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Current views regarding some aspects of the contraction of muscle and its regulation are reviewed. Recent work bearing on the segmental flexibility of myosin is related to cross-bridge movements and ATP metabolism in living muscle. Interaction of troponin with calcium, the process that initiates activation of actin-myosin interaction, is discussed in terms of Ca-induced conformational changes and protein-protein interactions.
The leaves of Cassia tora Linn. (Family: Caesalpiniaceae) were soxhlet extracted with methanol. The spasmogenic effects of the extract were evaluated on guinea pig ileum, rabbit jejunum and mice intestinal transit. Antinociceptive activity of the extract was also evaluated in the mice. The LD(50) values of the extract in mice were >2000 mg/kg i.p. and p.o. The extract contracted smooth muscles of guinea pig ileum and rabbit jejunum in a concentration-dependent manner. Atropine reversibly blocked this activity. Mepyramine also reduced the contractile amplitude due to the extract in a concentration-dependent manner. The extract increased intestinal transit in mice dose dependently. C. tora extract significantly (P<0.05) reduced the number of acetic acid induced abdominal constrictions in mice and the effect was comparable to that of aspirin (150 mg/kg i.p.). The extract also significantly (P<0.05) reduced the nociceptive response of mice to increased force (g). The effects were dose-dependent. The studies suggest that the use of C. tora, traditionally, as a purgative and in the treatment of other ailments is justifiable.
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The aim of the present experiment was to investigate the relative contribution of nitric oxide produced in endothelial cells to functional and reactive hyperaemia in the hindlimb of anaesthetized cats. Cats (2.5-3.4 kg) were anaesthetized with alphadalone-alphalaxone, and breathed spontaneously following tracheotomy. Left hindlimb blood flow was measured with a flow probe and hyperaemia responses were monitored following 10 s occlusion of the left external iliac artery and during 20 min stimulation of the sciatic and femoral nerves at 3 Hz. This was repeated following nitric oxide synthase inhibition with NG-nitro-L-arginine methyl ester (L-NAME, 100 mg kg-1, I.V.). Following L-NAME administration, baseline hindlimb blood flow and arterial blood pressure were restored by infusion of sodium nitroprusside (range, 0.3-2.25 micrograms kg-1 min-1, I.V.). Following arterial occlusion, L-NAME reduced the peak reactive hyperaemia (6.5 +/- 0.8 vs. 4.5 +/- 1.0 ml min-1 kg-1, P < 0.05) and blood flow repayment (9.9 +/- 2.3 vs. 6.1 +/- 2.6 ml, P < 0.05) responses. In contrast, the total functional hyperaemia response during hindlimb contraction was not altered (264.7 +/- 68.2 vs. 264.4 +/- 62.8 ml kg-1, n.s.). The results of the study suggest that the production of nitric oxide from endothelial cells does not contribute to functional hyperaemia in contracting skeletal muscle, but plays a large role in reactive hyperaemia. The results imply that flow-dependent dilatation of feed arteries is mediated by NO in reactive hyperaemia.
Rodent motor units, muscle fibers, and motor proteins undergo significant aging-related changes. Such changes include spatial organization and physiological properties of fast- and slow-twitch single motor units, regulation of contractile speed and force generation capacity at the muscle fiber level, and altered functional properties of the motor protein myosin. In addition to specific changes, there also appears to be a "disorganization" of the coordinated expression of contractile, sarcoplasmic reticular, and mitochondrial protein isoforms in aging skeletal muscle. This is suggested to have a strong impact on aging-related impairments in muscle function in addition to the changes in specific muscle proteins.
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Two types of Ca2+-sensitive protein complexes control the contraction of muscle: Troponin (TN) and tropomyosin (TM) are associated with the thin actin filaments, and a specific light chain is a regulatory subunit of myosin itself. Most muscles have both types of regulation. X-ray diffraction diagrams from whole muscle have shown changes in the position of tropomyosin and changes in the pattern of myosin crossbridge attachment associated with different states of the regulatory switches. The full interpretation of these diagrams is often ambiguous, however, and structural studies of the purified proteins provide essential information. Recent crystallographic results reveal that the TM molecule has unusual local domains of marginal stability, leading to extensive motions of the tropomyosin filaments. Electron microscopy of negatively stained thin filaments decorated with subfragments of scallop myosin yields unusually detailed images that show marked conformational changes in myosin crossbridges that are dependent on the presence or absence of the regulatory light chain. These observations suggest that both the special dynamic design of tropomyosin and the striking structural changes in the myosin crossbridges are significant clues for detailed models for the regulatory mechanism.
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When isometrically contracting muscles are subjected to a quick release followed by a shortening ramp of appropriate speed (V(o)), tension decays from its value at the isometric plateau (P(o)) to <0. 05 P(o) with the same time course as the quick part of the release; thereafter, tension remains at a negligible level for the duration of the shortening ramp. X-ray diffraction data obtained under these conditions provide evidence that 1) at V(o) very few heads form an actomyosin complex, while the number of heads doing so at P(o) is significant; 2) relative to rest the actin filament at V(o) is approximately 0.12% shorter and more twisted, while it is approximately 0.3% longer and less twisted at P(o); and 3) the myosin heads attaching to actin during force development do so against a thin filament compliance of at least 0.646 +/- 0.046% nm per P(o).
1. The present study explored the hypothesis that interleukin-6 (IL-6) might be locally produced in response to skeletal muscle contractions and whether the production might reflect the type of muscle contraction performed. Rats were anaesthetized and the calf muscles of one limb were stimulated electrically for concentric or eccentric contractions (4 x 10 contractions with 1 min of rest between the 4 series, 100 Hz). The contralateral muscles served as unstimulated controls. The mRNA levels for IL-6, the glucose transport protein GLUT-4 and beta-actin in the rat muscles (white and red gastrocnemius and soleus) were quantified by quantitative competitive RT-PCR. 2. The IL-6 mRNA level, measured 30 min after the stimulation, increased after both eccentric and concentric contractions and there were no significant differences in IL-6 mRNA levels between the different muscle fibre types. No significant increase in IL-6 mRNA level was seen in the unstimulated contralateral muscle fibres. 3. No increase in GLUT-4 mRNA level was detected, indicating that the increase in IL-6 mRNA level was not due to general changes in transcription. 4. We conclude that IL-6 is locally produced after muscle contraction, with no significant differences between different muscle fibre types. This local production of IL-6 is not due to general changes in transcription, since no changes in the level of GLUT-4 mRNA were found. The fact that increased IL-6 mRNA levels were seen after both concentric and eccentric contractions indicates that the production of IL-6 is not solely due to muscle damage, seen primarily after eccentric exercise.
The calculation of additional heat evolved during contraction of muscle is presented. It is supposed that the cause of arise of additional heat is the transformation in heat of the potential energy which accumulates in actin filament in the process of its torsion.
Hypercontraction or abnormal contraction of vascular smooth muscle is a major cause of diseases such as hypertension and vasospasm of the coronary and cerebral arteries. A better understanding of the mechanism of regulation of smooth muscle contraction should lead to improved treatments for such diseases. Recent studies have revealed important roles for the small GTPase Rho and its effector, Rho-associated kinase (Rho kinase) in Ca2+ independent regulation of smooth muscle contraction. The Rho-Rho-kinase pathway modulates the level of phosphorylation of the myosin light chain of myosin II, mainly through inhibition of myosin phosphatase, and contributes to agonist-induced Ca2+ sensitization in smooth muscle contraction. Rho-Rho-kinase mechanisms also participate in a variety of the cellular functions of non-muscle cells, such as stress-fibre formation, cytokinesis and cell migration. This review summarizes the role of the Rho-Rho-kinase pathway in contractile processes of smooth muscle and in non-muscle cell functions, and the pathophysiological implications of this pathway.
Although muscle contraction is one of the principal themes of biological research, the exact mechanism whereby the chemical free energy of ATP hydrolysis is converted into mechanical work remains elusive. The high thermodynamic efficiency of the process, above all, is difficult to explain on the basis of present theories. A model of the elementary effect in muscle contraction is proposed which aims at high thermodynamic efficiency based on an approximate equilibrium between chemical and mechanical forces throughout the transfer of free energy. The experimental results described in the literature support the assumption that chemimechanic equilibrium is approximated by a free energy transfer system based on the binding of divalent metal ions to the myosin light chains. Muscle contraction demonstrated without light chains is expected to proceed with a considerably lower efficiency. Free energy transfer systems based on the binding of ions to proteins seem to be widespread in the cell. By establishing an approximate chemimechanic equilibrium, they could facilitate biological reactions considerably and save large amounts of free energy. The concept of chemimechanic equilibrium is seen as a supplementation to the concept of chemiosmotic equilibrium introduced for the membrane transport by P. Mitchell.
Attention is drawn to experimental results from many laboratories which indicate that the isometric force (F) in the contraction of striated muscle fibers is linearly proportional to their variable cross-section area (A). Reversible swelling of intact, skinned, or glycinerated fibers can be induced by changes in tonicity, ionic strength or pH. In all cases where careful measurements of F and A are reported, the maximal isometric tension namely, T = F/A, is found around 1 kgf /cm2, even though F and A may change more than threefold for a given fiber at a certain length. These results seem to be independent of the fiber length or temperature. Thus, the isometric tension T in striated muscle does not depend on the number or the rate of the interacting cross-bridges. This result of constant isometric tension, which has so far received little attention, is however, a simple prediction of the hydraulic mechanism which is proposed for muscle contraction. Therefore, the hydraulic model, which is based on the hypothesis of vectorial flux of energetic protons deserves serious consideration.
To investigate the relationship between skeletal muscle metabolism and arteriolar dilations in the region local to contracting muscle fibers as well as dilations at remote arteriolar regions upstream, we used a microelectrode on cremaster muscle of anesthetized hamsters to stimulate four to five muscle fibers lying approximately perpendicular to and overlapping a transverse arteriole. Before, during, and after muscle contraction, we measured the diameter of the arteriole at the site of muscle fiber overlap (local) and at a remote site approximately 1,000 microm upstream. Two minutes of 2-, 4-, or 8-Hz stimulation (5-10 V, 0.4-ms duration) produced a significant dilation locally (8.2 +/- 2.0-, 22.5 +/- 2.4-, and 30.9 +/- 2.1-microm increase, respectively) and at the remote site (4.2 +/- 0.8, 11.0 +/- 1.1, and 18.9 +/- 2.7 microm, respectively). Muscle contraction at 4 Hz initiated a remote dilation that was unaffected by 15-min micropipette application of either 2 microM tetrodotoxin, 0.07% halothane, or 40 microM 18-beta-glycyrrhetinic acid between the local and upstream site. Therefore, at the arteriolar level, muscle contraction initiates a robust remote dilation that does not appear to be transmitted via perivascular nerves or gap junctions.
1. The present study examined the effect of muscle contraction on the rate of extracellular adenosine formation and on the distribution of 5' nucleotidase in primary rat skeletal muscle cells in culture. Experiments were also performed to determine whether the muscle cells release a metabolite upon contraction which may influence the extracellular production of adenosine. 2. Muscle contraction, induced by electrical stimulation, increased (P < 0.05) the rate of adenosine formation in the presence of physiological concentrations (2 and 5 microM) of adenosine monophosphate (AMP). Muscle contraction also led to an increase (P < 0.05) in the maximal rate of extracellular adenosine formation from 4.09 +/- 0.19 to 7.04 +/- 0.27 micromol (g protein)-1 min-1. Similarly, homogenates of contracted muscle cells had a higher (by 19.5 +/- 10.5 %; P < 0.05) AMP 5' nucleotidase activity than homogenates of control cells. 3. Addition of buffer from contracted cells to control cells induced an elevation (18.4 +/- 5.3 %; P < 0.05) in the rate of adenosine formation. The rate of adenosine formation was also increased with decreased intracellular adenylate charge (P < 0.05). 4. Cell homogenates treated with detergent had a higher (by 58.0 +/- 16.3 %; P < 0.05) AMP 5' nucleotidase activity than untreated homogenates, suggesting the existence of an enclosed pool of 5' nucleotidase within the muscle cells. The rate of adenosine formation in the detergent-treated homogenates was similar for electrically stimulated and non-electrically stimulated cells. 5. The present data show that muscle contraction induces an enhanced extracellular adenosine production via an increase in the activity of ecto AMP 5' nucleotidase. The activity of 5' nucleotidase can be elevated via a compound released by muscle cells during contraction and by alteration in intracellular adenylate charge. It is furthermore proposed that the extracellular adenosine formation is increased by translocation of 5' nucleotidase from an enclosed intracellular pool to the muscle membrane.