title: Electron microscopic evidence for the thick filament interconnections associated with the catch state in the anterior byssal retractor muscle of Mytilus edulis.
abstract: The anterior byssal retractor muscle (ABRM) of a bivalve mollusc Mytilus edulis is known to exhibit catch state, i.e. a prolonged tonic contraction maintained with very little energy expenditure. Two different hypotheses have been put forward concerning the catch state; one assumes actin-myosin linkages between the thick and thin filaments that dissociate extremely slowly (linkage hypothesis), while the other postulates a load-bearing structure other than actin-myosin linkages (parallel hypothesis). We explored the possible load-bearing structure responsible for the catch state by examining the arrangement of the thick and thin filaments within the ABRM fibers, using techniques of quick freezing and freeze substitution. No thick filament aggregation was observed in the cross-section of the fibers quickly frozen not only in the relaxed and actively contracting states but also in the catch state. The thick filaments were, however, occasionally interconnected with each other either directly or by distinct projections in all the three states studied. The proportion of the interconnected thick filaments relative to the total thick filaments in a given cross-sectional area was much larger in the catch state than in the relaxed and actively contracting states, providing evidence that the thick filament interconnection is responsible for the catch state.
authors.affiliation: Central Electron Microscopic Laboratory, School of Medicine, Teikyo University, 2-1-1 Kaga, Itabashi-ku, Tokyo 173-8605, Japan.
authors.forename: Ichiro
authors.initials: I
authors.lastname: Takahashi
authors.forename: Mitsuyo
authors.initials: M
authors.lastname: Shimada
authors.forename: Tsuyoshi
authors.initials: T
authors.lastname: Akimoto
authors.forename: Teruhiko
authors.initials: T
authors.lastname: Kishi
authors.forename: Haruo
authors.initials: H
authors.lastname: Sugi
mesh_terms: D008841:Actin Cytoskeleton / Q000502:physiology / Q000648:ultrastructure; D000818:Animals; D049872:Bivalvia / Q000502:physiology*; D017110:Freeze Substitution; D008854:Microscopy, Electron; D009119:Muscle Contraction / Q000502:physiology*; D009126:Muscle Relaxation / Q000502:physiology; D009132:Muscles / Q000502:physiology* / Q000648:ultrastructure; D009218:Myosins / Q000502:physiology* / Q000648:ultrastructure
chemical_list: D009218:Myosins