Parallel-absorption cell

The nearly parallel laser beam furthermore allows absorption spectroscopy experiments which realize a long light path at low pressures in the absorption cell without loss in intensity due to beam spread effects it also eliminates disturbing reflections from cell walls. 

It is well established today that drug absorption through the alimentary canal walls is a complex event, which involves, in many cases, parallel or sequent microprocesses at the apical membrane of the absorptive cell (enterocyte) or between them (paracellular absorption). In addition to the various types of diffusion processes across the enterocyte membrane, numerous specific proteins—transporters and efflux pumps—are involved in the intricate drug absorption process. In the following sections the various epithelial tissues of the different organs of the GI tract will be looked at briefly. A review of major drug absorption mechanisms across epithelial cells, as they are customary today will follow. 

Fig. 4.8. Basic geometry for the observation of the dispersed Faraday effect the polariser P is crossed with an analyser A, and propagation along an axis Oz is parallel to the field lines B in the region of the atomic absorption cell. Rotation of the plane of polarisation through an angle (p occurs as a result of magneto-optical birefringence (after J.-P. Connerade [161]).

The experimental arrangement is sketched in fig. 4.8. Plane polarised radiation is incident on an absorption cell placed between two crossed po-larisers in a magnetic field. The direction of the magnetic field is parallel to the direction of propagation. It is helpful to consider the three classical geometries associated with a+, 7r and a polarisations. These are... 

Sensitization of photogalvanic action by dyes which are themselves not capable of photoredox action has been demonstrated (7). Action spectra of solutions containing rhodamine 6G and two coumarin dyes in addition to thionine and methylene blue closely parallel absorption spectra, corresponding to the possible use of about 50% of the insolation spectrum and a theoretical maximum sunlight engineering efBciency of 7 %. It has been demonstrated that rhodamine 6G does, in fact, increase the power output of iron-thionine (or other thiazine) cells under white-light illumination an approximately 40% increase has been observed under illumination with 35 mWcm" (8). 

The excited state of a single extended polypeptide chain is made up of a band of states with an energy spread which depends on the magnitude of the coupling constants. The transition moment to any one of these states is the sum of all the individual transition moments in the molecule, each multiplied by a phase factor (Schellman and Schellman, 1964). In the important special situation of the polypeptide chain with a twofold screw axis (sheet structures), a group of two peptide groups in sequence can be considered as a unit cell. In this situation (Davydov, 1962) the allowed motions can be described in terms of transition moments that are either in phase with one another or 180° out of phase n radians). The former case produces a parallel absorption band, the latter a perpendicular band thus, the component of the transition moments in the direction of the molecular axis contributes only to the parallel band and the component perpendicular to this axis contributes to the perpendicular band. 

Figure 8. Electron micrograph of an intestinal absorptive cell 1 h after administration of a com oil meal. The apical cytoplasm contains numerous SER-bound droplets of resynthesized lipid (L). The RER has receded from the apical cytoplasm to predominate in the region of the cell above the Golgi complex. Golgi vacuoles (GV) are now numerous and filled with lipid (compare with Fig. 4). Completed chylomicra (CH) are observed in the intercellular spaces between adjacent absorptive cells. The chylomicra closely parallel the size of lipid droplets observed in Golgi vacuoles. Microvillus border (MV) nucleus (N). X 9355.

GIT, is considered to be lost from the absorption site, as is metabolic clearance and sequestration in various cell types and membranes (72,14). It is clear from Scheme I that the relative rates of the various processes will define the bioavailable fraction of the dose and understanding those factors which control pulmonary absorption kinetics is obviously the key to enhancing bioavailability via the lung. In a recent book (75) the molecular dependence of lung binding and metabolism was considered alongside the parallel processes of absorption, clearance and dissolution in the lung (14). Some key features of this work will be repeated as it relates to the systemic delivery of polypeptides. 

In vitro studies permit further isolation of parallel transport processes and can provide a reduction in experimental variability. Rate of absorption assessment can be measured as intestinal uptake or flux across an intestinal barrier at both the tissue and cell monolayer levels. Experimental variability is also reduced by the fact that a large number of tissue samples can be used from the same experi-... 

There are several approaches to estimating absorption using in vitro methods, notably Caco-2 and MDCK cell-based methods or using methods that assess passive permeability, for example the parallel artificial membrane permeation assay (PAMPA) method. These are reviewed elsewhere in this book. The assays are very useful, and usually have an important role in the screening cascades for drug discovery projects. However, as discussed below, the cell-based assays are not without their drawbacks, and it is often appropriate to use ex vivo and/or in vivo absorption assays. 

Based on experiments with linearly polarized light, Mayer also concluded that the photoreceptor is arranged in a dichroic fashion close to the cell wall. The electrical dipole moment for the absorption of bluelight lies parallel to the cell wall, but is probably random with respect to the normal of the cell wall. In the first experiment, the cells were irradiated with bright light. Clearly, the chloroplasts separate from the walls, which are parallel to the -vector and exhibit a banded pattern (Fig. 17, left). However, in weak polarized light the chloroplasts tended to move close to those walls parallel to the -vector (Fig. 17, right). 

For the in vitro test, the fibroblasts are allowed to form a half-confluent monolayer within 24 h. Different concentrations of the test chemical are then incubated for 1 h with two sets of cells in parallel (typically on 96-well plates, 104 cells per well, passage number <100). After the incubation with the test substances, one set is irradiated with a nontoxic dose of UVA light (5 J/cm2), while the other set is kept in the dark. Twenty hours after irradiation, cell viability is evaluated by measuring the uptake of NR for 3 h. After the end of the absorption process, excess NR is removed and the cells are treated with an NR desorption solution (ethanol/acetic acid) to extract the dye taken up by the cells. Subsequently, the optical density of the NR solution is measured at 540 nm. As positive control, a test with chlorpromazine is performed. 

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