Theory of muscle contraction II. Isotonic contraction

The theory of muscle contraction developed in Part I is extended to non-isometric cases. The basic feature of the approach is the strong viscous coupling of the movement of the counterionic (K⁺) layer with the movement of I-filaments. The surface conductance of the K⁺ layer governs the flux of H⁺ along the I-filaments which in turns regulates the rate of ATP hydrolysis. The energy output of the muscle becomes the function of its mechanical activity. By assuming linear dependence of the K⁺ layer's surface conductance on the velocity of shortening Hill's equation has been derived. With a set of reasonably chosen values of the basic parameters of the theory the values of Hill's constants have been computed. The theory has been also shown to provide the observed dependence of the isometric tension on the degree of the myofilamental overlap.     

Muscular contraction

A molecular theory of muscular contraction, based on the trigger action of the cross bridge between actin and myosin, is postulated. The formation of the cross bridge is followed by a transconformation in contractile protein producing work and liberating heat. The process possesses a mechanochemical character and utilizes the energy liberated by dephosphorylation of ATP. The equation of for tension dependence of muscle power is derived from the theory of reaction rates. The equation of is meaningful after elementary treatment; the physical meaning of the constants in these equations is explained. Quantitative analyses are corroborated by the experimental data.     

Cell locomotion forces versus cell contraction forces for collagen lattice contraction: an in vitro model of wound contraction

Cultured human dermal fibroblasts suspended in a rapidly polymerizing collagen matrix produce a fibroblast-populated collagen lattice. With time, this lattice will undergo a reduction in size referred to as lattice contraction. During this process, two distinct cell populations develop. At the periphery of the lattice, highly oriented sheets of cells, morphologically identifiable as myofibroblasts, show cell-to-cell contacts and thick, actin-rich staining cytoplasmic stress fibers. It is proposed that these cells undergoing cell contraction produce a multicellular contractile unit which reorients the collagen fibrils associated with them. The cells in the central region, referred to as fibroblasts, are randomly oriented, with few cell-to-cell contacts and faintly staining actin cytoplasmic filaments. In contrast it is proposed that cells working as single units use cell locomotion forces to reorient the collagen fibrils associated with them. Using this model, we sought to determine which of these two mechanisms, cell contraction or cell locomotion, is responsible for the force that contracts collagen lattices. Our experiments showed that fibroblasts produce this contractile force, and that the mechanism for lattice contraction appears to be related to cell locomotion. This is in contrast to a myofibroblast; where the mechanism for contraction is based upon cell contractions. Fibroblasts attempting to move within the collagen matrix reorganize the surrounding collagen fibrils; when these collagen fibrils can be organized no further and cell-to-cell contacts develop, which occurs at the periphery of the lattice first, these cells can no longer participate in the dynamic aspects of lattice contraction.     

Calcium in epithelial cell contraction

Epithelial morphogenesis in many organs involves asymmetric microfilament-mediated cellular contractions. Similar contractions, in terms of ultrastructure and cytochalasin B sensitivity, can be induced in the carcinoma cell line C-4II in culture. This line was used to compare total intracellular calcium levels ([Ca]i) in contracted monolayer fragments and in control cultures, and to determine whether epithelial cell contraction depends on influx of extracellular Ca. [Ca]i, defined as Ca not displaceable by lanthanum, was measured by atomic absorption spectrophotometry. Degrees of contraction were determined from shape changes of monolayer fragments. Detachment from the growth surface initiated cellular contractions and caused an immediate increase in [Ca]i, from 1.0 to 4.0-5.0 micrograms Ca/mg protein in early confluent cultures, and from 0.3 to 1.0-2.0 micrograms Ca/mg protein in crowded cultures. This increase was followed by a gradual decline in [Ca]i, though Ca levels remained higher than in controls and contraction progressed for 30 min. Contraction was inhibited completely by cold (7 degrees C) and by Ca-free medium, and in a dose-dependent manner by papaverine (2.5 x 10(-6) M-2.5 x 10(-4) M), lanthanum (1.0 x 10(-6) M-1.0 x 10(-4) M); and D-600 (1.0-2.0 x 10(- 4) M). The Ca ionophore {"type":"entrez-nucleotide","attrs":{"text":"A23187","term_id":"833253","term_text":"A23187"}}A23187 had no effect at 5.0 x 10(-6) M and was inhibitory at higher concentrations. The results provided direct evidence for increased [Ca]i in contracting epithelial cells, and suggest that Ca influx is required for such contraction to take place.     

Theory of muscle contraction: I. Isometric contraction

For each molecule of ATP hydrolyzed by the ATPase at the subfragment 1 of the heavy meromyosin, one H⁺ is produced and remains associated with the myosin heads until a contact with the G-actins of the I-filaments is established. This contact is brought about by the calcium ions released in the sarcomeres by the sarcoplasmic reticulum at the arrival of nerve impulses. A rapid flux of protons along the I-filaments towards the Z-membrane down the concentration gradient leads to the buildup of a diffusion potential which in turn causes a charge-compensating movement of the diffused cationic layer around the I-filaments in the opposite direction. The latter movement exerts a viscous drag on the actins and tends to move the I-filaments deeper into the inter-A-filament spaces towards the M-line. A consistent and straightforward theory of muscular contraction is developed on these lines. The value of the isometric tension in striated muscle fiber of frog at slack length calculated on the basis of this theory agrees well with the measured value.     

Role of swelling in muscle contraction

The two types of volume change occurring in muscle and in contractile protein systems have been defined. Theoretical examination has been made of the influence of hydrophobic group interactions upon these two volume changes. Three different contractile mechanisms have been proposed in which osmotic changes can occur. The most plausible mechanism for striated muscle does not require a cell membrane and can effect a load-sensitive division of energy between direct pull and lateral swelling.     

Ovarian contraction in various teleost families

Fishes from 12 species (a minimum of three fish of each species), representing nine families were examined for ovarian contraction. An ovary from each fish was removed, placed in a muscle bath and assessed for contractions using a force-displacement transducer. After at least 30 min of observation for spontaneous contractions, acetylcholine (ACh) was added to the muscle bath to assess the ovary's capacity for contraction. At least one third of the ovaries from each species contracted spontaneously or in response to 10⁻⁵ M ACh. Mean ± s . e . frequencies of spontaneous contractions ranged from 7·5 ± 2·0 h⁻¹ in the Atlantic silverside Menidia menidia to 380·3 ± 101·5 h⁻¹ in the sheepshead minnow Cyprinodon variegatus . Simultaneously tested ovaries from a single white perch Morone americana contracted at rates of 31·4 and 81·9 h⁻¹. This is consistent with independent generation of the contractile rhythm in the two ovaries. In three species (goldfish Carassius auratus , mummichog Fundulus heteroclitus and white perch), the tunica albuginea was removed from the ovary and found to be capable of independent contraction. This suggests that this membrane contributes to the observed contractions. The presence of ovarian contractions in this varied group of fish species indicates that the contractions play an important role in reproduction.