The contractile and control sites of natural actomyosin

Modulating factors of calcium-free contraction at low [MgATP]: a physiological study on the steady states of skinned fibres of frog skeletal muscle

Factors that modulate Ca(2+)-free contraction at low [MgATP] were examined by analysing steady tension development in skinned fibres of frog skeletal muscle. The commonly accepted bell-shaped relationship between steady tension and log (1/[MgATP]) was found to be highly susceptible to subtle experimental conditions at the higher [MgATP] side (right limb). The limb shifted to the right with increased fibre thickness, interrupted stirring of the bathing solution, increased temperature and fibre extension, although the effects of temperature and extension were marked only in thick fibres (cross-sectional area > 6000 microns 2). The shift of the right limb was reproduced by an addition of ADP to the bathing solution. These results, together with the extreme steepness of the right limb in thick fibres, suggest that a diffusion-dependent self-regenerative activation occurs in thick fibres in which ADP accumulation and ATP depletion positively feed back through further activation of the myofibrillar ATPase. Numerical simulation supported the hypothesis of the self-regenerative activation under poor diffusion conditions, and suggested that a small rise in temperature and fibre extension can trigger the self-regenerative process at the right limb. Consequently, ADP, temperature and fibre extension are deduced to be the primary potentiators of the activation at low [MgATP]. The high efficiency of ADP in shifting the limb suggests that the activating efficiency of the MgADP-bound actomyosin complex is higher than the nucleotide-free actomyosin complex.

Effect of ATP depletion on the isolated thick filament of limulus striated muscle

With the dynamic light scattering method, we have shown that calcium ions increase the high-frequency internal motions of isolated thick filaments of Limulus striated muscle in the presence of ATP (Kubota, K., B. Chu, Shih-fang Fan, M.M. Dewey, P. Brink, and D. Colflesh. J. Mol. Biol. 1983. 166:329 and Fan, S.-f., M.M. Dewey, D. Colflesh, B. Gaylinn, R. Greguski, and B. Chu. Biophys. J. 1985. 47:809). If ATP is removed from the suspending medium, an increase of high-frequency internal motions also has been observed with characteristics different from those of filaments suspended in a medium containing ATP and calcium ions. These internal motions appear whether calcium ions are present or not and are suppressed by trifluoperazine (TFP). The motions differ from the calcium ion-induced motions in that (a) an energy supply is not required; (b) they are insensitive to heat treatment (42 degrees C, 10 min) and (c) they are also insensitive to phenylmethylsulfonyl fluoride which blocks the motions in the presence of ATP and calcium ions (Fan, Shih-fang, M.M. Dewey, D. Colflesh, B. Gaylinn, R. Greguski, and B. Chu. 1985. Biochim. Biophys. Acta. 827:101). Electron micrographs of negatively stained thick filaments in an ATP-free medium show that the majority of crossbridges extend out from the backbone of the filament and optical diffraction patterns from these filaments lack layer lines arising from the crossbridges. The flexibility of the thick filaments suspended in ATP-free media increases.

Studies on the special properties of actomyosin in the gel form. II. An analysis of the turbidity changes seen in gel suspensions

The turbidity changes induced by MgATP in suspensions of actomyosin gel particles have been studied systematically over a wide range of MgATP concentrations at different temperatures with and without the regulatory proteins. An analysis of these changes distinguishes three separate protein-protein interactions in the gel: (1) The transient cyclic interactions between actin and myosin involved in the hydrolysis of MgATP and contraction. (2) Cross-links that cause turbidity in the original suspension. (3) Cross-links that cause the high turbidity usually associated with superprecipitation. In general, there was a good correlation between the half-time for reaching maximum turbidity during superprecipitation and the rate of hydrolysis. On the other hand, the actual magnitude of the turbidity increase was progressively diminished as the concentration of substrate was raised in the millimolar range. It appears that some essential phase of the superprecipitation process is limited by the same enzymatic step that limits the rate of hydrolysis. However, whether or not this phase leads to an increase in turbidity depends on the concentration of MgATP. Apparently, high physiological levels of MgATP (1-5 mM) inhibit the formation of the specific cross-links that cause high turbidity in the superprecipitate. It is proposed that MgATP interferes with these links when it can bind to a low-affinity site on myosin that is separate from the high-affinity active sites for hydrolysis. Thus, in contracting muscle, interfilament interactions analogous to those that increase turbidity and cause isodimensional shrinkage in the gel would be prevented by the high level of MgATP in the sarcoplasm. Observations relating the shortening of isolated myofibrils to their turbidity in suspension lend support to this interpretation.

Regulation of tension in the skinned crayfish muscle fiber. II. Role of calcium

Tension outputs were measured in skinned crayfish muscle fibers exposed to solutions variously buffered for both Mg-adenosine triphosphate (ATP) and Ca. Two types of data are shown, relating tension and substrate concentration with different levels of Ca present, or tension and calcium concentration at different levels of substrate. The data are fitted by curves calculated from a general equation for substrate inhibition. The equation is based on the schema that both tension and relaxation are induced by the substrate and that the relaxing effect of excess substrate is repressed by calcium. The physiological findings of the present work are similar to data obtained by others on biochemical model systems of the contractile proteins.

Regulation of tension in the skinned crayfish muscle fiber. I. Contraction and relaxation in the absence of Ca (pCa is greater than 9)

In isolated skinned crayfish muscle fibers bathed in solutions that were buffered to be virtually free of Ca(2+) (pCa 8-10) the substrate for both contraction and relaxation is the MgNTP complex. Tension increased up to 50% of the maximum capability of the fiber as the substrate MgATP increased to an optimum (pMgATP = 5.5). Relaxation was induced by further increases in MgATP. Similar bell-shaped curves of tension vs. pMgNTP were obtained with UTP and ITP, but optimum pMgUTP was about 4.5 and optimum pMgITP was about 2.6. The relation between equilibrium tension and pMgNTP is described by an equation analogous to that for the kinetics of enzymes regulated by substrate inhibition.

Rigor contraction and the effect of various phosphate compounds on glycerinated insect flight and vertebrate muscle

1. Glycerol-extracted flight muscle from the giant water-bug, Lethocerus cordofanus, undergoes contraction when deprived of ATP. This rigor contraction occurs in the presence of the calcium chelating agent EGTA, under conditions in which the predicted free calcium ion concentration is less than 5 x 10(-9)M. A mechanically similar contraction has been observed by depriving the muscle of Mg(2+), in the presence of 5 mM-ATP.2. In the rigor contraction tensions of up to 120 mN/mm(2) under isometric conditions, and shortenings of between 2(1/2) and 6% under isotonic conditions, have been observed at 20 degrees C.3. Muscles in rigor can be relaxed by the addition of ATP. The minimum concentration of ATP required to give full relaxation depends upon other ionic constituents of the solutions, and upon temperature, but is between 0.3 and 1 mM at 20 degrees C. Lower concentrations result in partial reduction of tension and stiffness.4. Pyrophosphate (PP) causes a reduction in the tension of muscle which has developed rigor under isometric conditions, but the stiffness, when measured at frequencies between 1 and 100 Hz, remains indistinguishable from that of rigor muscle. The stiffness when measured by applying slow length changes is comparable to that of ATP-relaxed muscle. It is suggested that in PP-relaxed muscle the cross-bridges remain in close proximity to, but are not rigidly attached to, the I filaments, resulting in a high viscous interaction between the two sets of filaments.5. The addition of low concentrations of ADP (less than about 0.7 mM) to rigor muscle in the absence of ATP causes an increase in tension. The effect is sigmoid, and is probably due to the formation of low concentrations of ATP throughout the body of the fibre by myokinase activity. Larger concentrations of ADP (about 5 mM) added to rigor muscle cause relaxation, probably due to the formation of higher ATP concentrations.6. AMP and inorganic orthophosphate have little effect upon the mechanical properties of rigor muscle.7. There is a delay of about 1 min before the onset of rigor contraction when fibres are transferred from an ATP-solution to one containing no ATP, due to the transfer of ATP in the body of the fibres. Both ATP hydrolysis by the fibres and diffusion of ATP into the bathing solution contribute significantly to the rate of depletion of ATP.8. Rabbit psoas muscle shows a similar rigor contraction in the presence of EGTA, and has mechanical properties similar to those described for Lethocerus flight muscle in the presence of pyrophosphate.

Structure and some contractile properties of fast and slow muscles of the chicken

1. A comparative study has been made of the structure of the posterior and anterior latissimus dorsi muscles (P.L.D. and A.L.D.) of the chicken. Some of the contractile properties of these muscles, particularly of the A.L.D. muscle, have also been examined.2. In confirmation of earlier work, it is found that structural differences between these two muscles mainly reside in the organization of the T-system and the sarcoplasmic reticulum.3. In the P.L.D. muscle the T-system and sarcoplasmic reticulum are tubular networks of regular pattern, with frequent regions of contact between them in the form of the familiar triads. As described earlier, two series of triads are present in each sarcomere, near the level of the A-I boundary.4. The presence in the A.L.D. muscle of both T-system and sarcoplasmic reticulum has been established here by the combined use of electron microscopic and histochemical techniques. The appearance of these structures is considerably less regular in this than in the P.L.D. muscle, and the area of contact between them relatively small: in terms of contact area per sarcomere, about 15% or less of that in the P.L.D. muscle. Regions of contact are mainly in the form of dyads, but a few triads are also present.5. Contractures of the A.L.D. muscle in high K fluids have been recorded and their modification due to variation of the external concentration of Ca, K and Mg examined. The K contracture consists of two phases: an initial transient and a later maintained phase. The response of these phases to variation in the external medium is markedly different.6. Application of Ca-free fluids causes contracture tension of the late phase to subside within a few minutes whilst that of the initial phase declines completely only after several hours. High Mg concentrations depress the later but not the early phase of contracture.7. Tentative interpretations of both the morphological and physiological differences described have been proposed.

Parallel response of myofibrillar contraction and relaxation to four different nucleoside triphophates

The Mg chelates of ITP, GTP, and UTP in addition to that of ATP were shown to be capable of causing complete relaxation of myofibrils as indicated by the complete inhibition of syneresis and the reduction of the NTPase activity to that of isolated myosin. For ITP and GTP the required concentrations were about 100 times higher than those for ATP, whereas UTP was maximally effective also in low concentrations (0.2 mM). For all NTP's the concentrations for relaxation were related to those necessary for contraction so that NTP concentrations which gave 80-90% maximal NTPase activity in the presence of Ca caused complete relaxation in the absence of Ca. Thus, these experiments do not support the view that relaxation is caused by the binding of NTP to a special inhibitory site but are quite compatible with the idea that relaxation depends on the extent to which the hydrolytic site is saturated with NTP. The Ca concentration required for contraction depends on the nature of the NTP base and its concentration; it is lower for ITP than for ATP and decreases with decreasing concentrations of ITP.