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First published online April 8, 2004
Journal of Experimental Biology 207, 1675-1681 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00934

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Marked load-bearing ability of Mytilus smooth muscle in both active and catch states as revealed by quick increases in load

Masao Mukou¹, Hirohiko Kishi¹, Ibuki Shirakawa¹, Takakazu Kobayashi², Katsutoshi Tominaga², Haruka Imanishi² and Haruo Sugi^1,*

¹ Department of Physiology, School of Medicine, Teikyo University, Itabashi-ku, Tokyo 173-8605, Japan
² Department of Electronic Engineering, Shibaura Institute of Technology, Minato-ku, Tokyo 108-8548, Japan

^* Author for correspondence (e-mail: sugi{at}med.teikyo-u.ac.jp)

Accepted 12 February 2004

The anterior byssal retractor muscle (ABRM) of the bivalve Mytilus edulis shows a prolonged tonic contraction, called the catch state. To investigate the catch mechanism, details of which still remain obscure, we studied the mechanical responses of ABRM fibres to quick increases in load applied during maximum active isometric force (P₀) generation and during the catch state. The mechanical response consisted of three components: (1) initial extension of the series elastic component (SEC), (2) early isotonic fibre lengthening with decreasing velocity, and (3) late steady isotonic fibre lengthening. The ABRM fibres could bear extremely large loads up to 10–15P₀ for more than 30–60 s, while being lengthened extremely slowly. If, on the other hand, quick increases in load were applied during the early isometric force development, the ABRM fibres were lengthened rapidly (`give') under loads of 1.5–2P₀. These findings might possibly be explained by two independent systems acting in parallel with each other; one is the actomyosin system producing active shortening and active force generation, while the other is the load-bearing system responsible for the extremely marked load-bearing ability as well as the maintenance of the catch state.

Key words: Mytilus edulis, smooth muscle, catch state, load-bearing ability, series elastic component, isotonic lengthening, parallel hypothesis

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