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First published online May 28, 2005
doi: 10.1242/10.1242/jcs.02368

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`Sarcomeres' of smooth muscle: functional characteristics and ultrastructural evidence

¹ Department of Pathology and Laboratory Medicine, St Paul's Hospital/Providence Health Care, University of British Columbia, 1081 Burrard Street, Vancouver, BC V6Z 1Y6, Canada
² Department of Medicine, St Paul's Hospital/Providence Health Care, University of British Columbia, 1081 Burrard Street, Vancouver, BC V6Z 1Y6, Canada
³ James Hogg iCAPTURE Centre for Cardiovascular and Pulmonary Research, St Paul's Hospital/Providence Health Care, University of British Columbia, 1081 Burrard Street, Vancouver, BC V6Z 1Y6, Canada

View larger version (174K): [in a new window]   Fig. 1. Electron micrograph of a longitudinal section of a trachealis cell bundle fixed at the in situ length (Lin situ) in the relaxed state. Bar, 5 µm.

View larger version (12K): [in a new window]   Fig. 2. The relationship between maximally shortened muscle length (normalized by the in situ length) and the isotonic load (expressed as a percentage of maximal isometric force). Solid circles are data obtained at plateaus of isotonic contractions under various loads and fitted with a linear line (solid). The dotted lines represent a reference length-force relationship from skeletal muscle [reproduced from Gordon et al. (Gordon et al., 1966)] with the sarcomere length of 2.0 µm superimposed on the in situ length of the trachealis. The open circles and filled squares are data obtained after quick stretches of magnitudes of 10% and 30% Lin situ, respectively, that show isometric forces at the stretched lengths and final shortened lengths (arrow) in isotonic contractions under a constant load of 30% Fmax.

View larger version (16K): [in a new window]   Fig. 3. Two-dimensional schematic representation of partial assemblies of contractile units in smooth muscle. (A) In a fully adapted muscle, the thick filaments are assumed to span the entire distance between the associated dense bodies. For simplicity, multiple attachments of thin filaments on one side of a dense body are not depicted. (B) Isometric contraction (1) and isotonic contractions (2,3) against different external loads. (C) Isometric contractions (1,2) at different lengths and isotonic contraction (3) against an isotonic load. Each numbered configuration represents an equilibrium (static) condition where external load equals the force produced by the contractile unit. The solid portion of the thick filaments represents the segment in between the dense bodies that overlaps with both of the thin filaments. It is assumed that only the cross bridges within the solid portion of the thick filaments can interact with the thin filaments properly to generate force. Length of the solid portion of the thick filaments in a contractile unit therefore correlates directly with the ability of the muscle to generate force or carry load during contraction. (See text for more details).

View larger version (128K): [in a new window]   Fig. 4. Examples illustrating the spatial arrangement of myosin filaments and dense bodies that could be interpreted as a standard design of contractile unit in smooth muscle: a myosin filament spanning the distance between two dense bodies on opposite sides of the filament. The longitudinal EM sections were obtained from porcine trachealis fixed at their in situ length and in the relaxed state. Length of the thick filaments and the distance between the dense bodies are between 1.8-2.2 µm. Large arrowheads indicate the relevant dense bodies; small arrowheads follow the myosin thick filaments. Bar, 0.1 µm.

View larger version (213K): [in a new window]   Fig. 5. Electron micrograph of a transverse section of porcine trachealis fixed at the in situ length and in the relaxed state. DB: dense body. The magnified portion shows a dense body surrounded by numerous thin filaments, intermediate filaments (small arrows) and thick filaments (large arrows). Bar, 1 µm.

View larger version (12K): [in a new window]   Fig. 6. The number of thick filaments within a 60-nm perimeter surrounding the edge of dense bodies measured (in micrographs such as that shown in Fig. 5) before and after a 30%-Lin situ length increase was imposed on the muscle. The decrease in number (mean±s.e.m.) immediately after the stretch is statistically significant (*, P<0.05). After adaptation at the stretched length, the number returned to the pre-stretch level.

View larger version (17K): [in a new window]   Fig. 7. Length-force relationships of trachealis preparations obtained at two adapted lengths. The data and fitted line (dashed) shown in gray depict the length-force relationship at Lin situ (from Fig. 2), and are shown here as a reference. The isotonic loads are expressed as a percentage of Fmax. The maximally shortened lengths at the various loads are normalized to Lin situ.

View larger version (18K): [in a new window]   Fig. 8. Force-velocity and force-power relationships of trachealis preparations obtained at 0.75 Lin situ (dashed curves) and at 1.5 Lin situ (solid curves). See text for curve fitting.

View larger version (38K): [in a new window]   Fig. 9. Upper panel: Schematic representation of a trachealis cell and the intracellular arrangement of contractile units at 0.75 Lin situ (left-hand rectangle), and the three possible rearrangements of the contractile units after the cell has been stretched and fully adapted at 1.5 Lin situ. Assuming that the kinetics of actomyosin crossbridge interaction are not affected by the contractile unit reconfiguration, all three models predict no change in isometric force with length doubling; all three models also predict an increase of 67% in shortening velocity, muscle power output, rate of ATP consumption and myosin thick filament density with length doubling [consistent with observations made by Kuo et al. (Kuo et al., 2003), and data in Fig. 8]. In all models, the number of contractile units in series has increased by 67% with length doubling. In Model A, the increase in cell length is associated with the appearance of non-overlap zones between the thick and thin filaments, and no change in the thick filament length. In Model B, the increase in cell length is associated with the appearance of non-contractile elements (instead of non-overlap zones) in series with the contractile units, and no change in the thick filament length. Although the non-contractile elements are placed at the ends of the cell in the drawing, they can be anywhere in the cell as long as they are in series with the contractile units. In Model C, the increase in cell length creates neither non-overlap zones nor non-contractile elements; instead, the thick filament length is increased by 20% and the number of contractile units in parallel decreased by 20%. Lower panel: Predictions by the models regarding the relationship between maximally shortened length and isotonic load. The data points are redrawn from Fig. 7. The model predictions are based on the linear regression line (solid) for the data collected at 0.75 Lin situ. The maximally shortened length under zero-load is assumed proportional to the number of dense bodies in series plus any serially connected non-contractile elements.

View larger version (88K): [in a new window]   Fig. 10. Electron micrograph (longitudinal section) of a trachealis cell showing dense bodies (arrows). The muscle was fixed at 1.5 Lin situ in the relaxed state. Bar, 1 µm.

View larger version (22K): [in a new window]   Fig. 11. Density of dense bodies measured from 4-µm cell longitudinal sections in micrographs such as that shown in Fig. 10. The dashed line indicates an increase of 33% from the average value measured at 0.75 Lin situ.

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