Carrier-mediated processes, transcellular

All chemical and mechanical phases of digestion are directed toward changing the ingested material (food or xenobiotics) into absorptive forms that can pass through mucosal cells into blood and lymphatic vessels. The process of absorption occurs via passive diffusion (para and transcellular) and carrier-mediated process (facilitated or active transport). About 90 % of absorption of nutrients occurs in the small intestine. 

This refers to the transport across the epithelial cells, which can occur by passive diffusion, carrier-mediated transport, and/or endocytic processes (e.g., transcytosis). Traditionally, the transcellular route of nasal mucosa has been simply viewed as primarily crossing the lipoidal barrier, in which the absorption of a drug is determined by the magnitude of its partition coefficient and molecular size. However, several investigators have reported the lack of linear correlation between penetrant lipophilicity and permeability [9], which implies that cell membranes of nasal epithelium cannot be regarded as a simple lipoidal barrier. Recently, compounds whose transport could not be fully explained by passive simple diffusion have been investigated to test if they could be utilized as specific substrates for various transporters which have been identified in the... 

Transport across the cell membrane may occur via different routes. Some of these transport processes are energy dependent and therefore termed active others are independent from energy, thus passive. Passive transport phenomena, for example, transcellular transport, are triggered by external driving forces, such as concentration differences, and do not require metabolic activity. However, generally, they are restricted to small lipophilic compounds. In contrast, active transport phenomena, such as active carrier-mediated transport or vesicular pathways, take course independent from external driving... 

Transcellular Drug Absorption—Nondiffusional Processes 1.5.5.1 Carrier-Mediated Approaches to Drug Absorption... 

Both processes exhibit classical saturation kinetics, since there are only a finite number of carrier molecules. Thus unlike passive absorption (paracellular or transcellular), where the rate of transport is directly proportional to the drug concentration (Figure 1.5, A), carrier-mediated transport is only proportional to the drag concentration at low concentrations of drug. At higher concentrations, the carrier mechanism becomes saturated and the rate of absorption remains constant (Figure 1.5, B). 

Other transcellular mechanisms of absorption include carrier-mediated transport and endocytic processes. Although it is well known that carrier-mediated transport systems exist for di- and tripeptides in the intestine, there is still no evidence for carrier-mediated transport of peptides across the vaginal mucosa, although prostaglandins have been demonstrated to utilize such a mechanism. Although there must be some type of endocytic transport of endogenous peptides into the epithelial cells in order to regulate proliferation, no receptor-mediated or bulk-fluid mechanisms have been reported. 

The nasal epithelium possesses selective absorption characteristics similar to those of a semipermeable membrane, i.e., it allows a rapid passage of some compounds while preventing the passage of others. The process of transportation across the nasal mucosa involves either passive diffusion, via paracellular or transcellular mechanisms, or occurs via active processes mediated by membrane-bound carriers or membrane-derived vesicles involving endo- or transcytosis. 

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