Calcium ion-dependent regulation of uterine smooth muscle thin filaments by caldesmon

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.