Microtubular

Because of the three-dimensional void structure of a packed column, the exact form of the velocity profile is not clearly defined as in the microtubular... 

Taxanes (paclitaxel, docetaxel) are derivatives of yew tree bark (Taxus brevifolia). They stabilize microtubules in the polymerized state leading to nonfunctional microtubular bundles in the cell. Inhibition occurs during G2- and M-phases. Taxanes are also radiosensitizers. Unwanted effects include bone marrow suppression and cumulative neurotoxicity. 

Centrosomes, also called the microtubule organizing centre, are protein complexes that contain two cen-trioles (ringlike structures) and y- tubulin. They serve as nucleation points for microtubular polymerization and constrain the lattice structure of a microtubule to 13 protofilaments. They are responsible for organizing the mitotic spindle during mitosis. 

In view of the importance of the motor dynein for microtubular function, this protein is currently considered as a new target for the development of cytostatic agents. 

Inside the typical smooth muscle cell, the cytoplasmic filaments course around the nuclei filling most of the cytoplasm between the nuclei and the plasma membrane. There are two filamentous systems in the smooth muscle cell which run lengthwise through the cell. The first is the more intensively studied actin-myosin sliding filament system. This is the system to which a consensus of investigators attribute most of the active mechanical properties of smooth muscle. It will be discussed in detail below. The second system is the intermediate filament system which to an unknown degree runs in parallel to the actin-myosin system and whose functional role has not yet been completely agreed upon. The intermediate filaments are so named because their diameters are intermediate between those of myosin and actin. These very stable filaments are functionally associated with various protein cytoarchitectural structures, microtubular systems, and desmosomes. Various proteins may participate in the formation of intermediate filaments, e.g., vimentin. 

Miura K, Koide N, Himeno S, Nakagawa I, Imura N. 1999. The involvement of microtubular disruption in methyhnercury-induced apoptosis in neuronal and nonneuronal cell lines. Toxicol Appl Pharmacol 160 279-288. 

Levis, S. and Deasy, P. (2002) Characterization of halloysite for use as a microtubular drug delivery system. Internal Journal of Pharmaceutics, 243, 125-134. 

The nature of the clustered phase is not well understood. One possible interpretation is that a cluster is composed of a relaxed divacancy whose inner surface is dressed with hydrogens however there is no direct NMR data which supports this identification (Reimer and Petrich, 1988). Alternatively, it has been suggested that the broad component of the NMR spectrum arises from hydrogen atoms lined up alone microtubular structural defects (Chenevas-Paule and Bourret, 1983). 

With regard to microtubular ultrastructure, micro filaments (5-7 run in diameter) are composed of filamentous actin. The tubule-like structures are formed by a, P-tubulin heterodimers. The wall is composed of 13 parallel protofilaments. Various microtubule-associated proteins and motor proteins (kinesin and dynein) are bound to the wall. The microtubule is a polar structure, i.e., plus and minus ends. 

The decreased contribution of film resistance for the microtubular electrode makes sense because the effective film thickness for the microtubular system is less than for the thin film control electrode. This is because the surface area of the microtubular current collector is eight times higher than the surface area of the planar current collector. (This factor is calculated from the membrane thickness and the density and diameter of the pores in the membrane.) Since the control and microtubular electrodes contain the same amount of TiS2, the eight times higher underlying surface area of the microtubular electrode means that the TiS2 film is effectively a factor of 8 thinner, relative to the control electrode. 

The decreased contribution due to slow electron transfer kinetics for the microtubular electrode is also attributable to the higher underlying surface area of the tubular current collector. Because the surface area is higher, the effective current density for the microtubular TiS2 is less than for the thin film TiS2, which has a conventional planar current collector. The decreased contributions of film resistance and slow electron transfer kinetics also account for the higher peak current density of the microtubular electrodes (Fig. 27). 

The reason for this loss in capacity with increasing scan can be clearly seen in the voltammograms in Fig. 30A and 30A. The peak separation, discussed in detail above, becomes larger as the scan rate is increased. The result of this enhanced distortion of the voltammetric wave is the inability to utilize the capacity of the electrode over the useful potential window of the electrode (3.0 to 1.5 V). As would be expected (see above), this distortion is less for the microtubular electrode, and this should result in higher capacities for this electrode. 

At the lowest scan rate employed, the microtubular electrode delivers an experimental capacity (256 21 mA hr g ) that is identical to the theoretical capacity. As scan rate is increased, capacity does fall off however, at any scan rate, the experimental capacity obtained from the microtubular electrode is greater than the capacity obtained at the control electrode. At the highest scan rate employed, the microtubular electrode delivers almost seven times the experimental capacity of the control electrode, even though both electrodes contain the same amount of TiS2. 

Finally, Fig. 33 shows the results of constant current discharge experiments at a microtubular electrode and a control electrode containing the same amount of TiS 2. Note that at this discharge current density, the microtubular electrode delivers 90% of its theoretical capacity. In contrast, as would be expected, the control electrode delivers significantly less capacity. 

FIG. 32. Discharge capacity versus scan rate for (A) TiS2 microtubular electrode (0.86 mg TiS2 cm ) and (B) TiS2 film control electrode (0.60 mg TiS2 cm ). 

The microtubular electrode concept described here also offers another possible advantage. In these concentric tubular electrodes, each particle of the Li intercalation material (the outer tube) has its own current collector (the inner metal microtubule). This could be an important advantage for Li+ intercalation materials with low electrical conductivity. This advantage was not demonstrated here because TiS2 has relatively high electronic conductivity. We have recently shown that electrochemical synthesis can be used to coat the gold microtubular current collector with outer mbes of a... 

Li+ intercalation material (V. M. Cepak and C. R. Martin, unpublished). These results, which will be the subject of a future paper, show that other synthetic methodologies, in addition to CVD, can be used to make micro-structured battery electrodes like those described here. In addition, the underlying microtubular current collector does not have to be Au. Microtubules composed of graphite [35] or other metals [1,3] (e.g., Ni) could be used. Finally, for the advantages noted above to be realized in practical cells, large-scale template-fabrication methods would have to be developed. 

Weber, K., Rathke, P. C., and Osborn, M. (1978) Cytoplasmic microtubular images in glutaraldehyde-fixed tissue culture cells viewed by electron microscopy and by immunofluorescence microscopy. Proc. Natl. Acad. Sci. USA 75, 1820-1824. 

Vesicular transport of bile acids has not been demonstrated under normal conditions, shown by using isolated rat hepatocyte couplets and fluorescently labelled bile acids. In these experiments confocal microscopy found no evidence of sequestering into clusters and colchicine disruption of microtubular function did not affect bile-acid transport. This makes it unlikely that vesicle transport plays a role and it is now believed that bile acids traverse the hepatocyte by diffusion through the cytosol while bound to soluble proteins. It is worth considering the caveat that fluorescently labelled bile acids, while very useful tools, do differ structurally from endogenous bile acids with increased hydro-phobicity leading to greater retention by cells. ... 

The mechanism of action of Catharanthus alkaloids undoubtedly relates to their effect on tubulin aggregation and consequent microtubule assembly and function. As microtubular function is intracellular, the alkaloids have to enter cells and remain within them to be effective. 

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