Influence of Ca-activated brevin on the mechanical properties of skinned smooth muscle

Solutions of purified brevin were applied to skinned thin bundles or isolated fibres of smooth muscle. This produced a sharp drop of isometric tension, an effect due to the severing effect of brevin on actin filaments, partially depleted from tropomyosin in skinned preparations. On skinned single fibres, brevin accelerates the speed of unloaded shortening. As no effect was detected on the myofibrillar ATPase turnover rate, brevin was thought to affect the viscosity of the cytoplasm. This was confirmed by analysis of the cytoplasm stiffness which decreased in the presence of brevin. It is proposed that Ca-activated brevin acts on actin-filamin gels, set in parallel to the contractile apparatus.