Critical dependence of calcium-activated force on width in highly compressed skinned fibers of the frog

Abstract

Force development by skinned frog semitendinosus fibers was studied at various levels of lateral compression to compare the results with intact fibers and to evaluate the limits on cross-bridge movements during isometric contraction. The skinned fibers were compressed osmotically using a high molecular weight polymer, dextran T500. Ca-activated force remained constant down to 58% of the fiber width (w0) after skinning, corresponding to a nearly twofold change in separation between the thin and thick filaments in the myofilament lattice. This agrees with the earlier result on intact fibers, and gives additional evidence that the cross-bridge mechanism for force generation is relatively insensitive to large changes in interfilament separation. Further compression, below 0.58 w0, produced a sharp drop in force, and the force was practically zero at a fiber width of 50%. The effect at high compression was the same at all pCa's, which indicates that the Ca sensitivity of the myofilaments is unaffected by radial compression. The stiffness of the fiber remained high in rigor in the presence of dextran, which indicates that the rigor cross-bridge attachment is not inhibited, and actually may be improved, with decreases in the interfilament space. Also, the drop in active force with the highest compression was similar when the compressed fibers were put in rigor before contraction, which suggests that the force drop also was not due to a hindrance to cross-bridge attachment. The results appear to exclude large motions such as tilting and rocking of the bridge as a rigid molecule, but suggest that at least some molecular movement is essential for force development; they also raise the possibility that there is a critical interfilament separation in the fiber, below which the cross-bridge cannot function.

Factors influencing the ascending limb of the sarcomere length‐tension relationship in rabbit skinned muscle fibres.

“…3 and 5), as previously suggested by Maughan & Godt (1981 b) and confirmed by Kawai & Schulman (1985). Recent findings by Gulati & Babu (1985) suggested that force was invariant over a similar range of fibre diameters, although the basis for this apparent discrepancy is unclear. An increase in tension with decreased filament lattice spacing cannot be explained on the basis of changes in the component vectors of cross-bridge force, since previous calculations indicated that the force acting parallel to the long axis of the thick filament remains virtually constant despite changes in lateral filament separation within the physiological range (Julian, Moss & Sollins, 1978;Schoenberg, 1980).…”

Length and myofilament spacing-dependent changes in calcium sensitivity of skeletal fibres: effects of pH and ionic strength

“…Dextran T-500 concentrations up to 8% caused both a slight increase (Gulati & Babu, 1985) and a slight decrease (Goldman & Simmons, 1986) of force in skinned frog fibres, while 3% T-500 caused a slight increase in glycerinated rabbit psoas fibres (Kawai & Schulman, 1985). However, we have measured diameter in resting fibres and some lattice spacing decrease undoubtedly occurred during maximal contraction, particularly without PVP (Kawai & Schulman, 1985;Matsubara et al, 1985).…”

“…Therefore, length-dependent changes in the spacing of charged filaments could alter the electrostatic potential at the surface of each myofilament by altering the degree of superposition of electrostatic fields from adjacent myofilaments (Stephenson & Wendt, 1984). This in turn could influence the sensitivity of the contractile apparatus to calcium ions by altering the binding constant of troponin C for calcium (Stephenson & Wendt, 1984), the concentration of calcium in the myofilament space (Godt, 1981), and/or actin-myosin kinetics or interaction (Eisenberg & Moos, 1968;Krasner & Maughan, 1984;Eisenberg & Hill, 1985;Gulati & Babu, 1985).…”

Pressure sensitivity of active tension in glycerinated rabbit psoas muscle fibres: Effects of ADP and phosphate

“…We argued that the reversible depression of steady tension in a maximally activated muscle fibre must be due to a decrease in the number of operating crossbridges and/or a decrease in force per crossbridge. Additionally, we favoured the view that such changes may be induced directly rather than by changes in filament lattice spacing or in filament lengths; the steady active tension is shown to be relatively insensitive to changes in filament lattice spacing (Gulati & Babu, 1985;Metzger & Moss, 1987) and significant changes in filament lengths were not expected under the conditions adopted in these experiments. The results reported here show that the presence of the products of the ATPase reaction, (ADP and phosphate) alter the sensitivity of the steady active tension to changes in hydrostatic pressure.…”