Absorptive pinocytosis

Materials may be absorbed by a variety of mechanisms. Depending on the nature of the material and the site of absorption, there may be passive diffusion, filtration processes, faciHtated diffusion, active transport and the formation of microvesicles for the cell membrane (pinocytosis) (61). EoUowing absorption, materials are transported in the circulation either free or bound to constituents such as plasma proteins or blood cells. The degree of binding of the absorbed material may influence the availabiHty of the material to tissue, or limit its elimination from the body (excretion). After passing from plasma to tissues, materials may have a variety of effects and fates, including no effect on the tissue, production of injury, biochemical conversion (metaboli2ed or biotransformed), or excretion (eg, from liver and kidney). 

Pinocytosis. This process by which particles are absorbed can be an important factor in the ingestion of particulate formulations of chemicals (e.g., dust formulations, suspensions of wettable powders, etc.) however, it must not be confused with absorption by one of the above processes, where the agent has been released from particles. 

Generally the smaller the molecule the more easily it will be taken up. Molecular weights below 500 are favorable for absorption molecular weights in the 1000s do not favor absorption. Generally solids have to dissolve before they can be absorbed. It is possible for small amounts of particles in the nanometer size range to be taken up by pinocytosis. The absorption of very large particles, several hundreds of micrometers in diameter, that are administered dry (e.g., in the diet) or in a suspension may be reduced because of the time taken for the particle to dissolve. This would be particularly relevant for poorly water-soluble substances. 

All the above transport mechanisms are only applicable to the absorption of small molecules, less than approximately 500 Da. There is evidence that larger molecules can be absorbed with low efficiency due to endocy tosis. Endocy tosis is defined as the internalization of plasma membrane with concomitant engulfment of extracellular material and extracellular fluid. The process can be divided into two types, pinocytosis and phagocytosis. 

All exposure pathways can ultimately result in the absorption of soluble substances across the body s membranes (skin, eyes, respiratory, or digestive tracts), by passive or active diffusion, active transport, or cellular pinocytosis/phagocytosis (the engulfment of foreign particles by cells). The proportion of a substance in contact with a membrane that is absorbed is a complex function of many factors, including the concentration and chemical form of the substance, the relative chemical conditions ambient on either side of the membrane, and the surface area of the membrane with which the substance is in contact. 

Three processes are involved in transcellular transport across the intestinal epithelial cells simple passive trans-port, passive diffusion together with an efflux pump, and active transport and endocytosis. Simple passive transport is the diffusion of molecules across the membrane by thermodynamic driving forces and does not require direct expenditure of metabolic energy. In contrast, active transport is the movement of molecules across the mem-brane resulting directly from the expenditure of metabolic energy and transport against a concentration gradient. Endocytosis processes include three mechanisms fluid-phase endocytosis (pinocytosis), receptor-mediated endocytosis, and transcytosis (Fig. 6). Endocytosis processes are covered in detail in section Absorption of Polypeptides and Proteins, later. 

The absorption method for medication particles engulfed by a cell and pulled across a membrane is pinocytosis. 

Absorption at cellular level occurs through passive transport, active transport, pinocytosis and facilitated diffusion... 

Cooper and Castle have proposed a three-step sequence to explain the vitamin B12 absorption in the gastrointestinal tract. In the first step, vitamin B12 binds with the intrinsic factor in gastric juice. The affinity of the vitamin for the intrinsic factor is thought to be greater than its affinity for proteins in the intestinal content consequently, the intrinsic factor (IF) successfully displaces the vitamin from its weaker bonds with other proteins. Calcium facilitates and EDTA inhibits the absorption of vitamin B12 by the everted intestine. On the basis of these and related findings, workers proposed the second step in vitamin Bi2 absorption. At that stage it is assumed that the intrinsic factor-vitamin B12 complex is trapped in the intestinal wall by the intermediate of calcium bonds and absorbed by pinocytosis. This stage of the absorption process probably is interfered with in sprue and steatorrhea where calcicum soaps are formed in the intestinal lumen. 

A third method of absorption is by pinocytosis cell drinking , in which cells have the capacity to engulf large molecules in solution or suspension. Such a process is particularly important in many newborn suckled mammals in which immunoglobulins present in colostrum are absorbed intact. 

Reference has already been made to the absorption of intact proteins, such as immunoglobulins, in the newborn animal by pinocytosis. 

Absorption of nutrients occurs mainly in the small intestine. Carbohydrates are absorbed as monosaccharides by active transport, a process involving carrier proteins. Amino acids and fatty acids are also absorbed by active transport, but emulsified triglycerides are absorbed by passive diffusion. Large molecules, especially the immunoglobulins present in colostrum, are absorbed by a process known as pinocytosis. Many minerals and vitamins require special processes of absorption. 

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