title: Calcium ion-dependent regulation of uterine smooth muscle thin filaments by caldesmon.
abstract: Native thin filaments were extracted from rabbit uterus by the procedure of Marston and Smith. The protein content was actin, tropomyosin, and caldesmon in molar ratios of 1:0.2:0.03. Some filamin, myosin, and calcium-binding protein were also present. The thin filaments activated skeletal or smooth muscle myosin magnesium adenosine triphosphatase at least 30-fold. Activation was regulated by Ca2+; maximum observed Ca2+ sensitivity was greater than 10 times. The thin filaments were dismantled into component proteins by the method of Smith and Marston. Actin and actin-tropomyosin-activated myosin magnesium adenosine triphosphatase, but the activation was not Ca2+-regulated. Added caldesmon inhibited adenosine triphosphatase activation by as much as 80%, with 50% inhibition at 1 caldesmon per 50 actin. Caldesmon inhibition was not Ca2+ dependent, but inhibition could be reversed by further addition of Ca2+ and calmodulin. It is concluded that the thin filaments of uterine smooth muscle are Ca2+ regulated and that this regulatory system could be involved in control of uterine smooth muscle contractility. A mechanism for thin filament regulation, mediated by caldesmon, is proposed.
authors.affiliation: Department of Cardiac Medicine, National Heart and Lung Institute, London, England.
authors.forename: S
authors.initials: S
authors.lastname: Marston
mesh_terms: D008841:Actin Cytoskeleton / Q000187:drug effects*; D000199:Actins / Q000032:analysis; D000255:Adenosine Triphosphate / Q000037:antagonists & inhibitors; D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D002148:Calmodulin-Binding Proteins / Q000032:analysis / Q000494:pharmacology*; D003599:Cytoskeleton / Q000187:drug effects*; D003720:Densitometry; D004591:Electrophoresis, Polyacrylamide Gel; D005260:Female; D009215:Myometrium / Q000032:analysis / Q000187:drug effects*; D009218:Myosins / Q000032:analysis; D011247:Pregnancy; D011817:Rabbits; D014335:Tropomyosin / Q000032:analysis
chemical_list: D000199:Actins; D002148:Calmodulin-Binding Proteins; D014335:Tropomyosin; D000255:Adenosine Triphosphate; D009218:Myosins; D002118:Calcium