incorporation of 32Pi into ATP has been found to be catalyzed by myosin only when and if it interacts with actin. This exchange reaction is inhibited in natural but not in desensitized actomyosin after removing of trace Ca2+ with ethyleneglycol bis(2-aminoethyl)-N,N'-tetraacetic acid (EGTA). In desensitized as well as in synthetic actomyosin the exchange reaction can be fully inhibited by the addition of troponin I (0.5 mg troponin I/mg actomyosin results in a 50% inhibition) or after replacing the Mg activator by CaCl2. The exchange rate is about 1:500 of the ATPase rate in presence of 2 mM phosphate. These results suggest the existence of an 'energy-rich' actin -- myosin -- nucleoside-diphosphate intermediate during the cross-bridge cycle.

Regulatory proteins of lobster striated muscle

The regulatory proteins of lobster muscles consist of tropomyosin and of troponin. Troponin contains a 17,000 chain weight component, two closely related components of about 30,000 and a 52,000 chain weight component. In addition to troponin, tropomyosin is required for the inhibition of the magnesium activated actomyosin ATPase activity in the absence of calcium and for the reversal of this inhibition by calcium. Lobster tropomyosin interacts with rabbit actin and lobster troponin interacts with rabbit tropomyosin. The 30,000 doublet component corresponds to the troponin-I of rabbit and inhibits the ATPase activity of actomyosin both in the presence and in the absence of calcium. The 17,000 component corresponds to the troponin-C of rabbit; it binds calcium and reverses the inhibition of the ATPase activity by troponin-I in the presence of calcium. No more than 1 mol of calcium is bound by a mole of troponin-C or by troponin. The 52,000 component interacts with tropomyosin and has been tentatively identified as troponin-T; however, it has not been demonstrated as yet that this component had a role in the regulation of lobster actomyosin.

Phosphorylation of troponin and the effects of interactions between the components of the complex

1. The troponin complex from skeletal muscle contains approximately 1 mol of phosphate/80000g of complex, covalently bound to the troponin T component. 2. On prolonged incubation of the troponin complex or troponin T with phosphorylase kinase the phosphate content of troponin T was increased to approx. 3mol/mol. 3. On prolonged incubation of troponin I with phosphorylase kinase up to 1.6mol of phosphate/mol were incorporated. 4. Phosphorylation of troponin I was greatly inhibited by troponin C owing to the strong interaction between these proteins. Thus in the troponin complex troponin T was the main substrate for phosphorylase kinase. The phosphorylation of isolated troponin T was also inhibited by troponin C. 5. Troponin I was phosphorylated when the troponin complex was incubated with a bovine cardiac 3':5'-cyclic AMP-dependent protein kinase. Troponin T either in its isolated form or in the troponin complex was not phosphorylated by bovine protein kinase to any significant extent under the conditions used. 6. If the troponin complex was dephosphorylated to 0.2mol/mol, or phosphorylated up to 2.5mol/mol there was no significant effect on the ability of normal concentrations to confer Ca(2+) sensitivity on the adenosine triphosphatase of densensitized actomyosin.

The interaction of the calcium-binding protein (troponin C) with bivalent cations and the inhibitory protein (troponin I)

1. The molecular weight of the calcium-binding protein of rabbit white skeletal muscle was estimated to be 18500 by sedimentation equilibrium and electrophoresis in sodium dodecyl sulphate. 2. Addition of 2 Ca(2+) ions per molecule produced reversible changes in the u.v.-absorption spectrum that are interpreted as arising from conformational changes in the structure of the protein. 3. Cd(2+) was almost as effective as Ca(2+) in producing the spectral changes. Other bivalent metal ions, particularly Mg(2+), were less effective. 4. Binding of Ca(2+) by the calcium-binding protein produced an increase in mobility to the anode on electrophoresis in 6m-urea at pH8.6. The Ca(2+)-saturated form of the protein was more retarded on gel filtration than the Ca(2+)-free form. 5. In the presence of Ca(2+) the calcium-binding protein formed an equimolar complex with the inhibitory protein. This complex was stable in 8m-urea and in the pH range 7.0-8.6. 6. An isotope-dilution method for the measurement of the content of calcium-binding protein in whole muscle is described. In rabbit psoas muscle the ratio of actin monomers to molecules of calcium-binding protein was approx. 7:1. Similar values were obtained for red skeletal and cardiac muscle. 7. Evidence is presented indicating that in the rabbit the inhibitory protein of the troponin complex of red skeletal and cardiac muscles is different from the inhibitory protein of white skeletal muscle.

A phosphorylated light-chain component of myosin from skeletal muscle

1. The low-molecular-weight components of myosin from rabbit skeletal muscle migrated as four bands on polyacrylamide-gel electrophoresis in 8m-urea but only as three in systems containing sodium dodecyl sulphate. The two bands of intermediate mobility in 8m-urea (Ml(2) and Ml(3)) had identical mobilities in sodium dodecyl sulphate. 2. The isolation of pure samples of all four low-molecular-weight components by DEAE-Sephadex chromatography is described. 3. The amino acid compositions of components Ml(2) and Ml(3) were identical. Further analyses showed the presence of 1 mol of phosphate/18500g of component Ml(2) and less than 10% of this amount in component Ml(3). Neither light component contained ribose. 4. Alkaline phosphatase from Escherichia coli converted component Ml(2) into Ml(3). Incubation with crude preparations of phosphorylase b kinase or protein kinase in the presence of ATP converted component Ml(3) into Ml(2). 5. Phosphorylation of component Ml(3) with the kinases isolated from skeletal muscle and [gamma-(32)P]ATP gave incorporation of (32)P only into component Ml(2) whether whole myosin or separated low-molecular-weight components were used. 6. High-voltage electrophoresis at pH6.5 and pH1.8 of a chymotryptic digest of (32)P-labelled component Ml(2) yielded one major radioactive peptide containing serine phosphate. 7. The amino acid sequence of this peptide was shown to be: Arg-Ala-Ala-Ala-Glu-Gly-Gly-(Ser,Ser(P))-Asn-Val-Phe. This sequence shows no obvious similarity to the site phosphorylated in the conversion of phosphorylase b into phosphorylase a by phosphorylase b kinase. 8. Evidence suggests that in vivo all the 18500-molecular-weight light chain is in the phosphorylated form. The extent of dephosphorylation that occurred during myosin extraction depended on the conditions employed.

A protein that binds specifically to the M-line of skeletal muscle is identified as the muscle form of creatine kinase

Published information on the properties of two proteins from chicken muscle, creatine kinase (MM-creatine kinase) and an M-line protein, suggested that they might be identical molecules. Different published procedures were used to purify the two proteins to homogeneity, and the properties of the two preparations were compared. Creatine kinase specific activity increased during purification of M-line protein, reaching a value comparable to that of purified MM-creatine kinase. The two proteins migrated identically in two electrophoretic systems and, after electrophoresis, both could be stained for creatine kinase activity. Double immunodiffusion tests with antibody prepared against MM-creatine kinase established the serological identity of the two protein preparations. Immunofluorescent studies showed that antiserum against MM-creatine kinase was bound in a regular pattern at the centers of the A-band regions of isolated myofibrils. These data show conclusively that the M-line protein and MM-creatine kinase are identical.

Evidence from electron microscope studies on actin paracrystals concerning the origin of the cross-striation in the thin filaments of vertebrate skeletal muscle

When purified F-actin is precipitated by Mg 2+ , it forms paracrystals consisting of regularly packed filaments that retain the double-helical structure characteristic of actin polymers. In negatively stained preparations the only axial periodicity observed is that of the actin polymers. In sections, the polymer structure is not resolved and the paracrystals do not appear cross-striated. The paracrystals formed from unpurified actin preparations likewise contain regularly packed filaments in which the F-actin structure is observed but, in addition, the assembly of filaments is crossed at fairly regular intervals (approximately 37 nm) by bands of unstained amorphous material. Sections of these paracrystals show a corresponding cross-striation (mean spacing 38 nm). Other proteins known to be present in unpurified preparations of actin were prepared separately and added in solution to purified F-actin; paracrystals were formed on Mg 2+ precipitation. The cross-striation was reproduced when a mixture of tropomyosin and troponin had been added to the actin. The addition of purified tropomyosin alone resulted in paracrystals that lacked the cross-striation. Comparison of the cross-striation in the paracrystals with that in the thin filament assembly of the myofibril (as seen in sections and in negatively stained I-segments) supports the conclusion that filaments resembling the natural ones have been synthesized from actin, tropomyosin and the troponin complex. It follows that the cross-striation in the thin myo­filaments can be attributed to the location of part or all of the troponin complex at sites spaced at regular intervals along the filaments.

The regulatory proteins of the myofibril. Separation and biological activity of the components of inhibitory-factor preparations

1. Inhibitory-factor preparations isolated from myofibrils were shown to consist principally of proteins with molecular weights of 37000 and 23000. Under certain preparative procedures an additional component of molecular weight 14000 was present. 2. The 23000-dalton protein, the inhibitory factor, was the major active component. Its activity was enhanced by tropomyosin. 3. The 14000-dalton component also possessed inhibitory activity, although less than that of the 23000-dalton component when compared on a molar basis. Its activity was not always enhanced by tropomyosin. The 14000-dalton component could not be detected in whole fresh myofibrils and the limited evidence available is compatible with its formation during the preparation of the troponin complex. 4. The 37000-dalton component could not replace the inhibitory factor, calcium-sensitizing factor or tropomyosin as components of the relaxing-protein system. 5. All three components had distinctive amino acid compositions, particularly in their cysteine content.