Thick filaments in a freeze-etched vertebrate smooth muscle

Summary In freeze-etched vertebrate smooth muscles cells of the mouse intestine thick filaments were observed, showing a main orientation in the long fiber axis and a random arrangement within the cell. The filaments strongly resemble that obtained by thin-sectioning technique of the same tissue.The authors wish to thank Dr. F. Wunderlich for helpful discussions and Mrs. I. Vocke for technical assistance. The work was supported in part by the Deutsche Forschungsgemeinschaft.     

Hypertrophy-induced increase of intermediate filaments in vascular smooth muscle

The distribution of filaments was studied in hypertrophied rabbit vascular smooth muscle. Hypertrophy was induced by partial ligation of the portal-anterior mesenteric vein. 14 d after ligation, there was an approximately threefold increase in the number of intermediate filaments per cross-sectional area, as compared to control values. The actin:intermediate:myosin filament ratio was 15:1.1:1 in control and 15:3.5:0.5 in hypertrophied portal-anterior mesentric vein vascular smooth muscle. Comparison of the filament ratios with the increase in volume density of the hypertrophied cells suggests that the number of myosin filaments per cell profile remained approximately the same as in controls, whereas the number of actin filaments increased in proportion to the increase in cell volume.     

Subunit exchange between smooth muscle myosin filaments

Filaments formed from phosphorylated smooth muscle myosin are stable in the presence of MgATP, whereas dephosphorylated filaments are disassembled to a mixture of folded monomers and dimers. The stability of copolymers of phosphorylated and dephosphorylated myosin was, however, unknown. Gel filtration, sedimentation velocity, and pelleting assays were used to show that MgATP could dissociate dephosphorylated myosin from copolymers containing either rod and myosin or dephosphorylated and phosphorylated myosin. Copolymers were typically formed by dialyzing monomeric mixtures into filament-forming buffer but, unexpectedly, could also be formed within minutes of mixing preformed rod and myosin minifilaments. This result suggested that molecules can rapidly and extensively exchange between filaments, presumably via the monomeric pool of myosin in equilibrium with polymer. An exchange of molecules between filaments was demonstrated directly by electron microscopy using gold-labeled streptavidin or antibody to detect the exchanged species. By this approach it was shown that smooth muscle myosin filaments, like other macromolecular assemblies, are dynamic structures that can readily alter their composition in response to changing solvent conditions. Moreover, because folded monomeric myosin is unable to polymerize, these experiments suggest a mechanism for the disassembly of the filament by MgATP.     

Disassembly and reconstitution of the Ca2+-sensitive thin filaments of vascular smooth muscle

The Ca2+-sensitive thin filaments of aorta smooth muscle have been, disassembled into their constituent proteins, actin, tropomyosin and a 120-kDa protein. The 120-kDa protein bound to aorta actin-tropomyosin and inhibited its ability to activate myosin MgATPase. This inhibition correlated with the binding of one 120-kDa protein molecule per 29 actin monomers. Upon the addition of calmodulin to the actin-tropomyosin-120-kDa protein complex, the inhibition was relieved in 10(-4) M Ca2+ but not 10(-9) M Ca2+. The full release of inhibition was not accompanied by a full release of 120-kDa protein binding to actin-tropomyosin. A fully active, Ca2+-sensitive aorta thin filament has thus been reconstituted from just four components: actin, tropomyosin, 120-kDa protein and calmodulin.     

Assembly of smooth muscle myosin into side-polar filaments

The in vitro assembly of myosin purified from calf aorta muscle has been studied by electron microscopy. Two types of filament are formed: short bipolar filament similar to those formed from skeletal muscle myosin, and longer "side-polar" filaments having cross bridges with a single polarity along the entire length of one side and the opposite polarity along the other side. Unlike the case with skeletal myosin filaments, antiparallel interactions between myosin molecules occur along the whole length of side-polar filaments. The side-polar structure may be related to the in vivo form of myosin in vertebrate smooth muscle.     

血管平滑肌细胞的钙动力学

血管平滑肌细胞外的Ca~(2+)通过多种通道进入细胞内。Ca~(2+)通道的本质是镶嵌在膜脂质双分子层中的糖蛋白,神经介质和药物可影响Ca~(2+)通道的功能。靠近胞膜的肌质网和胞膜内侧面的高亲和性Ca~(2+)结合位点是血管平滑肌细胞内储存和释放Ca~(2+)的主要部位。胞浆[Ca~(2+)]增高后在钙调蛋白的介导下引起血管收缩。高血压等血管性疾病的发生与其平滑肌细胞的钙动力学异常有关。     

Oxygen-dependent signaling in pulmonary vascular smooth muscle

The pulmonary circulation constricts in response to acute hypoxia, which is reversible on reexposure to oxygen. On exposure to chronic hypoxia, in addition to vasoconstriction, the pulmonary vasculature undergoes remodeling, resulting in a sustained increase in pulmonary vascular resistance that is not immediately reversible. Hypoxic pulmonary vasoconstriction is physiological in the fetus, and there are many mechanisms by which the pulmonary vasculature relaxes at birth, principal among which is the acute increase in oxygen. Oxygen-induced signaling mechanisms, which result in pulmonary vascular relaxation at birth, and the mechanisms by which chronic hypoxia results in pulmonary vascular remodeling in the fetus and adult, are being investigated. Here, the roles of cGMP-dependent protein kinase in oxygen-mediated signaling in fetal pulmonary vascular smooth muscle and the effects of chronic hypoxia on ion channel activity and smooth muscle function such as contraction, growth, and gene expression were discussed.     

Cardioprotective prostacyclin signaling in vascular smooth muscle

Prostacyclin plays an important cardioprotective role, which has been increasingly appreciated in recent years in light of adverse effects of COX-2 inhibitors in clinical trials. This cardioprotection is thought to be mediated, in part, by prostacyclin inhibition of platelet aggregation. Multiple lines of evidence suggest that prostacyclin additionally protects from cardiovascular disease by pleiotropic effects on vascular smooth muscle. Genetic deletion of the prostacyclin receptor in mice revealed an important role for prostacyclin in preventing the development of atherosclerosis, intimal hyperplasia, and restenosis. In vitro studies have shown these effects may be due to prostacyclin inhibition of vascular smooth muscle cell proliferation and migration. Prostacyclin has also been shown to promote vascular smooth muscle cell differentiation at the level of gene expression through the Gs/cAMP/PKA pathway. Recently identified single nucleotide polymorphisms in the prostacyclin receptor that compromise receptor function suggest that some genetic variations may predispose individuals to increased cardiovascular disease. Herein, we review the literature on the cardioprotective effects of prostacyclin on vascular smooth muscle, and the underlying molecular signaling mechanisms. Understanding the role of prostacyclin and other eicosanoid mediators in the vasculature may lead to improved therapeutic and preventative options for cardiovascular disease.