SMOOTH MUSCLE: AN ULTRASTRUCTURAL BASIS FOR THE DYNAMIC OF ITS CONTRACTION

Extrinsic and Intrinsic Factors Determine Expression Levels of Gap Junction-Forming Connexins in the Mammalian Retina

Gap junctions (GJs) are not static bridges; instead, GJs as well as the molecular building block connexin (Cx) proteins undergo major expression changes in the degenerating retinal tissue. Various progressive diseases, including retinitis pigmentosa, glaucoma, age-related retinal degeneration, etc., affect neurons of the retina and thus their neuronal connections endure irreversible changes as well. Although Cx expression changes might be the hallmarks of tissue deterioration, GJs are not static bridges and as such they undergo adaptive changes even in healthy tissue to respond to the ever-changing environment. It is, therefore, imperative to determine these latter adaptive changes in GJ functionality as well as in their morphology and Cx makeup to identify and distinguish them from alterations following tissue deterioration. In this review, we summarize GJ alterations that take place in healthy retinal tissue and occur on three different time scales: throughout the entire lifespan, during daily changes and as a result of quick changes of light adaptation.

Connexin36 Expression in the Mammalian Retina: A Multiple-Species Comparison

Much knowledge about interconnection of human retinal neurons is inferred from results on animal models. Likewise, there is a lack of information on human retinal electrical synapses/gap junctions (GJ). Connexin36 (Cx36) forms GJs in both the inner and outer plexiform layers (IPL and OPL) in most species including humans. However, a comparison of Cx36 GJ distribution in retinas of humans and popular animal models has not been presented. To this end a multiple-species comparison was performed in retinas of 12 mammals including humans to survey the Cx36 distribution. Areas of retinal specializations were avoided (e.g., fovea, visual streak, area centralis), thus observed Cx36 distribution differences were not attributed to these species-specific architecture of central retinal areas. Cx36 was expressed in both synaptic layers in all examined retinas. Cx36 plaques displayed an inhomogenous IPL distribution favoring the ON sublamina, however, this feature was more pronounced in the human, swine and guinea pig while it was less obvious in the rabbit, squirrel monkey, and ferret retinas. In contrast to the relative conservative Cx36 distribution in the IPL, the labels in the OPL varied considerably among mammals. In general, OPL plaques were rare and rather small in rod dominant carnivores and rodents, whereas the human and the cone rich guinea pig retinas displayed robust Cx36 labels. This survey presented that the human retina displayed two characteristic features, a pronounced ON dominance of Cx36 plaques in the IPL and prevalent Cx36 plaque conglomerates in the OPL. While many species showed either of these features, only the guinea pig retina shared both. The observed similarities and subtle differences in Cx36 plaque distribution across mammals do not correspond to evolutionary distances but may reflect accomodation to lifestyles of examined species.

Developmental changes in the expression level of connexin36 in the rat retina

Connexin36 (Cx36) is the major gap junction forming protein in the brain and the retina; thus, alterations in its expression indicate changes in the corresponding circuitry. Many structural changes occur in the early postnatal retina before functional neuronal circuits are finalized, including those that incorporate gap junctions. To reveal the time-lapse formation of inner retinal gap junctions, we examine the developing postnatal rat retina from birth (P0) to young adult age (P20) and follow the expression of Cx36 in the mRNA and protein levels. We found a continuous elevation in the expression of both the Cx36 transcript and protein between P0 and P20 and a somewhat delayed Cx36 plaque formation throughout the inner plexiform layer (IPL) starting at P10. By using tristratificated calretinin positive (CaR(+)) fibers in the IPL as a guide, we detected a clear preference of Cx36 plaques for the ON sublamina from the earliest time of detection. This distributional preference became more pronounced at P15 and P20 due to the emergence and widespread expression of large (>0.1 μm(2)) Cx36 plaques in the ON sublamina. Finally, we showed that parvalbumin-positive (PV(+)) AII amacrine cell dendrites colocalize with Cx36 plaques as early as P10 in strata 3 and 4, whereas colocalizations in stratum 5 became characteristic only around P20. We conclude that Cx36 expression in the rat IPL displays a characteristic succession of changes during retinogenesis reflecting the formation of the underlying electrical synaptic circuitry. In particular, AII cell gap junctions, first formed with ON cone bipolar cells and later with other AII amacrine cells, accounted for the observed Cx36 expressional changes.

Influence of dispersion in myosin filament orientation and anisotropic filament contractions in smooth muscle

A new constitutive model for the biomechanical behaviour of smooth muscle tissue is proposed. The active muscle contraction is accomplished by the relative sliding between actin and myosin filaments, comprising contractile units in the smooth muscle cells. The orientation of the myosin filaments, and thereby the contractile units, are taken to exhibit a statistical dispersion around a preferred direction. The number of activated cross-bridges between the actin and myosin filaments governs the contractile force generated by the muscle and also the contraction speed. A strain-energy function is used to describe the mechanical behaviour of the smooth muscle tissue. Besides the active contractile apparatus, the mechanical model also incorporates a passive elastic part. The constitutive model was compared to histological and isometric tensile test results for smooth muscle tissue from swine carotid artery. In order to be able to predict the active stress at different muscle lengths, a filament dispersion significantly larger than the one observed experimentally was required. Furthermore, a comparison of the predicted active stress for a case of uniaxially oriented myosin filaments and a case of filaments with a dispersion based on the experimental histological data shows that the difference in generated stress is noticeable but limited. Thus, the results suggest that myosin filament dispersion alone cannot explain the increase in active muscle stress with increasing muscle stretch.

The ultrastructure and contractile properties of a fast-acting, obliquely striated, myosin-regulated muscle: the funnel retractor of squids

We investigated the ultrastructure, contractile properties, and in vivo length changes of the fast-acting funnel retractor muscle of the long-finned squid Doryteuthis pealeii. This muscle is composed of obliquely striated, spindle-shaped fibers ~3 mum across that have an abundant sarcoplasmic reticulum, consisting primarily of membranous sacs that form 'dyads' along the surface of each cell. The contractile apparatus consists of 'myofibrils' approximately 0.25-0.5 microm wide in cross section arrayed around the periphery of each cell, surrounding a central core that contains the nucleus and large mitochondria. Thick myofilaments are approximately 25 nm in diameter and approximately 2.8 microm long. 'Dense bodies' are narrow, resembling Z lines, but are discontinuous and are not associated with the cytoskeletal fibrillar elements that are so prominent in slower obliquely striated muscles. The cells approximate each other closely with minimal intervening intercellular connective tissue. Our physiological experiments, conducted at 17 degrees C, showed that the longitudinal muscle fibers of the funnel retractor were activated rapidly (8 ms latent period following stimulation) and generated force rapidly (peak twitch force occurred within 50 ms). The longitudinal fibers had low V(max) (2.15 +/-0.26 L(0) s(-1), where L(0) was the length that generated peak isometric force) but generated relatively high isometric stress (270+/-20 mN mm(-2) physiological cross section). The fibers exhibited a moderate maximum power output (49.9 W kg(-1)), compared with vertebrate and arthropod cross striated fibers, at a V/V(max) of 0.33+/-0.044. During ventilation of the mantle cavity and locomotion, the funnel retractor muscle operated in vivo over a limited range of strains (+0.075 to -0.15 relative to resting length, L(R)) and at low strain rates (from 0.16 to 0.91 L(R) s(-1) ), corresponding to a range of V/V(max) from 0.073 to 0.42. During the exhalant phase of the jet the range of strains was even narrower: maximum range less than +/-0.04, with the muscle operating nearly isometrically during ventilation and slow, arms-first swimming. The limited length operating range of the funnel retractor muscles, especially during ventilation and slow jetting, suggests that they may act as muscular struts.

Ultrastructural basis of airway smooth muscle contractionThis article is one of a selection of papers published in the Special Issue on Recent Advances in Asthma Research

The sliding filament theory of contraction that was developed for striated muscle is generally believed to be also applicable to smooth muscle. However, the well-organized myofilament lattice (i.e., the sarcomeric structure) found in striated muscle has never been clearly delineated in smooth muscle. There is evidence that the myofilament lattice in some smooth muscles, such as airway smooth muscle, is malleable; it can be reshaped to fit a large range of cell dimensions while the maximal overlap between the contractile filaments is maintained. In this review, some early models of the structurally static contractile apparatus of smooth muscle are described. The focus of the review, however, is on the recent findings supporting a model of structurally dynamic contractile apparatus and cytoskeleton for airway smooth muscle. A list of unanswered questions regarding smooth muscle ultrastructure is also proposed in this review, in the hope that it will provide some guidance for future research.

Contractile filament architecture and force transmission in swine airway smooth muscle

It is well known that the cyclic interaction of myosin cross bridges with actin filaments is responsible for force and shortening generation in smooth muscle. The intracellular organization of contractile filaments, however, is still poorly understood. Here, we show electron microscopic and functional evidence that contractile filaments in airway smooth muscle lie parallel to the longitudinal axis of the cell bundle, in contrast to the obliquely arranged filaments depicted in conventional models. The parallel arrangement of contractile filaments is maintained despite the fact that individual cells are spindle-shaped. This is accomplished through filament attachment to membrane-associated dense plaques that are in turn connected to similar structures on neighboring cells. Intracellularly, the parallel arrangement is maintained despite the centrally located nucleus. This is accomplished by attachment of actin filaments to the nuclear envelope and making the nucleus a force transmitting structure. The results suggest that smooth muscle cells in tissue form a mechanical syncytium and are able to function properly only as a group.

Ordered distribution of membrane-associated dense plaques in intact quail gizzard smooth muscle cells revealed by freeze-fracture following treatment with cholesterol probes

The surface distribution of membrane-associated dense plaques in intact quail gizzard smooth muscle cells was investigated by freeze-fracture. Replicas of fractured smooth muscle cell plasma membrane showed caveola-free regions with few intramembrane particles, interspersed with caveola-populated areas with a higher intramembrane particle density. Electron microscopy of thin sections of quail gizzard smooth muscle revealed the regions free of caveolae to be occupied by membrane-associated dense plaques; anchoring sites for the contractile filaments of the cell. Demarcation between the caveola-populated and caveola-free regions on the relicated intramembrane surface was not clear and thus provided little information concerning the distribution of dense plaque sites. However, treatment of the smooth muscle tissue with the cholesterol-binding agents filipin or tomatin prior to freeze-fracture allowed the dense plaque sites to be easily observed as the sites remained free of the membrane deformations characteristic of these agents. The dense plaque sites consist of caveola-free oval areas juxtaposed in regular bands that traverse the long axis of the cell. The dense plaque sites on the freeze-fracture replica were confirmed by electron microscopy of thin sections of filipin-treated quail gizzard smooth muscle cells, which showed the plasma membrane associated with the dense plaques to be unaffected by the actions of filipin, whereas that of the caveola-populated region was severely deformed. The observations presented in this study provide evidence for a highly ordered distribution of dense plaques at the cell surface of intact quail gizzard smooth muscle cells and thus corroborate existing evidence for an organized substructure of smooth muscle cells.

Corkscrew-like shortening in single smooth muscle cells

The slower and more economical contraction of smooth muscle as compared to that of skeletal muscle may relate to the arrangement of its contractile apparatus. Because the arrangement of the contractile apparatus determines the manner in which a single smooth muscle cell shortens, shortening of a contracting cell was examined by tracking of marker bead movements on the cell surface by means of digital video microscopy. Smooth muscle cells were observed to freely shorten in a unique corkscrew-like fashion with a pitch of 1.4 cell lengths (that is, the length change required for one complete rotation of cell) at a rate of 27 degrees per second. Corkscrew-like shortening was interpreted in terms of a structural model in which the contractile apparatus or cytoskeleton (or both) are helically oriented within the cell. Such an arrangement of these cytoarchitectural elements may help to explain in part the contractile capabilities of smooth muscle.

Coalignment of the muscle cell and nucleus, cell geometry and Vv in the tunica media of monkey cerebral arteries, by electron microscopy

We undertook to ascertain how well aligned is the rod-shaped nucleus within the spindle-shaped cell of vascular muscle in order that we might use the darkly staining nucleus in histological sections to indicate precisely the directional alignment of the cell. We fixed cerebral arteries from five monkeys under physiological pressure and embedded portions of the tissue so that mid-plane longitudinal sections of the arteries were obtained; the circumferentially arranged muscle cells were cut in cross-section. From the electron micrographs we obtained the cross-sectional profile of the cell and its nucleus, determining that the centre of the nucleus was on average 9.5 +/- 5.8% (SD) away from the centre of the cell (expressed as a ratio of the cellular diameter). We calculated the alignment between the cell and nucleus to be from 0 to 3 degrees, and obtained a volume fraction of 59% for muscle tissue in the tunica media of these arteries.

Estimates of cellular mechanics in an arterial smooth muscle

Estimates of force generation or shortening obtained from smooth muscle tissues are valid for individual cells only if each cell is contracting homogeneously and if cells anatomically arranged in series are mechanically coupled. These two assumptions were tested and shown to be valid for the pig carotid media under certain conditions. Homogeneity of cellular responses in carotid strips was estimated from the motion of markers on the tissue during K+ -induced isometric contractions. When tissues were stretched to L0 (the optimum length for force generation), there was little marker movement on stimulation. However, considerable marker movement was observed on stimulation at shorter muscle lengths, reflecting localized shortening or stretching. The mechanical coupling of the very small cells in the media was determined by measuring the dependence of cell length on tissue length. Tissues were fixed with glutaraldehyde during isometric contractions at various tissue lengths (0.4--1.1 x L0). The fixed tissues were macerated with acid and the lengths of the dispersed cells were measured. Cell lengths were broadly distributed at all muscle lengths. However, the direct proportionality between mean cell length and muscle length (as a fraction of L0) indicated that cells which are anatomically in series are coupled force-transmitting structures. We conclude that valid estimates of cellular mechanical function in this preparation can be obtained from tissue measurements at lengths greater than about 0.9L0.

Mechanical properties of smooth muscle cells in the walls of arterial resistance vessels

1. Methods have been developed for measuring the dynamic mechanical response of arterial resistance vessels (i.d. 83--235 micrometer) with a time resolution of about 4 msec. 2. Observations of the microscope image of the smooth muscle cells in the walls of these vessels indicate that there is little intercellular compliance in this preparation, and that the mechanical properties of the activated preparation are a reflexion of the mechanical properties of the individual smooth muscle cells. 3. Under isometric conditions the force developed per unit cell area was about 350 mN/mm2. Under isotonic conditions the cells had a maximum velocity for shortening at 37 degrees C of about 0.17 lengths/sec. 4. Quick releases of activated vessels indicate that the instantaneous elastic characteristic of smooth muscle cells is approximately exponential. 5. The wall tension response to small (0.3%) square wave changes in circumference was proportional to the logarithm of the time following the start of each circumference change. 6. Active wall tension, deltaT, was varied by varying the Ca2+ concentration of the activating solution. Under these conditions the active dynamic stiffness, k, was proportional to deltaT, and was not temperature dependent. The active half response time, tau (the time, taken to recover half the tension change caused by a small change in circumference) was also proportional to deltaT, but here the constant of proportionality had a Q10 of about 1.8. 7. It is concluded that the quick release response and the square wave response are in part a function of the mechanical properties of the crossbridges between the contractile filaments. Calculations show that both these responses can be explained if it is assumed that there is a relatively compliant passive component in series with the crossbridges.

Arrangement of smooth muscle cells and intramuscular septa in the taenia coli

Bands of electron-dense material beneath the cell membrane of smooth muscle cells of the guinea-pig taenia coli provide attachment to thin myofilaments and to intermediate (10nm) filaments; about 50% of the cell membrane is occupied by dense bands in muscle cells transversely sectioned at the level of their nucleus, and between 50 and 100% in small cell profiles nearer the cell's ends. In addition to the known cell-to-cell junctions (intermediate contacts), more complex apparatuses anchor muscle cells together, either end-to-end or end-to-side of side-to-side. They consist of elaborate folds, invaginations and protrusions accompanied by large amounts of basal lamina material. In the end-to-end anchoring apparatuses numerous finger-like and laminar processes from the two cells interdigitate. Other muscle cells have a star-shaped profile in the last few microns of their length, or show longitudinal invaginations occupied by a thickened basal lamina and occasionally by collagen fibrils. The septa of connective tissue extend only for a few hundred microns along the length of the taenia. In taeniae fixed in condition of mild stretch the muscle cells form an angle of about 5 degrees with the septa. In muscles fixed during isotonic contraction the angle increases to about 29--22 degrees, and in longitudinal sections the muscle cells appear arranged in a herring-bone pattern. The collagen concentration in the taenia coli is 4--6 times greater than in skeletal and cardiac muscles. These various structures are discussed in terms of their possible role in the mechanism of force transmission.

Reorientation of myofilaments during contraction of a vertebrate smooth muscle

The purpose of the investigation was to determine whether filaments within smooth muscle cells changed their orientation (with respect to the main axis of the cell) during contraction. The stomach muscle of Bufo marinus was used, since its cells may be easily isolated, enabling direct observation in living cells. In addition to still micrography, cinemicrography was used to record continuously during contraction. Polarization microscopy revealed a change in birefringence after contraction, with relaxed cells exhibiting uniform birefringence while contracted cells displayed a discontinuous pattern. Movies revealed a progressive change in orientation of birefringent elements from nearly parallel to the cell's main axis in relaxed cells to increasingly larger angles to the cell's axis as contraction progressed. Phase-contrast microscopy revealed a change in filamentous components, from being parallel to the cell's axis in relaxed cells to being in an undulating or helical pattern during concentration. Cell shape tended to follow the configuration of the filamentous component. Electron microscopy of muscle strips corroborated the observations of living cells and substantiated the conclusion that filaments change their orientation from parallel to oblique (with respect to the cell's axis) during shortening with an undulating or helical pattern of filaments in shortened muscles.

The force generated by a visceral smooth muscle

1. Strips of taenia coli from the caecum of the guinea-pig were mounted in an organ bath at 37 degrees C; isometric contractions were elicited with 10(5)M carbachol. Each taenia was stretched to the length at which it produced the maximum active tension; it was then fixed and embedded for measurement of the transverse sectional area. 2. The maximal force produced ranged between 96-1 and 138-3 mN. This corresponded to a force of between 251 and 513 mN.mm(2) (mean: 416 +/-28 [n = 10]). Temperature changes in the range 23-38 degrees C had little effect on the maximal force output.3. When allowance is made for the extracellular space (about 32% of the transverse sectional area), for the non-muscular cells present in the taenia (about 5%), and for the non-contractile material present in the muscle cells (about 10%), the maximal force generated was about 734 mN.mm(2) of contractile material (or almost twice as large as in skeletal muscle).4. Electron microscopy revealed terminal apparatuses at the ends of muscle cells, anchoring the cells to the connective tissue, and cell-to-cell junctions (attachment plaques). In addition, many dense patches of dense bands, sites near the cell surface where filaments are seen to end, were scattered along the entire length of the muscle cell and lay close to bundles of collagen fibrils. 5. It is suggested that the production of such a large force by this smooth muscle is partly explained by the lateral attachment of some contractile units to sites along the entire cell length, which in their turn are anchored to the collagen network; the latter may be considered a sort of intramuscular tendon.

Isometric and isotonic length-tension relations and variaitonsin cell length in longitudinal smooth muscel from rabbit urinary bladder

Isometric and isotonic length-tension relations of longitudinal smooth muscle from rabbit urinary bladder were studied together with muscle cell length and tissue structure as revealed histologically. In vivo strip length at a bladder volume of 10 m1 is referred to as L10. The smooth muscle was relaxed by Ca2+-free solution and contracted by K+-high solution with different Ca2+-concentrations. Maximal active force, 12.5+/-0.4 N/cm2 (S.E., n =11), for wholestrips was attained at a length of 206+/-4% (S.E., n=5) of L10. Passive tension at this length was about 15% of total tension. After correction for amount of connective tissue, maximal active tension of pure muscle bundles was 19 N/cm2. Up to about 165% of L10 isometric and isotonic length-tension relations were identical; if the muscle was stretched beyond this, it failed to shorten isotonically to the same length as when contracting from a shorter starting length. This decreased shortening capacity was reversible if the muscle was shortened passively. The extent of shortening against zero load was dependent on degree of activation suggesting an internal resistance to shortening. A linear relationship was found between bladder radius and muscle cell length, indicating that no slippage occurs between the cells when the bladder is filled. Mean cell diameter in the nuclear regionat L10 was 7.2+/-0.2 mum (S.D.,n=10). Mean macimal active tension per cell was calculated to be 2.3-10(-6) N and occurred at a cell length of 655 mum.

Contraction of isolated smooth-muscle cells--structural changes

The contraction of isolated smooth-muscle cells has been correlated with evagination of the cell membrane, a marked change in myofilament orientation, and a decrease in cellular volume. Both localized and full contractions have been elicited in the same cell by varying the intensity of electrical stimulation. These results may be explained by a model of the smooth-muscle cell in which the contractile apparatus extends between attachment sites on the cell membrane that are relatively closely spaced.

Fine structure of smooth muscle cells grown in tissue culture

The fine structure of smooth muscle cells of the embryo chicken gizzard cultured in monolayer was studied by phase-contrast optics and electron microscopy. The smooth muscle cells were irregular in shape, but tended to be elongate. The nucleus usually contained prominent nucleoli and was large in relation to the cell body. When fixed with glutaraldehyde, three different types of filaments were noted in the cytoplasm: thick (150-250 A in diameter) and thin (30-80 A in diameter) myofilaments, many of which were arranged in small bundles throughout the cytoplasm and which were usually associated with dark bodies; and filaments with a diameter of 80-110 A which were randomly orientated and are not regarded as myofilaments. Some of the aggregated ribosomes were helically arranged. Mitochondria, Golgi apparatus, and dilated rough endoplasmic reticulum were prominent. In contrast to in vivo muscle cells, micropinocytotic vesicles along the cell membrane were rare and dense areas were usually confined to cell membrane infoldings. These cells are compared to in vivo embryonic smooth muscle and adult muscle after treatment with estrogen. Monolayers of cultured smooth muscle will be of particular value in relating ultrastructural features to functional observations on the same cells.

Obliquely striated muscle. 3. Contraction mechanism of Ascaris body muscle

Segments of the obliquely striated body muscle of Ascaris were fixed at minimum body length after treatment with acetylcholine and at maximum body length after treatment with piperazine citrate and then studied by light and electron microscopy. Evidence was found for two mechanisms of length change: sliding of thin filaments with respect to thick filaments such as occurs in cross-striated muscle, and shearing of thick filaments with respect to each other such that the degree of their stagger increases with extension and decreases with shortening. The shearing mechanism could account for great extensibility in this muscle and in nonstriated muscles in general and could underlie other manifestations of "plasticity" as well. In addition, it is suggested that the contractile apparatus is attached to the endomysium in such a way that the sarcomeres can act either in series, as in cross-striated muscle, or individually. Since the sarcomeres are virtually longitudinal in orientation and are almost coextensive with the muscle fiber, it would, therefore, be possible for a single sarcomere contracting independently to develop tension effectively between widely separated points on the fiber surface, thus permitting very efficient maintenance of isometric tension.