Myosin-mediated Ca++-regulation of actomyosin-adenosinetriphosphatase from porcine aorta

The nature of the Ca++-sensitive regulatory system for contraction of vascular smooth muscle is considered in detail. Smooth muscle actomyosin prepared from the medial layer of porcine aorta is analyzed chemically and its ATPase (adenosinetriphosphatase, EC 3.6.1.14) activities are investigated. The Mg++-ATPase of this vascular actomyosin is sensitive to the concentration of calcium in the range from 0.1 mM to 10 nM. The calcium sensitivity is maintained in the presence of excess pure actin from skeletal muscle and is abolished in the presence of pure skeletal myosin. It is concluded that the regulatory properties of this natural actomyosin from smooth muscle are in the myosin portion of the protein complex and are not bound to actin-tropomyosin as in skeletal muscle.