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First published online January 31, 2006
Journal of Experimental Biology 209, 668-676 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02033

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Effect of pH on the rate of myosin head detachment in molluscan catch muscle: are myosin heads involved in the catch state?

Marion Christine Höpflinger, Olena Andruchova, Oleg Andruchov, Herbert Grassberger and Stefan Galler^*

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

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

Accepted 13 December 2005

Moderate alkalisation is known to terminate the catch state of bivalve mollusc smooth muscles such as the anterior byssus retractor muscle (ABRM) of Mytilus edulis L. In the present study, we investigated the effect of moderate alkalisation (pH 7.2-7.7 vs control pH 6.7) on the myosin head detachment rate in saponin-skinned fibre bundles of ABRM in order to investigate the possible role of myosin heads in the force maintenance during catch. The detachment rate of myosin heads was deduced from two types of experiments. (1) In stretch experiments on maximally Ca²⁺-activated fibre bundles (pCa 4.5), the rate of force decay after stepwise stretch was assessed. (2) In ATP step experiments, the rate of force decay from high force rigor (pCa>8) was evaluated. The ATP step was induced by photolysis of caged ATP. We found that moderate alkalisation induces relaxation of skinned fibres in catch, thereby reducing both force and stiffness, whereas it does not accelerate the rate of myosin head detachment. This acceleration, however, would be expected if catch would be simply due to myosin heads remaining sustainably attached to actin filaments. Thus, the myosin heads may be less involved in catch than generally assumed. Catch may possibly depend on a different kind of myofilament interconnections, which are abolished by moderate alkalisation.

Key words: catch muscle, mollusc smooth muscle, Mytilus edulis, pH effect, caged ATP

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