Control of muscle contraction

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.

Reversible disaggregation of myofilaments in vertebrate smooth muscle

Strips of taenia coli from guinea pigs were incubated under isometric conditions in Krebs-Ringer bicarbonate saline (MKR) containing various concentrations of Ca(+2) and/or Mg(+2). Spontaneous or chemically induced contractile activity was abolished within 15 min of exposure to MKR containing Ca(+2) at concentrations below 10(-6) M; contractile activity was restored by reincubation in normal MKR after 1-2 h. Exposure of taenia coli to MKR containing Ca(+2) at concentrations below 10(-6) M for 1 h or more led to loss of thick and thin myofilaments from the sarcoplasm as observed with the electron microscope. Except for the loss of these two filament types, the cells contained all other structural features observed in preparations incubated in MKR containing Ca(+2) at its normal level (1.3 x 10(-3) M). The loss of thick and thin myofilaments in strips exposed to a Ca(+2) concentration below 10(-6) M was reversed by reincubation for 30 min in MKR containing normal Ca(+2) levels. The observed loss of thick and thin myofilaments in response to low Ca(+2) is interpreted as resulting from the disaggregation of some or all of the molecular components of these two filament types.

Membrane current and contraction in frog atrial fibres

1. Membrane current and mechanical activity were recorded from short segments of frog atrial muscle strips using a double sucrose gap voltage clamp arrangement. Experiments were performed at 4-7 degrees C. Two types of contraction were observed dependent upon the duration of the clamp.2. Short-lasting depolarizations caused a flow of Ca inward current, I(Ca), and development of a phasic contraction. Time to peak tension approximated 400 msec. Both I(Ca) and contraction, as functions of membrane potential, had a threshold of about - 40 mV and were maximal at inside positive potentials in normal Ringer fluid. Peak tension decreased at strong depolarizations.3. The minimum time of depolarization required for initiation of a phasic contraction was 40-70 msec. The time necessary for full activation of contraction was 200-300 msec and comparable to the period of time covered by the flow of I(Ca).4. There was no marked change in peak tension upon repetitive depolarization to the same membrane potential.5. Restoration of (phasic) contractility after a preceding contraction was strongly dependent on the level of membrane potential between conditioning and test pulse. Restoration was half complete at potentials around - 45 mV.6. Long-lasting depolarizations generated tonic (sustained) contractions superimposed on the phasic (transient) ones. Threshold potential for initiation of tonic contractions was usually positive to the threshold of phasic contractions. The time taken to attain the final level of tension ranged between 0.7 and 3 sec. Plateau tension, as a function of membrane potential, increased with increasing depolarization and reached a flat maximum at about + 50 mV in normal Ringer fluid.7. At membrane potentials near zero level, plateau tension developed by the tonic mechanism was about twice peak tension due to phasic contraction.8. Removal of Ca ions from the external medium resulted in an almost complete abolition of phasic contraction within 1-2 min and a gradual decrease of tonic contraction during the first 10 min. Application of a ;Ca inhibitor' to normal Ringer fluid caused a strong reduction of both I(Ca) and phasic contraction without affecting tonic contractions.9. It is concluded that phasic contractions are directly activated by the flow of I(Ca). Generation of tonic contractions may be attributed to a Ca transfer mechanism different from I(Ca) or a release of Ca from intracellular stores.

Role of the sarcoplasmic reticulum in glycogen metabolism. Binding of phosphorylase, phosphorylase kinase, and primer complexes to the sarcovesicles of rabbit skeletal muscle

Sarcoplasmic vesicles and beta-glycogen particles 30-40 mmicro in diameter were isolated from perfused rabbit skeletal muscle by the differential precipitation-centrifugation method. This microsomal fraction was subjected to zonal centrifugation on buffered sucrose gradients, in a B XIV Anderson type rotor, for 15 hr at 45,000 rpm in order to separate the two cytoplasmic organelles. Zonal profiles of absorbance at 280 mmicro, proteins, glycogen, and enzymatic activities (phosphorylase b kinase, phosphorylase b, and glycogen synthetase) were performed. Whereas the entire synthetase activity was found combined with the glycogen particles, 39% of phosphorylase and 53% of phosphorylase b kinase activities, present in the microsomal fraction, were recovered in the purified vesicular fraction (d = 1.175). This latter fraction consists of vesicles, derived from the sarcoplasmic reticulum, and of small particles 10-20 mmicro in diameter attached to the outer surface of the membranes. These particles disappear after alpha-amylase treatment. Incubation of the sarcovesicular fraction with (14)C-labeled glucose-1-phosphate confirms the localization of a polysaccharide synthesis at the level of the membranes. "Flash activation" of phosphorylase b, i.e. Ca "activation" of phosphorylase kinase followed by a conversion of phosphorylase b into a, was demonstrated in the purified sarcovesicular fraction. Moreover, the active enzymatic sites were detected on the membranes by electron microscopy. The presence of binding sites between the membranes of the sarcoplasmic vesicles and a glycogen-enzyme complex suggests that this association plays a role in the glycogenolysis during muscle contraction.

Deuterium oxide: inhibition of calcium release in muscle

Calcium release, measured as luminescence of the protein aequorin, was measured simultaneously with membrane potential and isometric tension in single muscle fibers of the barnacle (Balanus nubilus). Deuterium oxide inhibited calcium release and isometric tension but did not affect membrane potential, a result consistent with the postulate that deuterium oxide inhibits the coupling between excitation and contraction.

Action potential parameters affecting excitation-contraction coupling

In quantifying type B potentiation effects, given earlier merely qualitatively, it is found that Zn(2+), 1-50 microM, causes increases in action potential duration, twitch tension, and twitch contraction period time, which are all directly proportional to the log of the concentration. Hence, the duration of the action potential, i.e. the magnitude of its mechanically effective period, is a causal factor quantitatively determining the degree of mechanical activation in the isometric twitch. In higher concentrations of Zn(2+) up to 1000 microM, the spike duration and the contraction time continue to increase but the twitch tension is disproportionately smaller, evidently because the high zinc (500-1000 microM) raises the mechanical threshold of excitation-contraction (E-C) coupling and reduces the intrinsic strength of the contractile system. Eserine (1.5 mM) and also high Zn(2+) not only cause type B potentiation effects, but also slow the rise of the spike, thus causing retardation of the very onset of tension production, which is even greater for high Zn(2+) because of the raised mechanical threshold. This retardation is then succeeded by the faster tension output characteristic of type B potentiation resulting from spike prolongation. Thus, the changes in the consecutive, rising and falling phases of the action potential explicitly register their separate effects in the respective very earliest and directly following periods of twitch output; i.e., each phase of the action potential produces its own mechanical "transform." These transforms, and other effects, suggest that the release of activator Ca(2+) from the sarcoplasmic reticulum during E-C coupling can be graded in both the rate and the total amount of the release.

Structure of a calcium-binding carp myogen

The amino-acid sequence and three-dimensional structure of a calcium-binding protein prepared from carp muscle has been determined. This protein, designated carp-muscle calcium-binding protein B, is one of three closely related parvalbumins found in this tissue. The electron density map, calculated by heavyatom substitution crystallographic methods to 2.0-A resolution, reveals the orientation of most of the amino-acid side chains. The calcium coordination site consists of one glutamic- and three aspartic-acid carboxyl groups in a tetrahedral arrangement. The core of this spherical molecule is remarkably hydrophobic, with 8 of its 10 phenylalanine side chains packed in an approximate herringbone pattern. 52 of the 108 residues are in six alpha-helixes; there is no beta-pleated sheet. The acetylated amino-terminal alanine appears not to be accessible to solvent. All of the heavy-atom derivatives are bound at the sole cysteine. The properties of this protein suggest a relationship to troponin A of mammalian tissue.

Energetics of activation in frog and toad muscle

1. If activation heat reflects the operation of the calcium pump it should be independent of actomyosin activity. The semitendinosus preparation affords a technique for removing actomyosin activity since the muscle can be stretched till there is almost no overlap between the filaments.2. Heat production, H, in twitches and tetani of stretched muscle fits the relation H = A+M.P/P(ot) where P/P(ot) is the fraction of the optimal tension remaining at the stretched length and A and M are assumed to be the activation dependent and actomyosin dependent heat components.3. For twitches the A component is early and fast and constitutes 0.26 (S.D. 0.09) of the heat production at normal muscle lengths. Its time course is similar in both frog and toad muscle although both M and P are twofold slower in toad muscle. High concentrations of CO(2) slow only M and P(ot). The A component is associated with a normal recovery heat.4. The twitch-tetanus tension ratio, after correction for the extra shortening that occurs during a tetanus, does not vary with the degree of muscle stretch: it is thus probable that twitch activation does not vary with muscle stretch.5. Moderately hypertonic Ringer solution reduces M and P(ot) but not A, but strongly hypertonic solution also reduces A. Zn(2+), No(3) (-) and second shock potentiation of a twitch increase A, M and P(ot) in proportion to each other.

Excitation-contraction coupling in amphioxus muscle cells

1. Excitation-contraction coupling was studied in myotomal muscles of amphioxus, Branchiostoma californiense.2. The action potential of a muscle cell produces a twitch with a rise time of 30-40 msec at 11 degrees C and its Q(10) is about 2.2.3. The twitch increases in amplitude with increasing external Ca concentration and is abolished in Ca-free saline (1 mM-EGTA and 55.7 mM-MgCl(2)); the twitch amplitude is suppressed by Co or La ions.4. Caffeine at concentrations above 1 mM in the external saline causes a prolongation of the action potential and a contracture which lasts several minutes.5. After exposure to caffeine the responsiveness of the muscle to subsequent applications of caffeine recovers in normal saline in 20-30 minutes but not in Ca-free saline.6. The amplitude of the caffeine contracture is independent of the external Ca concentration and is unaltered after the twitch is eliminated in Ca-free saline.7. After exposure to caffeine a full-sized twitch can be obtained before the responsiveness to caffeine shows any significant recovery.8. It is concluded that the twitch is produced by the Ca influx resulting from the increased permeability of the muscle cell membrane to Ca during the action potential and that the Ca mobilized by caffeine is not necessary to the initiation of the twitch.9. Electronmicroscopy shows the existence of sarcoplasmic reticulum.

The effect of calcium on the force-velocity relation of briefly glycerinated frog muscle fibres

1. Twitch fibres were isolated from the semitendinosus muscles of frogs. The sarcolemma was made more permeable by a 30 min soak in a solution containing 47.3% glycerine (v/v), 2 mM-EGTA and 10 mM phosphate buffer, pH 7. This was followed by a 30-60 min soak in a solution containing the non-ionic detergent Lubrol-WX. The fibres were then placed in a relaxing medium containing (in mM): KCl, 100; MgCl(2), 1; ATP, 4; EGTA, 2; imidazole buffer, 10; pH 7.0.2. A piece of fibre about 1-2 mm long treated as described in (1) was attached to a servo apparatus. This apparatus made it possible either to hold fibre length constant giving isometric conditions, or alternatively to hold the force constant while measuring isotonic length changes. A special network made it possible to switch control from isometric to isotonic conditions so that afterloaded contractions with a shortening stop could be carried out.3. Contractions were induced at about 4 degrees C by lowering the pCa in the relaxing solution to various levels determined by the ratio of calcium and EGTA added. Contractions were never observed above pCa 7. The steady force generated reached a maximum over the range of pCa 6.09 to 5.49. The relationship between steady force generated and pCa is S-shaped and very steep, implying that multiple interacting binding sites for calcium are involved in the force generating process.4. The relative force-velocity relation is the same at pCa 6.09 and 5.49 where the steady force is at a maximum. The data points can be well fitted by a hyperbola in which the extrapolated value for V(max) is 2.39 muscle lengths/sec. The values obtained for the Hill parameters a/P(0) and b are within the range of those reported for living electrically excited frog muscle.5. The relative force-velocity points obtained at higher pCa values at which the steady force was on average 37% of that developed at pCa 5.49 can also be fitted by a hyperbola. However, the extrapolated value for V(max) is only 1.12 muscle lengths/sec. The value for a/P(0) is increased slightly and the value for b is markedly decreased.6. Evidence is presented against the possibility that an unrecognized fixed internal load is responsible for the change in the relative force-velocity relation obtained at high pCa.7. The relative force-velocity relation does not change appreciably over at least part of the range of sarcomere lengths in which the force generated varies linearly with overlap provided the pCa is held constant.8. The results support the view that lowering the pCa produces a mechanical state equivalent to that produced by tetanic electrical stimulation.9. Some models for calcium activation are discussed. It is concluded that a model based on calcium binding to troponin on the thin filaments is difficult to reconcile with all of the experimental evidence. There is additional evidence for believing that activating calcium may directly influence the cross-bridges.

Experimental alteration of the relationship between the external calcium concentration and the contractile force generated by auricular trabeculae isolated from the heart of the frog, Rana pipiens

1. The contractile strength generated by isolated frog auricular trabeculae has been determined by perfusion with high-K Ringer over a range of [Ca](o).2. Experiments are described in which the cubic relationship between the contracture tension and [Ca](o) has been changed to a square or a linear relationship.3. These results have been interpreted by proposing that three Ca compounds, whose concentrations are proportional to [Ca](o), act co-operatively at some stage of the process leading to the generation of tension.4. The change in contractile strength, determined by regular electrically evoked twitches, has been investigated at different temperatures and the results have been explained by assuming that the concentrations of the three hypothetical activating compounds vary at different rates when [Ca](o) is altered.5. The staircase response is supposed to develop as the consequence of an increase in the concentrations of the two activating Ca compounds with the slowest time constants.6. The possible physical representations of the hypothetical activating compounds are discussed.

Chemical change and energy output during muscular contraction

1. The production of heat and (internal) work and the changes in the amount of phosphocreatine (PCr), ATP, inorganic phosphate (Pi) and sometimes lactate have been measured from moment to moment during and after tetanic isometric contractions of isolated frog muscles at 0 degrees C.2. Heat production was measured by thermopiles and a novel apparatus was employed for freezing the muscles rapidly at a chosen instant so as to halt the chemical processes before analysis.3. Using unpoisoned muscles in oxygen, it was shown that neither oxidative recovery processes nor glycolytic ones led to appreciable restitution of PCr or ATP during 15 sec of contraction. However, clear signs of recovery processes could be seen within a minute. In our preparations artificial ;ageing' by storage at low temperature did not interfere with the capacity for glycolysis.4. Our clearest result was that the break-down of PCr was not nearly large enough to account for the rapid heat production during the first few sec of contraction. By the end of a 15 sec tetanus as much as 10 mcal/g remained unaccounted for.5. The source of this heat is not clear. At no time is there any sign of net break-down of ATP; indeed there appears to be a slight increase of ATP in the stimulated muscle.6. Break-down of PCr continues both during relaxation and during the minute following, while the muscle is at rest. Thus during contraction there is heat production without PCr break-down, while subsequently there is PCr break-down without heat production.

The dependence of the contractile force generated by frog auricular trabeculae upon the external calcium concentration

1. A method is described by which the solutions bathing single auricular trabeculae, isolated from the heart of the frog, can be rapidly altered while the tension generated and the membrane potential can be measured simultaneously.2. Changes of the [Ca](o) result in changes of the twitch strength similar to that reported for frog ventricle.3. At [Ca](o) of less than 1 mM, the isometric contracture tension generated during application of K-rich solutions, and the maximum rate of tension development, are proportional to [Ca](o) (3).4. This relationship is not the consequence of (a) the hypertonicity of the K-rich solutions, (b) the dependence of the membrane potential on [Ca](o), or (c) the facilitation due to a twitch response at the initiation of the contracture.5. Reduction of the [Na](o) increases the strength of the high-K contractures according to the ratio of [Ca](o)/[Na](o) (2); Na ions in the bathing medium are shown to competitively inhibit the potentiating action of Ca ions on the force generated during contractures.6. An equation is derived which assumes that three Ca compounds act co-operatively at some stage in the process of excitation-contraction coupling.7. Two hypotheses are discussed. The first proposes that the sarcoplasmic [Ca] established during depolarization of the muscle membrane depends upon [Ca](o) (3), and tension generated by the contractile elements on a first order reaction with ionic Ca. The second suggests that if the sarcoplasm [Ca] established during excitation is proportional to [Ca](o), then three Ca ions are required to activate the contractile response at the unit level.

Studies of the triad. II. Penetration of tracers into the junctional gap

Ferritin and Imferon molecules were introduced as tracers inside "skinned" muscle fibers to test which part of the triadic junction gap is freely exchangeable with the sarcoplasm. At least 50% of the T-system surface is freely accessible from the sarcoplasm. Of the remainder, 30% of the total T-system surface is covered by the junctional feet, and 20% in the center of the junction may or may not be accessible. The possibility is discussed that the triadic junction may not function as an electrical coupling.

Effects of calcium on the contraction of the hypodynamic frog heart

1. The time course of build-up or decline of twitch tension in response to enhancement or reduction of the external calcium concentration was determined in frog heart ventricles, in the hypodynamic state as well as under non-hypodynamic conditions.2. The tension build-up induced by high calcium was greatly slowed when the heart passed into the hypodynamic state, whereas the decline in low calcium was little altered by this condition.3. The tension decline consisted of two approximately exponential phases, an initial rapid phase (t((1/2)) between 3 and 10 sec) and a later slow phase (t((1/2)) between 50 and 180 sec).4. A similar composite time course of tension build-up only occurred under certain conditions: (a) in ventricles in which the hypodynamic state had not developed; (b) after a conditioning period of exposure to enhanced calcium or reduced sodium concentrations.5. The results are explained on the assumption that contraction is brought about by the co-operative action inside heart cells of two calcium compounds whose concentrations change at different rates after variation of the external calcium concentration.6. Formal relationships describing the dependence of twitch tension on the concentration of these two hypothetical compounds are obtained.A tentative explanation of the development of the hypodynamic state is also proposed.