Titin, the largest known protein in the body expressed in three isoforms (N2A, N2BA and N2B), is essential for muscle structure, force generation, conduction and regulation. Since the 1950s, muscle contraction mechanisms have been explained by the sliding filament theory involving thin and thick muscle filaments, while the contribution of cytoskeleton in force generation and conduction was ignored. With the discovery of insoluble protein residues and large molecular weight proteins in muscle fibers, the third myofilament, titin, has been identified and attracted a lot of interests. The development of single molecule mechanics and gene sequencing technology further contributed to the extensive studies on the arrangement, structure, elastic properties and components of titin in sarcomere. Therefore, this paper reviews the structure, isforms classification, elastic function and regulatory factors of titin, to provide better understanding of titin.
Connectin/genetics* ; Muscle Proteins/metabolism* ; Protein Isoforms/genetics* ; Sarcomeres/metabolism* ; Muscle Fibers, Skeletal/metabolism*

Irradiation which acts directly and produces the reactive oxygen radicals by ionizing water molecules, causes significant morbidity and mortality. The muscle is damaged by direct action, oxygen radicals and the alterations of microcirculation and metabolism after irradiation. The changes of SOD immunoreactivities in muscles of the rats after irradiation were observed. The ultrastructural changes of the irradiated muscles with the pretreatment of SOD (superoxide dismutase) or without were also investigated. A total of 60 healthy Sprague-Dawley male rats weighing from 200g to 250g were used as experimental animals. Under urethane (1.15g/kg. IP.2 times) anesthesia,30 Gy irradiation to lower extremities by PICKER-C9 Cobalt-60 teletherapy unit was done. 15,000 unit/kg of SOD was administered intraperitoneally 1 hour before irradiation. The experimental animals were sacrificed 1 day, 3 days, 7 days, 2 weeks and 4 weeks after irradiation. The superficial portions of the mid-belly of the rectus femoris muscles were obtained and sliced into portions, 2 mm in length, 1 mm in width and in thickness. The specimens were prepared by routine methods for the electron microscopic observation. All preparations were stained with uranyl acetate and lead citrate and observed with a Hitachi-600 electron microscope. The other parts of mid-belly of the rictus femoris muscles were sectioned in 14 micrometer thickness with cryostat at -20 degrees C. The immunoreactivities of SOD by use of antihuman Cu, Zn-and Mn-SOD antibodies were observed. The results were obtained as follows . 1. After irradiation, the immunoreactivities of SOD in the rictus femoris muscle were decreased. 2 weeks after irradiation, the immunoreactivities of Cu, Zn-SOD were trace, which was lowest.4 weeks after irradiation, the immunoreactivities were trace or weak. 1 day after irradiation, the immunoreactivities of Mn-SOD were trace, which was lowest. The immunoreactivities of Mn-SOD were increased gradually 4 weeks after irradiation, the immunoreactivities of Mn- SOD were moderate or weak. 2. The ultrastructural changes in the rectus femoris muscles of the rats were getting severer and severer after irradiation. 2 weeks after irradiation, unclear A band and I band, myofibrillolysis, increased and dilated cistemae of sarcoplasmic reticulum and mitochondria with dilated cristae and electron lucent matrix were seen. 4 weeks after irradiation, lysis of sarcomere and increased cisternae of sarcoplasmic reticulum were seen. 3. The ultrastructural changes in the rectus femoris muscles of the rats were getting worse and worse after 3 days of irradiation with the pretreatment of SOD. 2 weeks after irradiation with the pretreatment of SOD, myofibrillolysis, increased and dilated cisternae of sarcoplasmic reticulum and damaged mitochondria were seen. 4 weeks after irradiation with the pretreatment of SOD, the ultrastructures of rectus femoris muscles were recovered to normal. Consequently, after irradiation of 30 Gy, the immunoreactivities of SOD are decreased and SOD attenuates the reversible changes of ultrastructures in muscles.
Animals ; Antibodies ; Citric Acid ; Humans ; Lower Extremity ; Male ; Metabolism ; Microcirculation ; Mitochondria ; Mortality ; Muscles ; Quadriceps Muscle* ; Rats* ; Rats, Sprague-Dawley ; Reactive Oxygen Species ; Sarcomeres ; Sarcoplasmic Reticulum ; Superoxide Dismutase ; Urethane

Cardiotrophin-1 (CT-1) activates a distinct form of cardiac muscle cell hypertrophy in which the sarcomeric units are assembled in series. The aim of the study was to determine the expression pattern of sarcomeric contractile protein α-actin, specialized cytoskeletal protein α-actinin and mitochondrial uncoupling protein-2 (UCP2) in myocardial remodeling induced by chronic exposure to CT-1. Kunming mice were intraperitoneally injected with carboxy-terminal polypeptide (CP) of CT-1 (CT-1-CP, 500 μg·kg(-1)· day(-1)) for 1, 2, 3 and 4 week (s), respectively (4 groups obtained according to the injection time, n=10 each, with 5 males and 5 females in each group). Those injected with physiological saline for 4 weeks served as controls (n=10, with 5 males and 5 females). The heart tissues of mice were harvested at 1, 2, 3 or 4 week (s). Immunohistochemistry (IHC) and Western blotting (WB) were used to detect the distribution and expression of sarcomeric α-actin, α-actinin and mitochondrial UCP2 in myocardial tissues. IHC showed that α-actin was mainly distributed around the nuclei of cardiomyocytes, α-actinin concentrated around the striae and UCP2 scattered rather evenly in the plasma. The expression of α-actin was slightly greater than that of α-actinin and UCP2 in the control group (IHC: χ(2)=6.125; WB: F=0.249, P>0.05) and it gradually decreased after exposure to CT-1-CP. There was no significant difference in the expression of α-actin between the control group and the CT-1-CP-treated groups (χ (2)=7.386, P>0.05). But Western blotting revealed significant difference in the expression of α-actin between the control group and the 4-week CT-1-CP-treated group (F=2.912; q=4.203, P<0.05). Moreover, it was found that the expression of α-actinin increased stepwise with the exposure time in CT-1-CP-treated groups and differed significantly between CT-1-CP-treated groups and the control group (ICH: χ (2)=21.977; WB: F=50.388; P<0.01). The expression of UCP2 was initially increased (WB: control group vs. 1- or 2-week group, q values: 5.603 and 9.995, respectively, P<0.01) and then decreased (WB: control group vs. 3-week group, q=4.742, P<0.01; control group vs. 4-week group, q=0.558, P>0.05). It was suggested that long-term exposure to CT-1-CP could lead to the alteration in the expression of sarcomeric α-actin, α-actinin and mitochondrial UCP2. The different expressions of sarcomeric structure proteins and mitochondrial UCP2 may be involved in myocardial remodeling.
Actinin ; biosynthesis ; Actins ; biosynthesis ; Animals ; Cardiomegaly ; chemically induced ; metabolism ; pathology ; Cytokines ; adverse effects ; pharmacology ; Female ; Gene Expression Regulation ; drug effects ; Ion Channels ; biosynthesis ; Male ; Mice ; Mitochondrial Proteins ; biosynthesis ; Myocardium ; metabolism ; pathology ; Sarcomeres ; metabolism ; pathology ; Uncoupling Protein 2