A better understanding of normal physiological function of the muscle can help to find explanations or the pathophysiological mechanisms for several muscle abnormalities which are caused by genetic factors or environmental stress.
1.7.1 Skeletal muscle form and function
The main function of skeletal muscle is to generate force by contraction which is used for movement or to support the skeletal structures. Actin and myosin filaments are the main contractile proteins that are organized in pattern to give a striated appearance. Troponin I, C and T, tropomyosin, and alpha-actinin are considered as regulatory contractile proteins (Harvey and Marshall, 1986; Szczesna et al., 1996; Watanabe et al., 1997). The contraction of muscle is usually started by reception of excitatory electrical signal at the neuromuscular junction which induces the interaction of contractile proteins by depolarization of the muscle membrane in the presence of certain calcium level which called "excitation-contraction" coupling (EC).
Sarcoplasmic reticulum (SR) releases the stored calcium under regulated physiological conditions. EC coupling consists of three phases take place at triads which are specified sites at junctional constant between the T-tubule and sarcoplasmic reticulum. The first phase is voltage sensing which measures intra-membrane charge movements by charging sensor molecules located in the membrane of T-Tubule (high in dihydropyridine binding receptors, DHPRs). It is thought that DHPRs are responsible about the transmission the second phase. In the third phase calcium releases from SR in the myoplasm through different types of calcium channels that mediating the release of calcium from internal stores which leads to increase the level of myoplasmic calcium (Schneider, 1994).
Calcium has a different role in the body not just for muscle contraction but also it is responsible about regulating different metabolic and cellular activities (proteins-proteins interaction, phosphorylation, gene expression, cell growth, etc.) (Ashley, 1995; Berridge, 1997).
In addition, calcium is present as immobilized form which works as a reservoir and it is used according the requirements of organisms. Calcium is remarkably toxic because its concentration cannot be easily regulated by metabolic synthesis and degradation as other biological messengers. It is thought that the toxicity of calcium is a consequence of its regulatory role.
Hence, Increase the intracellular calcium level due to pathological effect can be explained by over-stimulation of multiple calcium-sensitive pathways.
Intracellular levels of phosphate are always high because phosphorylated compounds are continuously used to generate and regenerate energy. And so when the calcium level is very high, phosphate-calcium precipitate may occur which lead to inhibition of oxidative phophorylation and as a consequence mitochondrial dysfunction and physical damage. Because the dual characteristics of calcium between functionality and toxicity, intracellular free calcium is precisely regulated by different mechanisms. In normal conditions, cells keep cytosolic free calcium level 104-fold less than extracellular calcium due to very low permeability of the cell membrane as well as controlling the release, uptake and extrusion of calcium by mobile proteins and storage of calcium in the membrane-bound compartment (Ashley, 1995)
Cytosolic Ca+2 levels can be regulated by two groups of binding proteins. The first group of binding proteins works as high capacity storage for calcium, which are present in intracellular compartments and sarcoplasmic and endoplasmic reticulum such as calreticulin (CR) and calsequestrin (CSQ). While the second group of proteins are cytosolic proteins such as troponin C and parvalbumin which exhibit their action in the nucleous and the cytoplasm (Zimmer et al., 1995). Unused calcium ions cannot stay as free in the cytoplasm and should be stored by accumulation in a suitable complex form without generating calcium-phosphate precipitate (Pozzan et al., 1994). Because calcium has several roles in controlling different metabolic and biochemical reactions in the cell, it is important to keep intracellular free calcium homeostasis.
Hence, release, buffering and re-uptake of calcium are controlled by precise regulation in order to maintain calcium homeostasis (Konishi et al., 1991). Therefore, any failure in the system of calcium homeostasis leads to toxic effect and adverse consequences which must be taken into considerations in the pathology of the muscle and myopathies. In some cases due to excessive muscle contraction, calcium level is raised and if the energy depletion proceeds, then extrusion is restricted and increase of calcium may lead to exercise-induced myopathy.
Malignant hyperthermia (MH) is a pharmacogenetic hypermetabolic disorder due to an inherited defect in sarcoplasmic reticulum RyR or calcium release channels which leads to increase the concentration of cytosolic calcium. It is normally induced by exposure the animals to halothane anaesthesia separately or in concomitment with muscle relaxants. MH is usually characterized by several physiological disorders such as uncontrolled muscle contraction,
increase of metabolism, hyperthermia, and aberration in sarcolemmal membrane followed by release in intracellular muscle enzymes into the blood stream. Moreover, due to the increase of calcium level in the cells, muscle contraction increases sharply (hypermetabolism) in order to eliminate calcium from the cytoplasm by pumping (ATPase) into the SR, mitochondria, and by extrusion through the sarcolemma which generate high amount of energy (hyperthermia).
Sarcoplasmic reticulum ryanodine-sensitive calcium release channel (SR-RSCRC) plays a major role in regulating calcium release from storage as well as myoplasim calcium level during the process of excitation-contraction coupling. It is expressed by three distinct genes producing different isoforms (RyR1, RyR2, and RyR3) distributed in specific locations in the tissues.
Hence, SR-RSCRC disorders may lead to several muscle pathologies, lesion, muscle damage, and dysfunction. In addition, genetic selection as well as other factors like nutrition and environmental stressors may cause disruption in sarcoplamic calcium homeostasis which leads to muscle pathology (Mitchell, 1999).
1.7.2 Indicators of muscle cell damage
In the last years, different clinical indicators have been developed to understand the type, extent and origin of muscle abnormalities. A muscle biopsy is important in histological analysis tool to understand the morphological changes. In addition, blood analysis can be considered more desirable and less destructive. Changing in the integrity of sarcolemma due to muscle lesion or myopathy may lead to leakage of intracellular enzymes and metabolites into the bloodstream. Hence, measuring the activity and levels of different enzymes (lactate dehydrogenase (LDH), aspartate aminotransferase (AST), aldose, and creatine kinase (CK) in blood can be used as indicators for muscle pathology. Creatine (phospo) kinase (CK/CPK) is the most common enzyme that is often used as an indicator for myopathy due to very high activity in respect to other enzymes.
1.7.3 Mechanism of myopathy in poultry
It was found that commercial lines of chickens and turkey had a degree of myopathy rised with age while muscle damage in selected fast growing lines was higher than in their genetic predecessors (or traditional lines). Additionally, selected broilers exhibited higher muscle damage due to acute heat stress than control lines. Several studies that have been done by Mitchell and Sandercock (1994, 1995, 1997) to explain the mechanism of stress induced
myopathy and monensin myotoxicity by evaluating radioisotopic calcium (45Ca) uptake and CK efflux. They found that the increased level of intracellular calcium either by increased calcium crossing inside the cells (by specific calcium ionophores) or release calcium from sarcoplasmic stores leads to changes in membrane integrity and efflux of CK. It was found that both increase of calcium and sodium may increase the enzyme level and causing membrane damage due to activation of phopholipase (PLAs). Monensin increased the entry of sodium by sodium-proton exchange into the cells, which also allow the entrance of calcium by sodium-calcium exchange mechanism in the sarcolemma as well as calcium release into cytoplasm by SR calcium channel or ryanodine receptor. The disturbance in ion balance underlying the myopathies in poultry is consistent with the mechanisms proposed by mammals. Monensin A is a type of antibiotic produced by streptomyces cinnamonensis. It forms complexes with different monovalent cations such as Li+, Na+, K+, Rb+, Ag+, and Ti+. Recently, it was found that monensin may have ability to transport sodium ion through the membrane in both electrogenic and electroneutral conditions (Huczyński et al., 2007). Increase the level of sodium and calcium in the muscle of Duchenne muscular dystrophy patients reduced the concentration of potassium and magnesium (Jackson et al., 1985).
Based on the former previous studies, it was found the levels of Na, K, Mg and Ca based on inorganic matter content were higher in broiler lines than layers and traditional lines. Broiler lines had a higher content of creatine kinase four times than layer and traditional lines. In other hand, broiler lines exhibited lower initial and ultimate pH than layer and traditional lines (Sandercock et al., 2009). Increase of sodium concentration in selected broiler muscle in comparison to unselected lines indicates to presence of changes in muscle cation homeostasis, and as consequences a sign of the initiation of muscle degeneration (Sandercock and Mitchell, 2004).
Skeletal muscle has the ability to regenerate when exposes to stress like exercise, injury, and disease. Regeneration process is triggered by activating small population of quiescent stem cells (satellite cells) to proliferate, differentiate, and fuse into multinucleated myotubes (Montarras et al., 2005; Collins et al., 2005; Kuang et al., 2007). The presence of abnormalities during the regeneration process in proliferation or differentiation in satellite cells can lead to muscle dysfunction and can induce the appearance of muscle disease (Mitchell, 1999).