X-ray evidence for the elongation of thin and thick filaments during isometric contraction of a molluscan smooth muscle

Abstract

The elongation of thin and thick filaments during isometric contraction of a molluscan smooth muscle was studied by measuring spacing changes of meridional reflections in the medium-angle X-ray diffraction pattern. X-ray patterns from the anterior byssus retractor muscle of Mytilus edulis in the resting, active, and catch states were taken from the same part of a muscle bundle at a fixed specimen-to-detector distance, using imaging plates and 10 s exposure to synchrotron radiation. The third-order reflection (9.2 A) of the axial period of actin, and the fourteenth-order reflection (10.4 A) of the axial subunit-repeat of the thick filament are increased in spacing in the active and catch states. From accurately measured changes in the axial distance of the 9.2 A layer line from the origin, thin filament elongations in the active and catch states are estimated to be 0.48 and 0.32%, respectively, in a muscle that maintains a tension of 12.2 kg cm-2 in the active state and 9.8 kg cm-2 in the catch state. Thick filament elongations in the active and catch states are similarly estimated to be 0.33, and 0.28%, respectively, based on the axial shift of the 10.4 A reflection. The 0.48% elongation of the thin filament in the active state agrees with an elongation that is presumed by White and Thorson (1973) to estimate the lower limit of the thin-filament stiffness. It seems that in the catch state the activated and resting thin filament structures are intermixed. The activated parts of the thin filament are probably more elongated than the apparent value, 0.32%.

Invertebrate muscles: Thin and thick filament structure; molecular basis of contraction and its regulation, catch and asynchronous muscle

“…This work also showed that muscle activation is associated with a change in thin filament position (Svendsen, 1981(Svendsen, , 1982, that cross-bridges are likely present every 6 th actin monomer (Svendsen, 1981), and that the thin and thick filaments stretch during isometric contractions (Tajima et al, 1994). Electron microscopy is consistent with these and the asynchronous muscle data, showing that activation or rigor decrease thick filament 14.5 nm periodicity and cause the myosin heads to move away from the thick filaments (Vibert and Craig, 1985;Craig, 1989, 1992;Zhao and Craig, 2003a,b), that both heads attach to the thin filament at an acute angle, and that this binding is associated with elongation and bending of the lead head (Craig et al, 1980;Zhao and Craig, 2003b).…”

“…Given the range of physiological ion concentrations present in invertebrates, and between vertebrates and invertebrates, some of the measured differences between the vertebrate and invertebrate data could thus be due to differing responses to experimental procedures. Second, actin filament repeat length depends on Ca ++ concentration (Ruiz et al, 1998), whether the muscle is at rest or in rigor , and the tension the muscle is experiencing (Tajima et al, 1994). Given the importance of thin filament structure in force generation, it would seem useful to repeat this work using modern techniques to determine definitively how much variation actually exists in invertebrate thin filaments.…”

Elastic Properties of Isolated Thick Filaments Measured by Nanofabricated Cantilevers

“…One possibility is that thick filaments do actually lengthen during in situ contraction, the above results notwithstanding. We are not referring to the 0.3% length change reported in ABRM (Tajima et al, 1994) and other species, but to changes of the same large magnitude as found here. This possibility is not necessarily out of accord with the results cited above as the diffraction patterns come mainly from ordered regions of the filaments.…”

The overlap between the thin- and thick-filament reflections in the small-angle X-ray diffraction pattern from a molluscan smooth muscle

“…These re¯ections seem to appear in connection with the well known intensity increase of the 193 A Ê layer line which occurs relatively far from the meridian about the radial position, 0.02 A Ê À1 , as a result of activation of the thin ®laments (Vibert et al, 1972;Tajima & Amemiya, 1991). The intensities of the four re¯ections are approximately in proportion to the intensity of this 193 A Ê re¯ection.…”

“…Background-subtracted axial intensity pro®les in the resting and contracting states that were obtained by radial integration of the intensities within the region 2.5 Â 10 re¯ection because of an intensity decrease of the latter re¯ection (Tajima & Amemiya, 1991).…”