Control of muscle contraction

A kinetic study of muscular contractions

In the present work the kinetics of muscular contractions are studied, based on a three-state model which is introduced in Section I. The general properties of this class of models for muscular contractions are derived in Sections II-VI. The kinetic equations are solved for four particular examples in Sections VII-X. These analytical results should be useful for comparison with experimental data to determine the functional dependence of the rate contants alpha and beta on muscle length.

Cooperative interactions between calcium-binding sites on glycerinated muscle fibers. The influence of cross-bridge attachment

A double isotope technique and EGTA buffers were used to measure the binding of Ca2+ to rabbit psoas muscle fibers extracted with detergent and glycerol. These experiments were designed to test the effect of rigor complex formation, determined by the degree of filament overlap, on the properties of the Ca2+-binding sites in the intact filament lattice. In the presence of 5 mM MgCl2 (no ATP), reduction of filament overlap was associated with a reduced binding of Ca2+ over the entire range of free Ca2+ concentrations (5.10(-8)-2.10(-5) M). With maximum filament overlap (sarcomere length 2.1-2.2 micrometer) the maximum bound Ca2+ was equivalent to 4 mol Ca2+/mol troponin and there was significant positive interaction between binding sites, as shown by Scatchard and Hill plots. With no filament overlap (sarcomere length 3.8-4.4 micrometer) the maximum bound Ca2+ was equivalent to 3 mumole Ca2+/mol troponin and graphical analysis indicated a single class of non-interacting sites. The data provide evidence that when cross-bridge attachments between actin and myosin filaments are formed not only does an additional Ca2+ binding site appear, but cooperative properties are imposed upon the binding sites.

Regulation of tension in skinned muscle fibers: effect of high concentrations of Mg-ATP

The control of tension in skinned fibers by Mg-ATP and Ca described in previous publications has been studied at high substrate concentrations over a wide range of temperature and salt concentration. Curves of tension versus pCa shift systemically to the right as [Mg-ATP] increases. The maximum Ca-activated tension of a skinned fiber declines at sufficiently high substrate concentrations. This behavior is described by a generalization of the scheme given in the earlier reports.

Polymerizability of rabbit skeletal tropomyosin: effects of enzymic and chemical modifications

Polymerizability of tropomyosin was unaffected by the removal of the three terminal residues 282, 283, and 284 using carboxypeptidase A. However, when residue 281 was removed, polymerizability was abolished. These results are consistent with a 9-residue molecular head-to-tail overlap in polymerized tropomyosin, in which residue 281 plays a space-filling role at the center of the overlap core. In acetylation studies, loss of polymerizability closely paralleled the extent of acetylation of lysine-7, and this residue was more susceptible to acetylation than any other. The effect of acetylation on polymerizability was probably caused not only by cleavage of salt-bridge between lysine 7 epsilon-NH2 and residue 284 alpha-COOH but also by distortion of the overlap core by the N-acetyl group. Specific modification of methionine in tropomyosin indicated that, in addition to residue 281, methionine-8 is also involved in formation of the overlap core. Modified nonpolymerizable tropomyosins could still bind to F-actin, indicating that the head-to-tail polymerization of tropomyosin is not a prerequisite for actin binding, although the regularity of tropomyosin molecules along the actin helix is presumably disrupted.

Regulation of intracellular calcium in chick embryo fibroblast: calcium uptake by the microsomal fraction

The total membrane fraction of a chick embryo fibroblast (CEF) homogenate accumulates calcium in an energy-dependent manner. This activity can be dissociated into azide-sensitive and azide-insensitive components. The azide-sensitive component of calcium uptake is believed to represent mitochondrial calcium uptake. The azide-insensitive component of calcium uptake is enhanced by the presence of a calcium trapping agent such as oxalate, and cannot utilize, ADP, inorganic phosphate and a Krebs cycle substrate to support uptake. The distribution of the azide-insensitive calcium uptake in subcellular fractions suggests that this uptake occurs in other than mitochondrial membranes. The membranes most likely to contribute to the azide-insensitive component of calcium uptake are the endoplasmic reticulum and plasma membrane. A microsomal preparation from CEF cells is essentially devoid of the azide-sensitive calcium uptake activity. This microsomal activity is similar in characteristics to the sarcoplasmic reticulum of skeletal muscle. However the specific activity of CEF microsomal calcium uptake system is much less than that found in the skeletal muscle system. The transport of calcium by these membranes provide a mechanism for the regulation of cytosol calcium levels and may play a role in the control of movement and growth of cultured cells.

Ca-linked phosphorylation of a light chain of vertebrate smooth-muscle myosin

In vertebrate smooth muscle actomyosin and myofibrils a myosin light chain of molecular weight about 20,000 becomes phosphorylated at the same Ca2+ concentration as required to stimulate the actin-activated ATPase activity of myosin. Further, the degree of phosphorylation in the preparations as well as in various reconstituted actomyosins is proportional to their measured Ca2+ sensitivity. The phosphorylation process is very rapid and is essentially completed before the rise in ATPase activity. The enzyme responsible for the observed myosin phosphoylation is a specific myosin light chain kinase which is routinely co-purified with myosin. This kinase is normally present in actomyosin and its removal together with tropomyosin leads to a complete loss of the actin-activated ATPase activity. It is suggested that the Ca-dependent phosphorylation of the light chain via the light chain kinase represents the initial step in the activation of myosin that leads to contraction. Relaxation is probably effected by an as yet uncharacterised light chain phosphatase.

Cooperative interactions between the contractile proteins of cardiac and skeletal muscle

The calcium activation of the ATPase (ATP phosphohydrolase, EC 3.6.1.3) activity of cardiac actomyosin reconstituted from bovine cardiac myosin and a complex of actin-tropomyosin-troponin extracted from bovine cardiac muscle at 37 degrees C was studied and compared with similar proteins from rabbit fast skeletal muscle. The proteins of the actin complex were identified by polyacrylamide gel electrophoresis in sodium dodecyl sulfate. Half-maximal activation of the cardiac actomyosin was seen at a calcium concentration of 1.2 +/- 0.002 (S.E. of mean) muM. A hybridized reconstituted actomyosin made with cardiac myosin and the actin-tropomyosin-troponin complex extracted from rabbit skeletal muscle was also activated by calcium but the half-maximal value was shifted to 0.65 +/- 0.02 (S.E. of mean) muM Ca2+. Homologous rabbit skeletal actomyosin showed half-maximal activation at 0.90 +/- 0.01 (S.E. of mean) muM Ca2+ and the value for a hybridized actomyosin made with rabbit skeletal myosin and the actin-complex from cardiac muscle was found at 1.4 +/- 0.03 (S.E. of mean) muM Ca2+ concentration. Kinetic analysis of the Ca2+ activated ATPase activity of reconstituted bovine cardiac actomyosin indicated some degree of cooperativity with respect to calcium. Double reciprocal plots of reconstituted actomyosins made with bovine cardiac actin complex were curvilinear and significantly different than those of reconstituted actomyosins made with the rabbit fast skeletal actin complex. The Ca2+-dependent cooperativity was of a mixed type as determined from Hill plots for homologous reconstituted bovine cardiac and rabbit fast skeletal actomyosin. The results show that cooperative interactions in reconstituted actomyosins were greater when the actin-tropomyosin-troponin complex was derived from cardiac than skeletal muscle.

Ca+2-accumulating components in developing skeletal muscle

This ultrastructural study on the localization of Ca+2 in developing skeletal muscle indicates that the formation of calcium-accumulating components begins during embryonic development. Both oxalate and pyroantimonate techniques are used to localize Ca+2 in distinct cellular components of chick pectoral and sartorius muscles. Two major sites for Ca+2 accumulation are present in ultrathin sections of embryonic and post-embryonic muscles: the terminal cisternae of the sarcoplasmic reticulum and specific lines in the I-bands. Calcium oxalate-accumulating vesicles are present in the smallest recognizable myotubes at the twelfth day of incubation, but calcium-accumulating components are not seen at myofibrillar I-band sites until the fourteenth to seventeenth days of incubation. The fact that myofibrils first form and later in development accumulate a Ca+2-binding component suggests that this Ca+2-binding component is not necessary for the formation of myofibrils, but is added to myofibrils before hatching to serve a probable regulatory role in contraction.

Permeability of sarcoplasmic reticulum membrane. The effect of changed ionic environments on Ca2+ release

Permeability properties and the effects of a changed membrane potential on Ca2+ release of sarcoplasmic reticulum vesicles of rabbit skeletal muscle were investigated by Millipore filtration. The relative permeability of sarcoplasmic reticulum to solutes determined under conditions of isotope exchange at equilibrium and/or under conditions of net flow of solute and water into the vesicles was as follows: sucrose, Ca2+, Mn2+ less than gluconate-, choline+, Tris+ less than methanesulfonate- less than urea, glycerol, K+, Na+,Li+, Cl-. Transient membrane potentials were induced by rapidly changing the ionic environment of the vesicles. Knowledge of the relative permeation rates of the above ions allowed prediction of the direction and extent of membrane polarization. Osmotic effects in the polarization measurements due to the rapid influx of solute and water into the vesicles were minimized by using media containing a fast (K+ or Cl-) and a relatively slow (gluconate- or choline+) penetrating ion. 45Ca2+ efflux from vesicles derived from different parts of the sarcoplasmic reticulum structure was not appreciably changed when vesicles were made more positive inside (choline chloride leads to potassium gluconate) or more negative inside (potassium gluconate leads to choline chloride). These studies suggest that part or all of the ion-induced changes in sarcoplasmic reticulum membrane permeability, previously interpreted to indicate "depolarization" -induced Ca2+ release, may be due to osmotic effects.

Deuterium oxide effects on excitation-contraction coupling of skeletal muscle

2H2O (99.8%) Ringer's solution greatly reduces the twitch and tetanus of frog sartorius muscle and, as specially shown here, slows the onset features of the mechanical output of the twitch by: (a) increasing the time (LR) from stimulus to start of latency relaxation; (b) slowing the development of the latency relaxation, and (c) greatly decreasing the rate of onset of tension development. These changes reflect effects of 2H2O on excitation-contraction coupling and they represent the critical direct effects of 2H2O on muscle since it does not depress either the action potential or the intrinsic myofibrillar contractility. The increase in LR is attributed to slowed inward electrical propagation in the T-tubule. But the critical effect of 2H2O on frog muscle is to greatly depress mobilization of activator Ca2+. The depression of the Ca2+ mobilization and of its effects on the activation of contraction evidently result from (a) a lowered rate of release of Ca2+ from the sar coplasmic reticulum, as indicated by the slowed development of the latency relaxation, (b) a decreased amount of Ca2+ released in a twitch, and (c) a reduced speed of diffusion of the Ca2+ to the contractile filaments. The depressed mobilization of Ca2+ is apparently the essential cause of 2H2O's general depression of twitch and tetanus output.

The role of divalent cations in the regulation of microtubule assembly. In vivo studies on microtubules of the heliozoan axopodium using the ionophore A23187

Low concentrations of calcium and magnesium ions have been shown to influence microtubule assembly in vitro. To test whether these cations also have an effect on microtubules in vivo, specimens of Actinosphaerium eichhorni were exposed to different concentrations of Ca++ and Mg++ and the divalent cation ionophore A23187. Experimental degradation and reformation of axopodia were studied by light and electron microscopy. In the presence of Ca++ and the ionophore axopodia gradually shorten, the rate of shortening depending on the concentrations of Ca++ and the ionophore used. Retraction of axopodia was observed with a concentration of Ca++ as low as 0.01 mM. After transfer to a Ca++-free solution containing EGTA, axopodia re-extend; the initial length is reached after about 2 h. Likewise, reformation of axopodia of cold-treated organisms is observed only in solutions of EGTA or Mg++, whereas it is completely inhibited in a Ca++ solution. Electron microscope studies demonstrate degradation of the axonemal microtubular array in organisms treated with Ca++ and A23187. No alteration was observed in organisms treated with Mg++ or EGTA plus ionophore. The results suggest that, in the presence of the ionophore, formation of axonemal microtubules can be regulated by varying the Ca++ concentration in the medium. Since A23187 tends to equilibrate the concentrations of divalent cations between external medium and cell interior, it is likely that microtubule formation invivo is influenced by micromolar concentrations of Ca++. These concentrations are low enough to be of physiological significance for a role in the regulation of microtubule assembly in vivo.

Similarity of junctions between plasma membranes and endoplasmic reticulum in muscle and neurons

The structure of membranes at junctions between the plasma membrane and underlying cisterns of endoplasmic reticulum in amphioxus muscle and mouse cerebellar neurons was studied using the freeze-fracture technique. In amphioxus muscle, subsurface cisterns of sarcoplasmic reticulum form junctions with the surface membrane at the level of the sarcomere I bands. On the protoplasmic leaflet of the sarcolemma overlying these junctions were aggregates of large particles. On the protoplasmic leaflet of the membranes of cerebellar basket, stellate and Purkinie cells there were similar aggregates of large particles. In both tissues, the corresponding external membrane halves had arrays of pits apparently complementary to the aggregates of large particles. Cross fractures through junctions showed that the particle aggregates in neuronal and muscle membranes were consistently located over intracellular cisterns closely applied to the plasma membrane. Thus, a similar plasma membrane specialization is found at subsurface cisterns in mammalian neurons and amphioxus muscle. This similarity supports the hypothesis that subsurface cisterns in neurons, like those in muscle, couple some intracellular activity to the electrical activity of the plasma membrane.

Calcium and the muscarinic receptor

Pharmacological receptors may be viewed as composed of two linked functions, a recognition site through which the specificity and selectivity of ligand action is expressed and an amplification or catalytic site which translates the ligand-recognition site interaction into response. The successful elucidation of receptor mechanisms requires analysis of both sites. The muscarinic cholinergic receptors of guinea-pig ileal longitudinal smooth muscle will be discussed from this two function stand point and the source and utilization of Ca2+ in excitation-contraction coupling discussed.

Beef muscle troponin: evidence for multiple forms of troponin-T

A method is described for the purification of troponin from beef skeletal muscle. The resultant preparation differs from the troponin of rabbit skeletal muscle in that it contains at least two forms of the tropomyosin-binding component, Troponin-T: these are designated as the 37 000 and 40 000 dalton forms of Troponin-T on the basis of sodium dodecyl sulphate gel electrophoresis. Either of these Troponin-T forms may be used to reconstitute troponin by mixing with the appropriate amounts of the calcium-binding (Troponin-C) and and actomyosin ATPase-inhibitory (Troponin-I) components. These reconstituted troponins are shown to interact with tropomyosin and also to confer full calcium sensitivity on actomyosin ATPase. Despite the existence of proteolysis in troponin preparations, the experimental evidence indicates that the smaller form of Troponin-T is not derived from the 40 000 dalton species by limited degradation. Although both species of Troponin-T have been found routinely in troponin from beef skeletal muscle, only the larger form is detected in troponin preparations from beef cardiac muscle. Further studies are required in order to clarify the functional significance and differential distribution of these multiple forms of Troponin-T.

Differential effects of mercurial compounds on excitable tissues

Sarcoplasmic reticulum (SR), Ca2+ plus Mg2+-ATPase, and Ca2+-ionophore were obtained from white rabbit skeletal muscles. Methylmercury inhibited the Ca2+ plus Mg2+-ATPase and Ca2+-transport but had no effect on the Ca2+-ionophore. Mercuric chloride inhibited all three functions (i.e., ATPase, transport and ionophoric activity). The mechanism of HgCl2 inhibition of the Ca2+-ionophore was by competition with Ca2+ for Ca2+-ionophoric site whereas its inhibition of the enzyme and Ca2+-transport was due to the blockage of essential sulfhydryl (--SH) groups. Ca2+ plus Mg2+-ATPase and Ca2+-transport were more sensitive to methylmercury than to HgCl2. Acetylcholine receptor (AChR) was obtained for the electric organ of T. californica. Methylmercury inhibited the ACh binding to AChR WITH Ki = 5.7 - 10(-6) M. This effect was not due to mercuric ion alone since mercuric chloride up to 10(-4) M did not affect ACh binding to AChR. It is concluded that: the Ca2+ plus Mg2+-ATPase and Ca2+-transport contain --SH groups essential for their activity, and that the two functions are tightly coupled; the Ca2+-ionophore contains no --SH groups essential for its activity; CH3HgCl inhibition of Ca2+ plus Mg2+-ATPase and Ca2+-transport is partly due to its reactivity with --SH groups in hydrophobic environment; the Ca2+-transport is inhibited by HgCl2 through two processes, one which is the blockage of --SH groups and another which is the inhibition of the Ca2+-ionophoric site; and the inhibition of ACh binding to AChR is due to the blockage of --SH groups in hydrophobic environment, which is inaccessible to Hg2+. Our data present for the first time a molecular basis for the myopathy associated with mercurial compounds toxicity.

A non-linear voltage dependent charge movement in frog skeletal muscle

1. Voltage-clamp experiments were carried out using the three microelectrode technique. Using this method membrane current density at V1 is proportional to deltaV( = V2 - V1) where V1 and V2 are voltages at distances 1 and 21 from the end of a fibre. Voltage dependent sodium currents were blocked by tetrodotoxin, potassium by tetraethylammonium ions and rubidium. Contraction was blocked by adding sucrose, 467 mM. 2. The current deltaV (control) associated with a positive voltage step from a hyperpolarized conditioning voltage to the holding potential, -80 mV, showed two components, a capacitative transient which decayed rapidly and a maintained steady level...

Muscle structure and function--an explanation

The structure of vertebrate skeletal muscle is reviewed. The mechanism of muscular contraction and its control is then discussed from the point of view of molecular structure. Contraction takes place by a sliding filament mechanism produced by cross-bridges which form between thick and thin filaments. Control is exercised by tropomyosin and troponin. When the calcium concentration is low, these proteins interfere with the formation of cross-bridges and prevent contraction, but when the calcium concentration is increased, they no longer interfere and contraction proceeds.

Energy-dependent calcium uptake activity of microsomes from the aorta of normal and hypertensive rats

Energy-dependent calcium uptake activity of microsomes isolated from the rat aorta has been characterized. The microsomes consist of smooth membrane vesicles which in the presence of MG-ATP as an energy source continuously sequester calcium over a 60-min period. This calcium uptake is greatly stimulated by oxalate anion which serves as a calcium trapping agent. Unlike the calcium uptake of mitochondria this uptake is not inhibited by sodium azide. Sucrose density gradient analysis of the microsomal calcium uptake suggests that the system is associated with the sarcoplasmic reticulum. In presence of 5 mM Mg-ATP and 20 muM calcium approximately 38 nmol of calcium per mg of microsomal protein are taken up in 20 min. In the absence of ATP, less than 2 nmol of calcium per mg of protein are taken up in the first 2 min with no further uptake of calcium in subsequent time periods. When calcium uptake activity is plotted against calcium or ATP concentration of the medium, half maximal activity is calculated for 24.3 muM calcium and for 1.6 mM ATP. The calcium uptake characteristics of the rat aorta microsomes are compatible with a postulated role in the relaxation of the vascular smooth muscle and the provision of an intracellular calcium store for muscle contraction. Aorta microsomes from SHR rats (a genetic strain that is spontaneously hypertensive) have a significantly reduced uptake when compared with the corresponding nonhypertensive control strain. The level of calcium and ATP for half maximal activity of the rat aorta microsomal calcium uptake system is approximately the same in the SHR and the control strain. The rate of release of calcium from rat aorta microsomes is apparently identical in SHR strain and control. The calcium uptake activity of kidney and liver microsomes isolated from the SHR strain and control. The calcium uptake activity of kidney and liver microsomes isolated from the SHR rat appears to be identical to that found in the control strain.

The jumping mechanism of Xenopsylla cheopis. II. The fine structure of the jumping muscle

The ultrastructure of the trochanteral depressor muscle of the oriental rat flea is described. It is shown to be similar to that of the tubular leg muscles of other insects except in the volume and arrangement of the sarcoplasmic reticulum. The sarcoplasmic reticulum occupies approximately 18% of the volume of the muscle fibres. It is in three configurations:a regular array of parallel tubules opposite the A-band, a collar of sacculi involved in the formation of the dyads at the edge of the A-band and a loosely woven arrangement of tubules around the I-band. This tripartite arrangement of the sarcoplasmic reticulum and its large surface area is discussed in relation to the action of the muscle as the main propulsive muscle in the jump of the flea.

Metabolic changes associated with the slowing of relaxation in fatigued mouse muscle

1. The biochemical basis of the slowing of relaxation seen in fatigue has been examined using an isolated mouse soleus preparation. 2. Slowing of relaxation occurred during prolonged tetani under anaerobic conditions when ATP and PC fell and lactate accumulated. 3. Slowing of relaxation was also demonstrated with muscles poisoned with cyanide and iodoacetic acid when there was a fall in ATP and PC but no accumulation of lactate. During a period of anaerobic recovery following a fatiguing tetanus, relaxation became faster at a time when lactate was accumulating in the muscle. 4. It is concluded that the slowing of relaxation in fatigue is not a consequence of lactate accumulation, and a relationship is demonstrated between the ATP content of the muscle and the rate of relaxation in muscles fatigued by prolonged stimulation, 5. Rates of ATP turn-over in fresh muscle, and at intervals throughout a tetanus are consistent with the suggestion that the rate limiting step for myofibrillar ATPase may be directly related to the rate limiting step for the decay of tension during relaxation.

The comparative effects of (Ca2+) and (Mg2+) on on tension generation in the fibers of skinned frog skeletal muscle and mechanically disrupted rat ventricular cardiac muscle

The effects of intracellular Mg2+ on Ca2+-activated isometric tension generation in rat cardiac muscle fibers and frog skeletal muscle were compared. The membranous sarcolemmal barrier was removed from rat cardiac muscle fibers by mechanical disruption and from frog skeletal muscle by skinning. Tension was recorded in the fibers in bathing solutions of different Ca2+ concentrations and either 5 X 10(-5) M or 1 X 10(-3) M Mg2+ concentration (the same concentrations used in a previous study on single skinned frog skeletal muscle fibers [3]). The amount of Ca2+ required to activate the muscle increased with Mg2+ concentration for both rat ventricular muscle and frog skeletal muscle. These data indicate that intracellular Mg2+ concentration could strongly modulate Ca2+-activated tension in cardiac muscle and that very similar molecular mechanisms are responsible for Ca2+-activated tension in rat ventricular muscle and frog skeletal muscle. The possible sites of action of Mg2+ on Ca2+-activated tension are discussed.

Interaction of troponin components with F-actin and F-actin-tropomyosin complex

1. Both TN-T and TN-I components of troponin interact with F-actin, causing its precipitation at 0.1 M KC1 and neutral pH in a form of highly ordered paracrystals, although the ability of TN-I component to precipitate of F-actin is much weaker. 2. F-actin paracrystals obtained in the presence of both TN-T and TN-I components consist of parallel arrays of F-actin filaments, although the fine structure is in each case different. 3. In the presence of tropomyosin in the proportion equal to that in muscle, less TN-T or TN-I component is needed to obtain full precipitation of F-actin. 4. Paracrystals of F-actin-tropomyosin-TN-T component and F-actin-tropomyosin-TN-I component show regular transverse striation spaced at about 380 A intervals. 5. The TN-C component of troponin solubilizes all precipitates of F-actin with TN-T or TN-I components, regardless of the presence of tropomyosin. 6. The results show that both TN-T or TN-I components can bind independently to F-actin-tropomyosin complex with the same periodicity, similar to that of the whole troponin in the living muscle.

Calcium regulation of muscle contraction

Calcium triggers contraction by reaction with regulatory proteins that in the absence of calcium prevent interaction of actin and myosin. Two different regulatory systems are found in different muscles. In actin-linked regulation troponin and tropomyosin regulate actin by blocking sites on actin required for complex formation with myosin; in myosin-linked regulation sites on myosin are blocked in the absence of calcium. The major features of actin control are as follows: there is a requirement for tropomyosin and for a troponin complex having three different subunits with different functions; the actin displays a cooperative behavior; and a movement of tropomyosin occurs controlled by the calcium binding on troponin. Myosin regulation is controlled by a regulatory subunit that can be dissociated in scallop myosin reversibly by removing divalent cations with EDTA. Myosin control can function with pure actin in the absence of tropomyosin. Calcium binding and regulation of molluscan myosins depend on the presence of regulatory light chains. It is proposed that the light chains function by sterically blocking myosin sites in the absence of calcium, and that the "off" state of myosin requires cooperation between the two myosin heads. Both myosin control and actin control are widely distributed in different organisms. Many invertebrates have muscles with both types of regulation. Actin control is absent in the muscles of molluscs and in several minor phyla that lack troponin. Myosin control is not found in striated vertebrate muscles and in the fast muscles of crustacean decapods, although regulatory light chains are present. While in vivo myosin control may not be excluded from vertebrate striated muscles, myosin control may be absent as a result of mutations of the myosin heavy chain.