Filament Ultrastructure and Organization in Vertebrate Smooth Muscle

Abstract

Using a variety of preparative techniques for electron microscopy, we have obtained evidence for the disposition of actin and myosin in vertebrate smooth muscle. All longitudinal myofilaments seen in sections appear to be actin. Previous reports of two types of longitudinal filaments in sections are accounted for by technical factors, and by differentiated areas of opacity along individual filaments. Dense bodies with actin emerging from both ends have been identified in homogenates, and resemble Z discs from skeletal muscle (Huxley, 1963). In sections, short, dark-staining lateral filaments 15 25 A in diameter link adjacent actin filaments within dense bodies and in membrane dense pataches. They appear homologous with Z-disc filaments. Similar lateral filaments connect actin to plasma membrane. Dense bodies and dense patches, therefore, are attachment points and denote units analogous to sarcomeres. In glycerinated, methacrylate-embedded sections, lateral processes different in length and staining characteristics from lateral filaments in dense bodies exist at intervals along actin filaments. These processes are about 30 A wide and resemble heavy meromyosin from skeletal muscle. They also resemble heads of whole molecules of myosin in negatively stained material from gizzard homogenates. Intact single myosin molecules and dimers have been found, both free and attached to actin, even in media of very low ionic strength. Myosin can, therefore, exist in relatively disaggregated form. Models of the contraction mechanism of smooth muscle are proposed. The unique features are: (1) Myosin exists as small functional units. (2) Movement occurs by interdigitation and sliding of actin filaments.

A filamentous cytoskeleton in vertebrate smooth muscle fibers.

“…Direct counts of actin filaments reveal also that they are more abundant than filaments containing myosin: values of 15-60:1 are reported (43,50), although this particular issue is complicated by the difficulty in preserving the myosin-containing filaments in smooth muscle. Determining the abundance and distribution of the myosin filaments has been a subject of considerable difficulty in recent studies (28,40,44,50). The class of filaments with the largest diameter were earlier assumed to contain myosin only because of their overall resemblance to the thick myosin-containing filaments in striated muscles and because of the similarity in the form of "synthetic" filaments made from the myosin from both smooth and striated muscle (5,12,30,39).…”

“…When methods of preparation affording high resolution were developed, the myofilaments were found to constitute a single class with a diameter of 60-80 A that corresponds to the contractile protein F-actin (24,48). There was no evidence that myosin was present in filamentous form (13,40), although this protein could be isolated and induced to form filaments in vitro (25,29,48). Further work established that filaments resembling the myosin-containing filaments in striated muscles could be found in experimentally treated smooth muscle (5,30,39) and, more recently, when smooth muscles were fixed under "physiological" conditions, numerous thick myofilaments and X-ray reflections characteristic of myosin filaments were found (8,12,21,34,44,50).…”

The Organization of Contractile Filaments in a Mammalian Smooth Muscle

“…Several recent reports from three different groups (2)(3)(4)(5), who found both thick and thin filaments in sections of smooth muscle by electron microscopy, strongly indicated that a mechanism of sliding between thick and thin filaments may operate in smooth muscle as well as striated muscle . One team failed to find thick filaments but instead claimed to have located myosin molecules in sections, and proposed a sliding mechanism involving myosin dimers (6,7) . X-ray diffraction studies (8,40) have generally failed to detect myosin thick-filament reflections, although one study on guinea pig taenia coli did find evidence for ordered arrays of thin filaments (9) .…”

Cross-Striated Arrays of Filaments in Endothelium

“…Another explanation for the presence of only thin filaments could reside in the functional state of the endothelium at time of fixation, since it has recently been shown that relaxed smooth muscle contains solely actin filaments (11) . The possibility of actin-myosin interaction is not precluded, however, by the absence of myosin-like filaments, for there is some evidence that myosin in living smooth muscle may exist in a relatively disaggregated form that is still able to promote the sliding of interdigitated actin filaments (17) . In this regard, actomyosin that is antigenically similar to that of uterine smooth muscle has recently been demonstrated in vascular endothelium by the fluorescent antibody technique (1) .…”