Capillary membrane vesicle

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.)...

Anionic sites on the lumenal surface of pulmonary microvascular endothelium have been shown to bind cationic ferritin in isolated, perfused rat lung studies [196]. The cationic ferritin is taken up by vesicles and discharged into the capillary membrane. Similar anionic sites are also present on alveolar epithelial surfaces [25]. 

Most electron microscopic studies of BBB endothelial cells suggest the presence of relatively few observable endocytic vesicles in the cytoplasm of these cells compared with other endothelia. For example, the BBB contains only a fifth to a sixth of the endocytic profiles seen in muscle capillary endothelia [54], although they may increase to comparable levels with inflammation of the BBB [55]. However, when a comparison is made of the ability of capillary endothelia in a variety of different tissues to trancytose protein, there is a very poor correlation between the protein permeability of a microvessel and the number of observable endocytic profiles [54]. Brain capillary endothelia are very thin cells, the luminal and ablum-inal membranes only being separated by some 500 nm or less (5000 A), and caveoli are 50-80 nm in diameter and thus the events of transcytosis may be difficult to capture within the cell using conventional electron microscopical techniques. 

The third mechanism is peritubular extraction of peptides and proteins from postglomerular capillaries and intracellular metabolism. Experiments using iodi-nated growth hormone (125I-rGH) have demonstrated that, while reabsorption into endocytic vesicles at the proximal tubule is still the dominant route of disposition, a small percentage of the hormone may be extracted from the peritubular capillaries [79, 86]. Peritubular transport of proteins and peptides from the baso-lateral membrane has also been shown for insulin [87] and the mycotoxin ochra-toxin A [88]. 

The mechanism by w hich immunoUposomes penetrate across the BBB is not fully understood. It w as hypothesized that the process involves the binding of immunoliposomes to multiple capillary luminal membrane receptors, fusion of the liposomes with several vesicular pits into a large vesicle and transcytosis of this vesicle to the abluminal membrane border (Comford and Comford, 2002). Studies by electron microscopy support this hypothesis (Faustmann andDermietzel, 1985). 

In the heart, the lead salt has been found to be deposited along the plasma membrane and sarcotubular system of muscle cells, and precipitation was enhanced by adrenaline, glucagon and F. In contrast, histamine caused particularly intense staining at the plasma membrane and the membrane of the pinocytotic vesicles of endothelial cell capillaries. This raises the possibility that cyclic AMP formed in endothelial cells of the coronary capillaries under the influence of histamine may affect cardiac cells, thereby acting as a local hormone. 

The barrier effect is mainly due to the fact that the cells lining the walls of the capillaries present in the brain tissue are tightly joined, contrary to what prevails with capillaries in other tissues this leaves very little space between the cells for filtration of small-size, water-soluble molecules. Moreover, the cells of brain capillaries possess very few endocytotic vesicles, which in capillaries of other tissues engulf large molecules and serve as a transfer mechanism as a result, many neurotoxins, such as diphteria and tetanus toxins, are excluded. Furthermore, the capillaries of the brain are surrounded by prolongations of certain brain cells, thus forcing lipid-soluble chemicals to cross an additional lipid membrane. Finally, the intercellular fluid bathing the brain cells contains lower concentrations of proteins this results in a reduction of the movement of certain water-insoluble chemicals that are more easily transported when bound to proteins. 

In the case of interactions between inclusions in lipid bilayers (Figure 5.19) the elasticity of the bilayer interior must also be taken into account. The calculated energy of capillary interaction between integral membrane proteins turns out to be of the order of several Hence, this interaction can be a possible explanation of the observed aggregation of membrane proteins. The lateral capillary forces have been calculated also for the case of particles captured in a spherical (rather than planar) thin liquid film or vesicle. ... 

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