Inhibition of contraction in a molluscan smooth muscle by thiourea, an inhibitor of the actomyosin contractile mechanism

Abstract

The action of thiourea on a molluscan 'catch' muscle was investigated, to determine whether the actomyosin contractile system can be inactivated directly in the surviving muscle by a poison known to inactivate actomyosin in vitro. Thiourea in a concentration of 0$\cdot $5 to 0$\cdot $6 M abolishes reversibly all contractile responses after stimulation with acetylcholine, cathodal direct current, isotonic potassium chloride, caffeine, and chloroform. The inhibiting effect is not caused by blocking the excitation mechanism at the membrane, since although thiourea inhibits the acetylcholine-and KCl contraction, it does not inhibit depolarization by acetylcholine and KCl. It also inhibits caffeine contracture although the latter is independent of the polarization of the membrane. The action of thiourea is not due to an osmotic effect. Thiourea diffuses into the muscle cell: the inside concentration reaches half the outside concentration in about 6 min. The rate of outflux is similar. Since the degree of inhibition depends on the thiourea concentration inside rather than outside it may be concluded that the site of action lies inside. Unlike DNP in certain muscles, thiourea does not reduce the ATP-store in concentrations in which it produces full inhibition of contractile responses. Again, quite unlike DNP, it inhibits the ATP contraction of the isolated contractile apparatus and both the ATP-ase of fibrils and the tension development of actomyosin threads. The dependence of the inhibition on the inhibitor concentration is similar in isolated systems and in the surviving ABRM after equilibration with thiourea. Urea, which produced little inhibition of the ATP contraction in isolated systems, had very little effect on the contractility of living muscles, in concentrations of 0$\cdot $6M. The simpliest and probably the most plausible hypothesis to account for all these findings is that thiourea inhibits contractility in the living muscle by inhibiting the contractile and enzymic activity of the actomyosin contractile system. Such a direct action may or may not be implemented by an additional effect on the coupling between excitation and contraction.