Effects of curcumin on ion channels and pumps: A review.

Curcumin, an orange-yellow lipophilic polyphenolic molecule, is the active component of Curcuma longa, which is extensively used as a spice in most of the Asian countries. This natural compound is able to interact with a large number of molecular structures like proteins, enzymes, lipids, DNA, RNA, transporter molecules, and ion channels. It has been reported to possess several biological effects such as antioxidant, anti-inflammatory, wound healing, antimicrobial, anticancer, antiangiogenic, antimutagenic, and antiplatelet aggregation properties. These beneficial effects of curcumin are because of its extraordinary chemical interactions such as extensive hydrogen and covalent bonding, metal chelation, and so on. Therefore, the aim of this review was to outline the evidence in which curcumin could affect different types of ion channels and ion channel-related diseases, and also to elucidate basic molecular mechanisms behind it. © 2019 IUBMB Life, 2019.

The Investigation of the Antitumor Agent Toxicity and Capsaicin Effect on the Electron Transport Chain Enzymes, Catalase Activities and Lipid Peroxidation Levels in Lung, Heart and Brain Tissues of Rats.

Cisplatin is one of the most active cytotoxic agents in cancer treatment. To clarify the interaction with mitochondria, we hypothesize that the activities of mitochondrial electron transport chain (ETC) enzymes succinate dehydrogenase (SDH) and cytochrome c oxidase (COX), nucleotide levels, as well as levels of catalase (CAT) enzyme and membrane lipid peroxidation (LPO) can be affected by cisplatin. There was a significant decrease of both SDH and COX activities in the lung, heart, and brain tissues at the 1st day after cisplatin exposure, and the observed decreased levels of adenosine triphosphate (ATP) and adenosine diphosphate (ADP) in comparison with the control could be because of cisplatin-induced mitochondrial dysfunction. The investigations suggested that cisplatin inhibits SDH, COX, and ATP synthase. The higher LPO level in the studied tissues after 1 and 4 days post-exposure to cisplatin compared to control can be inferred to be a result of elevated electron leakage from the ETC, and reactive oxygen species (ROS) can lead to wide-ranging tissue damage such as membrane lipid damage. Consequently, it was observed that capsaicin may have a possible protective effect on ETC impairment caused by cisplatin. The activities of SDH and COX were higher in heart and brain exposed to cisplatin + capsaicin compared to cisplatin groups, while LPO levels were lower. The investigated results in the cisplatin + capsaicin groups suggested that the antioxidant capacity of capsaicin scavenges ROS and prevents membrane destruction.

Functional and molecular characterization of transmembrane intracellular pH regulators in human dental pulp stem cells.

OBJECTIVE: Homeostasis of intracellular pH (pHi) plays vital roles in many cell functions, such as proliferation, apoptosis, differentiation and metastasis. Thus far, Na+-H+ exchanger (NHE), Na+-HCO3- co-transporter (NBC), Cl-/HCO3- exchanger (AE) and Cl-/OH- exchanger (CHE) have been identified to co-regulate pHi homeostasis. However, functional and biological pHi-regulators in human dental pulp stem cells (hDPSCs) have yet to be identified. DESIGN: Microspectrofluorimetry technique with pH-sensitive fluorescent dye, BCECF, was used to detect pHi changes. NH4Cl and Na+-acetate pre-pulse were used to induce intracellular acidosis and alkalosis, respectively. Isoforms of pHi-regulators were detected by Western blot technique. RESULTS: The resting pHi was no significant difference between that in HEPES-buffered (nominal HCO3--free) solution or CO2/HCO3-buffered system (7.42 and 7.46, respectively). The pHi recovery following the induced-intracellular acidosis was blocked completely by removing [Na+]o, while only slowed (-63%) by adding HOE694 (a NHE1 specific inhibitor) in HEPES-buffered solution. The pHi recovery was inhibited entirely by removing [Na+]o, while adding HOE 694 pulse DIDS (an anion-transporter inhibitor) only slowed (-55%) the acid extrusion. Both in HEPES-buffered and CO2/HCO3-buffered system solution, the pHi recovery after induced-intracellular alkalosis was entirely blocked by removing [Cl-]o. Western blot analysis showed the isoforms of pHi regulators, including NHE1/2, NBCe1/n1, AE1/2/3/4 and CHE in the hDPSCs. CONCLUSIONS: We demonstrate for the first time that resting pHi is significantly higher than 7.2 and meditates functionally by two Na+-dependent acid extruders (NHE and NBC), two Cl--dependent acid loaders (CHE and AE) and one Na+-independent acid extruder(s) in hDPSCs. These findings provide novel insight for basic and clinical treatment of dentistry.

Metal Resistance and Its Association With Antibiotic Resistance.

Antibiotic resistance is recognised as a major global threat to public health by the World Health Organization. Currently, several hundred thousand deaths yearly can be attributed to infections with antibiotic-resistant bacteria. The major driver for the development of antibiotic resistance is considered to be the use, misuse and overuse of antibiotics in humans and animals. Nonantibiotic compounds, such as antibacterial biocides and metals, may also contribute to the promotion of antibiotic resistance through co-selection. This may occur when resistance genes to both antibiotics and metals/biocides are co-located together in the same cell (co-resistance), or a single resistance mechanism (e.g. an efflux pump) confers resistance to both antibiotics and biocides/metals (cross-resistance), leading to co-selection of bacterial strains, or mobile genetic elements that they carry. Here, we review antimicrobial metal resistance in the context of the antibiotic resistance problem, discuss co-selection, and highlight critical knowledge gaps in our understanding.

Presence of a thapsigargin-sensitive calcium pump in Trypanosoma evansi: Immunological, physiological, molecular and structural evidences.

In higher eukaryotes, the sarco-endoplasmic reticulum (ER) Ca(2+)-ATPase (SERCA) is characterized for its high sensitivity to low concentrations of thapsigargin (TG), a very specific inhibitor. In contrast, SERCA-like enzymes with different sensitivities to TG have been reported in trypanosomatids. Here, we characterized a SERCA-like enzyme from Trypanosoma evansi and evaluated its interaction with TG. Confocal fluorescence microscopy using BODIPY FL TG and specific anti-SERCA antibodies localized the T. evansi SERCA-like enzyme in the ER and confirmed its direct interaction with TG. Moreover, the use of either 1 µM TG or 25 µM 2',5'-di (tert-butyl)-1,4-benzohydroquinone prevented the reuptake of Ca(2+) and consequently produced a small increase in the parasite cytosolic calcium concentration in a calcium-free medium, which was released from the ER pool. A 3035 bp-sequence coding for a protein with an estimated molecular mass of 110.2 kDa was cloned from T. evansi. The corresponding gene product contained all the invariant residues and conserved motifs found in other P-type ATPases but lacked the calmodulin binding site. Modeling of the three-dimensional structure of the parasite enzyme revealed that the amino acid changes found in the TG-SERCA binding pocket do not compromise the interaction between the enzyme and the inhibitor. Therefore, we concluded that T. evansi possesses a SERCA-like protein that is inhibited by TG.

The mechanisms underlying fructose-induced hypertension: a review.

We are currently in the midst of an epidemic of metabolic disorders, which may, in part, be explained by excess fructose intake. This theory is supported by epidemiological observations as well as experimental studies in animals and humans. Rising consumption of fructose has been matched with growing rates of hypertension, leading to concern from public health experts. At this stage, the mechanisms underlying fructose-induced hypertension have not been fully characterized and the bulk of our knowledge is derived from animal models. Animal studies have shown that high-fructose diets up-regulate sodium and chloride transporters, resulting in a state of salt overload that increases blood pressure. Excess fructose has also been found to activate vasoconstrictors, inactivate vasodilators, and over-stimulate the sympathetic nervous system. Further work is required to determine the relevance of these findings to humans and to establish the level at which dietary fructose increases the risk of developing hypertension.

Anthocyanins suppress the secretion of proinflammatory mediators and oxidative stress, and restore ion pump activities in demyelination.

The aim of this study was to investigate the protective effect of anthocyanins (ANT) on oxidative and inflammatory parameters, as well as ion pump activities, in the pons of rats experimentally demyelinated with ethidium bromide (EB). Rats were divided in six groups: control, ANT 30 mg/kg, ANT 100 mg/kg, EB (0.1%), EB plus ANT 30 mg/kg and EB plus ANT 100 mg/kg. The EB cistern pons injection occurred on the first day. On day 7, there was a peak in the demyelination. During the 7 days, the animals were treated once per day with vehicle or ANT. It was observed that demyelination reduced Na(+),K(+)-ATPase and Ca(2+)-ATPase activities and increased 4-hydroxynonenal, malondialdehyde, protein carbonyl and NO2plus NO3 levels. In addition, a depletion of glutathione reduced level/nonprotein thiol content and a decrease in superoxide dismutase activity were also seen. The dose of 100 mg/kg showed a better dose-response to the protective effects. The demyelination did not affect the neuronal viability but did increase the inflammatory infiltrate (myeloperoxidase activity) followed by an elevation in interleukin (IL)-1ß, IL-6, tumor necrosis factor-α and interferon-γ levels. ANT promoted a reduction in cellular infiltration and proinflammatory mediators. Furthermore, ANT restored the levels of IL-10. Luxol fast blue staining confirmed the loss of myelin in the EB group and the protective effect of ANT 100 mg/kg. In conclusion, this study was the first to show that ANT are able to restore ion pump activities and protect cellular components against the inflammatory and oxidative damages induced by demyelination.

Effect of diabetes on the ion pumps of the bladder.

OBJECTIVE: To establish whether the activities of Na+/K+-adenosine triphosphatase (ATPase) and Ca2+-ATPases ion pumps in bladder smooth muscle are altered as a consequence of diabetes and, if so, how this might contribute to bladder cystopathy. Urinary bladder dysfunction is a common occurrence in patients with diabetes. Pressure generation requires calcium and cytosolic ATP. Activities of these pumps are responsible for calcium homeostasis. METHODS: Rat urinary detrusor muscle strips were suspended in organ baths containing Krebs solution for isometric tension recording. Tissue responses to the Na+/K+-ATPase pump inhibitor, ouabain, the plasma membrane Ca2+ ATPase inhibitor, vanadate, and the sarcoplasmic reticulum Ca2+ ATPase inhibitor, cyclopiazonic acid (CPA), were examined from normal and streptozocin-induced diabetic rats for 2, 4, and 12 weeks. RESULTS: Ouabain, vanadate, and CPA caused concentration-dependent contractions of bladder strips from diabetic and normal rats. The degree of contraction of diabetic bladder muscle was lower than that of controls. This reduction was a function of duration of diabetes. For ouabain, the reduction peaked at 2 weeks, with partial restoration to normal after diabetes induction. For vanadate and CPA, the reduction increased with the duration of diabetes. CONCLUSION: The ion pumps are important modulators of bladder smooth muscle tone, and in a rat model of streptozotocin-induced diabetes, the activity of these pumps is impaired. Although this is only a single model of diabetes, these findings suggest that a defect in these pumps may be an important component of the development of diabetic bladder cystopathy.

[Effects of ANP upon ion pump activity and gene expression in aortic smooth muscle cells from spontaneously hypertensive rats].

OBJECTIVE: To explore the effects of atrial natriuretic peptide (ANP) upon the activities of Na(+), K(+)-ATPase, Ca(2+)-ATPase and mRNA expression levels of Na(+), K(+)-ATPase alpha(1)-subunit and plasma membrane Ca(2+)-ATPase isoform 1 (PMCA1) in cultured thoracic aortic vascular smooth muscle cells (ASMCs) isolated from spontaneously hypertensive rats (SHR). METHODS: ASMCs isolated from 14-week-old male SHR and Wistar-Kyoto (WKY) rats were interference-cultured in different doses of ANP and Angiotensin II (AngII). The contents of ANP and AngII in supernatant from ASMCs were measured by radioimmunoassay. The activities of the above two ATPases were measured by biochemistry and enzymology. RT-PCR assay was employed to determine the relative levels of Na(+), K(+)-ATPase alpha(1)-subunit and PMCA1 mRNA in ASMCs. RESULTS: The ANP level of supernatant in SHR ASMCs was significantly lower than those from WKY control [(7.3 +/- 2.4) pg x 10(-6) cells vs (19.3 +/- 3.3) pg x 10(-6) cells, P < 0.01] while the content of AngII in SHR ASMCs was significantly higher than those from WKY control [(57 +/- 4) pg x 10(-6) cells vs (44 +/- 4) pg x 10(-6) cells, P < 0.01]. The activity of Na(+), K(+)-ATPase [(4.3 +/- 0.8) micromol x h(-1) x mg(-1) vs (5.3 +/- 1.0) micromol x h(-1) x mg(-1)], Ca(2+)-ATPase [(3.2 +/- 0.7) micromol x h(-1) x mg(-1) vs (4.5 +/- 0.7) micromol x h(-1) x mg(-1)] in ASMCs from SHR were significantly lower than those from WKY control (both P < 0.01). The mRNA expression of Na(+), K(+)-ATPase alpha(1)-subunit (0.524 +/- 0.025 vs 0.704 +/- 0.116), PMCA1 (0.193 +/- 0.030 vs 0.547 +/- 0.045) significantly decreased in ASMCs from SHR versus the WKY control (both P < 0.01). As compared with SHR control, exogenous ANP improved obviously the activities of Na(+), K(+)-ATPase, Ca(2+)-ATPase and expression of alpha(1)-subunit, PMCA1 mRNA in a does-dependent manner (P < 0.05-P < 0.01). Exogenous AngII (1 x 10(-9), 1 x 10(-8), 1 x 10(-7) mol/L) significantly repressed activities of Ca(2+)-ATPase and attenuated the expression of PMCA1 mRNA (P < 0.05-P < 0.01). Only AngII (1 x 10(-7) mol/L) significantly inhibited the activity of Na(+), K(+)-ATPase and attenuated the expression of Na(+), K(+)-ATPase alpha(1)-subunit mRNA (both P < 0.05). ANP antagonized the effects of AngII (1 x 10(-7) mol/L) upon the activities of two ATPases and the expression of Na(+), K(+)-ATPase alpha(1)-subunit PMCA1 mRNA (P < 0.05-P < 0.01). AngII (1 x 10(-7) mol/L) increased the Na(+), K(+)-ATPase activity and the expression of Na(+), K(+)-ATPase alpha(1)-subunit mRNA, repressed the Ca(2+)-ATPase activity and the expression of PMCA1 mRNA in ASMCs from WKY rat (P < 0.05-P < 0.01). ANP antagonized the effects of AngII (1 x 10(-7) mol/L) upon the activity of Ca(2+)-ATPase and the expression of PMCA1 mRNA (P < 0.05-P < 0.01), but did not antagonize the effects of AngII (1 x 10(-7) mol/L) upon the activity of Na(+), K(+)-ATPase and the expression of alpha(1)-subunit mRNA in ASMCs from WKY rats (P > 0.05). CONCLUSION: The decreased activities of Na(+), K(+)-ATPase and Ca(2+)-ATPase may be related to the abnormal autocrine of ANP and AngII in ASMC of SHR. ANP can antagonize the effects of AngII upon the activities of two ATPases and the expression of Na(+), K(+)-ATPase alpha(1)-subunit PMCA1 mRNA.

Stevioside and related compounds: therapeutic benefits beyond sweetness.

Stevioside, an abundant component of Stevia rebaudiana leaf, has become well-known for its intense sweetness (250-300 times sweeter than sucrose) and is used as a non-caloric sweetener in several countries. A number of studies have suggested that, beside sweetness, stevioside along with related compounds, which include rebaudioside A (second most abundant component of S. rebaudiana leaf), steviol and isosteviol (metabolic components of stevioside) may also offer therapeutic benefits, as they have anti-hyperglycemic, anti-hypertensive, anti-inflammatory, anti-tumor, anti-diarrheal, diuretic, and immunomodulatory actions. It is of interest to note that their effects on plasma glucose level and blood pressure are only observed when these parameters are higher than normal. As steviol can interact with drug transporters, its role as a drug modulator is proposed. This review summarizes the current knowledge of the pharmacological actions, therapeutic applications, pharmacokinetics and safety of stevioside and related compounds. Although much progress has been made concerning their biological and pharmacological effects, questions regarding chemical purity and safety remain unsolved. These issues are discussed to help guide future research directions.

[Long-term regulation of Na+, K+-ATPase pump in human lymphocytes: the role of JAK/STAT- and MAPK-signaling pathways].

In interleukin-2 (IL-2)-induced human blood lymphocytes, the Na+/K+ pump function (assessed by ouabain-sensitive Rb+ influx), the abundance of Na+, K+-ATPase alpha1-subunit (determined by Western blotting) and the alpha1- and beta1-subunits mRNA of Na+, K+-ATPase (RT-PCR), as well as the phosphorylation of STAT5 and STAT3 family proteins and ERK1/2 kinase have been examined. A 3.5-4.0-fold increase in the expression of alpha1- and beta1-subunits mRNA of Na+, K+-ATPase was found at 24 h of IL-2 stimulation. The inhibitors of JAK3 kinase (B-42, WHI-P431) was shown to decrease both the phosphorylation of STATs and the rise in the oubain-sensitive rubidium influx as well as the increased abundance of Na+, K+-ATPase alpha1-subunit. The inhibition of the protein kinases ERK1/2 by PD98059 (20 microM) suppressed the alpha1-subunit accumulation. All the kinase inhibitors tested did not alter the intracellular content ofmonovalent cations in resting and IL-2-stimulated lymphocytes. It is concluded that MAPK and JAK/STAT signaling pathways mediate the IL-2-dependent regulation of the Na+, K+-ATPase expression during the lymphocyte transition from resting stage to proliferation.

[Advances in the study of the microbial efflux pumps and its inhibitors development].

Drug resistant bacteria is an increasingly urgent challenge to public health. Bacteria adaptation and extensive abuse of antibiotics contribute to this dilemma. Active efflux of antibiotics is employed by the bacteria to survive the antibiotic pressure. Efflux pump is one of the hot spots of current drug related studies and ideal targets for the improvement of treatment. The efflux pumps and related mechanisms of action, regulation of expression and methodologies were summarized. Comparative genomics analyses were employed to elucidate the underlying mechanisms of action and evolution of efflux pump as exemplified by the Mycobacterium in our lab, which is a crucial re-emerging threat to global public health. The pathway and state-of-art drug development of efflux pump related drugs are included too.

Bacterial efflux systems and efflux pumps inhibitors.

It is now well established that bacterial resistance to antibiotics has become a serious problem of public health that concerns almost all antibacterial agents and that manifests in all fields of their application. Among the three main mechanisms involved in bacterial resistance (target modification, antibiotic inactivation or default of its accumulation within the cell), efflux pumps, responsible for the extrusion of the antibiotic outside the cell, have recently received a particular attention. Actually, these systems, classified into five families, can confer resistance to a specific class of antibiotics or to a large number of drugs, thus conferring a multi-drug resistance (MDR) phenotype to bacteria. To face this issue, it is urgent to find new molecules active against resistant bacteria. Among the strategies employed, the search for inhibitors of resistance mechanisms seems to be attractive because such molecules could restore antibiotic activity. In the case of efflux systems, efflux pump inhibitors (EPIs) are expected to block the pumps and such EPIs, if active against MDR pumps, would be of great interest. This review will focus on the families of bacterial efflux systems conferring drug resistance, and on the EPIs that have been identified to restore antibiotic activity.

The toxicity of H2O2 on the ionic homeostasis of airway epithelial cells in vitro.

Oxygen species may be formed in the air spaces of the respiratory tract in response to environmental pollution such as particulate matter. The mechanisms and target molecules of these oxidants are still mainly unknown but may involve modifications of the ionic homeostasis in epithelial cells. Cytosolic concentrations of Ca2+ (Fura2) and Na+ (SBFI) and short-circuit current (Isc) were followed in primary cultures of human nasal epithelial cells and in the cell line 16HBE14o- after exposure to H2O2 or *OH (H2O2 + Fe2+). Cells were grown on glass coverslips for ionic imaging or on permeable snapwell inserts for Isc studies. Exposure of the apical as well as the basal side of the cultures to H2O2 or *OH induced a concentration-dependent transient increase in Isc which is due to a transient secretion of Cl-. Cai also increased transiently with approximately the same kinetics. The response was dependent on the release of calcium from intracellular stores. Nai on the contrary increased steadily over more than an hour. When the apical membrane was permeabilized with gramicidin, *OH inhibited the Na+ current (a measure of Na(+)-K(+)-ATPase activity in the baso-lateral membrane). The arrest of the pump was significant after 30 min exposure to oxidant. On the other hand no increase in the apical or baso-lateral sodium conductances could be detected. The progressive arrest of the Na+/K(+)-pump may contribute to the sustained elevation of Nai. This strong modification in the cellular ionic homeostasis may participate in the stress response of the respiratory epithelium through alterations in signal transduction pathways.

Arsenic sensing and resistance system in the cyanobacterium Synechocystis sp. strain PCC 6803.

Arsenic is one of the most important global environmental pollutants. Here we show that the cyanobacterium Synechocystis sp. strain PCC 6803 contains an arsenic and antimony resistance operon consisting of three genes: arsB, encoding a putative arsenite and antimonite carrier, arsH, encoding a protein of unknown function, and arsC, encoding a putative arsenate reductase. While arsB mutant strains were sensitive to arsenite, arsenate, and antimonite, arsC mutants were sensitive only to arsenate. The arsH mutant strain showed no obvious phenotype under the conditions tested. In vivo the arsBHC operon was derepressed by oxyanions of arsenic and antimony (oxidation state, +3) and, to a lesser extent, by bismuth (oxidation state, +3) and arsenate (oxidation state, +5). In the absence of these effectors, the operon was repressed by a transcription repressor of the ArsR/SmtB family, encoded by an unlinked gene termed arsR. Thus, arsR null mutants showed constitutive derepression of the arsBHC operon. Expression of the arsR gene was not altered by the presence of arsenic or antimony compounds. Purified recombinant ArsR protein binds to the arsBHC promoter-operator region in the absence of metals and dissociates from the DNA in the presence of Sb(III) or As(III) but not in the presence of As(V), suggesting that trivalent metalloids are the true inducers of the system. DNase I footprinting experiments indicate that ArsR binds to two 17-bp direct repeats, with each one consisting of two inverted repeats, in the region from nucleotides -34 to + 17 of the arsBHC promoter-operator.

Effect of ouabain on CFTR gene expression in human Calu-3 cells.

We have previously shown that ouabain, which changes the electrochemical properties of cell membranes by inhibiting Na(+),K(+)-ATPase, induces the expression of multidrug resistance (MDR-1) gene in several human cell lines. Because the expressions of the MDR-1 and CFTR (which encodes the cAMP-activated Cl(-) channel associated with cystic fibrosis) genes are physiologically regulated in opposing directions, we wanted to determine whether ouabain also decreases CFTR transcripts and subsequently to analyze its mechanism of action. We found that the submicromolar concentrations of ouabain that increase MDR-1 mRNAs decrease the CFTR transcripts with analogous time-dependency in human pulmonary Calu-3 cells. By altering or reproducing the ouabain-induced changes in intracellular ionic activities (decreasing in external Na(+) or K(+) or using Na(+) ionophore), we show that the ouabain-induced regulations of both CFTR and MDR-1 transcripts depend on the Na(+)/K(+) pump inhibition but that the decrease in CFTR mRNAs also proceeds from cytoplasm reactions simultaneously activated by ouabain. These data, which emphasize the complex mechanism of action of ouabain, suggest that changes in intracellular ionic activities modulate CFTR/MDR-1 gene expressions.

Developmental changes in Cl(-)-ATPase activity in rat brains.

Developmental changes in brain Cl(-)-ATPase activity were examined using fetal, neonatal and adult rats. The Cl(-)-ATPase activity rapidly increased over 20 postnatal days to a level four-fold higher than that in an 18-day-old fetus. On Western blot analysis using an anti-Cl(-)-ATPase/pump 51 kDa subunit (ClP51) antibody, the amount of ClP51 protein increased in parallel with Cl(-)-ATPase activity. Immunohistochemistry using the same antibody showed Cl(-)-ATPase-like immunoreactivity on the cell membranes of neurons such as cerebral and hippocampal pyramidal cells and cerebellar Purkinje cells, where the immunoreactivity increased with developmental changes in the size and shape of the neurons. These findings suggest that neuronal Cl(-)-ATPase activity markedly increases during early postnatal development with an increase in the amount of Cl(-)-ATPase protein, which may support the formation of inwardly directed neuronal Cl(-) gradients.

Peroxynitrite activates K+-Cl- cotransport in human erythrocytes.

Peroxynitrite was found to induce the release of K+ via the Na+/Cl- cotransport system, as do other oxidants. Since peroxynitrite is formed in vivo, its presence could contribute to a pathological dehydration of red blood cells.

Probenecid-inhibitable efflux transport of valproic acid in the brain parenchymal cells of rabbits: a microdialysis study.

Delivery of valproic acid (VPA) to the human brain is relatively inefficient as reflected by a low brain-to-unbound plasma concentration ratio (< or =0.5) at steady state. Previous pharmacokinetic studies suggested that the unfavorable brain-to-plasma gradient is maintained by coupled efflux transport processes at both the brain parenchymal cells and blood-brain barrier (BBB); one or both of the efflux transporters are inhibitable by probenecid. The present study in rabbits utilized microdialysis to measure drug concentration in the brain extracellular fluid (ECF) of the cerebral cortex during steady-state i.v. infusion with VPA alone or with VPA plus probenecid. Probenecid co-infusion elevated VPA concentration in the brain tissue surrounding the tip of the microdialysis probe to a greater extent than in the ECF (230% versus 47%). Brain intracellular compartment (ICC) concentration was estimated. In control rabbits, the ICC concentration was 2.8+/-0.28 times higher than the ECF concentration. Probenecid co-infusion elevated the ICC-to-ECF concentration ratio to 4.2+/-0.44, which confirms the existence of an efflux transport system in brain parenchymal cells. The ECF-to-unbound plasma concentration ratio was well below unity (0.029), indicating an uphill efflux transport of VPA across the BBB. Co-infusion of probenecid did not have a significant effect on VPA efflux at the BBB as evidenced by a minimal change in the ECF-to-unbound plasma concentration ratio. This study suggests the presence of distinctly different organic anion transporters for the efflux of VPA at the parenchymal cells and capillary endothelium in the brain.

Isolation of current components and partial reaction cycles in the glial glutamate transporter EAAT2.

The kinetic properties of the excitatory amino acid transporter EAAT2 were studied using rapid applications of L-glutamate to outside-out patches excised from transfected human embryonic kidney 293 cells. In the presence of the highly permeant anion SCN(-), pulses of glutamate rapidly activated transient anion channel currents mediated by the transporter. In the presence of the impermeant anion gluconate, glutamate pulses activated smaller currents predicted to result from stoichiometric flux of cotransported ions. Both anion and stoichiometric currents displayed similar kinetics, suggesting that anion channel gating and stoichiometric charge movements are linked to early transitions in the transport cycle. Transporter-mediated anion currents were recorded with ion and glutamate gradients favoring either unidirectional influx or exchange. Analysis of deactivation and recovery kinetics in these two conditions suggests that, after binding, translocation of substrate is more likely than unbinding under physiological conditions. The kinetic properties of EAAT2, the dominant glutamate transporter in brain astrocytes, distinguish it as an efficient sink for synaptically released glutamate.