Mechanisms of excitation-contraction coupling and calcium liberation in striated muscles of vertebrates

In vertebrate skeletal muscle, the main part of excitation-contraction coupling occurs at the level of the triad, where membranes of T-system and of junctional SR are facing each other. From place to place, the junctional gap is bridged by "feet" structures which include the SR Ca2+ channel. Half of them are closely apposed to tubular intramembranous structures assumed to be DHP-sensitive voltage-sensors which are similar to tubular Ca2+ channels and act by controlling Ca2+ release from SR. During a twitch, the release of Ca2+ activator from SR is controlled both by voltage-sensors via the feet structures and by a tubular Na+ current via a Na+-induced Ca2+ release mechanism. During long-duration mechanical responses, additional mechanisms are involved: a Ca2+-induced Ca2+ release which can be activated by ICa; the release of Ca2+ from membrane, controlled by the operation of a Na+/Ca2+ exchanger and/or new arrangements of surface membrane charges. An IP3-mediated Ca2+ release could be involved too. All these mechanisms can be regulated by intracellular biochemical or ionic processes.     

Characterization of target organs for calcitonin in lower and higher vertebrates

The bindings of calcitonin was investigated in trout bone, kidney and gill and rat bone and kidney. Specific binding of calcitonin was observed in all tissues tested except fish kidney membranes. The affinity constants for the sites of high affinity-low capacity (in trout bone and rat kidney) or for the unique site (in trout gill and rat bone) were of the same order of magnitude (2.0-9.0 x 10(9) M-1), the number of binding sites per mg of protein being higher in rat bone homogenates than in other tissues. These studies strongly support the theory that the gill in fishes is likely to perform some of the functions of the kidney in mammals.     

Comparative studies of contractile proteins from the skeletal and cardiac muscles of lower vertebrates

The available evidence suggests that the properties of the contractile proteins from lower vertebrates are broadly similar to those found in skeletal and cardiac muscles of mammalian species. However, the proteins from ectotherms are generally more unstable on isolation.     

Comparative study of the properties of serine proteases of lower and higher vertebrates

Results of the comparative study of trypsin- and chymotrypsin-like serine proteases from pyloric caeca of salmon fishes and trypsin and chymotrypsin of bulls are presented in the paper. The hydrolytic activity of salmon proteases with respect to methyl ethers of N-benzoyl-L-leucine is 2.4 times higher than that of bull chymotrypsin, but with respect to methyl esters of N-benzoyl-L-tyrosine and N-benzoyl-L-arginine the activity of salmon proteases is 6.5 and 80 times lower than that of bull chymotrypsin and trypsin. Salmon proteases in contrast to bull trypsin and chymotrypsin hydrolyze but slightly N-glutaryl-L-phenylalanine para-nitroanilide. It shown that fish proteases are not absolutely specific to synthetic substrates, which is a result of their less pronounced (than in case of bull trypsin and chymotrypsin) differences in structures of binding centres. The study of the salmon protease interaction with some immobilized ligands has confirmed the higher affinity of enzymes to reagents with two space-separated aromatic rings in their composition. It is supposed that salmon proteases interact with such reagents through two sites: hydrophobic "pockets" and probably additional binding site of the active centre. The salmon protease preparation demonstrates higher resistance to inactivating action of formaldehyde within the range of concentrations 2-16% than bull chymotrypsin does.     

Nuclear glycoproteins in higher vertebrates

Nuclear glycoproteins recognized by Concanavalin A have been isolated from pig, rabbit and chicken tissues. Mono and bidimensional electrophoresis patterns of proteins loosely and tightly bound to DNA have been examined. The tissue specificity rather than species-specificity appears to be a quite general property of these proteins, suggesting for them a role in the mechanism of regulation of chromatin functions.     

The fine structure of striated muscles in teleosts

Summary The organization of skeletal and cardiac muscles of fishes described on the basis of observations carried out on several species of freshwater fishes (Tinca tinca, Misgurnus fossilis, Perca fluviatilis, Lebistes reticidatus) and marine fishes (Gobius minutus, Pleuronectes platessa, Ammodytes tobianus). Truncal, subcutaneous, extrinsic eye and cardiac muscles were used for study. Glutaraldehyde-fixed tissue was refixed in OsO₄, embedded in Epon and after polymerization, cut into ultra thin sections and examined by an electron microscope.White and red muscles were distinguished in the material examined. The latter was represented by subcutaneous muscles and small fibres of extrinsic eye muscles. Particular types of fibers differ from each other in their organization of the SR and localization of the T system tubules. In the most muscles the T system tubules are situated at the level of the Z line. In the small fibers of extrinsic eye muscles alone these tubules lie at the A-I junction.The myocardiac cells consist of a cylindrically shaped myofibril. In the middle of the cylinder is the nucleus, the remaining space being filled with numerous mitochondria. A loose sarcoplasmic network is twined around the myofibril.     

New aspects of obscurin in human striated muscles

Obscurin is a giant protein (700-800 kDa) present in both skeletal muscles and myocardium. According to animal studies, obscurin interacts with myofibrillar Z-discs during early muscle development, but is translocalised to be predominantly associated with the M-bands in mature muscles. The proposed function for obscurin is in the assembly and organisation of myosin into regular A-bands during formation of new sarcomeres. In the present study, the precise localisation of obscurin in developing and mature normal human striated muscle is presented for the first time. We show that obscurin surrounded myofibrils at the M-band level in both developing and mature human skeletal and heart muscles, which is partly at variance with that observed in animals. At maturity, obscurin also formed links between the peripheral myofibrils and the sarcolemma, and was a distinct component of the neuromuscular junctions. Obscurin should therefore be regarded as an additional component of the extrasarcomeric cytoskeleton. To test this function of obscurin, biopsies from subjects with exercise-induced delayed onset muscle soreness (DOMS) were examined. In these subjects, myofibrillar alterations related to sarcomerogenesis are observed. Our immunohistochemical analysis revealed that obscurin was never lacking in myofibrillar alterations, but was either preserved at the M-band level or diffusely spread over the sarcomeres. As myosin was absent in such areas but later reincorporated in the newly formed sarcomeres, our results support that obscurin also might play an important role in the formation and maintenance of A-bands.