Sodium nitroprusside potentiates hydrogen-sulfide-induced contractions in body wall muscle from a marine worm

Hydrogen sulfide (H2S) at concentrations of about 0.05 to 1 mmol.l(-1) appears to function as a gasotransmitter in vertebrates, analogous to nitric oxide (NO) and carbon monoxide, but the actions of H2S in invertebrate tissue have not been well studied. In this study, we investigated the role of H2S in modulating body wall muscle tone in the marine echiuran worm Urechis caupo (Echiuridae). We first determined that U. caupo body wall homogenates produce H2S upon addition of L-cysteine and pyridoxal-5'-phosphate (PLP), and that the rate is increased by addition of 2-mercaptoethanol, suggesting the presence of an activated L-serine sulfhydrase pathway. We then measured the contractile response of U. caupo body wall circular muscle strips to sodium hydrosulfide (NaHS)--which produces H2S in solution--and the NO donor sodium nitroprusside (SNP), both with and without subsequent application of acetylcholine (ACh). We found that NaHS alone stimulated contraction in muscle strips equivalent to about one-third the force of ACh alone, whereas SNP alone had no effect on muscle tone. However, simultaneous addition of NaHS with SNP elicited a much stronger contraction, reaching more than twice that of ACh alone, which could be increased further by subsequent application of ACh.     

Insect visceral muscle. Fine structure of the proctodeal muscle fibres

The fine structure of the longitudinal muscle fibres of the cockroach proctodeum was investigated by electron microscopy. The fibre is separated incompletely into fibrils, the resting sarcomere length is variable: about 5·8 to 7·3 μm, and the A- and I-bandings are not always clear in longitudinal sections. The ratio of thin and thick filaments at the overlapped region is about 4·1:1 when relaxed, and about 9·8:1 when fully contracted. The myofilament array is not well organized.The previously observed prolonged time course of muscle contraction seems to correlate with the present observations on the poorly developed sarcoplasmic reticulum and irregular distribution of transverse tubules. The Z-bands are irregularly aligned and discontinuous in longitudinal sections. The Z-band structure was studied in relation to the supercontractility. It was found that at maximal isotonic contraction (about 25 per cent rest length) the myofilaments pass through the expanded Z-regions.     

Molluscan visceral muscle fine structure. General structure and sarcolemmal organization in the smooth muscle of the intestinal wall of Buccinum undatum L.

Fine structure of intestinal muscle in the gastropod Buccinum undatum is described. Myofibrillar organization is typical of non-pseudostriated molluscan muscles. The dense body system is poorly developed but there are extensive attachment plaques. The sarcolemma is elaborately modified. Deep infoldings of the membrane give the cells an irregular outline. Such infoldings enclose extracellular matrix and are associated with attachment plaques. Arising from these and from the general sarcolemma are numerous tubular membranous invaginations ending blindly at varying depth in the sarcoplasm. These structures have a helical coat of particles on the cytoplasmic face. Associated with both types of invagination are subsarcolemmal vesicles. The possibility that the tubular invaginations are analogues of vertebrate smooth muscle caveolae or striated muscle T-tubules and that the vesicles are the corresponding sarcoplasmic reticulum is discussed. The occurrence of such structures in molluscan muscle and elsewhere is reviewed.     

A model of smooth muscle cell synchronization in the arterial wall

Vasomotion is a rhythmic variation in microvascular diameter. Although known for more than 150 years, the cellular processes underlying the initiation of vasomotion are not fully understood. In the present study a model of a single cell is extended by coupling a number of cells into a tube. The simulated results point to a permissive role of cGMP in establishing intercellular synchronization. In sufficient concentration, cGMP may activate a cGMP-sensitive calcium-dependent chloride channel, causing a tight spatiotemporal coupling between release of sarcoplasmic reticulum calcium, membrane depolarization, and influx of extracellular calcium. Low [cGMP] is associated only with unsynchronized waves. At intermediate concentrations, cells display either waves or whole cell oscillations, but these remain unsynchronized between cells. Whole cell oscillations are associated with rhythmic variation in membrane potential and flow of current through gap junctions. The amplitude of these oscillations in potential grows with increasing [cGMP], and, past a certain threshold, they become strong enough to entrain all cells in the vascular wall, thereby initiating sustained vasomotion. In this state there is a rhythmic flow of calcium through voltage-sensitive calcium channels into the cytoplasm, making the frequency of established vasomotion sensitive to membrane potential. It is concluded that electrical coupling through gap junctions is likely to be responsible for the rapid synchronization across a large number of cells. Gap-junctional current between cells is due to the appearance of oscillations in the membrane potential that again depends on the entrainment of sarcoplasmic reticulum and plasma membrane within the individual cell.     

Myosin filament structure in vertebrate smooth muscle

The in vivo structure of the myosin filaments in vertebrate smooth muscle is unknown. Evidence from purified smooth muscle myosin and from some studies of intact smooth muscle suggests that they may have a nonhelical, side-polar arrangement of crossbridges. However, the bipolar, helical structure characteristic of myosin filaments in striated muscle has not been disproved for smooth muscle. We have used EM to investigate this question in a functionally diverse group of smooth muscles (from the vascular, gastrointestinal, reproductive, and visual systems) from mammalian, amphibian, and avian species. Intact muscle under physiological conditions, rapidly frozen and then freeze substituted, shows many myosin filaments with a square backbone in transverse profile. Transverse sections of fixed, chemically skinned muscles also show square backbones and, in addition, reveal projections (crossbridges) on only two opposite sides of the square. Filaments gently isolated from skinned smooth muscles and observed by negative staining show crossbridges with a 14.5-nm repeat projecting in opposite directions on opposite sides of the filament. Such filaments subjected to low ionic strength conditions show bare filament ends and an antiparallel arrangement of myosin tails along the length of the filament. All of these observations are consistent with a side-polar structure and argue against a bipolar, helical crossbridge arrangement. We conclude that myosin filaments in all smooth muscles, regardless of function, are likely to be side-polar. Such a structure could be an important factor in the ability of smooth muscles to contract by large amounts.     

Some implications of helical fibres in worm cuticles

The cuticles of many worms contain a cylindrical trellis made of layers of collagen fibres wound round the body in left- and right-hand helices. This kind of extensible skeletal structure imposes various constraints on a worm's behaviour, and some of these are examined, using the nematomorph Parachordodes woltersforffii as the main example. Upon extension or shortening of the cuticle, neighbouring fibres in one layer will eventually touch and so limit further movement; this in turn affects the extent of shortening, elongation, bending and coiling of the whole worm. Simple equations relate: the angle between a fibre and the worm's long axis; fibre spacing and radius; worm body radius and radius of curvature. The relevance to the life of the worm of fibre arrangement, matrix properties and volume changes is discussed.     

Morphogenesis of body wall muscle fibers in Enchytraeus minutus

The helical muscle fibers of Enchytraeus minutus (Annelida, Enchytraeidae) have been investigated with respect to their development. We have shown that longitudinal fibers, in the cocoon, begin to develop as platymyarian fibers, then develop into shallow coelomyarian ones and finally, just before hatching, reach a flat circomyarian stage. After hatching, the fibers grow only in size due to an increase in contractile material in the helical sarcomere system. Indications are given as to the ways this process can take place.     

Fine structure of single fibres of human skeletal muscle

Summary Individual muscle fibres were separated from freeze-dried needle biopsies and classed as type I or type II fibres according to their myofibrillar ATP-ase. Portions of the same fibres were processed for electron microscopy and their fine structure examined. Type I fibres were found to have thicker Z-bands and more mitochondria and lipid droplets than the type II fibres.     

The effect of phallotoxins on the structure of F-actin in myosin-free ghost muscle fibres of rabbit

Using polarized UV fluorescent microscopy it has been shown that phallotoxins (phalloidin-sulfone, phalloidin-sulfoxide-B, phalloidin-sulfoxide-A and dithio-phalloidin) cause an increase in tryptophan fluorescence anisotropy of F-actin myofilaments in myosin-free ghost muscle fibres of rabbit. The results obtained are considered to be evidence of conformational changes in F-actin, induced by phallotoxins. These changes are irreversible to a significant extent, which points to a high degree of actin binding to both toxic and nontoxic phallotoxins.     

The fine structure of striated muscle fibres of tunica muscularis of the intestine in some teleosts

Summary The muscular layer of the fish oesophagus is made up exclusively of striated muscles, which form a circular and a longitudinal layer. The intestinal muscular layer of the tench, which is an exception among the vertebrates in this respect, consists of two layers of striated muscles and a layer of smooth ones. The organization of striated muscle fibres is described from the muscular layer of the alimentary canal in four species: the tench, crucian carp, gudgeon and stone-loach. The fibres of the oesophagus differ from each other in diameter, length of sarcomeres, glycogen content and, in the case of one species, organization of SR. On the basis of the organization of sarcomeres all the fibres examined must be counted among the slow-contracting ones (broad Z line). The sarcoplasmic reticulum is organized according to the Z type except for the fibres of the oesophageal muscles in which the SR represents the A-I type. Two types, en grappe and en plaque, of nerve endings have been observed. The presence of relatively large dense core vesicles seems to indicate their adrenergic origin.This work has been supported by the Polish Academy Sciences.     

Assembly of body wall muscle and muscle cell attachment structures in Caenorhabditis elegans

C. Elegans has four muscle quadrants that are used for locomotion. Contraction is converted to locomotion because muscle cells are anchored to the cuticle (the outer covering of the worm) by a specialized basement membrane and hemidesmosome structures in the hypodermis (a cellular syncytium that covers the worm and secretes the cuticle). To study muscle assembly, we have used antibodies to determine the spatial and temporal distribution of muscle and attachment structure components in wild-type and mutant C. elegans embryos. Myofibrillar components are first observed diffusely distributed in the muscle cells, and are expressed in some dividing cells. Later, the components accumulate at the membrane adjacent to the hypodermis where the sarcomeres will form, showing that the cells have become polarized. Assembly of muscle attachment structures is spatially and temporally coordinated with muscle assembly suggesting that important developmental signals may be passed between muscle and hypodermal cells. Analysis of embryos homozygous for mutations that affect muscle assembly show that muscle components closer to the membrane than the affected protein assemble quite well, while those further from the membrane do not. Our results suggest a model where lattice assembly is initiated at the membrane and the spatial organization of the structural elements of the muscle is dictated by membrane proximal events, not by the filament components themselves.     

Developmental changes in cell wall structure of phloem fibres of the bamboo Dendrocalamus asper

BACKGROUND AND AIMS: Bamboo culms have excellent physical and mechanical properties, which mainly depend on their fibre content and anatomical structure. One of the features which is known to contribute to the high tensile strength in bamboo is the multilayered structure of the fibre cell wall. The aim of this study was to characterize the development of the layered structure in fibre cell walls of developing and maturing culms of Dendrocalamus asper. METHODS: Cell wall development patterns were investigated in phloem fibre caps of vascular bundles in the inner culm wall areas of Dendrocalamus asper of three different age classes (<6 months old, 1 year old, 3 years old). A combination of light microscopy and image analysis techniques were employed to measure cell wall thickness and to determine number of cell wall layers, as well as to describe the layering structure of fibre walls. Two-dimensional maps showing the distribution pattern of fibres according to the number of cell wall layers were produced. KEY RESULTS: The cell walls of fibres in phloem fibre caps located in the inner part of the culm wall of D. asper developed rapidly during the first year of growth. Six different fibre types could be distinguished based upon their cell wall layering and all were already present in the young, 1-year-old culm. In the mature stage (3 years of age) the multilayering was independent of the cell wall thickness and even the thinner-walled fibres could have a large number of wall layers. The multilayered nature of cell wall structure varied considerably between individual cells and was not exclusively related to the cell wall thickness. Nevertheless, fibres at the periphery of the fibre bundles and immediately adjacent to the phloem elements exhibited a consistent and high degree of layering in their cell walls. CONCLUSIONS: The multilayered structure of fibre cell walls was formed mainly during the first year of growth by the deposition of new wall layers of variable thickness, resulting in a high degree of heterogeneity in the layering patterns amongst individual fibres. A degree of 'order' in the distribution of multilayered fibres within the caps does exist, however, with multilayered cell walls common in fibres adjacent to phloem elements and around the edge of the fibre cap. These findings confirm the observations, primarily in Phyllostachys viridi-glaucescens. The layering structure was not found to be specifically related to the thickness of the cell wall.     

Evaluation of the smooth muscle cell component and apoptosis in the varicose vein wall

This study was designed to evaluate the role of the smooth muscle cell and the apoptosis in the pathogenesis of the varicose vein. Segments of saphenous vein were obtained from healthy subjects and from those with varicose veins. The vein specimens were subdivided according to subject age (younger or older than 50 years) and according to the varicose vein source (distal or proximal). Morphological, ultrastructural, cell proliferation (anti-PCNA method) and cell death (TUNEL method) analysis were performed. The walls of healthy, control vein specimens acquired a more collagenous and papillomatous appearance with age. A slight increase in the number of TUNEL-positive cells was also observed in specimens from older subjects. The proportion of apoptotic cells was much greater in the varicose veins than in control specimens. Most cellular alterations were seen in proximal varicose segments obtained from young subjects. These specimens showed hypertrophic areas with a high degree of cellularity (both in the media and in the thickened intima). The highest proportion of apoptotic cells and collagenisation were also observed in these areas. The enhanced number of apoptotic cells in varicose veins observed mainly in proximal/young vein specimens could be responsible, at least in part, for the acceleration of the final fibrosclerotic process characteristic of the varicose vein wall.