Depressant effect of active shortening in the anterior byssus retractor muscle ofMytilus edulis

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Abstract
The effect of shortening during activity, previously characterized in vertebrate striated muscle, was investigated in the anterior byssus retractor muscle (ABRM) of the molluscMytilus edulis. This muscle is considered to have an essentially myosin‐linked Ca2+‐ regulatory system. Release steps of different amplitude were performed during isometric phasic contraction, and force redevelopment was recorded at a muscle length L1, defined as 90% of the muscle length at which a slight resting tension, approximately 1 mN, appeared in the presence of 2.5 × 10‐5M 5‐HT. Active shortening caused a graded depression of the contractile force without affecting the total duration of the mechanical response. Peak redeveloped force after muscle shortening of 0.06 L1and 0.18 L1was reduced by approximately 1.5% and 7.0%, respectively, of the isometric tension value at L1. The shortening effect was fully reversible, and had a lifetime of approximately 8 to 9 s. The depressant effect of active shortening was augmented at a reduced degree of activation of the muscle. The presence of caffeine and dantrolene and altered tonicity of the extracellular medium (0.9 T‐1.2 T) did not significantly affect the shortening induced depression obtained at maximum phasic activation of the preparation. The nature of the shortening effect is compared to that obtained in vertebrate striated muscle and is discussed on the basis of differences in Ca2+‐regulation of the contractile system in these two muscles.