Capillary depletion

Figure 2.4. In vivo measurement of blood-brain barrier (BBB) permeability, (a) Internal carotid artery perfusion technique (i) in the rat. Other branches of the carotid artery are ligated or electrically coagulated (o, occipital artery p, pterygopalatine artery). The external carotid artery (e) is cannulated and the common carotid artery (c) ligated. Perfusion time may range from 15 s to 10 min, depending on the test substance. It is necessary to subtract the intravascular volume, Vo, from (apparent volume of distribution), to obtain true uptake values and this may be achieved by inclusion of a vascular marker in the perfusate, for example labelled albumin. Time-dependent analysis of results in estimates of the unidirectional brain influx constant Ki (pi min which is equivalent within certain constraints to the PS product. BBB permeability surface area product PS can be calculated from the increase in the apparent volume of distribution Vd over time. Capillary depletion, i.e. separation of the vascular elements from the homogenate by density centrifugation, can discriminate capillary uptake from transcytosis. (b) i.v. bolus kinetics. The PS product is calculated from the brain concentration at the sampling time, T, and the area under the plasma concentration-time curve, AUC.

Capillary depletion. Here the largely nonsolvent liquid encompassed by the gel departs, leaving behind empty capillaries. Frequently this results in the formation of snowflake patterns that gradually fill in until the entire membrane becomes opaque. The reason for this is light scattering by the micrometre-sized empty... 

Water is, in most Instances, a strong nonsolvent making it a causative factor in the occurrence of blushing. After phase inversion and gelation but prior to capillary depletion, the membrane gel is still filled with the less volatile nonsolvent... 

As a direct consequence of Jurin s law, a nonwetting liquid such as mercury, which does not wet glass, forms a surface which is convex upwards hence the pressure just under the meniscus will be greater than at a planar surface. Thus, the level of a mercury surface in a capillary tube will be below that of the surrounding liquid a capillary depletion occurs (h < 0). Conversely with water which wets glass, a capillary rise (h > 0) occurs. 

The symptoms of parkinsonism are caused by a depletion of dopamine in the CNS. Dopamine, when given orally, does not cross the blood-brain barrier and therefore is ineffective The body s blood-brain barrier is a mesh-work of tightly packed cells in die walls of the brain s capillaries that screen out certain substances. This unique meshwork of cells in the CNS prohibits large and potentially harmful molecules from crossing into die brain. This ability to screen out certain substances lias important implications for drug dierapy because some drugs are able to pass through die blood-brain barrier more easily dian odiers. 

In region III near the tube center, viscous stresses scale by the tube radius and for small capillary numbers do not significantly distort the bubble shape from a spherical segment. Thus, even though surfactant collects near the front stagnation point (and depletes near the rear stagnation point), the bubble ends are treated as spherical caps at the equilibrium tension, aQ. Region... 

In standard FAB, the surface of the matrix solution is depleted of analyte and suffers from radiational damage during elongated measurements. Refreshment of the surface proceeds by diffusion (limited by the viscosity of the matrix) or evaporation. Continuous-flow fast atom bombardment (CF-FAB) continuously refreshes the surface exposed to the atom beam. [107,108] The same effect is obtained in slightly different way by the frit-fast atom bombardment (frit-FAB) technique. [109,110] In addition, both CF-FAB and frit-FAB can be used for online-coupling of liquid chromatography (LC, Chap. 12) [111] or capillary electrophoresis (CE) to a FAB ion source. [112]... 

Fig. 3. Adhesion F, capillary F, and depletion Fj forces, acting between colloidal particles and the substrate in the presence of liquid...

As the other methods, the Hummel-Dreyer method was first developed for chromatography and then adapted to capillary electrophoresis (21). A small sample of the ligand, dissolved in buffer, is injected into the capillary. The capillary as well as source and destination vial are filled with buffer containing the substrate. In the ligand-containing sample, the initial concentration of the free substrate is depleted to the equilibrium concentration (see Fig. 7a). The depletion peak moves with the velocity of the substrate, and its area corresponds to the bound substrate. As demonstrated in Fig. 7b, the... 

Fig. 4 Elution profiles for (A) propranolol (a), promethazine (b), and chlorprom-azine (c) applied separately on a 5-mm ILC column containing cytoskeleton-depleted red blood cell membrane vesicles entrapped in dextran-grafted agarose gel beads (1.4 /amol phospholipid, 0.5 mL/min) and (B), from left to right, acetylsalicylic acid, salicylic acid, warfarin, and pindolol on a capillary continuous bed containing liposomes immobilized by use of C4 ligands (1.0 /xmol phospholipid, 10 /xl./min). The elution volumes in the absence of lipid are shown (a0, b0, and c0, and the arrow, respectively). (Part A is reprinted with permission, with slight modification, from Ref. 26. Copyright 1999 Elsevier Science. Part B is reprinted with permission from Ref. 23. Copyright 1996 Elsevier Science.)...

M NaOH if migration times become erratic. Buffer in both reservoirs should be replaced periodically because ions become depleted and electrolysis raises the pH at the cathode and lowers the pH at the anode. The capillary inlet should be 2 mm away from and below the electrode to minimize entry of electrolytically generated acid or base into the column.30 Stored capillaries should be filled with distilled water. 

The left-hand side of the above equation gives the Ilkovic expression. It is thus possible to consider the right-hand side as correction terms to the Ilkovib equation. These corrections must take into account spherical diffusion, solution depletion in the neighbourhood of the drop due to previous drops, contact area with and shielding due to the capillary, and solution stirring. 

Deviation of a capillary from a vertical position changes its drop time considerably. The horizontally positioned DME was originally used by Smoler [9]. Shielding and depletion effects are diminished to some extent, and current oscillations are much smaller. Probably the best performance is observed when the electrode position deviates 45° from the vertical however, opinions about such electrodes vary [10]. In some experiments, electrodes with small mercury flow rates are helpful. For these electrodes, the probability of the appearance of polarographic maxima is diminished. When m is very low, the drop time is usually very long. In such cases, artificial regulation of the drop time should be used (see Sec. II.F). 

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