Passive transport mechanisms

Quantitative Approaches to Delineate Passive Transport Mechanisms in Cell Culture Monolayers... 

Adson A. (1992). Quantitative approaches to delineate passive transport mechanisms in cell culture monolayers. MSc Thesis, University of Kansas, Lawrence, KS. 

The enthusiasm for using Caco-2 cells and other epithelial cell cultures in studies of drug transport processes has been explained by the ease with which new information can be derived from these fairly simple in vitro models [7]. For instance, drug transport studies in Caco-2 cells grown on permeable supports are easy to perform under controlled conditions. This makes it possible to extract information about specific transport processes that would be difficult to obtain in more complex models such as those based on whole tissues from experimental animals. Much of our knowledge about active and passive transport mechanisms in epithelia has therefore been obtained from Caco-2 cells and other epithelial cell cultures [10-15]. This has been possible since Caco-2 cells are unusually well differentiated. In many respects they are therefore functionally similar to the human small intestinal enterocyte, despite the fact that they originate from a human colorectal carcinoma [16, 17]. 

As the Caco-2 cell model contains most of the important transporters, it can be used for the study not only of passive transport mechanisms but also of mechanisms involving transporters. This is a major advantage of the Caco-2 model when... 

For application of the BCS, an apparent passive transport mechanism can be assumed when one of the following conditions is satisfied ... 

Concerning the absorption theory for organic compounds in the alimentary tract, the pH-partition hypothesis has been accepted. Under normal physiological conditions, drug absorption from the lower alimentary tract is well described by the pH-partition hypothesis. Therefore, the passive transport mechanism is dominant in drug absorption from rectal mucous membranes. On the other hand, does any specific transport mechanism, such as... 

Transport by facilitated diffusion A large number of molecules and ions were shown to permeate membranes considerably faster than expected from their lipid-water partitioning behavior. This led to the recognition of additional transport mechanisms. Systematic investigations of permeability rates in membranes, reconstituted membranes, and membrane models as functions of the temperature of the nature and concentration of the permeant in the absence and in the presence of additives, suggested three different facilitated passive transport mechanisms ... 

For the passive transport mechanism (Fig. 1, Path 2b), there is a linear dependence for permeability on diffusivity and the partition coefficient between aqueous and membrane compartments [Eq. (1)]. Diffusivity, which can be calculated from molar volume, generally varies within 1 order of magnitude, while partitioning can range over many orders of magnitude. Hence, for molecules with passive transport as the dominant pathway, determination of the partition coefficient can be a useful estimate of permeability. 

Approximately 90% of filtered uric acid is reabsorbed in the proximal tubule, probably by both active and passive transport mechanisms. There is a close linkage between proximal tubular sodium reabsorption and uric acid reabsorption, so states that enhance sodium reabsorption (e.g., dehydration) also lead to increased uric acid reabsorption. The exact site of tubular secretion of uric acid has not been determined this too appears to involve an active transport process. Postsecretory reabsorption occurs somewhere distal to the secretory site. 

Passive transport mechanisms use no external energy source to bring about diffusion of a substance across a membrane. The most common passive transport systems are diffusion and facilitated transport. 

See also Membrane Proteins, Erythrocyte Membrane, Passive Transport Mechanisms, Ankyrin, Table 10.5... 

Passive Transport Mechanisms Active Transport Mechanisms... 

See also Thermodynamics of Transport Across Membranes, Passive Transport Mechanisms, Active Transport Mechanisms... 

See also Passive Transport Mechanisms, Sodium-Potassium Pump, Lipid Bilayer... 

The intestinal absorption discrimination between Ca and Sr takes place in the passive transport mechanism and not in the active one or of vitamin D dependent... 

The most attractive possibility Is that nickel promotes the absorption of Fe(III) per se by enhancing Its complexatlon to a lipophilic molecule. Evidence shows that both active and passive transport mechanisms have roles In Iron absorption. 

Passive transport mechanisms Mechanisms that use the forces of concentration differences (diffusion) or pressure differences (convection) to move substances from one site to another. 

It has been known for some time that different viruses have different effects on cell membranes. In particular, the enveloped viruses can cause early cell fusion as they enter the host cell or late cell fusion after replication of the virus. These phenomena apply particularly to the paramyxoviruses and have been dubbed fusion from without and fusion from within (Bratt and Gallaher, 1969). Other enveloped viruses can produce similar but usually less striking effects on cell fusion. Nonenveloped viruses can also cause drastic changes in cell membranes, usually resulting in alteration in the permeability barrier of the cell membrane. Still other viruses, or the same viruses under different conditions, can alter in a more subtle way the active or passive transport mechanisms of cells. Under some conditions, some viruses even stimulate the formation of cell membranes by inducing augmented synthesis of cellular membrane lipids. 

Stable hybrids with DNA and may enter cells through passive transport mechanisms. Finally, and potentially important for drug delivery, we have shown that the boranophosphates are nuclease resistant (12), like the phosphorothioates and methyIphosphonates. Short boranophosphate oligomers exhibit a high degree of stability to spleen phosphodiesterase and snake venom phosphodiesterase, indicating that they could have long half-lives in the cell. 

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