Unstirred water layer thickness

Avdeef, A., Nielsen, P. E., Tsinman, O. PAM PA - a drug absorption in vitro model. 11. Matching the in vivo unstirred water layer thickness by individual-well stirring in microtitre plates. Eur. J. 

Fagerholm, U., Lennernas, H., Experimental estimation of the effective unstirred water layer thickness in the human jejunum, and its importance in oral drug absorption. Eur. J. Pharm. Sci. 1995, 3, 247-253. 

Fagerholm U and Lennernas H (1995) Estimation of the Unstirred Water Layer Thickness in the Human Small Intestine, and Its Importance in Oral Drug Absorption. Eur J Pharm Sci 3 pp 247-253. 

Fagerholm, U. and Lennernas, H. (1995) Experimental estimation of the effective unstirred water layer thickness in... 

Lewis, L. D. and Fordtran, J. S., Effect of perfusion rate on absorption, surface area, unstirred water layer thickness, permeability and intraluminal pressure in the rat ileum in vivo. Gastroenterology, 68 1509-1516, 1975. 

The glycocalyx and the mucus layer make up the structure of the unstirred water layer (UWL) [73]. The thickness of the UWL is estimated to be 30-100 pm in vivo, consistent with very efficient stirring effects [74]. In isolated tissue (in the absence of stirring), the mucus layer is 300-700 pm thick [73]. The pH in the unstirred water layer is 5.2-6.2, and might be regulated independently of the luminal pH (Section 2.3). The mucus layer may play a role in regulating the epithelial cell surface pH [73]. 

The UWL permeability is nearly the same for drugs of comparable size, and is characterized by the water diffusivity (Daq) of the drug divided by twice the thickness of the layer (ftaq), Pu = Aiq / (2 h.Aq), in a symmetric permeation cell [40], The unstirred water layer permeability can be determined experimentally in a number of ways based on pH dependency of effective permeability [25,509,535-538], stirring rate dependence [511-514,552,578], and transport across lipid-free microfilters [25,546],... 

The method preferred in our laboratory for determining the UWL permeability is based on the pH dependence of effective permeabilities of ionizable molecules [Eq. (7.52)]. Nonionizable molecules cannot be directly analyzed this way. However, an approximate method may be devised, based on the assumption that the UWL depends on the aqueous diffusivity of the molecule, and furthermore, that the diffusivity depends on the molecular weight of the molecule. The thickness of the unstirred water layer can be determined from ionizable molecules, and applied to nonionizable substances, using the (symmetric) relationship Pu = Daq/ 2/iaq. Fortunately, empirical methods for estimating values of Daq exist. From the Stokes-Einstein equation, applied to spherical molecules, diffusivity is expected to depend on the inverse square root of the molecular weight. A plot of log Daq versus log MW should be linear, with a slope of —0.5. Figure 7.37 shows such a log-log plot for 55 molecules, with measured diffusivities taken from several... 

Unstirred water layer (2.4 mm thickness in PAMPA, when plates are... 

Previously, the unstirred water layer (UWL) adjacent to the intestinal lining was considered to be the rate-limiting step for intestinal Peff of high-permeability compounds [27, 73], However, several in vivo studies have clearly reported that the thickness of this UWL is significantly thinner than was previously assumed, since... 

The layer of water adjacent to the absorptive membrane of the enterocyte is essentially unstirred. It can be visualized as a series of water lamellas, each progressively more stirred from the gut wall toward the lumen bulk. For BCS class 2 compounds the rate of permeation through the brush border is fast and the diffusion across the unstirred water layer (UWL) is the rate-limiting step in the permeation process. The thickness of the UWL in human jejunum was measured and found to be over 500 pm [3]. Owing to its thickness and hydrophilicity, the UWL may represent a major permeability barrier to the absorption of lipophilic compounds. The second mechanism by which the UWL functions act as a barrier to drug absorption is its effective surface area. The ratio of the surface area of the UWL to that of the underlying brush border membrane is at least 1 500 [4], i.e., this layer reduces the effective surface area available for the absorption of lipophilic compounds and hence impairs its bioavailability. 

Despite these variables, it appears that the primary attribute of soluble fibers that inhibit cholesterol absorption is the ability to form a viscous matrix when hydrated. Many water-soluble fibers become viscous in the small intestine (Eastwood and Morris, 1992). It is believed that increased viscosity impedes the movement of cholesterol, bile acids, and other lipids and hinders micelle formation, thus reducing cholesterol absorption and promoting cholesterol excretion from the body. Consumption of viscous fibers was shown to increase the thickness of the unstirred water layer in humans (Flourie et al., 1984 Johnson and Gee, 1981) and reduce the amount of cholesterol appearing in the lymph of cannulated rats (Ikeda et al., 1989b Vahouny et al., 1988). Turley et al. (1991, 1994) reported that... 

In contrast to the stomach, where there is a protective pH-gradient from 7 to 1 between Ihc mucosa and the iumen hoe, there is only a slight pH gradient from about 6 (mucosa) to about 7 (lumen). It corresponds to the inner mucosal part of the unstirred water layer. The thickness of this layer is about 20 pm. The pH has no significant influence on the absorption of protein or peptide drugs. 

The artificial membranes are prepared by dispersing a 20% (w/v) dodecane solution of a lecithin mixture (PN 110669 pION, USA) on a microfilter disc (125 /xm thick, 0.45 /urn pores). The system mounted is adequately stirred to properly simulate in vivo situations, that is, to obtain the expected unstirred water layer (UWL) of 30-100 /urn thickness [48]. 

In vivo as well as in vitro permeability is reduced due to the aqueous boundary layer or unstirred water layer (UWL). Shaking can reduce the UWL using different shaking frequencies (Hidalgo et al. 1991 Karlsson and Artursson 1992). However, the experimental conditions need to be tested carefully to avoid damage of monolayers. In side-by side diffusion systems the UWL was 52 pm (Karlsson and Artursson 1992). In vivo thickness of UWL was found 40 pm (Strocchi and Levitt 1991). 

Molecules diffuse less readily across a given distance in a plasma membrane than in a cell wall or an adjacent unstirred water layer. For the previous numerical values, DjKj is 1 x 10-9 m2 s-1 in the aqueous solution and 2 x 10 1° m2 s-1 in the cell wall, but for a plasma membrane about 7 nm thick, DjKj is only 10-18 to 10-14 m2 s-1. Membranes do indeed provide very effective barriers to the diffusion of solutes. 

The aqueous boundary layer or the unstirred water layer (UWL) is a more or less stagnant layer, about 30-100 pm in thickness, composed of water, mucus, and glycocalyx adjacent to the intestinal wall that is created by incomplete mixing of the lumenal contents near the intestinal mucosal surface. The glycocalyx is made up of sulfated mucopolysaccharides, whereas mucus is composed of glycoproteins (mucin), enzymes. 

Fuse, K. Bamba, T. Hosoda, S. Effects of pectin on fatty acid and glucose absorption and on thickness of unstirred water layer in rat and human intestine. Dig. Dis. Sci. 1989, 34(7), 1109-1116. 

The thickness of the unstirred water layer of rabbit intestine in vitro has been found to vary between 115 and 334 /im depending on the rate of stirring of the bulk phase [58]. The thickness of the unstirred water layer of the rat small intestine in vivo has been reported to be even greater [60]. If the effective thickness of the unstirred water layer can reach values of around 500 jam, then it is evident that it is a far from negligible compartment of the total luminal volume at least in the case of the rat. 

It has recently been discussed [61-63] whether the diffusional barrier at the intestinal surface can be accounted for solely by an unstirred water layer. It has been proposed that the mucus layer overlying the enterocytes should be regarded as an important diffusion barrier for uptake of lipid solutes from the luminal contents. The mucus adherent to the rat duodenal wall has been found to be approximately 80 jam thick in the fasted state [64]. The intestinal mucus coat is formed by proteoglycans produced by goblet cells, but so far very little is known about the molecular structure of the mucus layer [65]. The possible interaction between mucus constituents and luminal lipid solutes needs to be investigated in detail, since it might reveal key factors which constitute the diffusional barrier of the small intestine. 

In this experiment, carried out in 96-well plates, a buffered donor solution containing the compound is separated from a buffered acceptor solution using a fdter support, which has been treated with a solution of egg lecithin (mostly phosphatidylcholine) dissolved in n-dodecane, thus making a kind of sandwich. The wells are usually stirred to minimize the thickness of the unstirred water layer (or aqueous boundary layer), the water-filled area immediately adjacent to the membrane that can sometimes constitute another formidable barrier to the permeation of compounds. The concentration of the compound in the acceptor well is measured, usually by UV detection, over the course of some hours and the permeability, Papp, can be calculated. ... 

A variety of studies have suggested that a major limitation to cholesterol absorption exists at the epithelial cell surface. Also, several lines of evidence (e.g.[24-26]) support the concept of an unstirred water layer (UWL) at the mucosal surface, which provides a major resistance to transport of long-chain fatty acids and cholesterol into the cell. The thickness, and therefore resistance, of this layer is variable and is a function of physical mixing[51]. 

Experimental data indicate that the diffusion of micelles through the unstirred water layer is the rate limiting step among the cell surface events of cholesterol absorption. Wilson et al. firstly showed that the reduction of thickness of the unstirred layer, obtained by vigorous stirring, increased the rate of absorption of relatively insoluble molecules like long chain fatty acids[36]. In... 

The cathodic reaction during corrosion of iron in sea water is oxygen reduction. Solubility of 02 from the air in sea water is 0.189 mol m 3 and the diffusion coefficient of 02 is 2.75 x 10 9 m2 s 1. The diffusion layer thickness in an unstirred solution is about 0.5 mm. (a) Estimate the corrosion current density of iron in sea water, (b) If iron is connected to the negative pole of an external... 

A layer of relatively unstirred water lies adjacent to all biological membranes. The boundary between the bulk water and this unstirred layer is indistinct but, nevertheless, it has a real thickness. During absorption, dmg molecules must diffuse across this layer and then... 

If the surfactant is soluble in water, the situation at t = 0 is not the equilibrium state, and the surfactant will dissolve in the aqueous phase. (Water wiU likely dissolve in the particle as well, but we are not presently concerned with this process.) A very thin layer of water next to the interface will quickly be saturated, and no more dissolution may occur until the surfactant is transported away from this thin layer [12]. The maximum surfactant concentration that can develop within this layer is equal to the solubility of the surfactant. The thickness of this layer is typically much smaller than the thickness of unstirred boundary layers. 

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