Comparative accuracy of glucose monitors

The objective of this study was to rationalize the shape of membrane permeability-lipophilicity curves, when considering, in addition to the usual transcellular route, a parallel diffusion pathway through aqueous pores as present in biological membranes. The theoretical influence of different pH in donor and acceptor compartment and the molecular weight on the permeability curves was studied. We combined and extended two previously proposed absorption models, namely one describing diffusion through a simple membrane (two stagnant aqueous and two organic layers in series, no pores) as the sum of the two distribution steps at both membrane interfaces, and a second theoretical model considering the sum of different diffusional resistances through stagnant layers and membrane, respectively. Under certain conditions the equivalence of the two-step distribution model and the diffusional resistance model can be demonstrated. Incorporation of an aqueous diffusion pathway leads to an extended two-step distribution model. This theoretical membrane permeation model will permit a more physicochemical-based interpretation of permeation data and shows that combined log D values and molecular weight are important determinants for membrane transport processes through, e.g. Caco-2 monolayers and the mucosal GI membranes. We have demonstrated that the well-known sigmoidal permeability-lipophilicity relationship should be considered as a molecular weight-dependent set of sigmoidal relationships.     

Application of asymmetric TPX membranes to transdermal delivery of nitroglycerin

In this study, asymmetric poly(4-methyl-1-pentene) (TPX) membranes, fabricated by the dry/wet inversion method, were applied to transdermal delivery of nitroglycerin (NTG), a drug for treating angina pectoris. The flux of NTG through the TPX membrane was measured in vitro by a Franz cell. The results indicate that the NTG flux through asymmetric TPX membranes is strongly dependent on the membrane structure, which can be varied by adding nonsolvents in the casting solution. By adding different kinds of nonsolvents and adjusting the added amounts, membranes with different NTG release rates can be fabricated. It was also found that, with suitable drug formula, the NTG dissolution rate of a prototype TPX patch is comparable to that of a commercial patch, Transderm-Nitro. In addition, the data of NTG flux through a composite of TPX membrane and pig skin are also presented.     

Flip-flop of doxorubicin across erythrocyte and lipid membranes

Doxorubicin, an anticancer drug, is extruded from multidrug resistant (MDR) cells and from the brain by P-glycoprotein located in the plasma membrane and the blood-brain barrier, respectively. MDR-type drugs are hydrophobic and, as such, enter cells by diffusion through the membrane without the requirement for a specific transporter. The apparent contradiction between the presumably free influx of MDR-type drugs into MDR cells and the efficient removal of the drugs by P-glycoprotein, an enzyme with a limited ATPase activity, prompted us to examine the mechanism of passive transport within the membrane. The kinetics of doxorubicin transport demonstrated the presence of two similar sized drug pools located in the two leaflets of the membrane. The transbilayer movement of doxorubicin occurred by a flip-flop mechanism of the drug between the two membrane leaflets. At 37 degrees, the flip-flop exhibited a half-life of 0.7 min, in both erythrocyte membranes and cholesterol-containing lipid membranes. The flip-flop was inhibited by cholesterol and accelerated by high temperatures and the fluidizer benzyl alcohol. The rate of doxorubicin flux across membranes is determined by both the massive binding to the membranes and the slow flip-flop across the membrane. The long residence-time of the drug in the inner leaflet of the plasma membrane allows P-glycoprotein a better opportunity to remove it from the cell.     

Antioxidant status and alpha1-antiproteinase activity in subarachnoid hemorrhage patients

Grafting of polyethylene glycol chains onto cellulosic membrane can be expected to reduce the interaction between blood (plasma protein and cells) and the membrane surface. Alkylether carboxylic acid (PEG acid) grafted high flux cellulosic membranes for hemodialysis, in which the polyethylene glycol chain bears an alkyl group at one side and a carboxyl group at the other side, have been developed and evaluated. PEG acid-grafted high flux cellulosic membranes with various grafting amounts have been compared with respect to platelet adhesion, the contact phase of blood coagulation, and complement activation in vitro. A new method of quantitating platelet adhesion on hollow-fiber membrane surfaces has been developed, which is based on the determination of lactate dehydrogenase (LDH) activity after lysis of the adhered platelets. PEG acid-grafted high flux cellulosic membranes showed reduced platelet adhesion and complement activation effects in grafting amounts of 200 ppm or higher without detecting adverse effects up to grafting amounts of 850 ppm. The platelet adhesion of a PEG acid-grafted cellulosic membrane depends on both the flux and grafting amounts of the membrane. It is concluded that the grafting of PEG acid onto a cellulosic membrane improves its biocompatibility as evaluated in terms of platelet adhesion, complement activation, and thrombogenicity.     

Transdermal delivery of peptides by iontophoresis

Transdermal administration by iontophoresis (enhanced transport via the skin using the driving force of an applied electric field) has been successfully demonstrated but no formal relationship between peptide sequence/structure and efficiency of delivery has been established. There are notable examples, such as the lipophilic leutinizing hormone releasing hormone (LHRH) analogs, Nafarelin and Leuprolide, that exhibit down-regulation of their own transport across the skin under the influence of an iontophoretic current. The hypothesis that this phenomenon is due to neutralization of the skin's net negative charge by these cationic peptides was examined with LHRH oligopeptides. The impact of these compounds on the electroosmotic flow of solvent into the skin, which is induced by iontophoresis and which contributes significantly to the electrotransport of large, positively charged ions, was examined and quantified. Close juxtaposition of cationic and lipophilic residues profoundly inhibited electroosmosis and, presumably, peptide flux. The results indicate that the lipophilicity of the oligopeptides facilitates van der Waals interactions with hydrophobic patches along the transport route, thereby permitting the positively charged oligopeptide to interact with carboxylate side chains that give the skin its net negative charge at neutral pH. The lipophilic, cationic oligopeptide, therefore, becomes anchored in the transport path, neutralizing the original charge of the membrane, and completely altering the permselective properties of the skin.     

Active transport of nitrofurantoin across a mouse mammary epithelial monolayer

The antibiotic nitrofurantoin is transported against an electrochemical gradient into milk. A monolayer of CIT3 cells, a subline of the Comma 1D normal mouse mammary epithelial cell line, transports [14C]-nitrofurantoin against a concentration gradient from the basal to the apical solution when grown on membrane filters. In a side-by-side diffusion chamber with well-stirred solutions on both sides, the transfer rate is 50% higher in the basal-to-apical than in the apical-to-basal direction. Nonlabeled nitrofurantoin (500 microM) in the basal chamber equalized the transport in both directions, suggesting that a specific transporter is responsible for the basal-to-apical increment in flux. From inhibition studies, the apparent affinity of this transporter for nitrofurantoin is 50 microM. Changes in pH between 6.4 and 7.8 had no effect on the active transport component of the flux but did affect the passive flux component. Passive flux of the nonionized molecule was 2.6 times faster than that of the ionized molecule, but the ionized molecule did appear to cross the membrane passively. Our findings show that nitrofurantoin is actively transported across a mammary epithelial cell monolayer by a transporter whose affinity for nitrofurantoin does not depend on the anionic charge on nitrofurantoin. The pH dependence of a parallel passive pathway suggests that both nonionized and ionized forms of nitrofurantoin cross the membranes of the mammary epithelial cell by passive diffusion.     

Recovery of uranium from phosphoric acid by means of supported liquid membranes

U(VI) was transported at 23 ± 1°C from 5–6 M phosphoric acid solutions through liquid membranes of kerosene solutions of di(2-ethylhexyl) phosphoric acid and trioctyl phosphine oxide (D2EHPA/TOPO) supported on porous polytetrafluoroethylene to a solution of phosphoric acid of equal or greater molarity containing ferrous ion as a reducing agent. The ferrous ion could be omitted when the higher molarity acid was used. The uranium flux was proportional to the U(VI) concentration. The overall resistivity of the membranes to uranium flux had a diffusional component that was proportional to the membrane thickness and an interfacial component that resulted from rate-limiting uranium complexation/decomplexation kinetics. The interfacial component accounted for over 80% of the resistivity of a membrane 75 μm thick. Increasing the temperature to 60°C only slightly diminished the interfacial resistivity. A theoretical model was constructed that accommodated data obtained from uranium transport through the membranes and through quiescent layers of phosphoric acid and D2EHPA/TOPO in kerosene. The average uranium flux from simulated solutions of wet-process phosphoric acid at 90% uranium transfer was estimated to be 1.3 × 10^?11 mol cm^?2 sec^?1, or 0.09 lb ft^?2 yr^?1. The flux was judged to be too low for supported liquid membranes to be competitive with liquid/liquid extraction for recovery of uranium from wet-process phosphoric acid.     

Electron capture gas chromatography of nanogram amounts of tertiary amines as trichloroethyl carbamates

The overall permeability of epithelial tissues to solutes is generally determined by analyzing net or unidirectional transepithelial fluxes in response to transepithelial differences of concentration and/or electrical potential using relations that describe diffusional movements across a single membrane. If the solute is uncharged and diffusional movements are transcellular, the overall transepithelial permeability coefficient is determined by the permeabilities of the two limiting cell membranes combinded in series. However, if the solute is charged and the pathway for transepithelial movement involves diffusional flows across at least two membranes arranged in series (i.e. transcellular transport), the value of the overall transepithelial permeability coefficient determined using relations that describe ionic diffusion across a single membrane is not an accurate measure of the permeabilities of the two limiting membranes combined in series. Further, if ionic diffusion is transcellular, permeability coefficients determined from studies of transepithelial fluxes are not only quantitatively incorrect but can also result in grossly erroneous interpretations of changes in transepithelial permeabilities and faulty inferences regarding the route of transepithelial ionic diffusion.     

Mathematical modelling of drug transport in emulsion systems

Two mathematical models for the prediction of drug transport in triphasic (oil, water and micellar) emulsion systems as a function of micellar concentration have been developed and these models were evaluated by comparing experimental and simulated data. Fick's first law was used to derive a transport model for hydrophilic drugs, assuming that the oil/water (o/w) partitioning process was fast compared with membrane transport and therefore drug transport was limited by the membrane. Consecutive rate equations were used to model transport of hydrophobic drugs in emulsion systems assuming that the o/w interface acts as a barrier to drug transport. Benzoic acid and phenol were selected as hydrophilic model drugs. Phenylazoaniline and benzocaine were selected as hydrophobic model drugs. Transport studies at pH 3.0 and 7.0 were conducted using side-by-side diffusion cells. According to the hydrophilic model, an increase in micellar concentration is expected to decrease drug transport rates. The effective permeability coefficients (Peff) of drugs were calculated using an equation relating Peff and the total apparent volume of drug distribution (determined experimentally using drug/membrane permeability and partition coefficient values). The hydrophobic model was fitted to the experimental data for the cumulative amount of model drug in the receiver cells using a weighted least-squares estimation program (PCNONLIN). The oil/continuous phase partitioning rates (k1) and the membrane transport rates (k2) were estimated. The goodness of fit was assessed from the correlation coefficients of plots of predicted versus experimental data. The predicted data were consistent with the experimental data for both the hydrophilic and hydrophobic models.     

Does routine follow up after head injury help? A randomised controlled trial

Excess surfactant present in emulsions can influence the rates of transport of incorporated drugs by micellar solubilization, alteration of the partitioning process and by drug-surfactant complexation. Cetyltrimethylammonium bromide (CTAB), a cationic surfactant was selected to investigate these phenomena as it forms relatively stable mineral oil-water (O-W) emulsions and has the potential for ionic interaction. Phenylazoaniline, benzocaine, benzoic acid and phenol were chosen as model drugs for this study. The emulsion critical micelle concentration (CMC) for CTAB determined using a combination of a membrane equilibrium technique and surface-tension measurement was 1.0% w/v in 10% v/v% O-W emulsion systems. Ionic interaction between model drugs and surfactants and drug hydrophobicity affected their transport rates in the emulsion systems. The transport rates of the lipophilic drugs (benzocaine and phenylazoaniline) and the ionized hydrophillic drug (benzoic acid, pH 7.0) in the emulsion systems increased with increasing CTAB concentration up to 0.5% w/v micellar concentration and then decreased at higher concentrations. The rate of transport of phenol was not affected by the presence of micellar phase. Ionic interaction between surfactant and model drugs affected transport rates of model drugs in emulsion systems. The micellar phase was considered to affect the overall transport rates of model drugs.     

Kinetics of anthracycline accumulation in multidrug-resistant tumor cells: relationship to drug lipophilicity and serum albumin binding

A multidrug-resistant Ehrlich ascites tumor cell line (EHR2/DNR+) was used to examine the membrane transport kinetics of lipophilic anthracycline derivatives in the presence of serum albumin. We present a model for theoretical data analysis with consideration of drug-albumin complex formation. For a set of five derivatives (doxorubicin, daunorubicin, 4-demethoxydaunorubicin, 4'-deoxy-4'-iododoxorubicin, and 13-dihydro-4'-deoxy-4'-iododoxorubicin), data were given on the rates of diffusional drug uptake, and membrane permeability coefficients of the noncharged molecules were estimated. Both the initial rates and the steady-state levels of drug uptake were found to decrease by addition of BSA at concentrations ranging from 5 to 75 mg/mL. For each drug, this effect of serum albumin could be accounted for by the altered distribution between free and protein-bound drug molecules in the bulk aqueous medium. A good fit of theoretical accumulation curves to the experimental data was obtained. It was concluded that a mathematical simulation method makes it possible to predict the uptake characteristics of lipophilic anthracycline compounds into tumor cells under serum conditions.     

The location of fluorescence probes with charged groups in model membranes

The location of commonly used charged fluorescent membrane probes in membranes was determined in order to: (1) investigate the relationship between the structure of hydrophobic molecules and their depth within membranes; and (2) aid interpretation of experiments in which these fluorescent probes are used to examine membrane structure. Membrane depth was calculated using parallax analysis, a method in which the quenching induced by lipids carrying a nitroxide group at different locations in the membrane is compared. Shallow locations were found for xanthene dyes (fluorescein, eosin, Texas Red and rhodamine) both in free form and when attached either to the headgroup of phospholipids or long hydrocarbon chains. The exact structure of the xanthene and the nature of its linkage to lipid had only a modest effect on membrane location, which ranged between 19 and 24 A from the center of the bilayer in a charged state. Thus, the location of these fluorophores largely reflects their intrinsic properties rather than the nature of the groups to which they are attached. Furthermore, cationic and anionic xanthene derivatives had similar depths, indicating the type of charge does not have a large effect on depth. Consistent with this conclusion, shallow locations were also found for other hydrocarbon chain-linked cationic (acridine orange and styrylpyridinium) and anionic (coumarin, anilinonaphthalenesulfonic acid (ANS), and toluidinylnaphthalenesulfonic acid (TNS)) charged probes. These all located at 16-18 A from the bilayer center. We conclude that both anionic and cationic molecules that are otherwise hydrophobic predominantly occupy shallow locations within the polar headgroup region of the bilayer no matter how hydrophobic the molecule to which they are linked. This depth is significantly shallower than that occupied by most previously studied uncharged polar molecules that locate near the membrane surface. Consistent with this conclusion, a 2-4 A deeper location was found for xanthene probes with no net charge. In other experiments, methods to avoid chemical reactions that can distort the measurement of depth by fluorescence quenching were developed.     

Application of Cross-Flow Microfiltration for the Treatment of Combined Sewer Overflow Wastewater

Application of cross-flow microfiltration with and without backpulsing is evaluated for the treatment of dilute primary sewage effluent simulating combined sewer overflow wastewater. Four alpha alumina ceramic membranes of various pores sizes (0.2–5.0?μm) were tested to understand the impact of cross-flow velocity and transmembrane pressure on the permeate water quality and flux rate. The 0.2 and 0.8?μm membranes produced a permeate water quality that is likely to be suitable for surface water discharge. The combination of permeate chemical and biological water quality and long-term flux rates suggest that a 0.2?μm membrane would be the most appropriate membrane for the treatment of combined sewer overflow wastewater within sewersheds.     

Molecular and cell biological analyses for intestinal absorption and renal excretion of drugs

Intestinal absorption and renal tubular secretion are transport processes determining the availability and the disposition of drugs in the body. In this review, our studies on the molecular and cell biological analyses of intestinal absorption and renal secretion of drugs are described. We evaluated the transepithelial transport and the cellular accumulation of peptide-like drugs such as beta-lactam antibiotics and bestatin (a dipeptide-like antineoplastic agent) in the human adenocarcinoma colon cell line, Caco-2, as an in vitro model for studying absorption mechanisms of these drugs. We have found that the transcellular transport of these peptide-like drugs is mediated by both the apically- and basolaterally-localized peptide transporters. To characterize molecular aspects of absorption of the peptide-like drugs, we studied cDNA cloning of H+/peptide cotransporters, PEPT1 and PEPT2, expressed in rats. The rat PEPT1 has been shown to mediate the H- coupled uphill transport of beta-lactam antibiotics across the brush-border membranes of the intestinal and renal epithelia. The rat PEPT2 is expressed predominantly in the kidney, but not in the intestine, mediating tubular reabsorption of the peptide-like drugs. We examined the transcellular transport of organic cations across monolayers of the kidney epithelial cell line, LLC-PK1. We have found that LLC-PK1 cells possess the H+/organic cation antiporter and the membrane potential-sensitive organic cation transporter in the apical and basolateral membranes, respectively, thereby tetraethylammonium (TEA) being transported unidirectionally from the basolateral to the apical side of the monolayers. We have isolated a cDNA encoding a rat kidney-specific organic cation transporter, OCT 2, which transports TEA in a H(+)-gradient independent manner, suggesting that OCT2 is localized to the basolateral membranes of renal tubular cells. In addition, a cDNA encoding a novel rat organic anion transporter, OAT-K1, has been cloned. OAT-K1 is expressed exclusively in the renal proximal tubules, and mediates the transport of methotrexate. Analyses of the molecular and cell biological mechanisms for drug absorption and secretion will provide information for the understanding of organ specific drug transport systems and for the development of drug design and/or drug delivery system.     

The charge of endotoxin molecules influences their conformation and IL-6-inducing capacity

The activation of cells by endotoxin (LPS) is one of the early host responses to infections with Gram-negative bacteria. The lipid A part of LPS molecules is known to represent the endotoxic principle; however, the specific requirements for the expression of biologic activity are still not fully understood. We previously found that a specific molecular conformation (endotoxic conformation) is a prerequisite for lipid A to be biologically active. In this study, we have investigated the interdependence of molecular charge and conformation of natural and chemically modified LPS and lipid A and its transport and intercalation into phospholipid membranes mediated by human LPS-binding protein, as well as IL-6 production after stimulation of whole blood or PBMCs. We found that the number, nature, and location of negative charges strongly modulate the molecular conformation of endotoxin. In addition, the LPS-binding protein-mediated transport of LPS into phospholipid membranes depends on the presence of net negative charge, yet charge is only a necessary, but not a sufficient, prerequisite for transport and intercalation. The biologic activity is determined mainly by the molecular conformation: only conical molecules are highly biologically active, whereas cylindrical ones are largely inactive. We could demonstrate that the net negative charge of the lipid A component and its distribution within the hydrophilic headgroup strongly influence the molecular conformation and, therefore, also the biologic activity.     

Fast ion-exchange HPLC of proteins using porous poly(glycidyl methacrylate-co-ethylene dimethacrylate) monoliths grafted with poly(2-acrylamido-2-methyl-1-propanesulfonic acid)

Porous poly(glycidyl methacrylate-co-ethylene dimethacrylate) monoliths with different porous properties grafted with poly(2-acrylamido-2-methyl-1-propanesulfonic acid) chains using cerium(IV) initiated free-radical polymerization have been prepared and used for the separation of proteins in ion-exchange HPLC mode. Because of the presence of the large pores that are typical of monolithic separation media which allow easy flow of all of the mobile phase, the efficiency of the columns does not deteriorate even at high flow velocities as a result of the specific morphology of the monoliths. Optimization of the chromatographic conditions such as the shape of the mobile phase gradient and the flow rate allows for very fast separation of three proteins in less than 1.5 min.     

High penetrance and pronounced variation in expressivity of GCH1 mutations in five families with dopa-responsive dystonia

A mathematical model is presented which examines the extent to which the intestinal epithelium is accessed by drug molecules. Morphological information from the literature for the jejunum, ileum, and colon of the rat and for human jejunum was incorporated. Perturbation theory was used to derive the limiting cases for total access to the entire epithelial surface, for transport by diffusion and by diffusion with convection, respectively. A parameter gamma = square root of (Ph2)/(Db) was identified to provide a measure of the ability of drug molecules to access the entire epithelial surface down to the crypt wells, where P is the cell permeability, D the aqueous diffusion coefficient, h the channel depth between the villi, and b is half the width of the idealized intervillous channel. When gamma < 1, diffusion is not a limitation and the entire surface is fully utilized for absorption of drug. This condition arises with drugs of low permeability and is more likely to be met with colonic than small intestinal epithelium. When gamma >/= 1, diffusion becomes a limitation and then not all of the epithelial surface is functionally accessible to drug molecules, a condition most likely to prevail with drugs of high permeability traversing the jejunum. Furthermore, water flux per se is predicted to have relatively little influence on enhancing surface accessibility. This simple, but quantitative approach showed that the ranking order of permeability jejunum >ileum> colon for low permeable drugs can at least in part be explained by the differences in surface amplification between these different epithelial regions. The analysis also indicates that for highly permeable drugs extreme caution should be exercised in extrapolating permeability measurements in vitro across various preparations and to events in vivo.     

Kinetic analysis of tetraethylammonium transport in the kidney epithelial cell line, LLC-PK1

PURPOSE: The aims of this study were to establish a kinetic means of analyzing the membrane transport of organic cations in renal epithelial cells, and to simultaneously evaluate drug interactions in apical and basolateral membranes. METHODS: Tetraethylammonium (TEA) transport was measured using LLC-PK1 cell monolayers grown on microporous membrane filters. After incubating the cells with unlabeled TEA or other drugs, apical or basolateral medium was changed to that containing labeled TEA, and transcellular transport and cellular accumulation were measured. Clearance from apical medium to cells (CL12), cells to apical medium (CL21), cells to basolateral medium (CL23) and basolateral medium to cells (CL32) were calculated based on a three compartment model. RESULTS: TEA was accumulated progressively in the monolayers from the basolateral side and was transported unidirectionally to the apical side. CL32 was greater than CL12 and CL23 was greater than CL21. Therefore, the rate limiting step of TEA transport from the basolateral to the apical medium was the cell-to-apical step. Co-incubation of TEA with procainamide decreased the transport parameters of TEA, CL12, CL21 and CL32, whereas that with levofloxacin decreased only CL12 and CL21, not affecting the parameters in basolateral membranes. CONCLUSIONS: Using a simple model, we analyzed the transport of organic cation in kidney epithelial cell line, LLC-PK1. This method can be useful for the analysis of cation transport and drug interactions in the apical and basolateral membranes of renal tubules.     

Transport of tricarballylate by intestinal brush-border membrane vesicles from steers

Tricarballylic acid is a non-metabolizable rumen bacterial fermentation product of the naturally occurring tricarboxylic acid trans-aconitic acid. The aim of the present study was to investigate intestinal absorption of tricarballylate using brush-border membrane vesicles (BBMVs) isolated from the proximal jejunum of steers by a Ca2+ precipitation method with subsequent differential centrifugation. Transport of tricarballylate was investigated indirectly (influence of tricarballylate on the uptake of 14C-labelled citrate) as well as directly (uptake of 3H-labelled tricarballylate). Citrate as well as tricarballylate uptake (at a concentration of 0.05 mmol l-1) was strongly stimulated by an inwardly directed initial Na+ gradient. Furthermore, transport of both tricarboxylates under Na+ gradient conditions was clearly enhanced by lowering the extravesicular pH from 7.8 to 5.6. The imposition of an inwardly directed H+ gradient (pH(out)/pH(in) = 5.6/7.8) further enhanced the intravesicular accumulation of citrate as well as of tricarballylate compared with pH(out)/pH(in) = 5.6/5.6. Unequivocal evidence for a common transport site for tricarballylate and citrate was obtained from 'cis-inhibition' and 'trans-stimulation' of Na(+)-dependent citrate uptake by tricarballylate. In further experiments the influence of different substances on the uptake of 3H-labelled tricarballylate was evaluated. Unlabelled tricarballylate, citrate, succinate as well as trans- and cis-aconitate significantly inhibited the accumulation of 3H-labelled tricarballylate by BBMVs. Tricarballylate uptake as a function of the tricarballylate concentration revealed a Na(+)-dependent saturable component (apparent kinetic parameters: maximal transport capacity (Vmax) = 119 pmol (mg protein)-1 (3s)-1; affinity constant (Km) = 0.097 mmol l-1) and a Na(+)-independent diffusional component (diffusion constant: 169 nl (mg protein)-1 (3s)-1). It is concluded that tricarballylate and citrate are transported across the intestinal brush-border membrane by a common, Na(+)-dependent transport mechanism. The stimulatory influence of a low extravesicular pH most probably indicates that the protonated forms of tricarboxylates are better transported than the trivalent species.     

Characterization of convective solvent flow during iontophoresis

During iontophoresis under neutral pH conditions, there is a net convective flow of volume (electroosmosis) from anode to cathode leading to the enhanced transport of dissolved polar (but uncharged) solutes in the same direction. The objective of this study was to address the following unresolved questions with respect to electroosmotic transport: [1] Whether the efficiency of electroosmotic transport is solute size-dependent and, if so, how severe is this dependence? [2] Is electroosmosis linearly related to current density in the same way that the iontophoretic flux of charged species appears to be? [3] Are positively charged permeants able to influence their own electrotransport across the skin (by modifying the net charge on the membrane and altering, as a result, the permselectivity) and, if so, why and to what extent? Electroosmosis was assessed from the iontophoreically driven fluxes of mannitol, sucrose and lactose across hairless mouse skin in vitro. It was found that:- (a) The electroosmotic transport rate of mannitol is similar to that of the disaccharides, sucrose and lactose, when examined under identical conditions. The dependence of electroosmotic flux upon molecular size requires study of solutes having a wider range of MW than those considered here. (b) Electroosmotic flow from anode-to-cathode increases with applied current density; similarly, convective flow in the opposite direction diminishes with increasing current density. Apparently, there is correlation between the net movement of solvent and the total flux of ions across the skin. (c) The permselectivity of skin can be 'neutralized' by driving, iontophoretically, a cationic, lipophilic peptide (specifically the leutinizing hormone releasing hormone (LHRH) analog, Nafarelin) into the membrane.(ABSTRACT TRUNCATED AT 250 WORDS)