Skeletal muscles muscle development

Dickerson JWT Widdowson EM (1960) Chemical changes in skeletal muscle during development. Biochem J, 74 247-257. 

Wang, D. Z., et al. 2001. The Mef2c gene is a direct transcriptional target of myogenic bHLH and MEF2 proteins during skeletal muscle development. Development 128 4623-4633. 

Several of the common enzyme measurements aimed at detecting hepatotoxicity are not specific to the liver, show a widespread tissue distribution, and are therefore affected by damage to extrahepatic tissue (e.g., alanine aminotransferase, aspartate aminotransferase, and lactate dehydrogenase following injury to cardiac or skeletal muscles). The development of troponin assays has provided an alternative to enzyme measurements as indicators of cardiotoxicity, but not for myotoxicity here, the timing and the methods of sample collection are particularly critical for the detection of cardiac or muscle damage (see Chapter 7). 

Ryall JG, Church JB, Lynch GS. (2010) Novel role for beta-adrenergic signalling in skeletal muscle growth, development and regeneration. Clin Exp Pharmacol Physiol 37, 397-101. 

NHE5. The distribution of this isoform is distinct, being in neuronal-rich areas of the central nervous system. Low levels have also been found in testis, spleen and skeletal muscle. Like the preceding isoforms, NHE5 is found in the plasma membrane and is internalised by clathrin-associated endocytosis into recycling endosomes. The normal role of NHE5 is unknown but its malfunction is speculated to contribute to the development of neurodegenerative disease. 

NFAT proteins are expressed in skeletal, cardiac, and smooth muscle and play important roles in the regulation of the development and differentiation of these tissues. In skeletal muscle, NFAT isoforms are expressed at different stages of development and regulate progression from early muscle cell precursors to mature myocytes. NFAT proteins have also been shown to control the expression of the myosin heavy chain and positively regulate muscle growth [1, 2]. 

Ryanodine is a neutral plant alkaloid from Ryania speciosa and was used as an insecticide. It also has been well known by the characteristic action on mammalian skeletal muscle of slowly developing, and intensive and irreversible contracture. Ryanodine binds specifically to the open RyR channel at the stoichiometry of 1 mol/mol homotetramer with a high affinity (ATD nM) and leads the channel to ryanodine modified state characteristic of long-lasting subconductance ( 50% of normal) opening. At higher concentration, it blocks the channel. 

In striated muscles, SR is well developed to surround the myofibrils and is divided into two parts, the terminal cisternae (TC) and longitudinal tubules (LT). TC forms triad (skeletal muscle) or dyad (heart) structure with transverse tubules. The ryanodine receptor is located only in the TC, whereas the Ca2+ pump/SERCA is densely packed in both TC and LT. 

Skeletal muscle is made up of many muscle fibers (Figure 1) each of which is a multinucleated cell that was formed during development by the fusion of many cells (myoblasts). Skeletal muscle is formed from precursor myoblasts which arise... 

Figure 1. Muscle development. A skeletal muscle fiber is formed by the fusion of many single cells (myoblasts) into a multinucleated myotube. Myotubes then develop into the muscle fiber (see text). Sarcomeres form in longitudinal structures called myofibrils. The repeating structure of the sarcomere contains interdigitating thick and thin filaments.

The gene for myophosphoiylase has been assigned to chromosome 1 lql3. The enzyme is a dimer of two identical 97 IcDa subunits and is the sole isoform present in skeletal muscle. Heart and brain also contain this isoform in addition to a distinct brain isoenzyme and a hybrid muscle/brain isoform. Smooth muscle also contains a phosphorylase isoform distinct from the muscle isoenzyme. If regenerating muscle fibers are present they also contain phosphorylase activity due to the presence, in fetal and developing muscle, of an isoform said to be identical with brain phosphorylase. 

Development and specialisation of skeletal muscle. Edited by D.F. Goldspink... 

The general picture of muscle contraction in the heart resembles that of skeletal muscle. Cardiac muscle, like skeletal muscle, is striated and uses the actin-myosin-tropomyosin-troponin system described above. Unlike skeletal muscle, cardiac muscle exhibits intrinsic rhyth-micity, and individual myocytes communicate with each other because of its syncytial nature. The T tubular system is more developed in cardiac muscle, whereas the sarcoplasmic reticulum is less extensive and consequently the intracellular supply of Ca for contraction is less. Cardiac muscle thus relies on extracellular Ca for contraction if isolated cardiac muscle is deprived of Ca, it ceases to beat within approximately 1 minute, whereas skeletal muscle can continue to contract without an extraceUular source of Ca +. Cyclic AMP plays a more prominent role in cardiac than in skeletal muscle. It modulates intracellular levels of Ca through the activation of protein kinases these enzymes phosphorylate various transport proteins in the sarcolemma and sarcoplasmic reticulum and also in the troponin-tropomyosin regulatory complex, affecting intracellular levels of Ca or responses to it. There is a rough correlation between the phosphorylation of Tpl and the increased contraction of cardiac muscle induced by catecholamines. This may account for the inotropic effects (increased contractility) of P-adrenergic compounds on the heart. Some differences among skeletal, cardiac, and smooth muscle are summarized in... 

A wide selection of monoclonal and polyclonal anti-Ca -ATPase antibodies have become available in recent years. Studies with these antibodies defined the localization of Ca " -ATPase in the sarcoplasmic reticulum of developing and mature skeletal muscles [60,262-270] and established a pattern of cross reactivity with various Ca -ATPase isoenzymes in the sarco(endo)plasmic reticulum [270-286] and in the plasma membrane [284,287-290] of skeletal, cardiac and smooth muscles. Antibodies have also proved useful in the quantitation of Ca -ATPase, both in muscles of diverse fiber types [291-294] and in COS-1 cells transfected with Ca -ATPase cDNA [97,103,126,127,129,215],... 

Pulmonary hypertension develops late in the course of COPD, usually after the development of severe hypoxemia. It is the most common cardiovascular complication of COPD and can result in cor pulmonale, or right-sided heart failure. Hypoxemia plays the primary role in the development of pulmonary hypertension by causing vasoconstriction of the pulmonary arteries and by promoting vessel wall remodeling. Destruction of the pulmonary capillary bed by emphysema further contributes by increasing the pressure required to perfuse the pulmonary vascular bed. Cor pulmonale is associated with venous stasis and thrombosis that may result in pulmonary embolism. Another important systemic effect is the progressive loss of skeletal muscle mass, which contributes to exercise limitations and declining health status. 

For over three decades, laboratory research has shown caffeine to be effective at mobilizing calcium in skeletal muscle. In vitro experiments have amply demonstrated that caffeine lowers the excitability threshold and extends the length of muscular contractions via calcium release from the sarcoplasmic reticulum.1012 Caffeine also inhibits calcium reuptake by the sarcoplasmic reticulum, perpetuating calcium availability for muscle work.1318 Also, caffeine promotes increased twitch tension development in muscles.1718... 

Pyruvate kinase (PK) is one of the three postulated rate-controlling enzymes of glycolysis. The high-energy phosphate of phosphoenolpyruvate is transferred to ADP by this enzyme, which requires for its activity both monovalent and divalent cations. Enolpyruvate formed in this reaction is converted spontaneously to the keto form of pyruvate with the synthesis of one ATP molecule. PK has four isozymes in mammals M, M2, L, and R. The M2 type, which is considered to be the prototype, is the only form detected in early fetal tissues and is expressed in many adult tissues. This form is progressively replaced by the M( type in the skeletal muscle, heart, and brain by the L type in the liver and by the R type in red blood cells during development or differentiation (M26). The M, and M2 isozymes display Michaelis-Menten kinetics with respect to phosphoenolpyruvate. The Mj isozyme is not affected by fructose-1,6-diphosphate (F-1,6-DP) and the M2 is al-losterically activated by this compound. Type L and R exhibit cooperatively in... 

Following the development of the motor program, neurons originating in the multimodal motor association areas transmit impulses by way of association tracts to neurons of the primary motor cortex. The primary motor cortex is located in the precentral gyrus, which is the most posterior region of the frontal lobe adjacent to the multimodal motor association areas (see Figure 6.3) this area initiates voluntary contractions of specific skeletal muscles. Neurons whose cell bodies reside here transmit impulses by way of descending projection tracts to the spinal cord, where they innervate the alpha motor neurons (which innervate skeletal muscles). 

In smooth muscle, myosin crossbridges have less myosin ATPase activity than those of skeletal muscle. As a result, the splitting of ATP that provides energy to "prime" the crossbridges, preparing them to interact with actin, is markedly reduced. Consequently, the rates of crossbridge cycling and tension development are slower. Furthermore, a slower rate of calcium removal causes the muscle to relax more slowly. 

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