Origin contractility

Muscle tissue is unique in its ability to shorten or contract. The human body has three basic types of muscle tissue histologically classified into smooth, striated, and cardiac muscle tissues. Only the striated muscle tissue is found in all skeletal muscles. The type of cells which compose the muscle tissue are known as contractile cells. They originate from mesenchymal cells which differentiate into myoblasts. Myoblasts are embryonic cells which later differentiate into contractile fiber cells. 

Myometrium of pregnancy is a unique example of visceral smooth muscle. Essentially quiet for the majority of gestation, a burst of rhythmic contractile activity is responsible for delivery of the human fetus. Endocrine signals originating from the fetus are likely responsible for the events that begin labour (McLean Smith 2001), but the mechanisms of intra- and intercellular signalling during the course of labour are not fully resolved. 

Following his extensive studies of nicotine, curare, and other chemicals acting on what is now known as the cholinergic nervous system, John Langley hypothesized that there must be receptive substances present in cells to explain the effects of nicotine alone and in combination with other chemicals. He wrote Since the accessory substance is the recipient of stimuli which it transfers to the contractile material, we may speak of it as the receptive substance (italics in original) of the muscle (Langley 1905). These conclusions followed from observations... 

Contractile properties in rodents can be measured either in vitro in a dissected muscle or in vivo in an intact preparation with an anesthetized animal (e.g., (19-20)). Measurements made under isometric conditions are perhaps most common and use the most straightforward setup. The addition of servomotors for dynamic control of muscle length allows simulation of dynamic conditions (eccentric, isotonic, etc.) that may be modified by disease or other processes (22, 23). For in vitro studies, the muscle is anchored by ligating the tendon (origin) to a support, for in vivo studies, the bone (femur) is clamped to prevent movement. The other tendon (insertion) is then coupled to a force transducer. In both cases, a recording electrode is also placed in contact with the muscle to record the compound action potential, and a stimulating electrode is used to stimulate the nerve or the muscle, as described below. 

More broadly, timolol therapy should be considered with caution in patients with any significant sign, symptom, or history for which systemic beta-blockade would be medically imwise.This includes disorders of cardiovascular or respiratory origin (e g., asthma, chronic bronchitis, and emphysema) as well as many other conditions. Spirometric evaluation after institution of timolol therapy may help to identify patients in whom bronchospasm develops after commencement of therapy. In general, however, patients with asthma and other obstructive pulmonary diseases should avoid this drug. Sympathetic stimulation may be essential to support the circulation in individuals with diminished myocardial contractility, and its inhibition by P-adrenoceptor antagonists may precipitate more severe cardiac feilure. 

This review will deal primarily with this contractile protein from human blood platelets, which we have named thrombosthenin in view of its origin and function. A brief review on blood platelets in general and their role in hemostatis will be included. The role of thrombosthenin for platelet function will be discussed and its properties compared with those of other contractile proteins. 

Ultracentrifugation of thrombosthenin as well as of thrombosthenin M led to conflicting results. As is known from the contractile proteins of muscular origin, poorly defined complexes of variable size may form very easily, making molecular weight determinations difficult. The same conditions were encountered with the contractile protein from human platelets (Bettex-Galland and von Tavel, unpublished observations, 1963). 

Today thrombosthenin is the only contractile protein of nonmuscular origin available in larger quantities. This alone makes it an interesting material from the point of view of comparative physiology. On the other hand, the high content of thrombosthenin in the blood platelets also suggests an important function of this material in physiological platelet activity. 

As mentioned previously, Hoffmaim-Berling (1954, 1956) was the first to demonstrate the presence of a contractile material of the actomyosin type in cells of nonmuscular origin. He did so by first proving the contractility of glycerol-extracted cells on addition of ATP, and later, in spite of the many difficulties involved, succeeded in extracting the contractile... 

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