Solute transporter

Eor pesticides to leach to groundwater, it may be necessary for preferential flow through macropores to dominate the sorption processes that control pesticide leaching to groundwater. Several studies have demonstrated that large continuous macropores exist in soil and provide pathways for rapid movement of water solutes. Increased permeabiUty, percolation, and solute transport can result from increased porosity, especially in no-tiUage systems where pore stmcture is stiU intact at the soil surface (70). Plant roots are important in creation and stabilization of soil macropores (71). 

Fig. 6. Solute transport in hemodialysis. Clearance vs solute mol wt for dialy2ers prepared from the two different membranes illustrated in Figure 5. Numbers next to points represent in min /cm calculated from equations 10 and 5. Data is in vitro at 37°C with saline as the perfusion fluid. Lumen flow, dialysate flow, and transmembrane pressure were 200 ml,/min, 500 mL/min, and 13.3 kPa (100 mm Hg) area = 1.6. Inulin clearance of the SPAN...

Diffusion plays an important part in peak dispersion. It not only contributes to dispersion directly (i.e., longitudinal diffusion), but also plays a part in the dispersion that results from solute transfer between the two phases. Consider the situation depicted in Figure 4, where a sample of solute is introduced in plane (A), plane (A) having unit cros-sectional area. Solute will diffuse according to Fick s law in both directions ( x) and, at a point (x) from the sample point, according to Ficks law, the mass of solute transported across unit area in unit time (mx) will be given by... 

Leigh, R.A. (1983). Methods, progress and potential for use of isolated vacuoles in studies of solute transport in higher plant cells. Physiologic Plantarum, 57,390-6. 

New membranes have been developed with significantly enhanced mass-transfer characteristics. These membranes include rotating membrane systems that use a torsional oscillation to produce shear rates as high as 150,000 s. Coiled hollow fibres that exploit Dean vortices to increase solute transport and reduce fouling are receiving attention (Zydney, 2000). 

Miller CW, Benson LV (1983) Simulation of solute transport in a chemically reactive heterogeneous system model development and application. Water Resourc Res 19 381-391 Moise X, Starinsky A, Katz A, Kolodny Y (2000) Ra isotopes and Rn in brines and ground waters of the Jordan-Dead Sea Rift Valley enrichment, retardation, and mixing. Geochim Cosmochim Acta 64 2371-2388... 

Green MA. Aller RC, Cochran JK, Lee C, Aller JY (2002). Bioturbation in shelf/slope sediments off Cape Hatteras, North Carolina The use of " Th, Chl-a, and Br to evaluate rates of particle and solute transport. Deep-Sea Res 1149(20) 4627-4644... 

The Zn-N3imide interaction has been used to selectively extract imide-containing nucleosides and nucleotides into lipophilic media (39). Hexadecyl-derivatized Zn2+-cyclen was shown to extract dT from an aqueous solution containing a mixture of C, A, and G nucleobases. The antiviral agent AZT (3 azido-3 deoxythymidine) could also be extracted into CHCI3 from neutral aqueous solutions. Transport across a lipophilic layer was also shown, using acidic conditions, to promote the release of dT and AZT (Fig. 9). 

In whole tissue or cell monolayer experiments, transcellular membrane resistance (Rm = Pm1) lumps mucosal to serosal compartment elements in series with aqueous resistance (R = P ). The operational definition of Lm depends on the experimental procedure for solute transport measurement (see Section VII), but its magnitude can be considered relatively constant within any given experimental system. Since the Kp range dwarfs the range of Dm, solute differences in partition coefficient dominate solute differences in transcellular membrane transport. The lumped precellular resistance and lumped membrane resistance add in series to define an effective resistance to solute transport ... 

Precellular solute ionization dictates membrane permeability dependence on mucosal pH. Therefore, lumenal or cellular events that affect mucosal microclimate pH may alter the membrane transport of ionizable solutes. The mucosal microclimate pH is defined by a region in the neighborhood of the mucosal membrane in which pH is lower than in the lumenal fluid. This is the result of proton secretion by the enterocytes, for which outward diffusion is slowed by intestinal mucus. (In fact, mucosal secretion of any ion coupled with mucus-restricted diffusion will provide an ionic microclimate.) Important differences in solute transport between experimental systems may be due to differences in intestinal ions and mucus secretion. It might be anticipated that microclimate pH effects would be less pronounced in epithelial cell culture (devoid of goblet cells) transport studies than in whole intestinal tissue. 

The intercept defines ion flux across the epithelia (J0) while the slope provides a measure of the magnitude of the shunt pathway (Js) under short-circuit OF = 0) conditions. However, as discussed in the following section, mediated solute transport may also be voltage-dependent. 

In addition to faster solute transport rates, the major experimental features of membrane-facilitated transport that distinguish it from membrane diffusion include (1) specificity and selectivity (2) saturability (3) inhibition, activation, and cooperativity (4) transmembrane effects and (5) greater temperature sensitivity than is characteristic of membrane diffusion [42],... 

If initial solute uptake rate is determined from intestinal tissue incubated in drug solution, uptake must be normalized for intestinal tissue weight. Alternative capacity normalizations are required for vesicular or cellular uptake of solute (see Section VII). Cellular transport parameters can be defined either in terms of kinetic rate-time constants or in terms of concentration normalized flux [Eq. (5)]. Relationships between kinetic and transport descriptions can be made on the basis of information on solute transport distances. Note that division of Eq. (11) or (12) by transport distance defines a transport resistance of reciprocal permeability (conductance). 

Control of nutrient transport dictates significant coupling between transported components in G1 epithelia. This complicates solute transport analysis by requiring a multicomponent description. Flux equations written for each component constitute a nonlinear system in which the coupling nonlinearities are embodied in the coefficients modifying individual transport contributions to flux. 

The coupled processes described by Eqs. (8), (14), (17), and (22) can be added in (20) as parallel solute transport pathways across the membrane. The phenomenological coefficients (Ly) describe the membrane permeability by these pathways [potential-dependent, Eq. (8) via membrane lipid partition and diffusion, Eq. (14) carrier-mediated, Eq. (17) and convectively coupled, Eq. (22)]. These pathways define parallel resistances through the intestinal barrier in series with precellular resistances to solute transport. 

The coupling of solute transport in the GI lumen with solute lumenal metabolism (homogeneous reaction) and membrane metabolism (heterogeneous reaction) has been discussed by Sinko et al. [54] and is more generally treated in Cussler s text [55], At the cellular level, solute metabolism can occur at the mucosal membrane, in the enterocyte cytosol, and in the endoplasmic reticulum (or microsomal compartment). For peptide drugs, the extent of hydrolysis by lumenal and membrane-bound peptidases reduces drug availability for intestinal absorption [56], Preferential hydrolysis (metabolic specificity) has been targeted for reconversion... 

In situ perfusion studies assess absorption as lumenal clearance or membrane permeability and provide for isolation of solute transport at the level of the intestinal tissue. Controlled input of drug concentration, perfusion pH, osmolality, composition, and flow rate combined with intestinal region selection allow for separation of aqueous resistance and water transport effects on solute tissue permeation. This system provides for solute sampling from GI lumenal and plasma (mesenteric and systemic) compartments. A sensitive assay can separate metabolic from transport contributions. 

Mucosal brush border membrane vesicles and basolateral membrane vesicles can be isolated to study solute uptake across specific enterocyte boundaries. These more isolated vesicle systems allow for investigation of solute transport across a particular membrane barrier and permit separation of membrane trans-... 

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