Regulation in Vascular Smooth Muscle

Abstract

The recently-reported five-fold activation of a natural actomyosin from vascular smooth muscle by increasing the Ca-ion concentration from 10⁻⁸M to 10⁻⁵M (SPARROW et al., Amer. J. Physiol. 219, 1366 (1970)) is possible at low ionic strength (~0.1) Mg ion concentrations, and is completely absent at < 2 mM. This Ca-sensitivity is similar to that of skeletal muscle in that it depends upon the presence of a regulatory protein system, and in the threshold Ca-concentrations required for activation. This regulatory system from arteries has been found to cross-react with desensitized actomyosin from skeletal muscle, and likewise, the regulatory system of skeletal muscle reacts with that of arterial muscle. Despite its Ca-sensitivity, the natural actomyosin prepared from arteries has been found to contain a slight excess of the inhibitor fraction (troponin B), which can be titrated away. The presence of Mg⁺⁺ and other insolubilizing cations, such as Ca⁺⁺, has been shown to promote the aggregation of myosin into thick filaments (SCHOENBERG, Tissue Cell1, 83 (1969)). Vascular natural actomyosin in a medium containing low concentrations of these ions shows the high solubility and absence of Ca-ions sensitivity characteristic of “tonoactomyosin”, and a “tonoactomyosin” preparation assayed in the presence of high Mg, shows the characteristic properties of a Ca-ion sensitive natural actomyosin.