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

Abstract

Smooth muscle cells line the walls of hollow organs and control the organ dimension and mechanical function by generating force and changing length. Although significant progress has been made in our understanding of the molecular mechanism of actomyosin interaction that produces sliding of actin (thin) and myosin (thick) filaments in smooth muscle, the sarcomeric structure akin to that in striated muscle, which allows the sliding of contractile filaments to be translated into cell shortening has yet to be elucidated. Here we show evidence from porcine airway smooth muscle that supports a model of malleable sarcomeric structure composed of contractile units assembled in series and in parallel. The geometric organization of the basic building blocks (contractile units) within the assembly and the dimension of individual contractile units can be altered when the muscle cells adapt to different lengths. These structural alterations can account for the different length-force relationships of the muscle obtained at different adapted cell lengths. The structural malleability necessary for length adaptation precludes formation of a permanent filament lattice and explains the lack of aligned filament arrays in registers, which also explains why smooth muscle is `smooth'.

Time course of isotonic shortening and the underlying contraction mechanism in airway smooth muscle

“…4. A linear relationship between length and force has been found to exist in ASM (18); this finding is consistent with the model proposed above.…”

“…1) in smooth muscle has provided some additional supporting evidence (18). In that study, it was assumed that the amount of overlap between the thick and thin filaments would decrease linearly as the muscle shortened; the data revealed a linear dependence of active force on muscle length (18), consistent with the model prediction.…”

“…1) in smooth muscle has provided some additional supporting evidence (18). In that study, it was assumed that the amount of overlap between the thick and thin filaments would decrease linearly as the muscle shortened; the data revealed a linear dependence of active force on muscle length (18), consistent with the model prediction. With the establishment of relationships between force and velocity and between length and force, we are able to integrate the information and predict the time course of muscle shortening under a constant load.…”

“…mathematical model; contractile unit; sliding-filament mechanism THE STRUCTURE OF THE CONTRACTILE unit in smooth muscle is unknown, despite the common belief that the cycling-crossbridge/sliding-filament mechanism of contraction, which operates in striated muscle, is also responsible for smooth muscle contraction. The predominant model for the sarcomere-equivalent contractile unit in smooth muscle is a side-polar myosin (thick) filament sandwiched by two oppositely oriented actin (thin) filaments, each attached to a dense body that is believed to function like a Z disk in striated muscle (18,21,27), as shown in Fig. 1.…”

“…The predominant model for the sarcomere-equivalent contractile unit in smooth muscle is a side-polar myosin (thick) filament sandwiched by two oppositely oriented actin (thin) filaments, each attached to a dense body that is believed to function like a Z disk in striated muscle (18,21,27), as shown in Fig. 1.…”