Properties of soleus muscle Z-lines and induced Z-line analogs revealed by dissection with Ca2+-activated neutral protease

Rat soleus muscle Z-lines and Z-line anomalies induced by neostigmine methyl sulfate (NMS) and cat soleus muscle Z-lines and Z-line anomalies induced by tenotomy were examined by electron microscopy before and after dissection of muscle fibers with Ca2+-activated neutral protease (CAF) to elucidate structural properties of Z-lines and related Z-line-type structures. In both normal and treated muscles, interdigitation of thin (6-7 nm) filaments, which were continuous with I-filaments (actin) from adjacent sarcomeres, was observed at the Z-line in longitudinal section. Both neostigmine methyl sulfate and tenotomy treatments induced muscle atrophy associated with Z-line degradation, streaming, and irregular distribution and accumulation of Z-line material and Z-rod formation. Tenotomized muscle also was characterized by the presence of N-line-like bands and I-Z-I brushes. CAF digestion removed the electron-dense covering material from Z-rods and revealed a backbone of actin filaments. The origin of Z-rods, their structural similarity to Z-lines in longitudinal and cross section, and their susceptibility to CAF indicate that Z-rods are directly related to native Z-lines and are probably lateral polymers of a basic Z-line unit. The regular square net alignment (22 nm) of I-filaments (actin) in cross sections of I-Z-I brushes which contain no N-lines suggests that the I-square net arrangement near the Z-line is determined by Z-filament-actin filament interaction rather than by the N-line or other factors. The results suggest that I-filaments (actin) penetrate the mammalian Z-line and are Z-line constituents and that the width of Z-lines and the length of Z-rods are determined by the amount of overlap of actin filaments. The perpendicular periodicity of Z-rods and the zigzag-oblique arrowheadlike appearance seen in longitudinal sections of Z-lines are attributed to alpha-actinin.