Complex stiffness of smooth muscle cytoplasm in the presence of Ca-activated brevin

F-actin stabilization increases tension cost during contraction of permeabilized airway smooth muscle in dogs

1. Dynamic actin reorganization involving actin polymerization and depolymerization may play an important functional role in smooth muscle. 2. This study tested the hypothesis that F-actin stabilization by phalloidin increases tension cost (i.e. ATP hydrolysis rate per unit of isometric force) during Ca2+-induced activation of Triton X-100-permeabilized canine tracheal smooth muscle. 3. Adenosine 5'-triphosphate (ATP) hydrolysis rate was quantified using an enzyme-coupled NADH fluorometric technique, regulatory myosin light chain (rMLC) phosphorylation was measured by Western blot analysis, and maximum unloaded shortening velocity (Vmax) was estimated by interpolation of the force-velocity relationship to zero load during isotonic loading. 4. Maximal activation with 10 microM free Ca2+ induced sustained increases in isometric force, stiffness, and rMLC phosphorylation. However, the increase in ATP hydrolysis rate initially reached peak values, but then declined to steady-state levels above that of the unstimulated muscle. Thus, tension cost decreased throughout steady-state isometric force. 5. Following incubation of permeabilized strips with 50 microM phalloidin for 1 h, the increases in isometric force and stiffness were not sustained despite a sustained increase in rMLC phosphorylation. Also, after an initial decline, tension cost increased throughout activation. Phalloidin had no effect on Vmax during steady-state isometric force or on rMLC phosphorylation. 6. These findings suggest that dynamic reorganization of actin is necessary for optimal energy utilization during contraction of permeabilized airway smooth muscle.

Involvement of the cytoskeleton in calcium-dependent stress relaxation of rat aortic smooth muscle

Rat aortic smooth muscle exhibits a remarkable capacity for stress relaxation, the release of tension following tissue stretch. Stress relaxation was markedly enhanced in contracted aortic rings compared with unstimulated tissue. The magnitude of stress relaxation in contracted aortic rings correlated well with the passive tension imposed on the tissue by stretching, but showed little relationship to changes in tissue length or to the level of tension developed in response to agonist stimulation prior to stretch. The enhancement of stress relaxation in precontracted tissue was not affected by intimal rubbing or treatment with L-NAME. By comparison, the removal of extracellular calcium markedly attenuated stress relaxation. In addition, the use of cytochalasin B to block actin polymerization inhibited stress relaxation, whereas colchicine, a drug used to cause microtubule disassembly, had no effect on the phenomenon. The results indicate that the enhanced stress relaxation in contracted tissue is a calcium-dependent process and is not due to passive tissue elastic properties. We suggest that stress relaxation may not involve cross-bridge formation but could be explained by the remodelling of a portion of the tension-bearing actin cytoskeleton.

X-ray diffraction study on mammalian visceral smooth muscles in resting and activated states

Structural changes of guinea pig taenia coli and rat anococcygeus muscle during contraction were studied by X-ray diffraction. The diffraction pattern of the taenia coli showed the 14.4-nm myosin reflection, the 5.9-nm actin layer-line and a diffuse equatorial peak at 1/11.4 nm-1. On application of carbachol, the muscle contracted and the intensity of the 14.4-nm reflection showed a concentration-dependent decrease: the maximum decrease was 24% at 2 x 10(-5) M. Such an intensity decrease was not observed in K-contracture (154 mM). The intensity of the 5.9-nm actin layer-line did not change appreciably on activation. The equatorial peak became broader during contraction. The 14.4-nm myosin reflection of the anococcygeus muscle was weak. Its intensity increased by 106% during contraction induced by 2 x 10(-5) M phenylephrine and by 75% during K-contracture. These results suggest that the number of myosin filaments may increase during contraction of rat anococcygeus muscle but not guinea pig taenia coli.

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.