Lincoln E Ford
Department of Medicine, Krannert Institute of Cardiology, Indiana University School of Medicine, Indianapolis, USA

https://doi.org/10.36866/pn.60.17

Some years ago, while Chun Seow and I were studying tension transients in skinned skeletal muscle, he asked how he might use our mechanical techniques to study smooth muscle. He had done his doctoral work on airway smooth muscle in Winnipeg and planned to return both to Canada and to smooth muscle after working with me. My response was that filament lattice plasticity facilitated by myosin filament evanescence might be a fruitful area.

The discovery of sliding filaments in skeletal muscle led to early searches for similar filaments in smooth muscle. Thin filaments were always seen, but thick-filament descriptions varied with respect to both shape and filament density. Some early workers (Kelly & Rice, 1968; Shoenberg, 1969) proposed that this variability resulted from thick filaments being evanescent, dissociating partially during relaxation and reforming upon activation. Attention turned elsewhere, however, and this issue disappeared nearly completely from the literature.

But the suggestion of thick filament evanescence was appealing. Muscle length changes in the walls of some hollow viscera are so large that they are unlikely to be accommodated by a fixed array of filaments, and filament evanescence could facilitate plastic adaptations to greater length ranges. Our first experiments confirmed that smooth muscle adapts to longer lengths by placing more contractile elements in series; developed force was nearly constant over a 3-fold range of length while velocity and compliance approximately doubled (Pratusevich et al. 1995). The constant force is predicted by the addition of new filaments in series allowing the thick and thin filaments to remain near their optimum overlap. The velocity and compliance increases are expected from the addition of new contractile elements in series.

Another prediction of the model derives from the consideration that thick filaments lengthen as they form. Since the crossbridges on each separate filament are arrayed in parallel, force should increase in proportion to filament lengthening. But fewer of the longer filaments are required to span the length of the muscle, so that there will be fewer of the longer filaments arranged in series. If the velocity of the individual crossbridges and filaments is independent of filament length, the overall muscle velocity will be proportional to the number of filaments in series. A second study confirmed that velocity declined during the rise in activation, and when correction was made for changes in the level of activation, the velocity decline was exactly balanced by a rise in force (Seow et al. 2000). This finding raised the question of whether filament formation is sufficiently rapid to account for the mechanical findings.

After returning to Canada, Chun Seow completed several quantitative electron microscope assessments of thick filament density made in steady states, confirming that thick filament mass is increased during contractures (Herrera et al. 2002) and at longer lengths (Kuo et al. 2003). Earlier quantitative studies on rat anococcygeus muscle (Godfraind-De Becker & Gillis, 1988; Gillis et al. 1988) also concluded that thick filament density increased during contractures when measured both by electron microscopy and by optical birefringence. Birefringence is a well­known property of striated muscle because the A-bands are defined by being anisotropic, i.e. birefringent, and it is now known that the birefringence is due to the myosin filaments. Thus, the strength of birefringence would be expected to reflect myosin filament density. Godfraind-De Becker and Gillis’ pioneering birefringence measurements had low time resolution, but their results suggested that this optical signal, if measured more rapidly, could be used to track filament formation if it occurs. Accordingly, we built the necessary apparatus to track birefringence electronically and extended the earlier findings to show that:

• birefringence was increased at longer lengths, as predicted by our mechanical experiments (Smolensky et al. 2005; Fig. 1);
• it increased during stimulation with about the same time course as force, confirming that filament formation is rapid enough to account for the velocity slowing.

All of our work has been done on the trachealis muscle of medium-sized mammals (dog, sheep, and pig), because Chun had done his doctoral work on this preparation. It was a fortunate choice; trachealis is composed of straight bundles of parallel cells with little connective tissue and is therefore ideal for mechanical studies, unlike many other smooth muscles, which have crisscrossed arrays of muscle bundles and much more connective tissue. The preparation is emphasized to point out that the filament evanescence and long length range of our preparation may not be detected easily in smooth muscles where large length changes are prevented by a heavy investment of connective tissue, or in muscles that do not normally relax fully when left unstimulated. Godfraind-De Becker & Gillis (1988), for example, failed to find the same increase in birefringence of taenia coli that they did in rat anococcygeus, and noted that the taenia coli showed spontaneous activation. Similarly, Herlihy & Murphy (1973) found a much shorter length range in arterial smooth muscle, which is heavily invested with elastic tissue.

Acknowledgements

Our work has been supported by USPHS Grant # RO1-HL52760. We thank Sir Andrew Huxley for many helpful suggestions and for his efforts to educate us in the mechanisms of birefringence.

References

Gillis J-M, Cao ML & Godfraind-De Becker A (1988). Density of myosin filaments in the rat anococcygeus muscle, at rest and in contraction II. J Muscle Res Cell Motil 9, 18-29.

Godfraind-De Becker A & Gillis J-M (1988). Analysis of the birefringence of smooth muscle anococcygeus of the rat at rest and in contraction I. J Muscle Res Cell Motil 9L, 9-17.

Helihy JT & Murphy RA (1973). Length-tension relationship of the hog carotid artery. Circulation Res 33, 275-283.

Herrera AM, Kuo K-H & Seow, CY (2002). Influence of calcium on myosin thick filament formation in intact airway smooth muscle. Am J Physiol Lung Cell Mol Physiol 286, L1161-L1168.

Kelly RE & Rice RV (1968). Localizations of myosin filaments in smooth muscle. J Cell Biol 37, 105-116.

Kuo KH, Herrera AM, Wang L, Pare PD, Ford LE, Stephens LE & Ford LE (2003). Structure-function correlation in airway smooth muscle adapted to different lengths. Am J Physiol Cell Physiol 285, C384-C390.

Pratusevich VR, Seow CY & Ford LE (1995) Plasticity in canine airway smooth muscle. J Gen Physiol 105, 73-94.

Seow CY, Pratusevich VR & Ford LE (2000). Series-to-parallel transition in the filamaent lattice of airway smooth muscle. J Appl Physiol 89, 869-876.

Shoenberg CF (1969). An electronmicroscope study of the influence of divalent ions on myosin filament formation in chicken gizzard extracts and homogenat. Tissue & Cell 1, 83-96.

Smolensky AV, Ragozzino J, Gilbert SH, Seow CY & Ford LE (2005). Length-dependent filament formation assessed from birefringence increases during activation of porcine tracheal muscle. J Physiol 563, 517-527.