Actomyosin inactivation by thiourea and the nature of viscous tone in a molluscan smooth muscle

The catch or viscous tone, a serotonin-sensitive reversible rigor in certain smooth molluscan muscles was studied in the anterior byssal retractor muscle of Mytilus edulis (ABRM) after inactivation of the contractile mechanism with thiourea (cf. Ruegg, Straub & Twarog 1963, preceding paper). Since a serotonin-sensitive reversible rigor could be produced in thioureapoisoned ABRM by CO$_{2}$ and/or cooling without any preceding contraction it may be concluded that tone is not simply a remainder of contraction or a failure to relax, and the mechanism producing tone must therefore be different from that producing contraction. There is evidence that-unlike the contractile mechanism-the tonic mechanism is not based on the actomyosin system but rather on a system associated with tropomyosin A (paramyosin) which is known to be abundant in catch muscle: First, in living ABRM serotonin-sensitive passive tension (tone) can be maintained even after equilibration of the muscle with sea water containing 0$\cdot $5 to 0$\cdot $6 M thiourea which penetrates the muscle readily and completely and which exerts a direct inhibiting effect on the contractile actomyosin system. Previous experiments have shown that isolated actomyosin structures (e.g. actomyosin threads) become completely relaxed and plasticized in the presence of 0$\cdot $5 M thiourea and ATP, and under these conditions the actomyosin ATP-ase and the actin-myosin interaction is inhibited. Secondly, after thiourea-induced inactivation of the actomyosin contractile system the thiourea-resistant passive tension and resistance to stretching is not only possible in surviving ABRM but also in (glycerol)-extracted fibre bundles suspended in the appropriate ATP medium. In these fibres passive tension may correspond to the viscous tone since it shows the same dependence on temperature and pH as serotonin-sensitive passive tension in surviving muscle; in the latter the intrafibre pH could be changed by changing the partial CO$_{2}$ pressure in the bath. Thus the tonic mechanism of surviving muscle appears to be operational in the form of a holding apparatus even in thiourea-poisoned extracted fibres where it may be investigated under simplified experimental conditions. This holding apparatus of extracted fibres appears to be associated with tropomyosin A. For unlike artificial actomyosin structures (actomyosin threads) the artificial tropomyosin A structures (tropomyosin A (paramyosin) threads) are barely plasticized by 0$\cdot $5 M thiourea and ATP and under these conditions the resistance to stretching of threads shows a very similar dependence on pH, temperature and concentration of unchelated ATP as the stretch resistance of thiourea-poisoned extracted ABRM fibre bundles; such a dependence on pH, ATP and temperature cannot be demonstrated with actomyosin structures or tropomyosin A-free extracted fibres of non-catch muscles