Discrimination of Ca2+-ATPase activity of the sarcoplasmic reticulum from actomyosin-type ATPase activity of myofibrils in skinned mammalian skeletal muscle fibres: distinct effects of cyclopiazonic acid on the two ATPase activities

Summary We have developed a procedure to discriminate actomyosin-type ATPase activity from Ca²⁺-ATPase activity of sarcoplasmic reticulum (SR) in mechanically skinned fibres, determining simultaneously their Ca²⁺-induced tension and accompanying ATPase activity. When they were treated with an alkaline CyDTA-containing solution of low ionic strength which was reported to remove troponin C, the fibres showed a considerable amount of Ca²⁺-dependent ATPase activity, in spite of having little or no Ca²⁺-induced isometric tension. The residual ATPase activity is ascribed to the Ca²⁺-ATPase activity of SR, because it is completely abolished by 1% CHAPS treatment for 10 min. This conclusion is also supported by the finding that the Ca²⁺-dependence of the ATPase activity is very similar to that of Ca²⁺-ATPase of SR isolated from rabbit skeletal muscle, and that the estimated activity is consistent with the reported values of direct determinations. On the other hand, treatment with a detergent such as CHAPS or Triton X-100 removes SR activities (ATPase and Ca-uptake), leaving Ca²⁺-induced tension and actomyosin-type ATPase activity unchanged. This procedure indicated that the contribution of Ca²⁺-ATPase activity of SR may be minimal in total steady-state ATPase activity of mechanically skinned mammalian skeletal muscle fibres. Successive CyDTA and CHAPS treatments eliminated both Ca²⁺-induced tension and ATPase activity, which were recovered by the addition of troponin C. Using these procedures, we also examined the effect of cyclopiazonic acid (CPA) which was reported to be a specific inhibitor of Ca²⁺-ATPase of SR. Ca²⁺-ATPase activity of SR in skinned fibres was inhibited completely by 10 m CPA and held to one-half by about 0.2 m. This effect was only partially reversible. CPA at 10 m or higher concentrations showed Ca²⁺-sensitizing action on myofibrils, which was readily reversible. CPA at 3 m inhibited almost completely the Ca²⁺-ATPase activity of SR, while it had no effect on either actomyosin-type ATPase or isometric tension of myofibrils.     

Effect of calcitonin gene-related peptide on contraction of striated muscle in the mouse

We have found ultrastructurally calcitonin gene-related peptide-like immunoreactivity in the axon terminal within the synaptic trough of neuromuscular junction of the mouse. We determined, using pharmacological means, with a phrenic nerve-diaphragm preparation, that this peptide enhances muscle contraction during stimulation of the nerve fibers or direct stimulation of the muscle. This effect is probably brought about via the receptor for this peptide not the acetylcholine receptor.     

Obscurin is required for the lateral alignment of striated myofibrils in zebrafish.

Obscurin/obscurin-MLCK is a giant sarcomere-associated protein with multiple isoforms whose interactions with titin and small ankyrin-1 suggest that it has an important role in myofibril assembly, structural support, and the sarcomeric alignment of the sarcoplasmic reticulum. In this study, we characterized the zebrafish orthologue of obscurin and examined its role in striated myofibril assembly. Zebrafish obscurin was expressed in the somites and central nervous system by 24 hours post-fertilization (hpf) and in the heart by 48 hpf. Depletion of obscurin using two independent morpholino antisense oligonucleotides resulted in diminished numbers and marked disarray of skeletal myofibrils, impaired lateral alignment of adjacent myofibrils, disorganization of the sarcoplasmic reticulum, somite segmentation defects, and abnormalities of cardiac structure and function. This is the first demonstration that obscurin is required for vertebrate cardiac and skeletal muscle development. The diminished capacity to generate and organize new myofibrils in response to obscurin depletion suggests that it may have a vital role in the causation of or adaptation to cardiac and skeletal myopathies.     

Heterotopic striated muscle on the surface of the brain

Summary Heterotopic tissue, identified by both light and electron microscopy as cardiac muscle, was detected at autopsy on the surface of the right superior temporal gyrus of a 73-year old female patient. The cells of the heterotopic tissue were interconnected by intercalated discs and the fibres contained regularly cross-striated, contracted, or relaxed myofibrils indicating a continuing contractile activity until the time of death.     

Tumors of striated muscle.

Three benign forms of muscle tumors and three variants of rhabdomyosarcoma are discussed from the point of view of their histology and behabior. Recent combined modality therapy has significantly improved the prognosis in at least the embryonal form of rhabdomyosarcoma.     

Effect of L-arginine-NO system on the erythrocyte ATPase activity and LPO

Activity of Na,K-ATPase in erythrocytes of children with asthmatic bronchitis and cerebral palsy rapidly changed under the effect of NO synthesis inhibitor L-NAME and did not depend on the plasma Ca²⁺ concentration. Enzyme activities correlated with the content of Mg²⁺; and a tendency toward accumulation of LPO substrates and primary molecular LPO products was noted. The relative content of Schiff bases increased almost 2-fold. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 128, No. 9, pp. 321–323, September, 1999     

Phosphorylation of tropomyosin in striated muscle

An historical perspective of the phosphorylation of tropomyosin is provided. The effects of this covalent modification on the properties of striated muscle tropomyosin are summarised. Technical hurdles and findings in other systems are also discussed.