Tropomyosin-paramyosin system and ‘prolonged contraction’ in a molluscan smooth muscle

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Abstract
Tonic molluscan muscles such as the anterior byssal retractor muscle of Mytilus ( ABRM ) are outstanding in two respects: First they contain, in even larger amounts than actin and myosin, tropomyosin- A (Bailey 1956; Rüegg 1961 a ) which constitutes a system of filaments, the tropomyosin-paramyosin system (cf. Hall, Jakus & Schmitt 1945; Hanson et al . 1957). Secondly, these muscles are able to maintain tension passively for prolonged periods without fatigue (holding function, catch). The capacity for prolonged (tonic) contraction has been related to the extremely slow relaxation of tonically contracted muscle fibres (Lowy & Millman 1963). During slow relaxation the stretch resistance is high and tension is maintained passively (Johnson & Twarog 1960; Jewell 1959; Lowy & Millman 1963). The slow tension decay after cessation of isometric contraction may then be attributed to the slow breaking of contractile actin-myosin linkages which are assumed to be formed in the preceding contraction-phase (Lowy & Millman 1963). Alternatively Johnson (1962) and Rüegg (1961) suggested that the relaxation of the contractile system is slowed down by a special colloidal system, the tropomyosin-paramyosin system, the stiffness of which was assumed to be increased in the catch and decreased after unlocking the catch with certain types of stimulation, e.g. by addition of serotonin. Subsequently it has been shown that the catch (i.e. serotonin sensitive stiffness) may also be produced in apparently resting ABRM when the muscle is immersed in a solution of increased CO 2 tension (Rüegg & Weber 1963). The caught state may then be regarded as a stretch-resistant state. If the catch is based on the tropomyosin-paramyosin system, tropomyosin- A structures ought to have a capacity to resist stretch and they should have this holding capacity in surviving as well as in glycerol extracted, dead muscle fibres. This possibility has been tested by stretch experiments with surviving and extracted ABRM fibre-bundles after inactivation of the actomyosin contractile system with thiourea. The latter has been shown to be an inhibitor of actinmyosin interaction in vitro (Portzehl 1961; Rüegg 1961 b ) and since it penetrates readily into muscle fibres, we have applied it also in the case of surviving ABRM in an attempt to inactivate the actomyosin system in situ (Rüegg, Straub & Twarog 1963).

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