Microtubules in mammalian heart muscle

Microtubules and desmin filaments during the onset of heart growth in the rat

Microtubules and intermediate desmin filaments were visualized in rat cardiomyocytes during the onset of heart growth by double immunolabelling of isolated myocytes, with specific antibodies raised against tubulin and desmin. Heart growth was stimulated either by mechanical overloading induced by aortic stenosis, or by injection of thyroxine into hypothyroid rats. In both experimental models, alterations in the microtubule pattern were observed soon after stimulation of growth whereas desmin filament organization remained unchanged. Microtubules were redistributed in arrays parallel to the long axis of the myocytes and were more numerous around the nuclei. Microtubules therefore appeared to be involved in the cellular events occurring during stimulated heart growth irrespective of the nature of the stimulus.

Effects of microtubule-disrupting agents on insulin binding and degradation in isolated cardiocytes from adult rat

Isolated muscle cells from adult rat heart have been used to study the effects of microtubule disruptive drugs on the maintenance of steady-state insulin binding to cardiac insulin receptors. Vinblastine, vincristine and podophyllotoxin significantly inhibited insulin binding (25-50%) in the presence of insulin (10(-8) mol/l). The effect of vinblastine was found to be time- and temperature-dependent and to be dependent on the amount of insulin bound to the cell. In the presence of cycloheximide (0.1 mmol/l) insulin binding decreased by 30%; this effect was found to be additive to the action of vinblastine. Treatment of cells with vinblastine significantly reduced the low-molecular mass material produced by receptor-mediated degradation of insulin. This effect was not additive to that of the lysosomotropic agent chloroquine. The results suggest involvement of microtubules in the intracellular transfer of insulin receptors from and to the plasma membrane.

Ultrastructure of terminally differentiated adult rat cardiac muscle cells in culture

Ventricular cardiac muscle cells isolated from adult rats were maintained in culture for 28 to 60 days and examined by transmission electron microscopy. In order to elucidate the detailed ultrastructure of the cultured myocytes, several different electron-dense stains were used. These included tannic acid, osmium ferrocyanide, osmium tetroxide (applied as a primary fixative), and lanthanum chloride, as well as more widely used stains such as osmium tetroxide, uranyl acetate, and lead citrate. Our results show that, compared to cultured neonatal rat myocytes, cultured myocytes derived from adult rats more closely resemble in vivo adult ventricular cells. The cultured adult myocytes contained typically distributed organelles such as nuclei, mitochondria, and elements of the sarcoplasmic reticulum. Myofilaments were well organized, and typical intercalated disks were observed between adjacent cells. Unlike cultured neonatal myocytes, the adult cells contained numerous residual bodies and a relatively well developed transverse tubular system. The transverse tubular system was identified by its continuity with the extracellular space (as indicated by the penetration of electron-dense extracellular tracers), location at or near the Z line, large lumenal diameter, and frequent participation in couplings with elements of the sarcoplasmic reticulum.

Filamentary rod-shaped granules in Clara cells of rats

The bronchiolar Clara cells of rats contain characteristic rod-shaped granules always surrounded by a unit membrane. These granules contain thin filaments about 9 to 10 nm in diameter lying in a pale matrix. Our morphological results suggest that the filamentary rod-shaped granules originate from the common, round-to-oval, electron-dense Clara cell granules, as we found different intermediate structures between these two kinds of granules. The electron-dense granules are digested by pepsin, whereas the filamentary rod-shaped granules are apparently not affected. The biochemical nature and the possible function of the filamentary rod-shaped granules are also discussed in relation to the secretory activity of the Clara cells.

Taxol induces postmitotic myoblasts to assemble interdigitating microtubule-myosin arrays that exclude actin filaments

Taxol has the following effects on myogenic cultures: (a) it blocks cell replication of presumptive myoblasts and fibroblasts. (b) It induces the aggregation of microtubules into sheets or massive cables in presumptive myoblasts and fibroblasts, but not in postmitotic, mononucleated myoblasts. (c) It induces normally elongated postmitotic myoblasts to form stubby, star-shaped cells. (d) It reversibly blocks the fusion of the star-shaped myoblasts into multinucleated myotubes. (e) It augments the number of microtubules in postmitotic myoblasts, and these are assembled into interdigitating arrays of microtubules and myosin filaments. (f) Actin filaments are largely excluded from these interdigitating microtubule-myosin complexes. (g) The myosin filaments in the interdigitating microtubule-myosin arrays are aligned laterally, forming A-bands approximately 1.5 micrometers long.

Membranes in the mitotic apparatus of barley cells

Membranes in the mitotic apparatus have been investigated ultrastructually in dividing cells of barley (Hordeum vulgare). After osmium tetroxide- potassium ferricyanide or ferrocyanide postfixation (OsFeCN) of material that had been fixed in glutaraldehyde in the presence of Ca(++), the nuclear envolope (NE)-endoplasmic reticulum (ER) complex is selectively stained, permitting observations on the cellular pattern and structural ramifications of this membrane system that have not been previously recognized. Specifically, it is observed that membrane system that have not been previously recognized. Specifically, it is observed that during mitosis the NE-ER forms a continuous membrane system that ensheathes and isolates the mitotic apparatus (MA). Elements of ER progressively accumulate in the region of the spindle pole, becoming most concentrated by early anaphase. Within the MA itself, there are striking spindle- membrane associations; in particular, tubular elements of predominantly smooth NE-ER invade the spindle interior selectively along kinetochore microtubules. The membrane elements at the pole and surrounding the MA consist of tubular reticulum and fenestrated lamellae. Membranes of the MA thus resemble in considerable detail the tubular network and fenestrated elements of the sarcoplasmic reticulum of muscle. It is suggested that the NE-ER of the dividing barley cell may function in one or both of the following ways: (a) to control the concentration of free Ca(++) in the MA and (b) to serve as an anchor to chromosome motion.

A new type of hereditary distal myopathy with characteristic sarcoplasmic bodies and intermediate (skeletin) filaments

A family with a hitherto unrecognized type of distal myopathy is described. The disorder appears to be of late onset and to be inherited through a dominant autosome. It has a more malignant course than the distal myopathies described earlier, from which it can be differentiated clinically by an early involvement of thenar muscles and hand flexors. The key to the correct diagnosis is provided by the morphological and immunohistological investigation of muscle biopsies, which show typical sarcoplasmic bodies and an abundance of intermediate-sized (skeletin) filaments.

Structures located at the levels of the Z bands in mouse ventricular myocardial cells

Within ventricular myocardial cells of the mouse, the myoplasmic regions located immediately adjacent to the Z lines of the sarcomeres contain a variety of structures. These include: (1) transversely oriented 10 nm ('intermediate') filaments that apparently contribute to the cytoskeleton of the myocardial cell; (2) the majority of the transverse elements of the T-axial tubular system; (3) specialized segments of the sarcoplasmic reticulum (SR) that are closely apposed to the sarcolemma or T-axial tubules (junctional SR); (4) 'extended junctional SR' ('corbular SR') that exists free of association with the cell membrane; (5) 'Z tubules' of SR that are intimately apposed to the Z line substance; and (6) leptofibrils. In addition, fasciae adherentes supplant Z lines where myofibrils insert into the transverse borders (intercalated discs) of the cells. The concentration of these myocardial components at the level of the Z lines suggests that a particular specialization of structural and physiological activities exists in the Z-level regions of the myoplasm. In particular, it appears that the combination of intermediate filaments, T tubules, and Z-level SR elements forms a series of parallel planar bodies that extend across each myocardial cell to impart transverse rigidity. The movement and compartmentation of calcium ion (Ca2+) would seem especially active near the Z lines of the myofibrils, in view of the preferential location there of Ca2+-sequestering myocardial structures such as T tubules, junctional SR, extended junctional SR and Z tubules.