Behaviour of the crossbridges in stretched or compressed muscle fibres

The filament lattice of striated muscle

The filament lattice of striated muscle is an overlapping hexagonal array of thick and thin filaments within which muscle contraction takes place. Its structure can be studied by electron microscopy or X-ray diffraction. With the latter technique, structural changes can be monitored during contraction and other physiological conditions. The lattice of intact muscle fibers can change size through osmotic swelling or shrinking or by changing the sarcomere length of the muscle. Similarly, muscle fibers that have been chemically or mechanically skinned can be compressed with bathing solutions containing very large inert polymeric molecules. The effects of lattice change on muscle contraction in vertebrate skeletal and cardiac muscle and in invertebrate striated muscle are reviewed. The force developed, the speed of shortening, and stiffness are compared with structural changes occurring within the lattice. Radial forces between the filaments in the lattice, which can include electrostatic, Van der Waals, entropic, structural, and cross bridge, are assessed for their contributions to lattice stability and to the contraction process.

Shape and length of myosin heads

There is controversy concerning the shape and length of myosin heads. In the present paper we try to analyse the data and to draw clear conclusions in this field. When the myosin heads are isolated (S1) from the rest of the molecule, their length is approximately 12 nm and their shape is close to that of a prolate ellipsoid with an axial ratio approximately 2.3 (in solution) or close to that of a comma when attached to F-actin (with a length of 12-13 nm). When the myosin heads are observed on a whole molecule, their length is approximately 19 nm and they are pear-shaped. Here we suggest that all these observations are compatible. We believe that, for a whole myosin molecule, a large part of the head-rod joint (S1/S2 joint) is measured with the head, owing to a particularly heavy staining or shadowing of this joint. On the other hand, S1 is probably built up of a head part plus the S1/S2 joint, which is not revealed by the usual techniques (hydrodynamics, X-ray and neutron scattering). Finally, the comma shape would be related to a flexible part in the head region of S1, which is significantly bent when S1 is attached to F-actin, but which would be less bent for S1 in solution. A similar bending also occurs in crystalline S1.

Velocity-induced modifications in the crossbridge and/or the actin filament behavior during shortening of muscle fibers

It has been shown experimentally that, in single muscle fibers, the force-velocity relation is more complicated than usually found (Edman, 1988a. Adv. expl. Med. Biol. 226, 643-652; 1988b, J. Physiol., Lond. 404, 301-321). In particular, there is a discontinuity in this relation for high loads (P/P0 greater than or equal to 0.8), i.e. for low velocities of shortening. Here the mathematical approach of the crossbridge behavior is used, independent of their mechanical role(s). This approach has been already presented in previous papers (Morel, 1984a,b, Prog. Biophys. molec. Biol. 44, 47-96, and references therein). It was found that, at P/P0 approximately 0.80, the force-velocity relation presents a reversal of curvature, which was compatible with the previous results obtained by Edman et al. (1976, Acta Physiol. Scand. 98, 143-156). However, by using extremely elegant techniques, Edman (1988a,b) found a different relation. Here, this problem is studied and it is shown that it is possible to fit the new relationship, provided it is assumed that shortening per se modifies the mechanical properties of the crossbridges and/or the actin filaments. For instance, the interval of attachment of a crossbridge increases with V and the constants of attachment f and detachment g decrease with V. Moreover, it is shown that this approach can predict the approximate constancy of the proportion of crossbridges attached to actin, irrespective of V. This is an old result presented by Podolsky et al. (1976, Proc. natn. Acad. Sci. U.S.A. 73, 813-817). This proportion is approximately 0.97.(ABSTRACT TRUNCATED AT 250 WORDS)