Thick filaments resembling myosin in electrophoretically-extracted vertebrate smooth muscle

Thick filaments in vertebrate smooth muscle

Smooth muscle cells of the mouse vas deferens fixed with 5% glutaraldehyde contained three types of filaments, namely, thin (50-80) A) filaments, intermediate (100 A) filaments and thick (120-180 A) filments. However, in 2 out of 16 experiments, under identical conditions, the cells did not contain thick filaments. With OSO4 fixation, thin filaments were not prominent, the most obvious being thick (120-250 A) and intermediate (100 A) filaments. After soaking in a modified Ringer solution under no applied tension for one hour, thick filaments (120-180 A) appeared prominently in smooth muscle cells of the mouse vas deferens and thin filaments were in ordered bundles. By 4 hours, thick filaments had increased in size and density, with thin filaments distributed randomly around them. After 8 hours in Ringer, thin filaments were diffuse and difficult to discern, while thick filaments were large (up to 300 A) and electron-dense. Intermediate (100 A) filaments were present in association with dark bodies. Physiological experiments indicated that the intracellular components responsible for the development of a mechanical response were still functional at this time. The presence of "thick filaments" is also reported in degenerating smooth muscle cells of the guinea-pig vas deferens in tissue culture.

Cytoplasmic filaments in developing and adult vertebrate smooth muscle

An extensive study of adult and developing smooth muscle has revealed the widespread occurrence of a distinct filament with an average diameter of about 100 A (termed the 100 A filament). Unlike that of myofilaments, their appearance in longitudinal section is uniform, but in transverse section they have a round profile, occasionally exhibiting a less electron-opaque core. The 100 A filaments are almost invariably preserved under a variety of fixation procedures, whereas myofilaments, particularly the thicker filaments, are preserved inconsistently. The 100 A filaments appear to be randomly oriented throughout the cytoplasm, either singly or in small groups, although they are sometimes concentrated in the juxtanuclear region of the smooth muscle cells. The intimate association of 100 A filaments with dark bodies, in both developing and adult smooth muscle cells, may indicate that these filaments either play a role in dark body formation or, at least, constitute a part of the dark body. The 100 A filaments are conspicuous in developing smooth muscle cells and occasionally form networks or clusters; they appear to decrease in relative number as maturation proceeds, but considerable numbers are still present in adult tissue.

Thick filaments in vascular smooth muscle

Two sets of myofilaments were demonstrated after incubation of strips of rabbit portal-anterior mesenteric vein under moderate stretch in a physiological salt solution. Thick filaments had a mean diameter of 18 nm and reached a maximum length of 1.4 microm with a mean length of 0.61 microm. In transverse sections, 2.5-5 nm particles were resolved as subunits of the thick filaments. Thin filaments had an average diameter of 8.4 nm and generally conformed to the structure believed to represent actin filaments in smooth and striated muscles. In the areas of maximum concentration there were 160-328 thick filaments/microm(2) and the lowest ratio of thin to thick filaments was 12:1. Thick filaments were present in approximately equal numbers in vascular smooth muscle relaxed by theophylline, in Ca(++)-free solution, or contracted by norepinephrine. The same preparatory procedures used with vascular smooth muscle also enabled us to visualize thick filaments in guinea pig and rabbit taenia coli and vas deferens.