||It has been suggested that this article be merged with Transcellular and Transcellular transport. (Discuss) Proposed since April 2012.|
Paracellular transport refers to the transfer of substances across an epithelium by passing through the intercellular space between the cells. It is in contrast to transcellular transport, where the substances travel through the cell, passing through both the apical membrane and basolateral membrane.
Capillaries of the blood–brain barrier have only transcellular transport, in contrast with normal capillaries which have both transcellular and paracellular transport.
The transcellular and paracellular pathway of transport is also important for the absorption of drugs within the gastrointestinal tract (GIT). The paracellular pathway allows the permeation of hydrophilic molecules that are not able to permeate through the lipid membrane for the transcellular pathway of absorption. However, this is only applicable for small molecules, as larger ones will not be able to fit through the pores in the tight junctions. The pore sizes for the jejunum, ileum, and human colon are 8 au (atomic units), 3 au, and 3 au, respectively. The tight junctions have a net negative charge, and are believed to preferentially transport positively-charged molecules. The vast majority of drug molecules are transported through the transcellular pathway, and the few which rely on the paracellular pathway of transportation typically have a much lower bioavailability; for instance, levothyroxine has an oral bioavailability of 40 to 80%, and desmopression (DDAVP) of 0.16%.
Structure of paracellular channels
Paracellular transport can be enhanced through the displacement of zona occludens proteins from the junctional complex by the use of permeation enhancers. Such enhancers include medium chain fatty acids (e.g. oleic acid), chitosans, zona occludens toxin, etc.
- Diagram at citracal.com
- Epithelial Transcellular Transport
- Physiology at MCG 1/1ch2/s1ch2_37
- Tubular Transport
- Anderson, J. M.; Van Itallie, C. M. (2009). "Physiology and Function of the Tight Junction". Cold Spring Harbor Perspectives in Biology 1 (2): a002584. doi:10.1101/cshperspect.a002584. PMC 2742087. PMID 20066090.
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