Pinocytosis fluid-phase

Pinocytosis is a property of all cells and leads to the cellular uptake of fluid and fluid contents. There are two types. Fluid-phase pinocytosis is a nonselective process in which the uptake of a solute by formation of small vesicles is simply proportionate to its concentration in the surrounding extracellular fluid. The formation of these vesicles is an extremely active process. Fi-... 

Compared with phagocytosis, pinocytosis appears to be a universal phenomenon in all cells, including phagocytes. Unlike phagocytosis, which is mediated by the serum opsonin, pinocytosis does not require any external stimulus. Pinocytosis is divided into two types fluid-phase pinocytosis and adsorptive pinocytosis (see Fig. 3B). Fluid-phase pinocytosis is a nonspecific, continuous process, and it is believed to be useful as a general process for transporting macromolecular constructs through epithelia, some endothelia, and into various blood cells. Adsorptive pinocytosis, in... 

The cellular uptake of AS-ODN is an energy-dependent process and takes place in a saturable and sequence-independent manner [120,121]. The exact mechanism of uptake remains controversial. From in vitro experiments, some authors have proposed that the uptake is endocytic and mediated by membrane receptor proteins. The receptor responsible for the cellular uptake of AS-ODNs was reported to consist of both a 30-kDa protein [122] and an 80-kDa membrane protein [121]. However, other workers have argued that AS-ODN binding to membrane proteins is relatively non-specific and is mostly charge associated, consistent with adsorptive endocytosis or fluid-phase pinocytosis [101]. As a result of these conflicting reports, it is unlikely that in vitro data can be safely extrapolated to what occurs in the intact organism. 

Fig. I. Endocytic pathways used by cells to internalize soluble macromolecules [25] fluid-phase pinocytosis (1), adsorptive pinocytosis (2), and receptor-mediated endocytosis (pinocytosis) (6). Each of these processes involves a formation of a sealed vesicle formed from the plasma membrane which encloses part of the extracellular medium. The internalization of a polymer-drug conjugate (P-D), and targeted polymer-drug conjugate ( => —P-D) is shown. Other abbreviations — = cell surface receptor/antigen 1 = clathrin molecule X = lysosomal enzyme. Fluid-phase pinocytosis (1) and adsorptive pinocytosis (2) are nonspecific processes which direct the macromolecule into the lysosomal compartment of the cell. Once P-D is internalized, whether by (1) or (2), the resulting endosome (3) is ultimately fused with a primary lysosome (4) forming a secondary lysosome (5). In the latter compartment P-D is in contact with several types of lysosomal enzymes. The membrane of (5) is impermeable to macromolecules. Consequently, the structure of P-D may be designed in such...

Solutes dissolved in the extracellular fluid, including large (soluble) macromolecules, may flow with the extracellular fluid into the invaginations and become internalized. This process, i.e. the uptake of macromolecules in solution, is known as fluid-phase pinocytosis. Alternatively, uptake may involve ... 

Figure 1.6 Schematic representation of fluid-phase pinocytosis and exocytosis...

The second subcategory of endocytosis is called fluid phase pinocytosis, which arises from entrapment of solutes by vesicles that invaginate from the cell surface. Importantly, the amount of material taken in by this route is proportional to a component s concentration in the extracellular environment. As such, pinocytosis is generally regarded as the means by which solutes enter cells nonspecifically. 

Compared with phagocytosis, fluid-phase pinocytic capture of molecules is relatively slower, being directly proportional to the concentration of macromolecules in the extracellular fluid. It is also dependent on the size of macromolecules in general, lower molar mass fractions are captured faster than the higher molar mass fractions. The magnitude of the rate of capture by adsorptive pinocytosis is higher than that by fluid-phase pinocytosis and relates to the nature of substrate-membrane interactions. 

With respect to the concentration factor, the molecules of the polymer can be found in the extracellular fluid either distributed randomly or forming some kind of concentration gradient with respect to the vicinity of the involved part of the cell surface. In the former case (fluid phase pinocytosis) , no interactions between the polymer and the cell membrane is assumed and differences in the amount of polymer absorbed by various cells or tissues will reflect differences in their pinocytic activity. In the latter case (adsorptive pinocytosis), the higher concentration of the polymer in the vicinity of the cell surface (i.e. in a layer of a thickness comparable with the vesicle... 

Munniksma et al, extensively studied the mechanism of the b vivo capture of I-labelled PVP. They confirmed that capture occurs by fluid-phase pinocytosis and measured the rates of pinocytosis by many cell types. The results they obtained on the blood clearance of I-labelled PVP using normal and nephrectomised rats again underline the importance of glomerular filtration in this process. 

Poly[N-(2-hydroxypropyl)methacrylamide] is another polymer developed as a prospective plasma expander and as such its rate of elimination from the bloodstream was of mterest. As mentioned in section 2, experiments were carried out using [ C]polymer and the results obtained show that clearance is clearly related to the molecular weight of the polymer. Recently, by using N-(2-hydroxypropyl)-methacrylamide copolymers whose tyrosine-containing side chams were labelled with [ I]iodide it has been shown that the pattern of blood clearance from rats is consistent with that reported for a similar molecular weight distribution of I-labelled PVP, i.e. these data show that N-(2-hydroxypropyl)methacrylamide copolymers are captured, like polyvmylpyrrolidone, non-selectively in vivo by fluid-phase pinocytosis. 

As mentioned above, HPMA copolymers containing approximately 2% oligopeptidyl side-chains have no affinity for the plasma membrane and enter cells by fluid-phase pinocytosis. Increasing the hydrophobicity of macromolecules appears to enhance, non-specifically, their rate of capture by rat visceral yolk sacs and we have shown that incorporation of 20% tyramine residues into the synthetic polymer poly-a-3"N(2-hydroxyethyl) -D,-L- aspartamide greatly enhances its rate of uptake by this tissue.More recently we have found that incorporation of tyrosine residues (approximately 18%) into HPMA oligopeptidyl side-chains produces the same results (Table 4). The existence of a correlation between the percentage tyrosine in the molecule and the rate of pinocytic uptake indicates that synthetic polymers can be designed in such a way as to control their non-specific affinity for membranes. 

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