The catch mechanism of molluscan smooth muscles

We have examined the arrangements of the thick and thin filaments of the smooth muscle of the adductor of the bivalve Cristaria plicata, Leach, in the catch state induced by acetylcholine, both in ultrathin sections and in separate filaments. The thick filaments are heavily bent and entangled and the thin filaments irregularly arranged. One thin filament may be attached to several thick filaments. When the muscle is in the relaxed state, the two kinds of filaments are seen to be in parallel. However, no difference could be observed in the periodic structure of the paramyosin core of the thick filaments in the relaxed state or in catch. Both cross-striated and checkerboard types coexist. It seems that phase transition is not directly related to catch. We propose that the disordered arrangements of the thick and thin filaments and the formation of three-dimensional anastomosed networks are the structural basis for catch, with a consequent loss of the capability of directional sliding of the filaments and requiring time to restore orderliness.