Mechanisms of fluid movement into, through and out of the brain: evaluation of the evidence

Interstitial fluid (ISF) surrounds the parenchymal cells of the brain and spinal cord while cerebrospinal fluid (CSF) fills the larger spaces within and around the CNS. Regulation of the composition and volume of these fluids is important for effective functioning of brain cells and is achieved by barriers that prevent free exchange between CNS and blood and by mechanisms that secrete fluid of controlled composition into the brain and distribute and reabsorb it. Structures associated with this regular fluid turnover include the choroid plexuses, brain capillaries comprising the blood-brain barrier, arachnoid villi and perineural spaces penetrating the cribriform plate. ISF flow, estimated from rates of removal of markers from the brain, has been thought to reflect rates of fluid secretion across the blood-brain barrier, although this has been questioned because measurements were made under barbiturate anaesthesia possibly affecting secretion and flow and because CSF influx to the parenchyma via perivascular routes may deliver fluid independently of blood-brain barrier secretion. Fluid secretion at the blood-brain barrier is provided by specific transporters that generate solute fluxes so creating osmotic gradients that force water to follow. Any flow due to hydrostatic pressures driving water across the barrier soon ceases unless accompanied by solute transport because water movements modify solute concentrations. CSF is thought to be derived primarily from secretion by the choroid plexuses. Flow rates measured using phase contrast magnetic resonance imaging reveal CSF movements to be more rapid and variable than previously supposed, even implying that under some circumstances net flow through the cerebral aqueduct may be reversed with net flow into the third and lateral ventricles. Such reversed flow requires there to be alternative sites for both generation and removal of CSF. Fluorescent tracer analysis has shown that fluid flow can occur from CSF into parenchyma along periarterial spaces. Whether this represents net fluid flow and whether there is subsequent flow through the interstitium and net flow out of the cortex via perivenous routes, described as glymphatic circulation, remains to be established. Modern techniques have revealed complex fluid movements within the brain. This review provides a critical evaluation of the data.

Ion transport across thin lipid membranes: a critical discussion of mechanisms in selected systems

There are now well-established examples of carriers and pores which facilitate the transfer of ions across thin lipid membranes. In the absence of such agents, lipid bilayer membranes are extremely impermeable to the common inorganic ions. Thus, the conductance of a pure lecithin + decane or glyceryl mono-oleate + decane membrane in M/10 NaCl is less than10−9Ω−1cm−2. However, on the addition of small lipid-soluble molecules such as valinomycin, or surface-active polypeptides such as gramicidin A, the conductance may become so high (> 10−1ω−lcm−2) that the resistance of the membrane merges into that of the aqueous phase. This review is concerned with the extent to which we now understand how these added substances transfer ions across lipid membranes. Attention has been concentrated on the simpler systems, i.e. the lipid-soluble ions, the 1–1 carriets, a simple pore and, with some loss of simplicity, a substance which prodeces interacting pores. Only molecules of known primary structure are discussed.

Fluid and ion transfer across the blood-brain and blood-cerebrospinal fluid barriers; a comparative account of mechanisms and roles

The two major interfaces separating brain and blood have different primary roles. The choroid plexuses secrete cerebrospinal fluid into the ventricles, accounting for most net fluid entry to the brain. Aquaporin, AQP1, allows water transfer across the apical surface of the choroid epithelium; another protein, perhaps GLUT1, is important on the basolateral surface. Fluid secretion is driven by apical Na+-pumps. K+ secretion occurs via net paracellular influx through relatively leaky tight junctions partially offset by transcellular efflux. The blood-brain barrier lining brain microvasculature, allows passage of O2, CO2, and glucose as required for brain cell metabolism. Because of high resistance tight junctions between microvascular endothelial cells transport of most polar solutes is greatly restricted. Because solute permeability is low, hydrostatic pressure differences cannot account for net fluid movement; however, water permeability is sufficient for fluid secretion with water following net solute transport. The endothelial cells have ion transporters that, if appropriately arranged, could support fluid secretion. Evidence favours a rate smaller than, but not much smaller than, that of the choroid plexuses. At the blood-brain barrier Na+ tracer influx into the brain substantially exceeds any possible net flux. The tracer flux may occur primarily by a paracellular route. The blood-brain barrier is the most important interface for maintaining interstitial fluid (ISF) K+ concentration within tight limits. This is most likely because Na+-pumps vary the rate at which K+ is transported out of ISF in response to small changes in K+ concentration. There is also evidence for functional regulation of K+ transporters with chronic changes in plasma concentration. The blood-brain barrier is also important in regulating HCO3- and pH in ISF: the principles of this regulation are reviewed. Whether the rate of blood-brain barrier HCO3- transport is slow or fast is discussed critically: a slow transport rate comparable to those of other ions is favoured. In metabolic acidosis and alkalosis variations in HCO3- concentration and pH are much smaller in ISF than in plasma whereas in respiratory acidosis variations in pHISF and pHplasma are similar. The key similarities and differences of the two interfaces are summarized.

Interaction of the breast cancer resistance protein with plant polyphenols

Multidrug transporters influence drug distribution in vivo and are often associated with tumour drug resistance. Here we show that plant-derived polyphenols that interact with P-glycoprotein can also modulate the activity of the recently discovered ABC transporter, breast cancer resistance protein (BCRP/ABCG2). In two separate BCRP-overexpressing cell lines, accumulation of the established BCRP substrates mitoxantrone and bodipy-FL-prazosin was significantly increased by the flavonoids silymarin, hesperetin, quercetin, and daidzein, and the stilbene resveratrol (each at 30 microM) as measured by flow cytometry, though there was no corresponding increase in the respective wild-type cell lines. These compounds also stimulated the vanadate-inhibitable ATPase activity in membranes prepared from bacteria (Lactococcus lactis) expressing BCRP. Given the high dietary intake of polyphenols, such interactions with BCRP, particularly in the intestines, may have important consequences in vivo for the distribution of these compounds as well as other BCRP substrates.

Ion movements in gramicidin pores. An example of single-file transport

Experimental results on ion movement through gramicidin membrane channels are presented and discussed in terms of ion transport in the simplest single-file pore (for review see Urban, B.W. and Hladky, S.B. (1979) Biochim. Biophys. Acta 554, 410-429). Single-channel conductance and bi-ionic potential data for Na+, K+, Cs+, NH4+ and Tl+ are used to assign values to the rate constants of the model. Not all of the rate constants can be determined uniquely and simplifications are introduced to reduce the number of free parameters. The simplified model gives good quantitative fits to the experimental results for Na+, K+, Cs+ and NH4+. For Tl+, although the model accounts qualitatively for the salient features of the results, the quantitative agreement is less satisfactory. Predictions calculated from the model and the fitted rate constants are compared with independent data from blocking and tracer-flux measurements. In agreement with experiment, the model shows that only Tl+ blocks the Na+ conductance significantly. Furthermore, the exponent, n, in the tracer flux ratio rises, as observed, well above unity. The values for the rate constants suggest internal consistency of the model in that entry is always slower to singly occupied pores than to empty pores while exit is always faster from doubly as compared to singly occupied pores. The agreement between model prediction and experimental results suggests that the main features of ion transport in the gramicidin channel arise from cation-cation interaction in a single-file pore.

Elimination of substances from the brain parenchyma: efflux via perivascular pathways and via the blood-brain barrier

This review considers efflux of substances from brain parenchyma quantified as values of clearances (CL, stated in µL g-1 min-1). Total clearance of a substance is the sum of clearance values for all available routes including perivascular pathways and the blood-brain barrier. Perivascular efflux contributes to the clearance of all water-soluble substances. Substances leaving via the perivascular routes may enter cerebrospinal fluid (CSF) or lymph. These routes are also involved in entry to the parenchyma from CSF. However, evidence demonstrating net fluid flow inwards along arteries and then outwards along veins (the glymphatic hypothesis) is still lacking. CLperivascular, that via perivascular routes, has been measured by following the fate of exogenously applied labelled tracer amounts of sucrose, inulin or serum albumin, which are not metabolized or eliminated across the blood-brain barrier. With these substances values of total CL ≅ 1 have been measured. Substances that are eliminated at least partly by other routes, i.e. across the blood-brain barrier, have higher total CL values. Substances crossing the blood-brain barrier may do so by passive, non-specific means with CLblood-brain barrier values ranging from < 0.01 for inulin to > 1000 for water and CO2. CLblood-brain barrier values for many small solutes are predictable from their oil/water partition and molecular weight. Transporters specific for glucose, lactate and many polar substrates facilitate efflux across the blood-brain barrier producing CLblood-brain barrier values > 50. The principal route for movement of Na+ and Cl- ions across the blood-brain barrier is probably paracellular through tight junctions between the brain endothelial cells producing CLblood-brain barrier values ~ 1. There are large fluxes of amino acids into and out of the brain across the blood-brain barrier but only small net fluxes have been observed suggesting substantial reuse of essential amino acids and α-ketoacids within the brain. Amyloid-β efflux, which is measurably faster than efflux of inulin, is primarily across the blood-brain barrier. Amyloid-β also leaves the brain parenchyma via perivascular efflux and this may be important as the route by which amyloid-β reaches arterial walls resulting in cerebral amyloid angiopathy.

Potential difference and the distribution of ions across the human red blood cell membrane; a study of the mechanism by which the fluorescent cation, diS-C3-(5) reports membrane potential

1. The mechanism by which the fluorescent, cationic dye diS-C3-(5) responds to the membrane potential of red blood cells has been investigated. 2. The dye in aqueous solution absorbs most strongly at 650 nm. Addition of white, haemoglobin-free membranes red shifts the absorption maximum ca. 20 nm, while addition of membrane-free cell lysate results in the appearance of a new dye absorption peak at 590 nm. Thus the dye binds both to cell membranes and to cell contents. The component of the cytoplasm which binds the dye is non-dialysable, presumably haemoglobin. 3. Dye added to a suspension of intact cells shows a strong absorption at 590 nm indicating that the dye has bound to the cell contents and that the membrane is permeable to the dye. 4. The amount of dye which partitions into (and on to) the cells can be determined, as reported by Sims, Waggoner, Wang & Hoffman (1974), from the fluorescence of the dye remaining in the supernatant after the cells are centrifuged to the bottom of the suspension. In most conditions the proportion of the cell associated dye which is either free inside the cell or bound to the outside face of the membrane is negligible compared to the proportion bound to the cell contents. 5. On the assumption that the dye is not actively transported, the ratio of the equilibrium dye activities inside and outside the cell, ai/ao, is determined by the membrane potential according to the Nernst relation. Driving the membrane potenial negative then increases the cell associated dye by increasing the activity ratio and hence ai and the amount of dye bound to cell contents. 6. At the known Donnan equilibrium potential the internal dye activity can be calculated from the external activity. An empirical relation between cell associated dye and internal activity has been determined by measuring the dye partition between cells and medium at different external activities. 7. Using this empirial relation, and providing that any changes in cell composition do not affect the dye binding, the internal activity at any potential can be calculated from the measured amount of cell associated dye. The external activity can be estimated fluorimetrically. The membrane potential is then calculated from the activity ratio. 8. The membrane potenial of cells has been altered by adding valinomycin in the presence of different K gradients. Under the conditions used, the 'constant field' permeability for K-Val is 15-20 times that of Cl. 9. Dye binding to haemoglobin is influenced by pH and thus dye partitioning into cells changes with intracellular pH. Increasing intracellular pH increases the amount of dye partitioned, while decreasing pH decreases this amount. 10. When large potentials are produced with valinomycin there is no change in intracellular pH. This result indicates that in red blood cells intracellular pH is determined by the external pH and the Cl concentration ratio and not by the membrane potentials. 11. DiS-C3-(5) can be used to estimate potentials across resealed ghost membranes...

Modulatory effects of plant phenols on human multidrug-resistance proteins 1, 4 and 5 (ABCC1, 4 and 5)

Plant flavonoids are polyphenolic compounds, commonly found in vegetables, fruits and many food sources that form a significant portion of our diet. These compounds have been shown to interact with several ATP-binding cassette transporters that are linked with anticancer and antiviral drug resistance and, as such, may be beneficial in modulating drug resistance. This study investigates the interactions of six common polyphenols; quercetin, silymarin, resveratrol, naringenin, daidzein and hesperetin with the multidrug-resistance-associated proteins, MRP1, MRP4 and MRP5. At nontoxic concentrations, several of the polyphenols were able to modulate MRP1-, MRP4- and MRP5-mediated drug resistance, though to varying extents. The polyphenols also reversed resistance to NSC251820, a compound that appears to be a good substrate for MRP4, as predicted by data-mining studies. Furthermore, most of the polyphenols showed direct inhibition of MRP1-mediated [3H]dinitrophenyl S-glutathione and MRP4-mediated [3H]cGMP transport in inside-out vesicles prepared from human erythrocytes. Also, both quercetin and silymarin were found to inhibit MRP1-, MRP4- and MRP5-mediated transport from intact cells with high affinity. They also had significant effects on the ATPase activity of MRP1 and MRP4 without having any effect on [32P]8-azidoATP[alphaP] binding to these proteins. This suggests that these flavonoids most likely interact at the transporter's substrate-binding sites. Collectively, these results suggest that dietary flavonoids such as quercetin and silymarin can modulate transport activities of MRP1, -4 and -5. Such interactions could influence bioavailability of anticancer and antiviral drugs in vivo and thus, should be considered for increasing efficacy in drug therapies.

The glymphatic hypothesis: the theory and the evidence

The glymphatic hypothesis proposes a mechanism for extravascular transport into and out of the brain of hydrophilic solutes unable to cross the blood-brain barrier. It suggests that there is a circulation of fluid carrying solutes inwards via periarterial routes, through the interstitium and outwards via perivenous routes. This review critically analyses the evidence surrounding the mechanisms involved in each of these stages. There is good evidence that both influx and efflux of solutes occur along periarterial routes but no evidence that the principal route of outflow is perivenous. Furthermore, periarterial inflow of fluid is unlikely to be adequate to provide the outflow that would be needed to account for solute efflux. A tenet of the hypothesis is that flow sweeps solutes through the parenchyma. However, the velocity of any possible circulatory flow within the interstitium is too small compared to diffusion to provide effective solute movement. By comparison the earlier classical hypothesis describing extravascular transport proposed fluid entry into the parenchyma across the blood-brain barrier, solute movements within the parenchyma by diffusion, and solute efflux partly by diffusion near brain surfaces and partly carried by flow along "preferred routes" including perivascular spaces, white matter tracts and subependymal spaces. It did not suggest fluid entry via periarterial routes. Evidence is still incomplete concerning the routes and fate of solutes leaving the brain. A large proportion of the solutes eliminated from the parenchyma go to lymph nodes before reaching blood but the proportions delivered directly to lymph or indirectly via CSF which then enters lymph are as yet unclear. In addition, still not understood is why and how the absence of AQP4 which is normally highly expressed on glial endfeet lining periarterial and perivenous routes reduces rates of solute elimination from the parenchyma and of solute delivery to it from remote sites of injection. Neither the glymphatic hypothesis nor the earlier classical hypothesis adequately explain how solutes and fluid move into, through and out of the brain parenchyma. Features of a more complete description are discussed. All aspects of extravascular transport require further study.

Ion transport in the simplest single file pore

A kinetic scheme is developed to describe single-file transport through pores containing up to two ions which may be of different species. The solution for the fluxes in terms of rate constants for entry, exit, and transfer is derived without specific assumptions about symmetry or the voltage and activity dependence of the constants. For a symmetrical pore the relation between the slope conductance at zero applied potential and ion activity can have two distinct regions in which the conductance increases linearly. Zero current or reversal potentials depend on the absolute values of the activities as well as their ratios. The use of this theory to describe the cation fluxes through the pores formed by gramicidin A will be considered in a subsequent paper. Here the model is discussed for a number of more specific assumptions, most extensively the following combination: (1) while entry to a pore is less likely when the pore is already occupied at the opposite end, this entry is still rapid; (2) exit is much more rapid when the pore is occupied by two ions; and (3) transfer from one end to the other of a singly occupied pore is rapid. With these assumptions and for a range of concentrations over which the fluxes are proportional to ion activities, the model predicts a flux ratio exponent nearly equal to 2, blocking by impermeant ions, rectification due to blocking particles on one side only, relief of block by increase in the permeant ion concentration on the opposite side, and anomalous variations of the conductance and zero current potential with mole ratio when the total concentration of the two permeants is held constant.

Activation of beta-catenin signalling by GSK-3 inhibition increases p-glycoprotein expression in brain endothelial cells

This study investigates involvement of beta-catenin signalling in regulation of p-glycoprotein (p-gp) expression in endothelial cells derived from brain vasculature. Pharmacological interventions that enhance or that block beta-catenin signalling were applied to primary rat brain endothelial cells and to immortalized human brain endothelial cells, hCMEC/D3, nuclear translocation of beta-catenin being determined by immunocytochemistry and by western blot analysis to confirm effectiveness of the manipulations. Using the specific glycogen synthase kinase-3 (GSK-3) inhibitor 6-bromoindirubin-3'-oxime enhanced beta-catenin and increased p-gp expression including activating the MDR1 promoter. These increases were accompanied by increases in p-gp-mediated efflux capability as observed from alterations in intracellular fluorescent calcein accumulation detected by flow cytometry. Similar increases in p-gp expression were noted with other GSK-3 inhibitors, i.e. 1-azakenpaullone or LiCl. Application of Wnt agonist [2-amino-4-(3,4-(methylenedioxy) benzylamino)-6-(3-methoxyphenyl)pyrimidine] also enhanced beta-catenin and increased transcript and protein levels of p-gp. By contrast, down-regulating the pathway using Dickkopf-1 or quercetin decreased p-gp expression. Similar changes were observed with multidrug resistance protein 4 and breast cancer resistance protein, both known to be present at the blood-brain barrier. These results suggest that regulation of p-gp and other multidrug efflux transporters in brain vasculature can be influenced by beta-catenin signalling.

Functional and molecular characterization of a volume-sensitive chloride current in rat brain endothelial cells

1. Volume-activated chloride currents in cultured rat brain endothelial cells were investigated on a functional level using the whole-cell voltage-clamp technique and on a molecular level using the reverse transcriptase-polymerase chain reaction (RT-PCR). 2. Exposure to a hypotonic solution caused the activation of a large, outward rectifying current, which exhibited a slight time-dependent decrease at strong depolarizing potentials. The anion permeability of the induced current was I- (1.7) > Br- (1.2) > Cl- (1.0) > F- (0. 7) > gluconate (0.18). 3. The chloride channel blocker 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB, 100 microM) rapidly and reversibly inhibited both inward and outward currents. The chloride transport blocker 4,4'-diisothiocyanatostilbene-2, 2'-disulphonic acid (DIDS, 100 microM) also blocked the hypotonicity-induced current in a reversible manner. In this case, the outward current was more effectively suppressed than the inward current. The volume-activated current was also inhibited by the antioestrogen tamoxifen (10 microM). 4. The current was dependent on intracellular ATP and independent of intracellular Ca2+. 5. Activation of protein kinase C by phorbol 12,13-dibutyrate (PDBu, 100 nM) inhibited the increase in current normally observed following hypotonic challenge. 6. Extracellular ATP (10 mM) inhibited the current with a more pronounced effect on the outward than the inward current. 7. Verapamil (100 microM) decreased both the inward and the outward hypotonicity-activated chloride current. 8. RT-PCR analysis was used to determine possible molecular candidates for the volume-sensitive current. Expression of the ClC-2, ClC-3 and ClC-5 chloride channels, as well as pICln, could be shown at the mRNA level. 9. We conclude that rat brain endothelial cells express chloride channels which are activated by osmotic swelling. The biophysical and pharmacological properties of the current show strong similarities to those of ClC-3 channel currents as described in other cell types.

ABC transporters and drug resistance in parasitic protozoa

Parasitic protozoa are responsible for a wide spectrum of diseases in humans and domestic animals. The main line of defence available against these organisms is chemotherapy. However, the application of chemotherapeutic drugs has resulted in the development of resistance mechanisms, which limit the number of antiprotozoal drugs that are effective in the treatment and control of parasitic diseases. Knowledge about the resistance mechanisms involved may allow the development of new drugs that minimise or circumvent drug resistance or may identify new targets for drug development. This review focuses on the role of protozoal ATP-binding cassette (ABC) transporters in drug resistance. These membrane proteins mediate the ATP-dependent transport of a wide variety of chemotherapeutic drugs away from their targets inside the parasites. The genome sequence of Plasmodium falciparum and Plasmodium yoelii has recently been completed, and the sequencing of other parasitic genomes are now underway. As a result, many new membrane transporters belonging to the ABC superfamily are being discovered. We review the ABC transporters in major parasitic protozoa, including Plasmodium, Leishmania, Trypanosoma and Entamoeba species. Transporters with an established role in drug resistance have been emphasised, but newly discovered transporters with a significant amino acid sequence identity to established ABC drug transporters have also been included.

Polarized distribution of nucleoside transporters in rat brain endothelial and choroid plexus epithelial cells

This study investigated mRNA expression and protein localization of equilibrative and concentrative nucleoside transporters (ENTs, CNTs) in primary cultures of rat brain endothelial cells (RBEC) and rat choroid plexus epithelial cells (RCPEC). Reverse transcriptase PCR analysis revealed that RBEC and RCPEC contained mRNA for rENT1, rENT2 and rCNT2 and for rENT1, rENT2, rCNT2 and rCNT3, respectively. Immunoblotting of membrane fractions of RBEC, fresh RCPEC and primary cultures of RCPEC revealed the presence of rENT1, rENT2 and rCNT2 proteins in all samples. Measurement of [14C]adenosine uptake into cells grown as monolayers on permeable plastic supports revealed a polarized distribution of Na+-dependent adenosine uptake in that CNT activity was associated exclusively in membranes of RBEC facing the lower chamber (which corresponds to the surface facing the interstitial fluid in situ) and in membranes of RCPEC facing the upper chamber (which corresponds to the surface facing the cerebrospinal fluid in situ). In both RBEC and RCPEC, adenosine uptake from the opposite chambers was Na+-independent and partially inhibited by nitrobenzylthioinosine, indicating the presence of the equilibrative sensitive transporter rENT1.

Transporters involved in regulation of intracellular pH in primary cultured rat brain endothelial cells

Fluid secretion across the blood-brain barrier, critical for maintaining the correct fluid balance in the brain, entails net secretion of HCO(3)(-), which is brought about by the combined activities of ion transporters situated in brain microvessels. These same transporters will concomitantly influence intracellular pH (pH(i)). To analyse the transporters that may be involved in the maintenance of pH(i) and hence secretion of HCO(3)(-), we have loaded primary cultured endothelial cells derived from rat brain microvessels with the pH indicator BCECF and suspended them in standard NaCl solutions buffered with Hepes or Hepes plus 5% CO(2)/HCO(3)(-). pH(i) in the standard solutions showed a slow acidification over at least 30 min, the rate being less in the presence of HCO(3)(-) than in its absence. However, after accounting for the difference in buffering, the net rates of acid loading with and without HCO(3)(-) were similar. In the nominal absence of HCO(3)(-) the rate of acid loading was increased equally by removal of external Na(+) or by inhibition of Na(+)/H(+) exchange by ethylisopropylamiloride (EIPA). By contrast, in the presence of HCO(3)(-) the increase in the rate of acid loading when Na(+) was removed was much larger and the rate was then also significantly greater than the rate observed in the absence of both Na(+) and HCO(3)(-). Removal of Cl(-) in the presence of HCO(3)(-) produced an alkalinization followed by a resumption of the slow acid gain. Removal of Na(+) following removal of Cl(-) increased the rate of acid gain. In the presence of HCO(3)(-) and initial presence of Na(+) and Cl(-), DIDS inhibited the changes in pH(i) produced by removal of either Na(+) or Cl(-). These are the expected results if these cells possess an AE-like Cl(-)/HCO(3)(-) exchanger, a 'channel-like' permeability allowing slow influx of acid (or efflux of HCO(3)(-)), a NBC-like Cl(-)-independent Na(+)-HCO(3)(-) cotransporter, and a NHE-like Na(+)/H(+) exchanger. The in vitro rates of HCO(3)(-) loading via the Na(+)-HCO(3)(-) cotransporter could, if the transporter is located on the apical, blood-facing side of the cells, account for the net secretion of HCO(3)(-) into the brain.

cGMP and glutathione-conjugate transport in human erythrocytes

The nature of cGMP transport in human erythrocytes, its relationship to glutathione conjugate transport, and possible mediation by multidrug resistance-associated proteins (MRPs) have been investigated. MRP1, MRP4 and MRP5 are detected in immunoblotting studies with erythrocytes. MRP1 and MRP5 are also detected in multidrug resistant COR-L23/R and MOR/R cells but at greatly reduced levels in the parent, drug sensitive COR-L23/P cells. MRP4 is detected in MOR/R but not COR-L23/R cells. Uptake of cGMP into inside-out membrane vesicles prepared by a spontaneous, one-step vesiculation process is shown to be by a low affinity system that accounts for more than 80% of the transport at all concentrations above 3 micro m. This transport is reduced by MRP inhibitors and substrates including MK-571, methotrexate, estradiol 17-beta-d-glucuronide, and S(2,4-dinitrophenyl)glutathione (DNP-SG) and also by glibenclamide and frusemide but not by the monoclonal Ig QCRL-3 that inhibits high-affinity transport of DNP-SG by MRP1. It is concluded that the cGMP exporter is distinct from MRP1 and has properties similar to those reported for MRP4. Furthermore the evidence suggests that the protein responsible for cGMP transport is the same as that mediating low-affinity DNP-SG transport in human erythrocytes.

Thickness fluctuations in black lipid membranes

Because a black lipid membrane is compressible, there will be spontaneous fluctuations in its thickness. Qualitative arguments are given that the preferred configuration of the membranes is flat and that thickness fluctuations are smaller in amplitude than the differences in mean thickness observed using different hydrocarbon solvents. Fluctuations with short characteristic lengths will not be large as a result of the large amounts of oil-water contact these would entail. Quantitative analysis based on an extension of the treatment for soap films, predicts that the root mean square (rms) amplitude for fluctuations of wavelength longer than approximately 10 nm is negligible for glyceryl monooleate membranes with squalene (less than 3%) but may be approximately 20% with n-decane. rms fluctuations of 20% would lead to a discrepancy between the rms thickness of the core and the mean reciprocal thickness of only 6%.

A quantitative resolution of the spectra of a membrane potential indicator, diS-C3-(5), bound to cell components and to red blood cells

Cationic cyanine dyes have been widely used to measure electrical potentials of red blood cells and other membrane preparations. A quantitative analysis of the binding of the most extensively studied of these dyes, diS-C3-(5), to red blood cells and their constituents is presented here. Absorption spectra were recorded for the dye in suspensions of isolated red cell membranes and in solutions of cell lysate. The dependence of the spectra on the concentrations of dye and cell constituents shows that the dye binds to these membranes as monomers with an absorbance maximum at 670 nm instead of 650 nm as for free aqueous dye and the dye binds to oxyhaemoglobin partly as monomer but primarily as dimer, with absorbance maxima ca. 670 and 595 nm, respectively. Quantitative estimates are derived for all binding constants and extinction coefficients. These estimates are applied to suspensions of whole cells to predict the dye binding, absorbance spectra, and calibration curves of binding and fluorescence vs. membrane voltage. Satisfactory agreement is found with binding and absorbance data for whole cells at zero membrane potential and with the binding and fluorescence data reported by Hladky and Rink (J. Physiol. (London) 263:287, 1976) for cells driven to positive and negative potentials using valinomycin. The marked tendency of oxyhaemoglobin to bind dye as dimer is not shared by some other proteins tested, including deoxyhaemoglobin and oxymyoglobin.

Interactions of mefloquine with ABC proteins, MRP1 (ABCC1) and MRP4 (ABCC4) that are present in human red cell membranes

Human erythrocyte membranes express the multidrug resistance-associated proteins, MRP1, MRP4 and 5, that collectively can efflux oxidised glutathione, glutathione conjugates and cyclic nucleotides. It is already known that the quinoline derivative, MK-571, is a potent inhibitor of MRP-mediated transport. We here examine whether the quinoline-based antimalarial drugs, amodiaquine, chloroquine, mefloquine, primaquine, quinidine and quinine, also interact with erythrocyte MRPs with consequences for their access to the intracellular parasites or for efflux of oxidised glutathione from infected cells. Using inside-out vesicles prepared from human erythrocytes we have shown that mefloquine and MK-571 inhibit transport of 3 microM [(3)H]DNP-SG known to be mediated by MRP1 (IC(50) 127 and 1.1 microM, respectively) and of 3.3 microM [(3)H]cGMP thought but not proven to be mediated primarily by MRP4 (IC(50) 21 and 0.41 microM). They also inhibited transport in membrane vesicles prepared from tumour cells expressing MRP1 or MRP4 and blocked calcein efflux from MRP1-overexpressing cells and BCECF efflux from MRP4-overexpressing cells. Both stimulated ATPase activity in membranes prepared from MRP1 and MRP4-overexpressing cells and inhibited activity stimulated by quercetin or PGE(1), respectively. Neither inhibited [alpha-(32)P]8-azidoATP binding confirming that the interactions are not at the ATP binding site. These results demonstrate that mefloquine and MK-571 both inhibit transport of other substrates and stimulate ATPase activity and thus may themselves be substrates for transport. But at concentrations achieved clinically mefloquine is unlikely to affect the MRP1-mediated transport of GSSG across the erythrocyte membrane.

Beta amyloid effects on expression of multidrug efflux transporters in brain endothelial cells

ABC (ATP Binding Cassette) efflux transporters at the blood-brain barrier, P-glycoprotein (ABCB1), multidrug resistance associated protein 4 (ABCC4) and breast cancer resistance protein (ABCG2), are important for protecting the brain from circulating xenobiotics. Their expression is regulated by signals from surrounding brain tissue that may alter in CNS pathologies. Differences have been reported in transporter expression on brain vasculature of Alzheimer's subjects where raised levels of β-amyloid (Aβ) occur. The present study examines in vitro the effects of Aβ using immortalised brain endothelial cells (hCMEC/D3). Significantly lower expression of ABCB1 but not ABCC4 or ABCG2 was found following exposure to Aβ(1-42) peptide but not its scrambled equivalent. This was evident at both protein and transcript level and was reflected in lower transcriptional activity of the ABCB1 promoter as judged from the luciferase reporter gene assay and in decreases in ABCB1-mediated efflux of rhodamine 123. Aβ exposure also affected Wnt/β-catenin signalling, decreasing levels of β-catenin protein, reducing activation of TOPFLASH and increasing transcript levels of endogenous inhibitor, Dkk-1. Application of Wnt3a reversed the Aβ-induced changes to ABCB1 protein. These results suggest that Aβ may impair Wnt/β-catenin signalling at the blood-brain barrier but that activation of this pathway may restore ABCB1.

Ion transporters in brain endothelial cells that contribute to formation of brain interstitial fluid

Ions and water transported across the endothelium lining the blood–brain barrier contribute to the fluid secreted into the brain and are important in maintaining appropriate volume and ionic composition of brain interstitial fluid. Changes in this secretion process may occur after stroke. The present study identifies at transcript and protein level ion transporters involved in the movement of key ions and examines how levels of certain of these alter following oxidative stress. Immunohistochemistry provides evidence for Cl⁻/HCO₃⁻ exchanger, AE2, and Na⁺, HCO₃⁻ cotransporters, NBCe1 and NBCn1, on brain microvessels. mRNA analysis by RT-PCR reveals expression of these transporters in cultured rat brain microvascular endothelial cells (both primary and immortalized GPNT cells) and also Na⁺/H⁺ exchangers, NHE1 (primary and immortalized) and NHE2 (primary cells only). Knock-down using siRNA in immortalized GPNT cells identifies AE2 as responsible for much of the Cl⁻/HCO₃⁻ exchange following extracellular chloride removal and NHE1 as the transporter that accounts for most of the Na⁺/H⁺ exchange following intracellular acidification. Transcript levels of both AE2 and NHE1 are increased following hypoxia/reoxygenation. Further work is now required to determine the localization of the bicarbonate transporters to luminal or abluminal membranes of the endothelial cells as well as to identify and localize additional transport mechanisms that must exist for K⁺ and Cl⁻.

Plasmodium falciparum expresses a multidrug resistance-associated protein

Plasmodium falciparum proteins that efflux toxic metabolic products such as oxidised glutathione (GSSG) are possible targets for anti-malarial drug development. Proteins capable of transporting GSSG and glutathione conjugates include the multidrug resistance-associated transporters (MRPs). A gene, PFA0590w, encoding a MRP homologue, has been identified in P. falciparum. Here we show the presence of full-length mRNA (5.5 kb) of this PfMRP in trophozoites by RT-PCR and Northern blotting. A polyclonal anti-PfMRP antibody generated against two unique, hydrophilic peptides in the predicted sequence produced a strong immunoreactive protein band of 210-215 kDa on Western blots of schizonts of chloroquine-sensitive and chloroquine-resistant strains, confirming expression of PfMRP protein. Using confocal microscopy the protein was seen to be localised at the edge of the schizonts with no obvious staining of the food vacuole. We suggest that PfMRP may act as the GSSG transporter in the parasite plasma membrane.