Transport paracellular

Some form of local desquamation of intestinal epithelium seems to be the most common explanation of how particles, of any size, might be absorbed via the usually tight junctions between epithelial cells (Luckey, 1974 Csaky, 1984). One hypothesized mechanism for the extrusion of epithelial cells is a combination of the pressure from reproducing epithehal cells, which loosens the surrounding cells, and variations in lymphatic pressure in the lacteals acting on the epithelium. A decrease in lymphatic pressure could then, in theory, draw particulate matter into the site through 

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The blood-brain barrier (BBB) forms a physiological barrier between the central nervous system and the blood circulation. It consists of glial cells and a special species of endothelial cells, which form tight junctions between each other thereby inhibiting paracellular transport. In addition, the endothelial cells of the BBB express a variety of ABC-transporters to protect the brain tissue against toxic metabolites and xenobiotics. The BBB is permeable to water, glucose, sodium chloride and non-ionised lipid-soluble molecules but large molecules such as peptides as well as many polar substances do not readily permeate the battier. 

Fig. 9 Schematic representation depicting the movement of molecules from the absorbing (mucosal or apical) surface of the GIT to the basolateral membrane and from there to blood. (A) transcellular movement through the epithelial cell. (B) Paracellular transport via movement between epithelial cells. (Q Specialized carrier-mediated transport into the epithelial cell. (D) Carrier-mediated efflux transport of drug out of the epithelial cell. (Copyright 2000 Saguaro Technical Press, Inc., used with permission.)...

C. Paracellular Transport—Equivalent Pore and Circuit Theory... 

Solute uptake can also be evaluated in isolated cell suspensions, cell mono-layers, and enterocyte membrane vesicles. In these preparations, uptake is normalized by enzyme activity and/or protein concentration. While the isolation of cells in suspension preparations is an experimentally easy procedure, disruption of cell monolayers causes dedifferentiation and mucosal-to-serosal polarity is lost. While cell monolayers from culture have become a popular drug absorption screening tool, differences in drug metabolism and carrier-mediated absorption [70], export, and paracellular transport may be cell-type- and condition-depen-dent. 

PG Ruifrok, WEM Mol. Paracellular transport of inorganic and organic ions across rat ileum. Biochem Pharmacol 32 637-640, 1983. 

III. PARACELLULAR TRANSPORT KINETICS A. Morphology of the Paracellular Route... 

Table 6 Estimations of Electrical and Molecular Size Restriction Factors Influencing the Paracellular Transport of Charged Permeants...

Artursson P, C Magnusson. (1990). Epithelial transport of drugs in cell culture. II. Effect of extracellular calcium concentration on the paracellular transport of drugs of different lipophilicities across monolayers of intestinal epithelial (Caco-2) cells. J Pharm Sci 79 595-600. 

Penetration enhancers are low molecular weight compounds that can increase the absorption of poorly absorbed hydrophilic drugs such as peptides and proteins from the nasal, buccal, oral, rectal, and vaginal routes of administration [186], Chelators, bile salts, surfactants, and fatty acids are some examples of penetration enhancers that have been widely tested [186], The precise mechanisms by which these enhancers increase drug penetration are largely unknown. Bile salts, for instance, have been shown to increase the transport of lipophilic cholesterol [187] as well as the pore size of the epithelium [188], indicating enhancement in both transcellular and paracellular transport. Bile salts are known to break down mucus [189], form micelles [190], extract membrane proteins [191], and chelate ions [192], While breakdown of mucus, formation of micelles, and lipid extraction may have contributed predominantly to the bile salt-induced enhancement of transcellular transport, chelation of ions possibly accounts for their effect on the paracellular pathway. In addition to their lack of specificity in enhancing mem-... 

Another limitation of the Caco-2 monolayers is their colonic origin and tight paracellular pathway, which tend to lead to underestimations in permeability to paracellularly transported compounds [97]. This is likely to be correct for small compounds (MW < 150) - i.e., compounds smaller than normal drugs - but it remains to be seen to what extent the Caco-2 model gives false-negative predictions of the fraction absorbed for polar drugs of normal size in humans where para-... 

The paracellular pathway, between the epithelial cells, is both size- (MW, volume) and charge-dependent [60, 109, 110]. In general, compounds that are limited to paracellular transport are not efficiently absorbed due to the small available absorptive area and the restriction by tight junctions. The molecular weight cut-off seems to be around 400 g mol-1 and 300 g mol-1 for the small and large intestine respectively, and 300 g mol-1 for the Caco-2 cell monolayers [60], which shows the more colonic nature of the Caco-2 monolayer model. Compounds with a... 

Fig. 5.5. Example of using the Caco-2 monolayers to evaluate paracellular transport of an unknown compound. Two markers, mannitol and propranolol are added together with the unknown compound to the apical side and transport of each of the molecules is measured both in the absence and in the...

Although the absence of paracellular transport across the BBB impedes the entry of small hydrophilic compounds into the brain, low-molecular-weight lipophilic substances may pass through the endothelial cell membranes and cytosol by passive diffusion [7]. While this physical barrier cannot protect the brain against chemicals, the metabolic barrier formed by the enzymes from the endothelial cell cytosol may transform these chemicals. Compounds transported through the BBB by carrier-mediated systems may also be metabolized. Thus, l-DOPA is transported through the BBB and then decarboxylated to dopamine by the aromatic amino acid decarboxylase [7]. 

Passage through the epithelium and endothelial cellular barriers likely represents the greatest challenge to absorption. Although the molecular details remain unclear, this absorption process appears to occur via one of two possible means transcytosis or paracellular transport (Figure 4.6). 

Figure 4.6 Likely mechanisms by which macromolecules cross cellular barriers in order to reach the bloodstream from (in this case) the lung. Transcytosis entails direct uptake of the macromolecule at one surface via endocytosis, travel of the endosome vesicle across the cell, with subsequent release on the opposite cell face via exocytosis. Paracellular transport entails the passage of the macromolecules through leaky tight junctions found between some cells...

In a second approach, Sugano et al. [138] tried to consider paracellular transport in addition to transcellular permeation. The prediction of the paracellular transport potential was based on size and charge parameters together with artificial membrane permeability in relation to known human absorption values. Other groups have focused on the paracellular route by modification of the assay [26],... 

A1 Rat fetal intestinal epithelial cells Temperature-sensitive differentiation More suited to study paracellular transport (leakier pores) than Caco-2... 

Another drawback is the lack of correlation for paracellularly transported compounds. For example, some low molecular weight hydrophilic compounds (e.g., ranitidine, atenolol, furosemide, and metformin) showed poor permeability in the Caco-2 model despite an absorption larger than 50% in humans [168], This is due to the smaller size of the paracellular channels (controlled by tight junctions) in the Caco-2 model compared to human small intestine [146],... 

An alternative method for assessing cell layer integrity is through the use of hydrophilic paracellular transport markers (e.g., radiolabeled D-mannitol or fluorescein-Na+), which passively traverse cells by the paracellular route. Small amounts of compound required for in vitro conjunctival cell culture transport experiments make this approach well suited for screening purposes. Relative absorption index of a series of pharmacologically active molecules can be ranked against known markers for the identification of candidates with potential absorption problems, which is a reliable tool to select drug candidates with optimal characteristics. 

An example of RUI analysis method using [3H]adenosine is shown in Figure 14.3B. The RUI value of [3H]adenosine is greater than that of [3H]d-mannitol (used as a paracellular transport marker) and significantly reduced by 30% in the presence of 2 mM unlabeled adenosine and thymidine, while 2 mM cytidine has no effect. Thus, the nature of the inhibition shown by this approach confirms the carrier-mediated transport of adenosine from the blood to the retina across the BRB in vivo [27],... 

One of the main functions of epithelia is to control water and solutes, compartmentalized by the regulation of transport across the epithelium from body interior to exterior (or vice versa). Deviations from the meticulously regulated movement of water and solutes across the epithelial barrier can lead to states of disease and can be detrimental to life. Fluids can traverse epithelia by one of two routes through the cells (transcellular transport) or between cells (intercellular or paracellular transport) (Figure 15.1A). 

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