Structural origin of the series elastic component in horseshoe crab skeletal muscle fibers

Abstract

The structural origin of the highly compliant series elastic component (SEC) in arthropod skeletal muscle was investigated with intact muscle fibers and glycerinated myofibril bundles from the telson depressor muscle of a horseshoe crab Tachypleus tridentatus. In electrically stimulated muscle fibers, the extension of SEC at the maximum isometric force P₀ was about 6% of the slack fiber length L₀ (sarcomere length, 7 μm), i.e. about 210 nm per half-sarcomere, being too large to be explained by the cross-bridge and the thin filament elasticities. High-speed cinematography of the fibers during a quick release showed uniform distribution of the SEC in each sarcomere. In Ca²⁺-activated myofibril bundles, the A-band length was observed to increase transiently by about 10% during Ca²⁺-activated contraction. The above elastic A-band lengthening in the myofibril bundles amounted to 5–6% of the initial sarcomere length in the isometric condition, being almost equal to the extension of SEC at P₀ in isometrically contracting muscle fibers. Since the A-band lengthening is known to result from misalignment of the thick filaments, these results indicate that the highly compliant SEC mostly originates from the thick filament misalignment in each A-band during isometric force generation, and titin network providing elastic restoring force.

Introduction

The behavior of active skeletal muscle can be explained by postulating an elastic component in series with a contractile component [3]. During the isometric force development, the contractile component shortens internally by stretching the series elastic component (SEC) until the force exerted by the contractile component balances with the force in the SEC. Thus, the degree of extension of the SEC under the maximum isometric force (P₀) is equal to the minimum amount of quick release required to drop the isometric force from P₀ to zero. By carefully removing the external stray compliance, Jewell and Wilkie [7] showed that SEC in vertebrate skeletal muscle is distributed along the entire muscle length, and its degree of extension under P₀ is about 1% of the slack muscle length (L₀). This was confirmed by Huxley and Simmons [4] on single muscle fibers, and they put forward a contraction model in which the SEC (or the instantaneous elasticity) largely resides in the cross-bridges; each cross-bridge has an elastic link extending from the thick filament, and the link is extended by about 10 nm, i.e. about 1% of the length of a half-sarcomere, when the cross-bridge exerts its full force.

On the other hand, Suzuki and Sugi [11] presented evidence that the SEC resides not only in the cross-bridges but also in the thin filament in the myofilament nonoverlap region, i.e. the I-band in each sarcomere. Recently, it has been established that, in vertebrate skeletal muscle fibers, about 50% of the SEC extension at P₀ (5 nm per half-sarcomere) originates from elastic extension of the thin filaments in the I-band, while the rest 50% of the SEC extension (also 5 nm per half-sarcomere) originates form the cross-bridges [5], [11], [12], [15].

In some arthropod skeletal muscles, the sarcomere length at L₀ is very long (5–10 μm) compared to that in vertebrate skeletal muscle fibers (about 2 μm) [6]. If the cross-bridge and thin filament elasticities are assumed to be the same in all types of skeletal muscle, the extension of the cross-bridges at P₀ is the same (5 nm per half-sarcomere) irrespective of the sarcomere length, while the extension of the thin filaments in the I-band is 5×I/0.2 nm, where I/0.2 is the I-band length per half-sarcomere at L₀ relative to the corresponding value in vertebrate skeletal muscle fibers. In crayfish abdominal extensor muscle fibers, the sarcomere length at L₀ is about 9 μm while the A-band length is about 6 μm [13]. The extension of the SEC at P₀ is estimated on the above basis to be (5+5×1.5/0.2)=42.5 nm per half-sarcomere, i.e. about 0.9% of L₀. Contrary to this estimation, the extension of the SEC at P₀ was about 2% of L₀, i.e. about 90 nm per half-sarcomere, and this compliant SEC distributed uniformly along the fiber length, indicating that there would be an additional source of the SEC in each sarcomere [9], [13]. Unfortunately, irregular striation patterns of crayfish fibers made it difficult to further study the structural origin of the SEC.

The present experiments were undertaken to investigate the structural origin of the highly compliant SEC in arthropod skeletal muscle fibers, using the telson depressor muscle fibers of a horseshoe crab Tachypleus tridentatus, which exhibited a reasonably uniform striation pattern along their entire length. It will be shown that the highly compliant SEC in the telson depressor muscle fibers largely originates from elastic misalignment of the thick filaments in the A-band.