Z band dynamics as a function of sarcomere length and the contractile state of muscle

Abstract

The Z band in skeletal muscle has two distinct structural states--a relaxed (small square or ss) form and a maximally activated (basket weave or bw) form. We have examined by electron microscopy and optical diffraction Z lattice forms and dimensions and A band spacings in relaxed, tetanized, stretched, and stretched-and-tetanized rat soleus muscle. We have tested the independent contributions of passive load, active tension, and sarcomere length to Z band state. As the A band spacing decreased with increasing load and increasing sarcomere length in the untetanized muscles, the Z lattice remained in the ss form and the Z spacing changed only slightly. Computer-enhanced images from digitized electron micrographs showed that the ss Z lattice resisted deformation regardless of load or method of stretching. In contrast, when the muscle was tetanized at sarcomere lengths of up to 2.7 microns, the Z lattice assumed the bw form and the Z spacing was increased by 20%. Regardless of lattice form, Z spacing did not vary significantly with sarcomere length. Images from freeze-substituted preparations showed both lattice forms comparable to those in images from glutaraldehyde-fixed muscles. Thus, Z band state appears to be a function of the presence (or absence) of active tension. Our previous three-dimensional model is compatible with these observations and with the sub-structures revealed by computer-enhanced images of both lattice forms.

The muscle Z band: lessons in stress management

“…The bw lattice also is the most common in rigor muscle (Edwards et al, 1989). Results from other studies by these investigators, looking at the effect of various parameters (sarcomere length, passive load and active tension, fixation protocol) on Z band lattice structure, are also consistent with the interpretation that only active tension influences the transition from ss to bw (Goldstein et al, 1987). The transition in lattice structure is accompanied by an increase in Z spacing from an average of 20-2I nm in ss to an average of 24nm in bw (Goldstein et al, 1986(Goldstein et al, , 1987.…”

“…In the square lattice a large unit cell (ls, large square) of 22 nm and a small unit cell (ss, small square) of 11 nm are evident (Landon, 1970). The ss and bw lattices differ not only in visual appearance but more impoffantly, they differ in interaxial filament spacing (Z spacing: Goldstein et al, 1986Goldstein et al, , 1987. Both have been shown lattices (Schroeter following section, cardiac and skeletal muscle Z bands to have nearly identical bw and ss et al, 1991).…”

Similar features in Z bands of both skeletal and cardiac muscle revealed by image enhancement

“…These observations also apply to the bw form. In agreement with previous studies (Edwards et al, 1989;Goldstein et al, 1986Goldstein et al, ,1987Goldstein et al, ,1988Goldstein et al, ,1989, the enhanced images reveal a larger Z spacing for the bw form than for the ss form. An interesting feature seen only by image enhancement is the differing appearance of nearest neighbor axial filaments seen in both cardiac and skeletal muscle.…”

Flies deficient in Muscleblind protein model features of myotonic dystrophy with altered splice forms of Z-band associated transcripts

“…The Z-band deWnes the lateral boundaries of the sarcomere and functions to transmit the tension generated by contraction/relaxation activities between successive sarcomeres along a muscle (Goldstein et al 1987(Goldstein et al , 1988. Disruption of the Z-band is observed in several forms of muscular dystrophy, though in most cases the underlying molecular basis for this is unknown.…”