Contractile process

In the presence of calcium, the primary contractile protein, myosin, is phosphorylated by the myosin light-chain kinase initiating the subsequent actin-activation of the myosin adenosine triphosphate activity and resulting in muscle contraction. Removal of calcium inactivates the kinase and allows the myosin light chain to dephosphorylate myosin which results in muscle relaxation. Therefore the general biochemical mechanism for the muscle contractile process is dependent on the avaUabUity of a sufficient intraceUular calcium concentration. 

Hartshome, D.J. (1987). Biochemistry of the contractile process in smooth muscle. In Physiology of the Gastrointestinal Tract. 2nd Edition (Johnson. L.R. ed.), pp. 423-482, Raven Press, New York. Hille, B. (1992). Ionic Channels of Excitable Membranes. 2nd edn., Sinauer Associates, Sunderland, MA. 

The lactate accumulation during intense anaerobic exercise was implicated as a possible fatigue factor as early as the beginning of this century (Fletcher and Hopkins, 1907). A detailed presentation of the role of H" and Pi in the contractile process has been presented in the section on fatigue in isolated fibers. 

The discovery that ATP was not only the source of the energy required for muscle contraction but was apparently directly involved in the contractile process was an enormous stimulus to biochemists and muscle biologists. In the early 1950s attempts were made to determine if the ATP was hydrolyzed to initiate the contraction or was merely involved in the recovery process. Because of the speed with which contraction occurs, experiments had to be performed with amphibian... 

Skeletal muscle comprises about 40% of body mass and plays an important part in life, but specific pathological problems relating directly to the contractile process are rare. Perhaps the foetus does not survive with defective skeletal muscles. 

The effect of a given adrenomimetic drug on a particular type of effector cell depends on the receptor selectivity of the drug, the response characteristics of the effector cells, and the predominant type of adrenoceptor found on the cells. For example, the smooth muscle cells of many blood vessels have only or predominantly a-adrenoceptors. The interaction of compounds with these adrenoceptors initiates a chain of events in the vascular smooth muscle cells that leads to activation of the contractile process. Thus, norepinephrine and epinephrine, which have high affinities for a-adrenoceptors, cause the vascular muscle to contract and the blood vessels to constrict. Since bronchial smooth muscle contains p2-adrenoceptors, the response in this tissue elicited by the action of p2-adrenoceptor agonists is relaxation of smooth muscle cells. Epinephrine and isoproterenol, which have high affinities for p2-adrenoceptors, cause relaxation of bronchial smooth muscle. Norepinephrine has a lower affinity for p2-adrenoceptors and has relatively weak bronchiolar relaxing properties. 

Nisoldipine. In vitro studies show that the effects nf nisoldipinc, 1,4-dihydn>-2. 6-dimethyl-4-(2-nitrophenyl)-3.S-pyridinecarbnxylie acid methyl 2-methyIpropyl ester (Sular), on contractile processes are selective, with greater potency on va.scular smooth muscle than nn cardiac muscle. 

Another candidate receptor for signal transduction through the cADPR pathway is the 5-HT2A receptor in tracheal smooth muscle. Here 5-HT in the absence of extracellular Ca + can mobilize Ca + without increasing IP3 formation, an effect that is blocked by prior incubation with ryanodine (Watts etal., 1994). It will be interesting to see if cADPR plays a role in transducing the effects of these two receptor types linked to the contractile processes in smooth muscle. 

Felodipine is a calcium-channel-blocking agent, which inhibits movement of calcium ions across cell membrane in systemic and coronary vascular smooth muscle, altering contractile process. 

As was initially shown by Sobieszek (Sobieszek 1977) that phosphorylation of the LC20 increases the actomyosin ATPase. Using more defined systems consisting of purified proteins this was then confirmed by a number of authors and it was clearly shown that the actin activated myosin MgATPase increased when LC20 was phosphorylated (for review Hart-shorne and Mrwa 1982, Kamm and Stull 1985). Besides activation of the contractile process, phosphorylation of LC20 can also influence the assembly of smooth muscle myosin into filaments in vitro (see chapter 2). 

Proteins are biopolymers having a molecular weight arbitrarily greater than 5,000 dal tons. They are essential for functions in cellular structure, catalysis, metabolic regulation and contractile processes. They also play an important part in the defensive and protective mechanisms in animals. 

Aggregation and adherence Aggregation and adherence are two closely related characteristics of the PMNL. Although poorly understood these mechanisms seem to be of importance in the inflammatory response. Thus, adherence of leukocytes to the endothelium of microvessels and the sticking phenomena are considered initial hallmarks of an inflammatory process in vivo [297]. As in chemotaxis and degranulation these actions involve a contractile process of the cell. 

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