In this review, our current knowledge on the structural proteins of vertebrate skeletal muscle is briefly outlined. Structural proteins include the contractile proteins (actin and myosin), the major regulatory proteins (troponin and tropomyosin), the minor regulatory proteins (M-protein, C-protein, F-protein, I-protein, and actinins), and the scaffold proteins (connectin, desmin, and Z-protein). In addition, the relative turnover rates of the muscle proteins (M-protein greater than or equal to troponin greater than soluble protein as a whole greater than tropomyosin not equal to alpha-actinin greater than myosin greater than 10S-actinin greater than actin) are discussed. The changes in the turnover of muscle proteins are compared in denervated and dystrophic muscles. The properties of the various proteases in muscle, including alkaline protease, calcium-activated neutral protease (CANP), and acidic protease (cathepsins), and the structural alterations of myofibrils by these proteases are also described. Finally, the role of proteases and their inhibitors in diseased muscle is summarized, with focus on CANP and its inhibitors, leupeptin and E-64.

Actins↗

PubMedDate not supplied

Regulatory mechanism in arterial smooth muscle contraction.

The regulatory mechanism in the aortic actomyosin system was studied. Superprecipitation of desensitized aortic myosin B was not exhibited even in the presence of Ca2+, but was observable only in the presence of native tropomyosin and Ca2+. Reconstituted actomyosin composed of pure aortic myosin and pure skeletal actin did not show superprecipitation. Addition of aortic native tropomyosin and Ca2+ caused a marked superprecipitation. The ATPase of reconstituted actomyosin was enhanced three- or fourfold by aortic native tropomyosin and Ca2+. The extent of superprecipitation of aortic myosin B did not show a biphasic type of response to Mg-ATP concentration. Thus, aortic native tropomyosin induces a real activation of the myosin, actin, and ATP system in the presence of Ca2+, in contrast with the case of skeletal native tropomyosin, which induces the depression of skeletal myosin-actin-ATP interaction in the absence of Ca2+.

Actomyosin↗

PubMedDate not supplied

Calcium ion in cardiac contractility.

Under physiological conditions where the intracellular Ca ion concentration does not exceed 3 X 10(-6) M, the sarcoplasmic reticulum plays a major role in the relaxation process of cardiac muscle; mitochondria do not take up a significant amount of Ca ion during this process. If cardiac muscle undergoes maximum contraction, in which the intracellular Ca ion concentration should reach 10(-4) M, the role of mitochondria in reducing intracellular Ca ion becomes appreciable. The relationship of the tension developed by cardiac glycerinated muscle fibers to the Ca ion concentration resembles the relationship of the amount of bound Ca of cardiac troponin to the Ca ion concentrations, being less steep in its slope compared with those of fast and slow skeletal muscles. This gentle slope seems to reflect the great diversity of affinities for Ca ion of the two Ca-binding sites of cardiac troponin, one being about 100 times that of the other.

Animals↗