Urea permeability

Sands, JM, Flores, FX, Kato, A, Baum, MA, Brown, EM, Ward, DT, Hebert, SC and Harris, HW, 1998, Vasopressin-elicited water and urea permeabilities are altered in IMCD in hypercalcemic rats, Am J Physiol 274 F978-985... 

Table 20. Ratios of urea permeability to NaCl permeability of stereocomplex membranes3155...

Figure 3 Correlation between urea permeability and electrical resistance. Each point represents the average of data collected at a given temperature for an individual HEM sample. Units of P are cm/sec, units of /f are k icm. [Reprinted with permission from Peck et al. (1995). Copyright 1995 by American Chemical Society.]...

Figure 4 Permeability coefficients for each permeant (urea, mannitol, sucrose, and raffinose) normalized by urea permeability coefficient for a fntted-glass membrane system [Peck et al., (1994)] (hatched bars) HEM (solid bars) and predictions based upon a correlation proposed by Potts and Guy (1992).

In addition to increasing the water permeability of the collecting duct and the urea permeability of the inner medullary collecting duct, V2-receptor activation also increases NaA transport in the thick ascending limb and collecting duct. Increased Na transport in the thick ascending limb is mediated by three mechanisms that affect the Na -K -2CI symporter rapid phosphorylation of the symporter, translocation of the symporter into the luminal membrane, and increased... 

W. Lee, K. Saito, S. Furusaki, T. Sugo and K. Makuuchi, Design of Urea-Permeable Anion-Exchange Membrane hy Radiation-Induced Graft Polymerization, J. Membrane Sci., 81 (1993) 295. 

Lee, W., Saito, K., Furusaki, S. et al. 1993. Design of urea-permeable anion-exchange membrane by radiation-induced graft polymerization. J Membr Sci. 81 295-305. 

When the properties of bacteria containing Urel and those with Urel deleted are compared in a medium of pH 2.5 at different urea concentrations, it is found that with Urel present, the organisms are able to survive pH 2.5, with half-maximal survival seen at a urea concentration of 1 mM within the physiologic range of gastric juice urea. With the deletion mutants, there is no survival in acid until a concentration of 100 mM urea is reached, and half-maximal survival is seen at a urea concentration of 250 mM. Hence, there is an approximately 300-fold increase of urea permeability in H. pylori due to expression of Urel at an external pH of 2.5. Because Urel is in the inner membrane and plays no role in the function of the putative surface urease, the latter compartment does not play a role in acid survival in vitro. 

Figure 9.16 Fractional volume change showing volume versus time for a urea permeability experiment. The vesicle was composed of SOPC, with an area of 5981 pm and a volume of 42 951 pm. The experiment was conducted at 25°C.

The volume change can therefore be used as a reporter of urea concentration in the vesicle and so determine its permeability coefficient for lipid vesicle membranes of varying composition. Preliminary experiments indicate that urea has a permeability coefficient of 6.8 x 10 cms in SOPC, and 1.4 x 10 cms in DOPC at 25 C. These values compare extremely well with the earlier reported values of urea permeability (4.0 x 10" cms" at 25°C in eggPC [33], or 1.4 x 10 cms at 25°C in DOPC [87]). These experiments now lay the founding protocols for experiments aimed at quantifying drug transport across giant vesicle bilayers. 

In one version of the urea electrode, shown in Figure 11.16, an NH3 electrode is modified by adding a dialysis membrane that physically traps a pH 7.0 buffered solution of urease between the dialysis membrane and the gas-permeable... 

Urea derivatives These compounds have lower permeability //-Butanol CH5CH2CH2CH2OH 0.01... 

It has been known for some years that gramicidin forms transmembrane ion channels in lipid bilayers and biological membranes and that these channels are assembled from two molecules of the polypeptide 213). The channels are permeable specifically to small monovalent cations [such as H+, Na+, K+, Rb+, Cs+, Tl+, NH4+, CHjNHj, but not (CH3)2NH2+J and small neutral molecules (such as water, but not urea). They do not allow passage of anions or multivalent cations 21 n. 

The permeability coefficients of urea and mannitol decrease with increasing molecular size (Table 5). Similar observations are made for the protonated methylamine and atenolol and between the acetate and hippurate anions. By comparing permeability coefficients among permeants of like charges, e.g., urea and mannitol, or acetate and hippurate, one observes... 

Figure 14 Observed permeability coefficients of urea and mannitol across monolayers of rat alveolar epithelial cells in primary culture in the Transwell system are correlated with transepithelial electrical resistance and days in culture.

Capillaries are the site of exchange between blood and the interstitial fluid surrounding tissue cells. Tissues with a higher metabolic rate have a more extensive capillary network, that is, a greater number of capillaries per unit area. Because of extensive branching of these vessels, the cells of the body are typically within 20 pm of the nearest capillary. Consequently, the distance that substances must travel between blood and the cells is minimized. Capillaries are permeable to water and small water-soluble substances, such as glucose, amino acids, lactic acid, and urea, and impermeable to proteins. 

Pulmonary delivery currently represents the most promising alternative to parenteral delivery systems for biopharmaceuticals. Delivery via the pulmonary route moved from concept to reality in 2006 with the approval of Exubera, an inhalable insulin product (Chapter 11). Although the lung is not particularly permeable to solutes of low molecular mass (e.g. sucrose or urea), macromolecules can be absorbed into the blood via the lungs surprisingly well. In fact, pulmonary... 

Uremia results in increased permeability of the blood-brain barrier to sucrose and insulin K+ transport is enhanced whereas Na+ transport is impaired. There is an increase in brain osmolarity in acute renal failure due to the increase in urea concentrations. However, in contrast to acute renal failure, the increase in osmolarity in chronic renal failure results from the presence of idiogenic osmoles in addition to urea. CBF is increased in uremic patients but CMR02 and CMR are decreased. In the brains of rats with acute renal failure, ATP, phosphocreatine and glucose are increased whereas AMP, ADP and lactate are decreased, most probably as a result of decreased energy demands. 

Gryns (1896), Hedin (1897), and especially Overton (1900) looked at the permeability of a wide range of different compounds, particularly non-electrolytes, and showed that rates of penetration of solutes into erythrocytes increased with their lipid solubility. Overton correlated the rate of penetration of the solute with its partition coefficient between water and olive oil, which he took as a model for membrane composition. Some water-soluble molecules, particularly urea, entered erythrocytes faster than could be attributed to their lipid solubility—observations leading to the concept of pores, or discontinuities in the membrane which allowed water-soluble molecules to penetrate. The need to postulate the existence of pores offered the first hint of a mosaic structure for the membrane. Jacobs (1932) and Huber and Orskov (1933) put results from the early permeability studies onto a quantitative basis and concluded molecular size was a factor in the rate of solute translocation. 

Under normal conditions, the transcellular route is not considered as the preferred way of dermal invasion, the reason being the very low permeability through the corneocytes and the obligation to partition several times from the more hydrophilic corneocytes into the lipid intercellular layers in the stratum corneum and vice versa. The transcellular pathway can gain in importance when a penetration enhancer is used, for example, urea, which increases the permeability of the corneocytes by altering the keratin structure. 

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