Summary and Discussion

Abstract

Recent investigations have indicated that the elements leading to contraction are of the same nature in smooth muscle as well as in skeletal and cardiac muscle. The contractile proteins in smooth muscle are thus actin and myosin. The myosin component is, however, in some respects different in smooth and skeletal muscle. It has thus a lower ATP-splitting ability, a fact which may explain, that smooth muscle contracts at a slower rate than skeletal muscle. In skeletal muscle, the contraction of actomyosin and its AT Pase activity can be triggered by increasing the Ca⁺⁺ concentration up to 10⁻⁵M and canceles by EDTA. Under the same conditions, the tonoactomyosin of the vascular smooth muscle shows on the other hand no contraction and no ATPase activity except for the case when 10 mM Mg⁺⁺ are added. Further, in skeletal muscle the contractile and ATP-splitting activity of actomyosin is inhibited by a protein complex of tropomyosin B and troponin, which has been discovered by EBASHI and coworkers. Ca⁺⁺-ions in a low concentration are able to block the inhibition produced by this protein complex, which explain the contractile actions of Ca⁺⁺-ions.