Distribution of mass in relaxed frog skeletal muscle and its redistribution upon activation

Abstract

Five orders of equatorial reflection were recorded from both relaxed and fully activated intact frog sartorius muscle using synchrotron x-ray radiation. Electron density maps of the myofilament lattice in axial projection were calculated from the integrated intensities by Fourier synthesis, using all possible phase combinations. These maps were evaluated systematically in terms of their compatibility with electron microscopically and biochemically derived properties of the lattice structure and with the minimum wavelength principle. For the relaxed state, one phase combination emerged as most consistent with these constraints: it shows a thick filament with a compact core surrounded by an annular shell of density. The distribution of mass suggests that the S-2 moiety of the myosin molecule is an integral part of the thick-filament backbone and the S-1 moiety makes up the shell and is tilted or slewed around the backbone. For the active state, there are two feasible maps, which differ according to whether or not the activation process is associated with phase inversion in two of the reflections. Both maps represent patterns of redistribution of mass upon activation in which the thick-filament backbone is practically unaffected and there is movement of density from the annular shell towards the thin filaments. In addition to this outward radial flux of density from the thick-filament periphery, the pattern of net mass transfer involves a pronounced azimuthal component in both cases. The total net mass transfer is equivalent to approximately 20% (no phase change) or approximately 40% (with phase change) of the S-1 mass. From the observed systematic increase in peak widths of the higher orders, the size of the crystalline domain in the myofilament lattice in the relaxed sartorius is estimated to be greater than 650 nm and the variations in myofilament lattice spacing among different myofibrils to be about +/- 3%. Furthermore, in the activated state, the equilibrium positions of the myofilaments are no longer well ordered, but are distributed statistically about the lattice points with a standard deviation of approximately 3 nm.

X-ray diffraction study of the structural changes accompanying phosphorylation of tarantula muscle

“…3). These changes are similar fo the ones that occur in other muscles (Huxley, 1968;Miller& Tregear, 1972;Wray et al, 1974;Millman & Bennet, 1976;Wakabayashi & Namba, 1981;Padr6n & Huxley, 1984;Yu et al, 1985) which have been interpreted as a movement of the myosin heads toward the actin filaments.…”

“…Figure 3 shows the averaged intensity ratios I10/Ill and I10/I20 obtained in each assayed condition. These changes have been interpreted in other muscles as a movement of the myosin heads toward the actin filaments (Huxley, 1968;Miller & Tregear, 1972;Wray et al, 1974;Millman & Bennet, 1976;Wakabayashi & Namba, 1981;Padr6n & Huxley, 1984;Yu et al, 1985). The mechanical and structural responses of the muscles fo relax and rigor solutions indicate that they were properly skinned.…”

Impact of osmotic compression on sarcomere structure and myofilament calcium sensitivity of isolated rat myocardium

“…There is an appreciable amount of lattice spacing inhomogeneity ( d ϭ ⌬d 10 /d 10 ) that stays relatively constant between 0% and 6% dextran compression but increases at higher lattice compression. Likewise, s , which can be related to the amount of liquid-like disorder in the myofilament lattice (41), is relatively low (ϳ1%) but also does not vary greatly between 0% and 6% dextran and increases at higher levels of lattice compression. If disorder is the cause of the weak outer reflections, the amount of disorder appears to be about the same over the range of osmotic pressure that yields a physiological lattice spacing, and thus this phenomenon is unlikely to be the cause of the observed changes in myofilament calcium sensitivity.…”

“…The I 1,1/I1,0 intensity ratio can be used to estimate shifts of mass (presumably cross-bridges) from the region of the thick filament to region of the thin filament. The widths of the pseudo-Gaussian peaks representing the X-ray reflections, as used in the fitting procedure, will vary systematically with distance from the center depending on the relative amount of heterogeneity in interfilament spacing among the myofibrils (as estimated by the parameter d) and the amount of paracrystalline (liquid-like) disorder (as estimated by the parameter s) of the myofilaments in the hexagonal lattice (41). These parameters, d and s, can therefore be used as a measure of changes in the relative amounts of each kind of disorder.…”

Titin strain contributes to the Frank–Starling law of the heart by structural rearrangements of both thin- and thick-filament proteins

“…4B), even though, upon stretch, it shows a similar relative change as the change in the WT muscles. Various authors (15,22) have noted that a decrease in the I 1,1 /I 1,0 ratio can be due to a reduced radial ordering of the thin filament relative to the thick filament, such that a decrease in the I 1,1 /I 1,0 does not necessarily require radial movement of the cross-bridges, or, alternatively, that it could be due merely to the removal of actin out of the bare zone in the center of the thick filament upon stretch (23). The 2D ED maps calculated from the higher order equatorial data (Fig.…”