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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

View larger version (26K): [in a new window]   Fig. 1. Diagram of experimental arrangement. The byssal end of the preparation was hooked to the lower lever of the displacement transducer with a stainless steal wire connector, while the shell end was held with the extension of the force transducer. The lever was initially fixed in position by stops 1 and 2, and after the full development of ACh-induced isometric force, stop 2 was removed to load the preparation with the spring, hooked between the upper lever and the micromanipulator.

View larger version (12K): [in a new window]   Fig. 2. Mechanical responses of the ABRM fibres to quick increases in load. The load on the preparation increased from P0 to P (2.3P0) in A, and from Px (0.5P0) to P (4.5Px) during the catch state in B. In this and the subsequent figures, the upper and lower traces show length and force changes, respectively. Bottom broken lines represent zero force level. Records A and B were obtained from the same preparation. Inset illustrates the three phases (1–3) of the mechanical response as well as the method to determine the amplitude of the early isotonic lengthening L. SEC, series elastic component.

View larger version (7K): [in a new window]   Fig. 3. Marked load-bearing ability of the ABRM fibres during the maximum active force generation (A) and during the catch state (B). The load was increased from P0 to P (8P0) in A, and from Px (0.5P0) to P (16Px) in B. Records A and B were obtained from the same preparation.

View larger version (8K): [in a new window]   Fig. 4. `Give' in the ABRM fibres following quick increases in load applied during the maximum active force generation (A) and during the early phase of ACh-induced force development (B). The amount of load P was 15P0 in A and 1.5P0 (4Px) in B. Records A and B were obtained from different preparations.

View larger version (14K): [in a new window]   Fig. 5. Dependence of the extension of the series elastic component SEC (A), the amplitude of the early isotonic lengthening L (B), and the velocity of the late isotonic shortening V (C) on the amount of load P. Filled and open circles represent data points during the maximum active force generation and during the catch state, respectively. All data points are obtained from the same preparation.

View larger version (9K): [in a new window]   Fig. 6. Segmental length change of an ABRM fiber bundle (L0=2.4 cm) at 1 s, 10 s and 50 s after a quick increase in load from P0 to 6.5P0, applied during the maximum isometric force generation. The preparation was divided into five consecutive segments designated a, b, c, d and e.