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First published online October 18, 2006
Journal of Experimental Biology 209, 4319-4328 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02501

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The catch state of mollusc catch muscle is established during activation: experiments on skinned fibre preparations of the anterior byssus retractor muscle of Mytilus edulis L. using the myosin inhibitors orthovanadate and blebbistatin

Oleg Andruchov, Olena Andruchova and Stefan Galler^*

Department of Cell Biology, University of Salzburg, Hellbrunnerstrasse 34, A-5020 Salzburg, Austria

^* Author for correspondence (e-mail: Stefan.Galler{at}sbg.ac.at)

Accepted 4 August 2006

Catch is a holding state of muscle where tension is maintained passively for long time periods in the absence of stimulation. The catch state becomes obvious after termination of activation; however, it is possible that catch linkages are already established during activation. To investigate this, skinned fibre bundles of the anterior byssus retractor muscle of Mytilus edulis were maximally activated with Ca²⁺ and subsequently exposed to 10 mmol l^-1 orthovanadate (V_i) or 5 µmol l^-1 blebbistatin to inhibit the force-generating myosin head cross-bridges. Repetitive stretches of about 0.1% fibre bundle length were applied to measure stiffness. Inhibitor application depressed force substantially but never resulted in a full relaxation. The remaining force was further decreased by moderate alkalization (change of pH from 6.7 to 7.4) or by cAMP. Furthermore, the stiffness/force ratio was higher during exposure to V_i or blebbistatin than during partial Ca²⁺ activation producing the same submaximal force. The increased stiffness/force ratio was abolished by moderate alkalization or cAMP. Finally, the stretch-induced delayed force increase (stretch activation) disappeared, and the force recovery following a quick release of the fibre length, was substantially reduced when the force was depressed by V_i or blebbistatin. All these findings suggest that catch linkages are already established during maximal Ca²⁺ activation. They seem to exhibit ratchet properties because they allow shortening and resist stretches. In isometric experiments a force decrease is needed to stress the catch linkages in the high resistance direction so that they contribute to force.

Key words: catch muscle, mollusc smooth muscle, Mytilus edulis, skinned muscle preparations, myosin inhibitors, calcium activation

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