Voltage-gated chloride channel blocker DIDS as an acaricide for Varroa mites

The Varroa mite is a primary driver behind periodical losses of honey bee colonies. These mites require honey bees for food and reproduction and, in turn, elicit physiological deficiencies and diseases that compromise colony health. Current acaricides for Varroa mite control, such as Apistan® (the pyrethroid tau-fluvalinate), CheckMite⁺® (the organophosphate coumaphos), and Apivar® (the formamidine amitraz) target the nervous system, can have adverse health effects on honey bees, and have limited effectiveness due to reported resistance issues. New target sites are needed to circumvent these obstacles in Varroa mite management, and voltage-gated chloride channels (VGCCs) are promising candidates due to their important role in the maintenance of nerve and muscle excitability in arthropod pests. Toxicological analysis of Varroa mites sensitive to tau-fluvalinate and coumaphos and Varroa mites with reduced sensitivity to these acaricides showed a significant increase in metabolic detoxification enzyme activities for the latter. Acetylcholinesterase activity in the Varroa mites exhibiting reduced mortality to coumaphos was significantly less sensitive to coumaphos-oxon compared to coumaphos-sensitive Varroa mites, which suggests target-site insensitivity to the acaricide. Voltage-gated chloride channel blocker DIDS had significantly greater field efficacy compared to Apistan® and CheckMite⁺® against Varroa mites from honey bee hives where tau-fluvalinate and coumaphos were observed to be ineffective, respectively. These data suggest that DIDS, and potentially other stilbene chemistries, might serve as candidates for continued field efficacy testing of alternative acaricides in apiaries where Apistan®- and CheckMite⁺® efficacy has been. reduced or lost for Varroa mites.

Movement of acid equivalents across the mammalian smooth muscle cell membrane

Factors affecting intracellular pH (pHi) in the smooth muscle of guinea pig ureter have been investigated using pH-sensitive microelectrodes. Associated acids and bases appear to have free passage across the cell membrane but results suggest very low permeability to charged acid equivalents, thus implicating carrier-mediated movements in many of the observed pHi transients. Recovery from acidosis in the nominal absence of CO2 was inhibited by removal of Na+ and by the presence of amiloride, indicating that Na+/H+ exchange was responsible. The presence of CO2 resulted in a faster recovery from acidosis but, since intracellular buffering power was not increased, not a substantially faster effective extrusion of protons. Surprisingly, amiloride no longer caused discernable inhibition. Recovery from moderate acidosis remained Na+ dependent but was not inhibited by DIDS or acetazolamide or by the absence of Cl-, suggesting a dominant Na+-, and HCO3(-)-dependent mechanism unlike any hitherto described. Recovery from alkalosis was inhibited by DIDS and Cl(-)-free conditions, indicating that Cl-/HCO3- exchange was involved. Results suggest reversal of this mechanism on extreme acidosis. Experiments in vascular smooth muscle with fluorescent indicators confirm the presence of Na+/H+ exchange but provide conflicting evidence about the presence and properties of the HCO3(-)-dependent mechanism.

Regulation of intracellular pH in cardiac muscle

Intracellular pH (pHi) in sheep cardiac Purkinje fibres is controlled by sarcolemmal Na+/H+ and Cl-/HCO3- exchange. At normal pHo (7.4), Na+/H+ exchange mediates an acid efflux whenever pHi falls and Cl-/HCO3- exchange mediates an equivalent acid influx in response to a rise in pHi. Intracellular pH is also influenced by Ca2+i, which can activate force development leading to the anaerobic production of lactic acid. This is evident after an increase in stimulation rate which reversibly reduces both pHi and extracellular surface pH (pHs). Rate-dependent pHi changes are inhibited following inhibition of glycolysis, indicating that they are caused by accumulation of lactic acid. In some cases, the efflux of lactic acid may provide a faster method for recovery of pHi from a metabolic acidosis than that provided by Na+/H+ exchange. Finally, direct pHi measurement in isolated mammalian ventricular myocytes suggests that the intrinsic intracellular buffering power (beta) of ventricular tissue may be considerably lower than previously believed. An accurate knowledge of beta is essential for calculating net membrane fluxes of acid equivalents from changes in pHi.

Incorporated sarcolemmal fish oil fatty acids shorten pig ventricular action potentials

BACKGROUND

Omega-3 polyunsaturated fatty acids (omega3-PUFAs) from fish oil reduce the risk of sudden death presumably by preventing life-threatening arrhythmias. Acutely administered omega3-PUFAs modulate the activity of several cardiac ion channels, but the chronic effects of a diet enriched with fish oil leading to omega3-PUFA-incorporation into the sarcolemma on membrane currents are unknown.

METHODS

Pigs received a diet either rich in omega3-PUFAs or in omega9-fatty acids for 8 weeks. Ventricular myocytes (VMs) were isolated and used for patch-clamp studies.

RESULTS

omega3-VMs contained higher amounts of omega3-PUFAs and had a shorter action potential (AP) with a more negative plateau than control VM. In omega3 VMs, L-type Ca(2+) current (I(Ca,L)) and Na(+)-Ca(2+) exchange current (I(NCX)) were reduced by approximately 20% and 60%, respectively, and inward rectifier K(+) current (I(K1)) and slow delayed rectifier K(+) current (I(Ks)) were increased by approximately 50% and 70%, respectively, compared to control. Densities of rapid delayed rectifier K(+) current, Ca(2+)-activated Cl(-) current, and Na(+) current (I(Na)) were unchanged, although voltage-dependence of I(Na) inactivation was more negative in omega3 VMs.

CONCLUSIONS

A fish oil diet increases omega3-PUFA content in the ventricular sarcolemma, decreases I(Ca,L) and I(NCX), and increases I(K1) and I(Ks), resulting in AP shortening. Incorporation of omega3-PUFAs in the sarcolemma may have consequences for arrhythmias independent of circulating omega3-PUFAs.

Volume-activated Cl− secretion and transepithelial vinblastine secretion mediated by P-glycoprotein are not correlated in cultured human T84 intestinal epithelial layers

The relationship between the P-glycoprotein-mediated vinblastine secretion and cell-swelling activated Cl- secretion (conductance) in intact epithelial layers of human colonic adenocarcinoma T84 cells has been investigated. Whereas vinblastine secretion is effectively inhibited by 100 microM 1,9-dideoxy-forskolin, volume-stimulated Cl- secretion is unaffected. In contrast, 100 microM 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS) inhibited the volume-stimulated Cl- secretion, but was without effect upon transepithelial vinblastine secretion. In addition, it was noted that some epithelial layers failed to express a volume-stimulated Cl- secretion but maintained a normal level of secretory vinblastine flux.

Cl/HCO3 antiport affects H-lactate symport activity at the basolateral pole of jejunum enterocyte

BACKGROUND/AIMS

Under normal conditions the jejunal tract of the rat intestine absorbs HCO3. A basolateral Cl/HCO3 exchange, evidenced by means of membrane vesicles, could be involved in this process. Aim of this study was to investigate the anion exchange activity in the whole jejunal tract, where various transport systems could interact.

METHODS

In the jejunal tract of rat intestine everted and incubated in vitro, the experimental conditions set up minimized loss of CO2 from the serosal solution, where pH and pCO2 were determined together with fluid and electrolyte transintestinal transport.

RESULTS

The serosal pCO2 increase and pH decrease, evident during the experiment, could be antagonized by enhancing the 4,4-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) concentrations in the serosal fluid. Moreover high DIDS concentration affected fluid, sodium, lactate, bicarbonate and, although in the opposite direction, chloride transport, whilst they were ineffective on K flux.

CONCLUSION

These results give evidence that in the basolateral membrane the inhibition of Cl/HCO3 antiport causes a diminution of lactic acid movement. Therefore we can hypothesize that Cl/HCO3 antiport facilitates basolateral H-lactate symport in order to carry endogenous lactic acid towards the blood stream.

[Structural modification of erythrocyte membranes during oxidative stress and activity of membrane bound NADH-methemoglobin reductase]

The activity of NADH-methemoglobin reductase (metHb-reductase) in membranes isolated from human erythrocytes treated with phenylhydrazine at its sublytic concentration was studied. A decrease in the activity of membrane-bound metHb-reductase was shown to depend on the concentration of phenylhydrazine. Simultaneously, an increase in the level of membrane-bound methemoglobin and a change in the fluorescence parameters of membrane-bound 4,4'-diisothiocy-anatostilbene-2,2'-disulfonic acid were registered. In the case when Hb-free erythrocyte ghosts were treated with 0.2-2.0 mM phenylhydrazine, the activity of metHb-reductase did not change. The obtained results indicate that the inhibition of the activity of membrane-bound metHb-reductase by phenylhydrazine-induced oxidative stress in human erythrocytes is not caused by the direct action of the oxidant on the enzyme. The reason for this is the interaction of the products of hemoglobin oxidation with erythrocyte membrane (protein band 3) and structural changes in membrane proteins.

Mechanism of band 3 dimer dissociation during incubation of erythrocyte membranes at 37 degrees C

The mechanism of dissociation of the stable dimer of band 3 was investigated during the incubation of isolated erythrocyte membranes or resealed ghosts at 37 degrees C. The kinetics of changes in the structural and functional integrity of the membrane domain of band 3 (MDB3) were measured and correlated with the change in the Stokes radius of band 3. MDB3 integrity was determined as follows: (1) by measuring the fluorescence emission spectrum of 4, 4'-di-isothiocyanostilbene-2,2'-disulphonate (DIDS) bound covalently to MDB3; (2) by measuring the number of DIDS covalent binding sites present after incubation of unlabelled resealed ghosts; and (3) by measuring the anion transport V(max) by using the same resealed ghosts. Incubation of membranes at 37 degrees C caused the dissociation of band 3 dimers to monomers but only after a lag period lasting approx. 50 h. The observation of such a lag implies that dissociation involves a sequence of molecular events beginning with some type of initial process. We have discovered that this initial process involves a conformation change in MDB3. There was a shift in the fluorescence spectrum for DIDS-labelled band 3 and a decrease in the DIDS binding capacity and transport activity of the unlabelled protein. Incubation of membranes at 4 degrees C inhibited the conformational change in MDB3 and the dissociation of dimers. Furthermore, no conformational change in MDB3 was observed when erythrocytes were incubated at 37 degrees C. We suggest that MDB3 unfolding is the molecular event responsible for the subsequent dissociation of stable dimers of band 3 to monomers during the incubation of erythrocyte membranes at 37 degrees C. The monomers so generated are either not functional in anion exchange or they have an attenuated functionality. The absence of a conformational change for band 3 in erythrocytes might imply that haemolysis perturbs the membrane structure and somehow predisposes band 3 to the conformational change that occurs during incubation at 37 degrees C.

pH-dependent fusion of synaptosomal membrane studied by fluorescence quenching method

We have found that both the synaptic vesicles (SV) and synaptic plasma membrane vesicles (SPM) have an activity to fuse with phosphatidylcoline/phosphatidylserine liposomes in a pH-dependent manner. The activity increases with decreases in extravesicular pH. At a pH lower than 4.0, the activity is almost steady at its maximum value, and there was a rapid drop around pH 5.5. The pH-dependent fusion was inhibited by proteolysis with trypsin; hence, at least in part, some membrane proteins play an important role in these pH-dependent fusion processes. To find specific markers, we screened various protein modifiers and found that anion channel blockers, stilbene derivatives (DIDS and SITS) and glibenclamide, affected the fusion process. DIDS and SITS decreased the fusion activity with an IC50 of 180 and 300 microM, respectively, whereas glibenclamide, on the contrary, increased it. From the results of an autoradiogram using 3H-tagged DIDS, a 30 kDa DIDS-binding protein was identified in the synaptic plasma membrane, which is possible to be responsible for the pH-dependent fusion.

Low pH-induced hemolysis of erythrocytes is related to the entry of the acid into cytosole and oxidative stress on cellular membranes

HCl-induced lysis of mammalian erythrocytes, pretreated with DIDS, which is a specific inhibitor of the anion transport in their membranes, was markedly delayed. After acidification of a suspension of DIDS-inhibited cells, hemolysis was initiated by addition of a protonophore (Na-salicylate) at any moment chosen by will. These findings revealed that low-pH hemolysis depended on the rate of the transfer of acid equivalents into cytosole. Erythrocyte acid resistance was studied in a group of mammals and found to be inversely related to the rate of monovalent anion exchange in membranes which supported the above observations. In human erythrocytes, the critical level of cytosole acidification was found to be about pH 5.7 by measuring the acid equivalent absorbed by cells prior to hemolysis. HCl-induced hemolysis was also studied in human erythrocyte ghosts resealed with one-sixth of the initial hemoglobin content of cells. During the prelytic interval the ghosts suspended in isotonic NaCl/sucrose media shrunk, indicating an increase in ion permeability. The increase in prelytic permeability and hemolysis were strongly delayed in ghosts prepared from DIDS-treated cells, suggesting a uniform mechanism of lysing in cells and their ghosts. The prelytic increase in ion permeability was measured by the corresponding rate of ghost shrinkage and was found to be pH-dependent, with a high value below pH 3.4 and a very low one above pH 4.0. Compared to cells, the prelytic barrier impairment in ghosts had more mild character although it required greater concentration of cytosolic H+. While finally complete, hemolysis of cells was strongly delayed in the presence of catalase (500-1500 U/ml) and superoxide dismutase (200-600 U/ml) in hemolytic media. In conclusion, the acid-induced hemolysis could be associated with an oxidative injury of membranes, mainly triggered by the entry of acid equivalents into the cytosole.

High-pressure-induced hemolysis of hereditary spherocytic erythrocytes is not suppressed by DIDS labeling

Hemolytic properties of human erythrocytes in hereditary spherocytosis (HS) were examined under hydrostatic pressure or hypotonic conditions. In the hypotonic buffer, HS erythrocytes were more fragile than normal erythrocytes, and the osmotic fragility was similarly enhanced if both erythrocytes were treated with 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS), an anion transport inhibitor. On the other hand, the hemolysis of HS erythrocytes at 200 MPa was almost the same degree as that of normal cells. Upon DIDS treatment, the hemolysis of normal erythrocytes at 200 MPa was suppressed by about 35%, whereas that of HS erythrocytes was not affected. The absence of a suppressive effect of DIDS on high-pressure-induced hemolysis is likely to be HS-specific and may be a reflection of the underlying defects of band 3-cytoskeleton interactions in HS erythrocytes.

Functional evidence for a monocarboxylate transporter (MCT) in strial marginal cells and molecular evidence for MCT1 and MCT2 in stria vascularis

The transport of lactate, pyruvate and other monocarboxylates across plasma membranes of metabolically active cells such as strial marginal cells (SMC) may be important under aerobic conditions as well as under ischemic and hypoxic conditions. This study addresses the question whether SMC from the gerbil contain a membrane transport mechanism for monocarboxylates. The type of transporter was identified in functional studies by monitoring uptake of monocarboxylates into SMC through measurement of the cytosolic pH (pHi) with the pH-sensitive dye 2',7'-bis-(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Further, subtypes of the functionally identified transporter which are present in stria vascularis were identified as transcripts by cloning and sequencing the reverse-transcription polymerase chain reaction (RT-PCR) products. All functional experiments were performed under nominally HCO3--free conditions. The monocarboxylates acetate and pyruvate (each 20 mM) induced an acidification of pHi. In contrast, the dicarboxylate malonate (20 mM) had no significant effect on pHi. Alpha-cyano-4-hydroxycinnamate (CHC; 5 mM), a blocker of H+/monocarboxylate cotransporter (MCT), reduced reversibly the acidification induced by 5 mM pyruvate. In contrast, 1 microM DIDS, a blocker of band-3 protein, had no significant effect on the acidification induced by 20 mM acetate. The presence of the transcripts for each of the MCT subtypes, MCT1 and MCT2, was determined by RT-PCR of stria vascularis from gerbil. RT-PCR performed with primers for the MCT1 and MCT2 subtypes on total RNA from stria vascularis revealed PCR products of the predicted sizes. Sequence analysis confirmed that amplified MCT1 and MCT2 cDNA fragments encoded a nucleotide sequence of MCT1 and MCT2, respectively. These observations suggest that SMC contain a MCT and that stria vascularis contains RNA for the subtypes MCT1 and MCT2 subtypes.

Regulation of intracellular pH in proximal tubules of avian loopless reptilian-type nephrons

In proximal tubules isolated from chicken superficial loopless reptilian-type nephrons, intracellular pH (pHi), measured with pH-sensitive fluorescent dye 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein, was approximately 7.1-7.2 under control conditions (N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid-buffered medium with pH 7.4 at 37 degrees C), and was reduced to approximately 6.9 in response to NH4Cl pulse. The rate of recovery of pHi (control value approximately equal to 5 x 10(-3) pH U/s) from this acid level was 1) significantly decreased by removal of Na+ or both Na+ and Cl- from the bath or addition of 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (0.25 mM) to the bath, 2) significantly increased by high bath K+ (75 mM), and 3) unchanged by removal of Cl- alone from the bath or addition of ethylisopropylamiloride (1 mM) or Ba2+ (5 mM) to the bath. Resting pHi was 1) significantly decreased by Na+ or simultaneous Na+ and Cl- removal, 2) significantly increased by high K+, and 3) unchanged by Cl- removal alone or addition of Ba2+. The data do not fit the concept of pHi regulation by the most commonly suggested basolateral transporters (Na+/H+ exchanger, Na(+)-dependent and Na(+)-independent Cl-/HCO3- exchangers, or Na(+)-HCO3(-)-CO3(2-) cotransporter).

Lys515-Lys492 cross-linking by DIDS interferes with substrate utilization by the sarcoplasmic reticulum ATPase

Sarcoplasmic reticulum (SR) Ca2+ ATPase was derivatized with 4,4'-diisothiocyanatostilbene-2,2'-sulfonic acid (DIDS), and complete enzyme inactivation was produced with a molecular stoichiometry of one DIDS per ATPase. It was determined by peptide analysis and sequencing that Lys492 and Lys515 were the ATPase residues derivatized by DIDS. Lack of electrophoretic resolution of the two peptide fragments that result from a single tryptic cut at Arg505 demonstrated that the two derivatized residues were cross-linked. Cross-linking of Lys492 and Lys515 by DIDS interfered with ATPase utilization of both ATP and p-nitrophenyl phosphate substrates, whereas derivatization of only Lys515 with fluorescein isothiocyanate interfered with ATPase utilization of ATP but not of p-nitrophenyl phosphate. Cross-linking with DIDS implies a distance of approximately 13 A between Lys492 and Lys515, which corresponds to the length of ATP bound in an extended configuration. Therefore, within the groove of the nucleotide binding domain, the ATP substrate is positioned with the adenosine moiety near Lys515 and its terminal phosphate near Lys492.

Lysosomal destabilization via increased potassium ion permeability following photodamage

Isotonic K2SO4 solution protected lysosomes osmotically during a 20 min incubation, but lost its protective effect if the lysosomes were initially photooxidized after sensitization with Methylene blue. Increasing K2SO4 concentration promoted the latency loss of photodamaged lysosomes, but did not impair the integrity of unirradiated lysosomes. The results indicate that the photodamage enhanced lysosomal ionic permeability, with osmotic imbalance over the lysosomal membrane. Out of the decreased latency induced by the photodamage, 32% was prevented by the addition of 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid to the incubation solution, suggesting that electroneutral K+/SO4(2-) co-uptake plays a role in the lysosomal destabilization. The photooxidation increased lysosomal H+/K+ exchange, which was confirmed by monitoring the H+ leakage with the pH sensitive probe p-nitrophenol and examining the K+ entry by membrane potential measurements. Addition of K2SO4 to a lysosomal suspension lowered the delta pH of photodamaged lysosomes, presumably due to an increase in the exchange of internal H+ for external K+. Out of the photodamage-induced lysosomal latency loss, 50-60% was prevented by either lowering the external pH or preincubating the lysosomes with methylamine to elevate their internal pH. The results suggest that the photodamage-promoted K+/H+ exchange plays a major role in lysosomal osmotic destabilization.

Human erythrocyte metabolism is modulated by the O2-linked transition of hemoglobin

The metabolic behaviour of human erythrocytes has been investigated with particular regard to the effect of their oxygenation state. Experiments performed at high phosphate concentration (80 mM) within the pH range 7.0-7.8 on erythrocytes at high (HOS) and low (LOS) oxygen saturation showed that at any pH value: (1) glucose consumption was independent of the oxygenation state; (2) pentose phosphate pathway (PPP) flux was about 2 times higher in the HOS than in the LOS state. At low phosphate concentration (1.0 mM) the PPP flux doubled in HOS as well as in LOS erythrocytes, whereas the decrease in glucose consumption was more marked in the HOS state. Metabolism of LOS erythrocytes approached that of HOS erythrocytes under the following conditions: (1) erythrocytes having band 3 modified by 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid; (2) CO-saturated erythrocytes. These data support the hypothesis of a modulation of the relative rates of PPP and glycolysis achieved through competition between deoxy-hemoglobin (deoxy-Hb) and glycolytic enzymes for the cytoplasmic domain of band 3.

Different regulation by pHi and osmolarity of the rabbit ileum brush-border and parietal cell basolateral anion exchanger

1. The purpose of this study was to look for evidence of a pH-sensitive modifier site on the parietal cell basolateral anion exchanger, determine the pH range in which allosteric regulation takes place, investigate the effect of the osmolarity on internal pH (pHi) dependence and compare it with that of the ileum brush-border anion exchanger. 2. When the pHi in parietal cell basolateral membrane (BLM) vesicles was increased, the rate of Cl(-)-gradient-driven 36Cl- uptake increased from 6.03 +/- 2.24 to 38.09 +/- 3.33 nmol (mg protein)-1 with the steep increase in anion exchange rates occurring within a narrow pH range between pHi 7.0 and 7.5. This was due to allosteric activation by internal OH- and not due to a change in driving force, since the driving force for maximal exchange rates was provided by the outwardly directed Cl- gradient. 3. The pHi dependency curve of parietal cell BLM anion exchange rates was shifted to the left by 0.25 pH units by increasing the osmolarity of the intra- and extravesicular solutions from 300 to 380 mosmol l-1. Thus cell shrinking may activate the parietal cell anion exchanger without a change in pHi and without phosphorylation of the anion exchanger protein. 4. In ileum brush-border membranes, the pHi-dependent increase in the rate of Cl(-)-gradient-driven 36Cl- uptake was more gradual and the half-maximal anion exchange rate was attained at lower pHi (pH 6.5). Increasing the osmolarity from 300 to 500 mosmol l-1 had no effect on pH dependence. 5. We conclude that the parietal cell basolateral and ileum brush-border anion exchangers possess an internal modifier site for allosteric activation by OH-, but the pH range in which allosteric regulation occurs differs between the two exchangers, as does the effect of an increase in osmolarity. Since current evidence suggests that both the parietal cell basolateral and the ileum brush-border anion exchanger are encoded by the AE2 gene, the differences in pHi dependence between the two may be due to alternative splicing, post-transcriptional modification, or the different membrane environment. 6. The pHi range for allosteric activation found in this study would suggest that for both the ileum and the parietal cell anion exchanger, but especially for the latter, a potentiating effect of the allosteric activation and the HCO3- availability occurs within the physiological pHi range and can cause dramatic increases in maximal anion exchange rates with increasing pHi.

Identification of cone classes in Xenopus retina by immunocytochemistry and staining with lectins and vital dyes

The aim of the present study was to determine the number of cone classes in the Xenopus retina. We examined the dimensions and staining properties of cones, utilizing two monoclonal antibodies, COS-1 and OS-2, developed by Szel and Rohlich (1985). Living cones also were reacted with the plant lectins peanut agglutinin (PNA) and wheat germ agglutinin (WGA) and with a fluorescent stilbene dye, DIDS, which binds selectively to red-sensitive cones (Kleinschmidt, 1991; Kleinschmidt & Harosi, 1992a,b). Three cone populations were distinguished based on differences in size and staining properties. Eighty-eight percent of all cones were stained strongly by COS-1, PNA, and DIDS, but weakly by OS-2. The group of cones stained by COS-1 had the largest mean dimensions of outer segment length, width, and oil droplet diameter. COS-1 negative cones were divisible into two groups: a subclass of miniature cones (approximately 4% total cones) was stained strongly by OS-2, PNA, and DIDS. The balance, constituting approximately 9% total cones, were of intermediate size, were not stained by PNA and reacted weakly to OS-2 and DIDS. WGA stained all cones. Large, COS-1+ cones appear to be red-sensitive and belong to the class of anion-tunable cone pigments. We suggest that the intermediate size, COS-1 negative cones are blue-sensitive based on the finding that blue-sensitive chromatic horizontal cells connect to them preferentially (Witkovsky et al., work in progress).(ABSTRACT TRUNCATED AT 250 WORDS)

N-terminal protein sequence analysis of the rabbit erythrocyte lactate transporter suggests identity with the cloned monocarboxylate transport protein MCT1

An improved purification for the rabbit erythrocyte lactate transporter, using aminoethyl-Sepharose chromatography, is described. The process of purification of the 40-50 kDa transporter, labelled with 4,4'-diisothiocyanostilbene-2,2'-disulphonate (DIDS), was followed by Western blotting with anti-DIDS antibodies [Poole, R. C. and Halestrap, A. P. (1992) Biochem. J. 283, 855-862]. Fractions highly-enriched in transporter were further purified by SDS/PAGE and the 40-50 kDa DIDS-labelled polypeptide was subjected to N-terminal protein sequencing. This analysis identified the first 16 amino acids of the protein. With the exception of one conservative substitution, this protein sequence is identical to the N-terminal protein sequence predicted from a cDNA isolated from Chinese hamster ovary cells that encode a monocarboxylate transporter, MCT1 [Kim Garcia, C., Goldstein, J. L., Pathak, R. K., Anderson, R. G. W. and Brown, M. S. (1994) Cell 76, 865-873]. This observation, along with similarities in functional properties, leads us to conclude that lactate transport in rabbit erythrocytes is mediated by the MCT1 monocarboxylate transporter isoform.

Reconstitution, identification, and purification of the Torpedo californica electroplax chloride channel complex

A voltage-gated chloride channel was identified in the electric organ of the marine ray Torpedo californica by White and Miller (J. Biol. Chem. 254, 10161-10166 (1979)). The experiments reported here concern the purification and identification of this channel which was accomplished by solubilization of electric organ plasma membranes and reconstitution of the channel into vesicles made of phosphatidylethanolamine, phosphatidylserine, and cholesterol. Channel activity was measured in these vesicles by assaying 36Cl- uptake against an outwardly directed chloride chemical gradient as described by Garty et al. (J. Biol. Chem. 258, 13094-13099 (1983)). Maximal uptake occurred by 15 s. Addition of valinomycin after 10 min released intravesicular 36Cl- suggesting that chloride is moving through a channel. Channel activity was inhibited by DIDS (K0.5 of 56 mM) and NBD chloride (K0.5 of 176 mM). In a 40 lipid/1 protein (w/w) reconstitution, approx. 30% of the vesicles contained a functional chloride channel, based upon uptake done in the presence of chlorotriphenyltin (an anion ionophore), indicating that the Torpedo electric organ is an enriched source as shown by White and Miller (Biophys. J. 35, 455-462 (1981)). The chloride channel was purified approx. 40-fold by sedimentation velocity. In this purified preparation, four polypeptides (210, 95, 55, and 40 kDa) were visible by silver-staining after nonreducing SDS-PAGE. Of the four polypeptides, the largest (210 kDa) is not sufficient for Cl- channel activity by itself, but it is labeled by DIDS, an inhibitor of channel activity. Channel activity was approx. 20-fold greater in material that bound to concanavalin A compared to the concanavalin A flow-through; all four polypeptides were present in the bound materia. It is possible that some of these polypeptides are subunits of the chloride channel.

Band 3, the anion exchanger of the erythrocyte membrane, is also a flippase

The transbilayer reorientation (flip-flop) of the long-chain amphiphilic anion DENSA (5-(N-decyl)aminonaphthalene-2-sulfonic acid) in the erythrocyte membrane was studied by fluorescence spectroscopy. DENSA intercalates into the membrane at a high membrane/water partition coefficient (3.2.10(5)) and rapidly reorients from the outer to the inner layer in a first order process (k = 0.11 min-1, 37 degrees C, pH 7.4) leading to a steady-state distribution inner:outer layer of about 30:70. The activation energy of the fully reversible and symmetric flip process is about 110 kJ/mol. DIDS and various other established covalent and non-covalent inhibitors of anion transport via the erythrocyte anion exchanger, band 3 (AE 1), suppress the flip to a minimum of about 30-35% of the control. The flip is also inhibited by Cl- with a half maximal inhibitory concentration equal to that required for the inhibition of the exchange flux of ordinary anions via band 3. These findings indicate the involvement of a band 3 mediated (DIDS-sensitive) component of the flip and a DIDS-insensitive one, possibly involving, at least to some extent, simple transbilayer 'diffusion'. This latter component is stimulated by diamide, an SH oxidant known to increase the permeability of the membrane lipid domain of the erythrocyte. Alcohols (butanol, hexanol) accelerate both flip components. Papain treatment, known to inhibit 'ordinary' anion exchange, accelerates both flip and flop. The results suggest that band 3 protein, besides being a conventional transporter of anions, can act as a flippase translocating anionic, membrane-intercalated amphiphiles approaching the transporter from the lipid domain. The flippase mode of operation of band 3 must, however, differ in its mechanism from the conventional exchange mode.

Properties of osmolyte fluxes activated during regulatory volume decrease in cultured cerebellar granule neurons

Efflux pathways for amino acids, K, and Cl activated during regulatory volume decrease (RVD) were characterized in cultured cerebellar granule neurons exposed to hyposmotic conditions. Results of this study favor diffusion pores (presumably channels) over energy-dependent transporters as the mechanisms responsible for the efflux of these osmolytes. The selectivity of osmolyte pathways activated by RVD was assessed by increasing the extracellular concentrations of cations, anions, and amino acids to such an extent that upon opening of the pathway, a permeable compound will enter the cell and block RVD by reducing the efflux of water carried by the exit of intracellular osmolytes. The cationic pathway was found selective for K (and Rb), whereas the anionic pathway was rather unselective being permeable to Cl, nitrate, iodine, benzoate, thiocyanate, and sulfate but impermeable to gluconate. Glutamate and aspartate as K but not as Na salts were permeable through the anion channel. RVD was slightly inhibited by quinidine but otherwise was insensitive to known K channel blockers. RVD was inhibited by 4,4'-diisothiocyanostilbene-2-2'-disulfonic acid (DIDS), niflumic acid, and dipyridamole. Gramicidin did not affect cell volume in isosmotic conditions but greatly accelerated RVD, suggesting that cell permeability to Cl is low in isosmotic conditions but increases markedly during RVD making K permeability the rate limit of the process. The permeability pathway for amino acids activated during RVD as permeable to short chain alpha- and beta-amino acids, but excluded glutamine and basic amino acids.

Effects of cross-linking of membrane proteins on vesiculation induced by dimyristoylphosphatidylcholine in human erythrocytes

To study the effect of cross-linking of membrane proteins on vesiculation of human erythrocytes by dimyristoylphosphatidylcholine (DMPC), red cells were treated with diamide at atmospheric pressure or 100 MPa and then incubated with DMPC in buffers of pH 6.5-8.5. Irrespective of buffer pH, the amount of released vesicles increased upon cross-linking of membrane proteins but approached the control level upon reduction of the cross-linking by dithiothreitol. Similar enhancement of vesicle release was also observed in N-ethylmaleimide-treated red cells. Hemolysis during vesiculation was observed only in red cells treated with diamide at 100 MPa. Furthermore, the composition of membrane proteins in released vesicles was analyzed by SDS-PAGE. Membrane vesicles released from intact red cells or the cells treated with diamide at atmospheric pressure contained band 3 as a major membrane protein. On the other hand, membrane vesicles from red cells treated with diamide at 100 MPa contained protein 4.1 in addition to band 3 and the orientation of these proteins was similar to that in intact cells. These results indicate that the amount and membrane protein composition of DMPC-induced vesicles are much affected by chemical modification of SH-groups in red cell membrane proteins.

The proton Bohr factor of native and crosslinker treated hemoglobins--its possible significance for the efficacy of hemoglobin based artificial oxygen carriers

Especially the (alkaline) proton Bohr effect seems to provide an important self regulating mechanism of the organism to deliver specifically oxygen into tissues suffering from O2 deficit. In this way these tissues switch from aerobic to anaerobic metabolism, get lactacid, thereby shifting oxygen hemoglobin binding curve to the right and thus facilitating the oxygen release. The higher the absolute value of the proton Bohr factor (: delta logP50/ delta pH) is the better this mechanism works. To get one characteristic number the proton Bohr factor at pH 7.1 is taken. This pH in blood is about a lower limit for organism and human blood has at this pH its maximum proton Bohr factor which is about -0.5. When designing a hemoglobin based artificial oxygen carrier such a high or even a higher proton Bohr factor should be aimed at. But bringing human hemoglobin into extracellular milieu decreases the said proton Bohr effect down to -0.31; about the same values have bovine and porcine hemoglobin under these conditions. Before native hemoglobin can be used as an artificial oxygen carrier outside the red blood cells, they must be crosslinked; otherwise they are cleared quickly by the kidneys. Reaction of human and bovine hemoglobin with the crosslinkers DIBS (2,5- diisothiocyanatobenzenesulfonate) and DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulfonate) decreases the proton Bohr effect once again substantially down to about -0.1 irrespective of the degree of polymerization (monomer and hyperpolymer). Proton Bohr factors of reaction products from various hemoglobins and different crosslinkers evaluated from measurements of other investigators largely confirm the findings of this study.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of the polymerization step alone on oxygen affinity and cooperativity during production of hyperpolymers from native hemoglobins with crosslinkers

The aim of this study was to find out how the polymerization per se changes oxygen affinity (P50) and cooperativity (n50) of various soluble huge hyperpolymers prepared from native hemoglobins by crosslinking. Increase of cooperativity would be expected considering natural hemoglobin networks. Those hyperpolymers with molecular weights of some 10(6) g/mol are candidates for artificial oxygen-carrying blood additives rather than volume substitutes. Human and bovine hemoglobin reacted with several crosslinkers (2,5-diisothiocyanatobenzenesulfonate (DIBS); 4,4'-diisothiocyanatostilbene-2, 2'-disulfonate (DIDS); 1,3-butadiene diepoxide (BUDE); glutaraldehyde (GDA)) in concentrated (case 1) and diluted (case 2) hemoglobin solutions. With high concentration hyperpolymer and with low concentration only monomer products were obtained. P50 and n50 of the products were determined at pH = 7.4, PCO2 = 40 mmHg, temp. = 37 degrees C. The difference of properties in both cases are regarded as the influence of polymerization per se. Considering this difference we found with almost all combinations of hemoglobin and crosslinker an increase of O2 affinity, with DIBS and DIDS cooperativity was not changed and with BUDE and GDA it was decreased. As compared with native hemoglobin loss of cooperativity is considerable in any combination and condition, but comparing human and bovine hemoglobin the first seems to maintain better cooperativity. In contrast bovine hemoglobin as compared with human hemoglobin maintains better or even decreases its O2 affinity upon reaction with the crosslinkers forming both, monomer and hyperpolymer products, especially in the deoxy state. DIBS and DIDS react very similarly.(ABSTRACT TRUNCATED AT 250 WORDS)

Chloride transport by human placental microvillous membrane vesicles

Unidirectional uptake of chloride by microvillous membrane vesicles prepared from human term placentas was studied over a range of membrane potentials in the presence and absence of chloride transport inhibitors alone and in combination at maximally effective concentrations. At 0 mV, inhibition of chloride uptake by 0.1 mM DIDS, 0.5 mM DPC, and 0.5 mM DPC plus 0.1 mM DIDS was similar, suggesting a common action upon an anion exchanger; neither 0.1 mM furosemide nor 0.1 mM bumetanide alone had any effect. An inside-positive membrane potential was created by imposing an inwardly directed potassium ion gradient in the presence of valinomycin. Total chloride uptake increased with increasing membrane potential (0, 4.6, 17.3, 25.8 and 32.0 mV). The inhibition of uptake by DPC and DPC/DIDS increased with the membrane potential. The effect of DPC compared to DPC/DIDS was significantly different at 4.6, 17.3 and 25.8 mV, suggesting a degree of additivity of inhibition. Neither furosemide nor bumetanide had any effect at any potential. There was a significant increase in inhibition due to DIDS alone until the membrane potential reached 25.8 mV. But there was no significant difference between the level of inhibition at 32 mV as compared to 0 mV, providing evidence of a DIDS-sensitive conductance similar to that previously seen in patch clamp studies. We suggest that uptake of chloride across the microvillous membrane of the human placenta may be by at least three different pathways; an electroneutral, DIDS-sensitive anion exchanger, a DPC-sensitive chloride conductance and a DIDS-sensitive chloride conductance.

Basolateral membrane Cl- and K+ conductances of the dark-adapted chick retinal pigment epithelium

1. We characterized the basolateral membrane Cl- and K+ conductances of the dark-adapted chick neural retina-retinal pigment epithelium (RPE)-choroid preparation. Conventional microelectrodes were used to measure apical (V(ap)) and basolateral (Vba) membrane voltage, and double-barreled Cl- and K+ selective microelectrodes were used to follow the time course and magnitude of ion concentration changes outside the basolateral (basal) membrane. 2. In response to a fivefold decrease in basal [Cl-]o, Vba rapidly depolarized by 6.4 +/- 0.7 (SE) mV, and the apparent resistance of the basolateral membrane (Rba) increased. The Cl- channel blocker 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS) suppressed the Vba depolarization by 40% and blocked the Rba increase. Estimates of the relative Cl- conductance (transference number, TCl) from the DIDS-sensitive component of the Cl- diffusion potential gave an average value for TCl of 0.22 +/- 0.03. 3. Further evidence for a Cl- conductance was obtained by measuring changes in intracellular Cl- activity (aCli) induced by transtissue current. Depolarizing Vba elevated aiCl, whereas hyperpolarizing Vba had the opposite effect, consistent with conductive Cl- movement across the basal membrane. TCl estimated from these data averaged 0.23 +/- 0.02. 4. In response to a sixfold increase in basal [K+]o, Vba depolarized 6.1 +/- 0.8 mV. The amplitude of this K+ diffusion potential was inhibited 44 and 67% by 5 and 10 mM Ba2+, respectively. TK was estimated to be 0.61 +/- 0.05. 5. The rapid c-wave membrane hyperpolarizations in response to the light-evoked decrease in subretinal [K+]o were used to calculate the equivalent resistances of the apical membrane (R(ap)), basolateral membrane (Rba), and the paracellular shunt pathway (Rs). They were 152 +/- 10, 615 +/- 38, and 138 +/- 7 omega.cm2 (n = 11 tissues), respectively. From these data the equivalent electromotive force for the basal (Eba) and apical (Eap) membranes were estimated to be -45 +/- 2 and -77 +/- 1 mV, respectively. This estimate of Eba is in the range of that predicted from our estimates of TCl and TK, indicating that, in the dark-adapted chick retina, the resting conductance of the basal membrane can largely be accounted for by the Cl- and K+ conductances described here.

Functional consequences of heterologous expression of the cystic fibrosis transmembrane conductance regulator in fibroblasts

We studied the consequences of cystic fibrosis transmembrane conductance regulator (CFTR) expression in NIH-3T3 fibroblasts as a model for the effects of virally transduced CFTR expression in non-epithelial cells. Fibroblasts were infected with a retrovirus vector that contained the human CFTR and neor cDNAs. We selected and expanded G418-resistant clones that encompassed a range of CFTR expression. CFTR-mediated Cl-conductance function was measured as whole cell current, and CFTR protein was quantitated by immunoblot analysis. Overall, there was a good relationship between CFTR protein levels and CFTR-mediated Cl- conductance. Some clones had consistently high basal levels of CFTR-mediated Cl- conductance. This variation in function was partially explained by CFTR protein levels and was not due to clonal variation in cAMP metabolism. High levels of CFTR expression were associated with depolarization of fibroblast membrane potential. The CFTR-expressing clones with the largest basally active CFTR Cl- conductances and the most depolarized membrane potentials also exhibited slower growth rates. These results suggest that potential side effects of gene replacement therapy for cystic fibrosis include functional consequences of CFTR expression in non-epithelial cells.

Bicarbonate-stimulated [14C]butyrate uptake in basolateral membrane vesicles of rat distal colon

BACKGROUND

The mechanism of short-chain fatty acid (SCFA) absorption by the colon is not known. The aim of these experiments was to identify the transport mechanisms present in the basolateral membrane to develop an overall model of colonic SCFA absorption.

METHODS

These studies determined the uptake of [14C]butyrate (used as a model SCFA) by basolateral membrane vesicles prepared from rat distal colonic mucosa.

RESULTS

Significantly higher [14C]butyrate uptake under an acidic environment (extravesicular pH [pHo] = intravesicular pH [pHi] = 5.5) than that under alkaline environment (pHo = pHi = 7.5) indicates the presence of nonionic diffusion. In the absence of a pH gradient (pHo/pHi = 7.5/7.5), outward gradients of bicarbonate significantly stimulated [14C]butyrate uptake. Additional presence of a pH gradient (pHo/pHi = 6.0/7.5) further enhanced the bicarbonate gradient-stimulated [14C]butyrate uptake that was not inhibited by voltage clamping but was inhibited substantially by an anion exchange inhibitor 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) (82%). Both increasing extravesicular butyrate concentration and intravesicular bicarbonate concentration saturated bicarbonate/pH gradient-stimulated [14C]butyrate uptake with an apparent Michaelis constant (Km) for butyrate of 6.9 mmol/L and an apparent Km for bicarbonate of 27.4 mmol/L.

CONCLUSIONS

Butyrate uptake by basolateral membrane vesicles represents both nonionic diffusion and a carrier-mediated SCFA-bicarbonate exchange process that differs from the SCFA-bicarbonate exchange recently identified in apical membrane vesicles. Thus, two distinct carrier-mediated anion exchange processes located in apical and basolateral membranes mediate transcellular SCFA transport in colonocytes.

Lysis of human monocytic leukemia cells by extracellular adenosine triphosphate: mechanism and characterization of the adenosine triphosphate receptor

The present study shows that extracellular adenosine triphosphate (ATP) has the capacity to mediate dose-dependent lysis of the monocytic leukemia cell line THP-1. The lysis, assessed by 51Cr release, was found to be selective for ATP, because adenosine diphosphate (ADP) or other nucleotides were less effective in their ability to lyse the cells. The amount of 51Cr released was particularly enhanced by the stimulation of the cells with 1,000 U/mL of interferon gamma (IFN-gamma) for 3 days, and the sensitivity was time and dose dependent. Analysis of the mechanism of lysis indicated that the fully ionized form, ATP4-, mediated the lysis, because the addition of cation chelators or the absence of the divalent cations, Ca2+ and Mg2+, in the culture medium of a 6-hour 51Cr release assay increased the percent specific lysis. Therefore, the ATP receptors on THP-1 cells were classified as P2Z purinoceptors. Moreover, it is shown here that the Ca2+/calmodulin complex plays a role in the regulation of the lysis by extracellular ATP of THP-1 cells, because antagonists of this complex, such as trifluoperazine or KN-62, were found to inhibit the ATP-mediated cell lysis.

DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid) inhibits an early step of protein translocation across the mammalian ER membrane

Protein translocation across the endoplasmic reticulum (ER) membrane of yeast can be inhibited by agents believed to specifically affect the transport of ATP through the membrane (Mayinger, P. and Meyer, D.I. (1993) EMBO J. 12, 659-666), suggesting the involvement of a translocation component in the lumen of the ER that binds ATP. We demonstrate that one of the inhibitors, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), also affects the translocation of proteins into mammalian microsomes. Translocation is blocked at the point of transfer of the nascent chain from the signal recognition particle (SRP) into the ER-membrane. We also confirm that photoaffinity-labelling of microsomes with 8-azido-ATP inhibits the same early step of protein translocation. Since this step is reported to not require ATP, these results raise the possibility that, in both cases, factor(s) other than ATP-binding components of the translocation machinery are perturbed.

Identification of the putative hexose-phosphate translocator of amyloplasts from cauliflower buds

Starch synthesis in amyloplasts isolated from cauliflower buds is strongly inhibited by the addition of micromolar concentrations of 4,4'-di-isothiocyano-2,2'-stilbenedisulphonic acid (DIDS). Using [3H]DIDS it was possible to label specifically a 31.6 kDa membrane protein of the envelope fraction of isolated amyloplasts. The intensity of the radioactive label was decreased in the presence of glucose 6-phosphate or dihydroxyacetone phosphate, indicating that this protein might be the amyloplastic hexosephosphate translocator.

Intracellular alkalinization stimulates bile flow and vesicular-mediated exocytosis in IPRL

Intracellular pH recovery from an acute alkaline load in rat hepatocytes is mediated by a Cl(-)-HCO3- exchanger, which is electroneutral, Na+ independent, and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) sensitive. Stimulation of this Cl(-)-HCO3- exchanger requires intact microtubules, suggesting that vesicular transport may be required to activate this exchanger. To determine if intracellular alkalinization stimulates biliary HCO3- excretion and bile flow in the intact liver by vesicle-mediated exocytosis, isolated perfused rat livers (IPRL) were alkalinized by two protocols. Isohydric changes in CO2 and HCO3- concentrations induced transient increases in bile flow by 36% (P < 0.01), which were abolished by DIDS (0.01 mM), inhibited by pretreatment with colchicine (P = 0.01), but not affected by membrane depolarization with the K(+)-channel blocker BaCl2 (1 mM). Similarly, perfusion with 20 mM NH4Cl produced a 42% increase in bile flow (P < 0.01) and a 26% increase in biliary HCO3- excretion. Both the increases in bile flow and HCO3- excretion were almost completely blocked by DIDS and inhibited by pretreatment with colchicine (P < 0.01). Biliary excretion of horseradish peroxidase was also increased during intracellular alkalinization with either protocol (P < 0.01). These findings suggest that intracellular alkalinization stimulates bile flow and biliary HCO3- excretion. Microtubule-dependent vesicular-mediated exocytosis is involved in this response.

NH4+ metabolism and the intracellular pH in isolated perfused rat liver

The effects of NH4+ on the intracellular pH (pHi) and on the ATP content in isolated perfused rat liver were studied by 31P n.m.r. spectroscopy. In the initial phase of perfusion an average pHi of 7.29 +/- 0.04 was estimated. The presence of low (0.5 mmol/l) and high (10 mmol/l) doses of NH4Cl induced significant intracellular acidification by -0.06 +/- 0.03 and -0.11 +/- 0.03 pH unit respectively. This effect was in contrast with the transient intracellular alkalinization observed in preliminary studies on isolated hepatocytes, which was caused by a passive entry of NH3 by non-ionic diffusion and subsequent conversion into NH4+. During application of 0.5 mmol/l NH4Cl the liver released 0.54 +/- 0.06 mumol of urea/min per g into the perfusate. When the intracellular availability of HCO3- was decreased by acetazolamide (0.5 mmol/l) or by removal of HCO3- from the perfusion medium, the decrease in pHi by NH4Cl application was significantly lower than under control conditions. Furthermore, synthesis of urea was significantly inhibited by the decrease in intracellular HCO3-. Under these conditions, 10 mmol/l NH4Cl caused the transient alkalinization that was expected because of the passive uptake of uncharged NH3. Therefore, it is concluded that the intracellular acidification induced by NH4Cl is caused by the continuous utilization of intracellular HCO3- via the synthesis of urea. This metabolic effect on pHi dominates the effects of passive NH3 entry. The rate of urea formation depends on continuous efflux of H+, which is strictly limiting the degree of intracellular acidification within a small range. If the extrusion of H+ by the Na+/H+ exchanger was inhibited by amiloride (0.5 mmol/l) during the NH4Cl application, the decrease in pHi was amplified and the formation of urea was significantly inhibited. The application of NH4Cl at 0.5 or 10 mmol/l decreased the ATP content by 11% or 22% respectively.

Kinetics of conformational changes associated with inhibitor binding to the purified band 3 transporter. Direct observation of allosteric subunit interactions

Subunit interaction effects were identified for isolated human erythrocyte band 3, the anion exchanger, by observing both static and stopped-flow kinetic protein fluorescence changes associated with inhibitor binding to the intramonomeric stilbenedisulfonate site. We measured the rate of conformational changes associated with reversible binding of H2DIDS (4,4'-diisothiocyanodihydrostilbene-2,2'-disulfonate). The rate of H2DIDS release was also measured. As a test for subunit interactions, we studied the effect of partial labeling of the band 3 monomer population with H2DIDS on the equilibrium and kinetics of H2DIDS reversible binding to the remaining monomers. The results showed biphasic kinetics for control band 3, with a pseudo-first-order ligand dependence for the fast phase followed by a slow ligand-independent relaxation. A second-order "on" rate constant for the fast phase was determined to be (1.2 +/- 0.1) x 10(7) M-1 s-1, while the associated "off" rate constant was found to be 1.1 +/- 0.5 s-1. From these kinetic constants, we calculated a Kd value of 95 +/- 50 nM, which is in excellent agreement with the Kd value determined at thermodynamic equilibrium (110 +/- 9 nM). Covalent labeling of 75% of the band 3 monomer population with H2DIDS changed the kinetics of the fast phase, slowing the apparent rate by changing the order of the reaction from pseudo-first-order to zero-order. Partial labeling did not affect the ligand-independent relaxation. Separate measurements of the H2DIDS "off" rate also showed a biphasic time course, with a 20-fold difference in apparent rate constants.(ABSTRACT TRUNCATED AT 250 WORDS)

Band 3 HT, a human red-cell variant associated with acanthocytosis and increased anion transport, carries the mutation Pro-868-->Leu in the membrane domain of band 3

1. We have studied band 3 HT, a human red-cell band 3 variant with increased M(r), which is associated with abnormal red-cell shape (acanthocytosis) and increased anion-transport activity. 2. We have shown that the increased M(r) does not result from the presence of the band 3 Memphis mutation, and that the variant band 3 is covalently labelled by 4,4'-di-isothiocyanato-1,2-diphenylethane-2,2'-disulphonic acid (H2DIDS) less readily than normal. 3. cDNA cloning studies show that band 3 HT results from the mutation Pro-868-->Leu, and the possible significance of the mutation in the altered anion-transport activity and cytoskeleton binding properties of band 3 HT is discussed.

4,4'-Di-isothiocyanatostilbene-2,2'-disulphonic acid ('DIDS') activates protein kinase C and Na+/H+ exchange in human platelets via alpha 2A-adrenergic receptors

Most agonists stimulate platelets by inducing Ca2+ mobilization, Ca2+ influx and protein kinase C (PKC) activation leading to Na+/H+ exchange, exposure of fibrinogen-binding sites and aggregation. In contrast, previous studies showed that adrenaline induces exposure of fibrinogen-binding sites and aggregation without appreciable changes in cystolic Ca2+ content or PKC activity. In the present study we investigated platelet responses mediated via alpha 2A-adrenergic receptors, using 4,4'-di-isothiocyanatostilbene-2,2'-disulphonic acid (DIDS), which is known to bind to this type of receptor. Addition of DIDS (2-20 microM) induced (i) a rise in cytosolic pH of 0.23 +/- 0.05 pH unit (n = 5) as detected by BCECF fluorescence, due to activation of the Na+/H+ exchanger, (ii) a 3.5-4-fold increase in the phosphorylation of the 47 kDa protein, a major substrate of PKC, (iii) exposure of 81,072 +/- 7293 (n = 3) binding sites for 125I-fibrinogen per platelet, and (iv) irreversible aggregation. These responses occurred without changes in cytosolic [Ca2+], secretion of dense-granule contents and enhanced phosphoinositide metabolism, and were not affected by inhibition of thromboxane A2 generation (30 microM indomethacin). The alpha 2A-adrenergic-receptor antagonists oxymetazoline (500 microM) and yohimbine (1 mM) completely abolished DIDS-induced responses. Inhibition of PKC (1 microM staurosporine) prevented phosphorylation of the 47 kDa protein, the increase in Na+/H+ exchange and exposure of fibrinogen-binding sites. Thus our present data suggest that activation of PKC is an early event in DIDS-induced platelet activation via the alpha 2A-adrenergic receptor, which precedes any of the other known signal-transducing sequences.

Ionic mechanisms underlying the depolarizing and hyperpolarizing afterpotentials of single spike in guinea-pig cingulate cortical neurons

Intracellular recordings and hybrid single-microelectrode voltage-clamp techniques were used to study the ionic mechanisms underlying the afterdepolarization and the subsequent slow afterhyperpolarization that followed a single action potential in layers V/VI neurons of the guinea-pig anterior cingulate cortex in in vitro slices. Both the afterdepolarization and afterhyperpolarization were markedly suppressed in size by addition of Co2+ or Cd2+, reduction in extracellular Ca2+, and intracellular EGTA injection. On the other hand, elevation of extracellular Ca2+ concentration augmented the amplitudes of the afterpotentials. The afterdepolarization amplitude was selectively depressed by the stilbene derivatives, 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonate, disodium 3H2O, and 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid, disodium salt. Reduction in external Cl- and intracellular Cl- injection enhanced the afterdepolarization amplitude without affecting the afterhyperpolarization. The null potentials for the afterdepolarizations recorded with K acetate- and Cs acetate-electrodes were -68 and -63 mV, respectively. The slope of the null potential obtained with K acetate electrodes or Cs acetate electrodes was 49 and 53 mV, respectively, per log unit of the external Cl- concentration. Reduction in external K+ markedly depressed the afterdepolarization and augmented the afterhyperpolarization in size, whereas rise in external K+ markedly augmented the afterdepolarization and depressed the afterhyperpolarization. The null potential for the afterhyperpolarization recorded with K acetate electrodes was -94 mV. The slope of the null potential was 57 mV per log unit of the external K+ concentration. Reduction in extracellular Na+ concentration slightly depressed both the amplitudes of the afterdepolarization and afterhyperpolarization. A hybrid voltage-clamp analysis revealed a slow decaying inward current and a subsequent outward current that followed an action potential. Both the amplitudes of the inward current corresponding to afterdepolarization and the outward current corresponding to afterhyperpolarization were suppressed by addition of Co2+. Reduction in extracellular Cl- concentration augmented the inward current amplitude without significantly affecting the outward current. These results indicate that the afterdepolarization is mainly due to an increase in a Ca(2+)-activated Cl- conductance, while the afterhyperpolarization is mainly generated by an activation of Ca(2+)-mediated K+ conductance.

Volume-regulatory amino acid transport in erythrocytes of the little skate, Raja erinacea

Skate erythrocytes swell in a hypotonic medium and then reduce their volume mainly by releasing the beta-amino acids taurine and beta-alanine. Although these amino acids exhibit a net efflux, Na(+)-independent influx is also increased. Both the reduction in cell volume and increase in amino acid transport are inhibited by several inhibitors of band 3-mediated anion transport, including 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) [L. Goldstein and S. R. Brill, Am. J. Physiol. 260 (Regulatory Integrative Comp. Physiol. 29): R1014-R1020, 1991]. The objective of the present investigation was to further characterize the mechanism of volume-activated amino acid transport. Na(+)-independent amino acid uptake was studied because of the ease in controlling amino acid concentrations. Na(+)-independent taurine uptake was observed to be linear over a range of 0.1-15 mM and was not inhibited by 10 mM beta-alanine, suggesting that the transporter may be a channel rather than a carrier. The uptake of a variety of amino acids was examined to characterize the size of the putative channel. Glycine, beta-alanine, taurine, proline, gamma-aminobutyric acid (GABA), and threonine exhibited volume-activated transport that was DIDS inhibited, whereas aspartic acid, leucine, methionine, and ornithine were not transported. On the basis of the size of these amino acids, it appears that molecules containing eight or fewer major atoms and having a molecular mass of < 125-131 Da are transported during volume activation but larger molecules are not. We estimate the size of the channel to be 5.7-6.3 A in diameter.

Na(+)-dependent sulfate transport in opossum kidney cells is DIDS sensitive

Sulfate transport was examined in OK/E cells, a clonal subline of opossum kidney cells that express several differentiated functions of the proximal tubule. Extracellular Na+ stimulated [35S]sulfate uptake five- to sixfold. Hill analysis demonstrating the dependence of sulfate uptake on Na+ concentration yielded a Hill coefficient of 1.5 and a Michaelis constant (KNa+) of 23 mM. Na(+)-dependent sulfate uptake was increased by lowering the pH from 7.4 to 6.4, decreased by raising the pH to 8.4 and inhibited by a 10-fold molar excess of SO3(2-), S2O3(2-) and CrO4(2-), but not by phosphate. The Na(+)-mediated component of sulfate uptake was saturable and kinetic parameters were estimated [Michaelis constant (Km) = 2.4 +/- 0.2 mM and maximum velocity (Vmax) = 125 +/- 15 pmol.mg protein-1.min-1]. Omitting extracellular Cl- resulted in a significant increase in the affinity of the carrier for sulfate (Km = 0.5 mm), without changing Vmax, consistent with competitive inhibition by Cl-. Na(+)-dependent sulfate uptake in OK/E cells was also inhibited by HCO3- [half-maximal inhibitory concentration (IC50) = 7 mM], 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS, IC50 = 0.9 microM), 0.5 mM picrylsulfonic acid and 0.1 mM ethacrynic acid, but not by 1 mM amiloride. Na(+)-dependent, DIDS-sensitive sulfate uptake was also expressed in the parental OK cell line and was not influenced by serum or 3,3',5-triiodo-L-thyronine. We conclude that Na(+)-dependent sulfate uptake in OK/E cells observes many of the features of Na(+)-sulfate cotransport in the renal brush-border membrane and provides a useful model to investigate the regulation of renal sulfate transport.

Proton (or hydroxide) fluxes and the biphasic osmotic response of human red blood cells

Upon exposure of human red blood cells to hypertonic sucrose, the fluorescence of the potentiometric indicator 3,3'-dipropylthiadicarbocyanine iodide, denoted diS-C3(5), displays a biphasic time course indicating the rapid development of an inside-positive transmembrane voltage, followed by a slow DIDS (4,4'-diisothiocyano-2,2'-disulfonic acid stilbene)-sensitive decline of the voltage. In addition to monitoring membrane potential, proton (or hydroxide) fluxes were measured by a pH stat method, cell volume was monitored by light scattering, and cell electrolytes were measured directly when red cells were shrunken either with hypertonic NaCl or sucrose. Shrinkage by sucrose induced an initial proton efflux (or OH- influx) of 5.5 mu eq/g Hb.min and a Cl shift of 21-31 mu eq/g Hb in 15 min. Upon shrinkage with hypertonic NaCl, the cells are initially close to Donnan equilibrium and exhibit no detectable shift of Cl or protons. Experiments with the carbonic anhydrase inhibitor ethoxzolamide demonstrate that for red cell suspensions exposed to air and shrunken with sucrose, proton fluxes mediated by the Jacobs-Stewart cycle contribute to dissipation of the increased outward Cl concentration gradient. With maximally inhibitory concentrations of ethoxzolamide, a residual proton efflux of 2 mu eq/g Hb.min is insensitive to manipulation of the membrane potential with valinomycin, but is completely inhibited by DIDS. The ethoxzolamide-insensitive apparent proton efflux may be driven against the electrochemical gradient, and is thus consistent with HCl cotransport (or Cl/OH exchange). The data are consistent with predictions of equations describing nonideal osmotic and ionic equilibria of human red blood cells. Thus osmotic equilibration after shrinkage of human red blood cells by hypertonic sucrose occurs in two time-resolved steps: rapid equilibration of water followed by slower equilibration of chloride and protons (or hydroxide). Under our experimental conditions, about two-thirds of the osmotically induced apparent proton efflux is mediated by the Jacobs-Stewart cycle, with the remainder being consistent with mediation via DIDS-sensitive HCl cotransport (or Cl/OH exchange).

A volume-sensitive Cl- conductance in a mouse neuroblastoma x rat dorsal root ganglion cell line (F11)

Whole cell currents were recorded in F11 cells, a mouse neuroblastoma (NG18TG2) x rat DRG hybrid cell line, using pipette and bath solutions intended to isolate any chloride conductance pathways. When recording with a pipette solution which was 40 mmol.kg-1 hypotonic to the bath solution, all cells showed a transient rise in input conductance which peaked 5.3 +/- 0.4 min after breaking into the cell and returned to the basal state 11.7 +/- 1.2 min later. At the peak of the effect, cell conductance had increased approximately sixfold. The use of short (300 ms) duration voltage steps at the peak of the conductance increase evoked whole-cell currents which were time-independent and had an outwardly rectifying current/voltage relationship. Ion substitution experiments showed that the whole-cell currents were carried by chloride ions and that the anion selectivity sequence of the conductance was I > Br > Cl > F > acetate. The stilbene derivative 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS) caused a reversible, 51% inhibition of the chloride currents. In cells which had already undergone this transient rise in conductance, whole-cell currents with identical properties could be activated by changing to a very hypotonic bath solution. Coincident with current activation, this manoeuvre caused a visible swelling of the cell. The increase in conductance and the cell swelling were both reversed by returning to the normal bath solution. In contrast, when a very hypotonic pipette solution was used, little or no increase in cell conductance was observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the chloride carrier in the plasmalemma of the alga Valonia utricularis: the inhibition by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid

The effect of the anion transport inhibitor 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) on the Cl(-)-transport system located in the plasmalemma of cells of the giant marine alga Valonia utricularis was studied by using the charge pulse relaxation technique. Analysis of the biphasic relaxation patterns in terms of the kinetic model published previously (Wang, J., Wehner, G., Benz, R. and Zimmermann, U. (1991) Biophys. J. 59, 235-248) demonstrated that extracellular DIDS dramatically reduced the translocation rate, KAS, of the Cl(-)-carrier complex (maximal inhibition 79%). The translocation rate of the free carrier molecules, KS, as well as the total surface concentration of the carrier, No, were not affected. A Hill-plot of DIDS inhibition on KAS yielded an half-maximal inhibition concentration (IC50) of 3.9 x 10(-5) M and Hill-coefficient of 1.61, suggesting a co-operative binding of the inhibitors to the Cl(-)-carrier. The maximal inhibition of DIDS was dependent on the extracellular Cl(-)-concentration. This inhibition was not competitive to chloride, since it increased and did not decrease with increasing chloride concentration. The DIDS effect decreased with increasing pH-value (investigated pH range between 6.5 and 10). Intravascular DIDS or SITS (4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid) had no effect on the biphasic voltage relaxation pattern. These results showed that the binding sites of DIDS must be located on the outer surface of the plasmalemma of V. utricularis and, in turn, supported previous conclusions that the Cl(-)-carrier (which is assumed to be part of the turgor-pressure-sensing mechanism) is only located in the outer membrane.

Uptake of riboflavin by intestinal basolateral membrane vesicles: a specialized carrier-mediated process

The mechanism of riboflavin (RF) uptake by intestinal basolateral membrane vesicles (BLMV) was examined in this study. BLMV were isolated by an established Percoll-gradient methodology from rabbit small intestine. Uptake of riboflavin was mainly the result of transport of the substrate into an osmotically active intravesicular space with less binding to membrane surfaces. Uptake of RF with time was similar in the presence of a Na+ and a K+ gradient (out > in) and was not significantly influenced by changes in incubation buffer pH. The initial rate of uptake of riboflavin as a function of concentration was saturable in both jejunal and ileal BLMV and occurred with apparent Km values of 5.0 microM and 4.4 microM and Vmax values of 91.6 and 60.8 pmol/mg protein per 5 s, respectively. Unlabeled riboflavin and the structural analogues lumiflavin, isoriboflavin and 8-aminoriboflavin all caused significant inhibition (but to different degrees) in the uptake of [3H]riboflavin. On the other hand, 8-hydroxyriboflavin, lumichrome, lumazine and D-ribose failed to inhibit [3H]riboflavin uptake. Trans-stimulation of [3H]riboflavin efflux from preloaded BLMV by unlabeled riboflavin or lumiflavin was also observed. Altering transmembrane electrical potential by anion substitution and valinomycin-induced K+ diffusion did not affect the riboflavin uptake process. These results demonstrate the existence of a specialized carrier-mediated mechanism for riboflavin uptake by intestinal BLMV. Furthermore, the system appears to transport the vitamin by a process which is Na(+)- and pH-independent, and electroneutral in nature.

Inhibition of gastric acid secretion in vivo and in vitro by an inhibitor of Cl(-)-HCO3- exchanger, 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid

The mode of action of 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), an inhibitor of the Cl(-)-HCO3- exchanger, on gastric acid secretion has been studied in vitro and in vivo. In the mouse isolated whole stomach preparation, DIDS (100 microM-1 mM) inhibited gastric acid secretion induced by histamine or bethanechol in a concentration-dependent fashion. On the other hand, DIDS, at a concentration enough to reduce the stimulated acid secretion, did not inhibit basal acid secretion in the resting preparation. In the perfused stomach of urethane-anesthetized rats, DIDS (1-10 mg/kg i.v.) inhibited gastric acid secretion stimulated by histamine, bethanechol or tetragastrin, whereas DIDS did not inhibit basal acid secretion. In pylorus-ligated rats, DIDS (3-30 mg/kg) administered intraduodenally also inhibited gastric acid output as well as gastric juice volume when administered immediately after ligation. When injected 6 h before ligation, DIDS inhibited the gastric acid secretion. However, this potency was weak in comparison with that observed when DIDS was administered immediately after ligation. These results demonstrate that DIDS can inhibit stimulated gastric acid secretion in vitro and in vivo, probably through its inhibitory effect on the Cl(-)-HCO3- exchanger.

Co-expression of an anion conductance pathway with Na(+)-glucose cotransport in rat renal brush-border membrane vesicles

Brush-border membrane vesicles were prepared from superficial rat renal cortex by a Mg(2+)-precipitation technique. The initial (20 s) [14C]glucose uptake rate from solutions containing 100 mmol/l Na (salt) was found to be dependent upon the anion composition of the medium; in comparison to gluconate-containing medium (0.46 +/- 0.05 nmol/mg protein), Cl- accelerated the initial rate to 1.47 +/- 0.21 nmol/mg protein (n = 4 preparations, +/- SEM). This enhancement was reduced by 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB, 0.5 mmol/l), but was unaffected by 4,4'-diisothiocyanatostilbene 2,2'-disulphonate (DIDS, 0.5 mmol/l). When membrane vesicles were pre-equilibrated with 100 mmol/l K (salt) and 100 mmol/l mannitol and glucose uptake was measured from a solution containing 100 mmol/l Na gluconate and 100 mmol/l mannitol in the presence of 80 mumol/l valinomycin (to generate an outward K+ diffusion electrical p. d.), it was found that intravesicular KCl depressed the initial glucose uptake compared to K gluconate. NPPB (0.5 mmol/l) increased the initial glucose uptake with intravesicular KCl towards values seen in K gluconate vesicles. In conditions where the only driving force for glucose uptake was established by an inward anion gradient (Nao = Nai) it was found that inward Cl- gradients could drive uphill glucose transport and that this was sensitive to NPPB (0.5 mmol/l), but insensitive to DIDS. We conclude that a Cl- conductance co-exists with Na-cotransport in rat renal brush-border membrane vesicles prepared from superficial renal cortex and this may function to regulate the activity of electrogenic transport systems at this membrane.

Endogenous production of superoxide by rabbit lungs: effects of hypoxia or metabolic inhibitors

We find spontaneous light emission from isolated Krebs-Henseleit-perfused rabbit lungs when the light-emitting super-oxide trap lucigenin is added to the perfusate. Lucigenin light emission appears to be specific for superoxide anion, because light emission from the lung caused by a superoxide-generating system is abolished by superoxide dismutase but not by catalase or dimethylthiourea. We also studied the relative sensitivity of lucigenin photoemission to superoxide and to H2O2 in vitro. Lucigenin photoemission is three to four orders of magnitude more sensitive to superoxide than to H2O2 and probably cannot detect H2O2 in concentrations thought to occur in biological systems. Basal lucigenin photoemission by the lung is oxygen dependent, because severe hypoxia completely inhibits light emission. Superoxide dismutase reduces basal photoemission by 50%, and administration of the low-molecular-weight superoxide scavenger 4,5-dihydroxy-1,3-benzene disulfonic acid (tiron) inhibits basal photoemission by approximately 90%. These observations suggest that endogenous superoxide production is primarily intracellular and that approximately half of the superoxide reaches the extracellular space. Superoxide transport may involve anion channels, because the anion channel blocker 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid increases photoemission, suggesting intracellular accumulation of superoxide. A cytochrome P-450 inhibitor, SKF 525A, or the mitochondrial transport inhibitor antimycin decreased basal photoemission by approximately 50%, suggesting that cytochrome P-450-mediated reactions and perhaps mitochondrial function contribute to basal superoxide production in the isolated perfused lung. Endogenous superoxide production may be important in regulation of pulmonary vascular reactivity and may contribute to the pathogenesis of lung reperfusion injury.

P2-purinergic receptors in human breast tumor cells: coupling of intracellular calcium signaling to anion secretion

ATP increases intracellular Ca++ ([Ca++]i) by activating different P2-purinoreceptors. Because ATP increases Cl- secretion in cystic fibrosis (CF)-affected epithelia, the current study was designed to establish the link between these two events. Studies were done in epithelial, human MCF-7 breast tumor cells in which the presence of mRNA transcripts encoding CF transmembrane conductance regulator was initially established. Changes in [Ca++]i were measured in single cells by fluorescence microscopy; anion transport was measured by 125I efflux. ATP stimulated concentration-dependent increases in [Ca++]i and 125I efflux from MCF-7 cells. The relative order of agonist potency of various selective P2-purinoreceptor agonists in increasing [Ca++]i and 125I efflux was: UTP > or = ATP > ADP = AMP; 2-chloro-ATP, 2-methylthio-ATP and AMP-phencyclidine were considerably less potent than ATP. The Ca++ ionophore ionomycin increased both intracellular [Ca++]i and 125I secretion. Exposing cells to the intracellular chelator ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetra-acetic acid (EGTA)-acetoxymethylester decreased (AM) decreased ATP- and ionomycin-stimulated 125I efflux. Extracellular EGTA did not alter the Ca++ response to ATP, but inhibited the response to ionomycin. The chelator inhibited both ATP- and ionomycin-induced 125I secretion. Exposure of cells to nifedipine did not affect the responsiveness of MCF-7 cells to ATP. The anion transport antagonist 4,4'-diisothiocyananatostilbene-2,2'-disulfonic acid partially inhibited ATP- and cationophore-stimulated increases in [Ca++]i and 125I secretion. The data suggest that activation of P2 receptors in MCF-7 cells leads to an increase in anion transport as a result of the ability of ATP to increase [Ca++]i; moreover, anion channel antagonists may produce their inhibitory effect on 125I secretion, in part, by blocking agonist-induced intracellular Ca++ signaling.

Anion transport blockers inhibit DL-2-amino-4-phosphonobutyrate responses induced by quisqualate in the rat cerebral cortex

1. Depolarizing responses to DL-2-amino-4-phosphonobutyrate (AP4) and related amino acids have been studied in the rat cerebral cortex slice following the application of quisqualate (Quis). 2. Before exposure to Quis, 500 microM DL-AP4 had little or no effect. However, following a single application of 40 microM Quis for 2 min, DL-AP4 produced depolarizing responses. With repeated applications of DL-AP4, there was a decline in response amplitude. A second application of Quis restored the depolarizing potency of DL-AP4 to a level above that for the first DL-AP4 response after the first Quis application. With a sequence of alternate applications of Quis and DL-AP4, the amplitude of DL-AP4 responses became maximal after the second Quis application. Responses to DL-AP4 could also be induced by the application of 1 microM Quis for 60 min, but were smaller in amplitude. 3. Responses to the normally inactive amino acids L-cysteine (Cys), L-cystathionine (CTN) and L-alpha-aminoadipate (AA) were also induced once Quis was applied. These responses were also maximized after a second application of Quis, except those to L-Cys, which failed to reach a plateau after three Quis applications. 4. The co-application of DL-AP4 with the first Quis application depressed the subsequent mean DL-AP4 response by 47%. Re-application of Quis restored the amplitude of DL-AP4 responses to levels comparable to control. L-alpha-AA also suppressed the induction of DL-AP4 responses, when co-applied with the first Quis exposure, reducing mean response amplitude by 98%. Unlike DL-AP4, however, the effect with L-alpha-AA persisted so that DL-AP4 responses were significantly suppressed compared to control, even after further applications of Quis. 5. The effects of the anion transport blockers, 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) and 4-acetoamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid (SITS) on the induction process and the DL-AP4 responses themselves were examined. DIDS (100 microM) significantly inhibited the DL-AP4 responses, and to a lesser extent the induction of the responses by 40 microM Quis (2 min), while SITS (300 microM) only inhibited the DL-AP4 responses. However, the induction of responses by 1 microM Quis (60 min) was significantly affected by this concentration of SITS. 6. DIDS (100 microM) had no effect on responses to alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate (AMPA), but selectively potentiated those to Quis. Examination of the full concentration-response curve for Quis revealed that, while the Rmax remained constant, the Hill slope was increased and the EC50 was decreased in the presence of DIDS. SITS (300 microM), however, antagonized responses to AMPA, and had little effect on responses to Quis except at the highest concentration of Quis tested (20 microM), where a potentiation was observed, suggesting that it is a non-NMDA receptor antagonist.7. These observations indicate that the production of depolarizing responses to a number of amino acids, including DL-AP4, in the cerebral cortex is mediated via an anion transport mechanism sensitive to DIDS and SITS, and that the exchange of DL-AP4 for a sequestered excitatory amino acid receptor agonist, probably Quis, could underlie the production of these responses. Indeed, Quis is apparently sequestered via a similar process. However, the involvement of such a process in the induction of these responses remains inconclusive.

Chloride channels activated by osmotic stress in T lymphocytes

We have used whole-cell and perforated-patch recording techniques to characterize volume-sensitive Cl- channels in T and B lymphocytes. Positive transmembrane osmotic pressure (intracellular osmolality > extracellular osmolality) triggers the slow induction of a Cl- conductance. Membrane stretch caused by cellular swelling may underlie the activation mechanism, as moderate suction applied to the pipette interior can reversibly oppose the induction of Cl- current by an osmotic stimulus. Intracellular ATP is required for sustaining the Cl- current. With ATP-free internal solutions, the inducibility of Cl- current declines within minutes of whole-cell recording, while in whole-cell recordings with ATP or in perforated-patch experiments, the current can be activated for at least 30 min. The channels are anion selective with a permeability sequence of I- > SCN- > NO3-, Br- > Cl- > MeSO3- > acetate, propionate > ascorbate > aspartate and gluconate. GCl does not show voltage- and time-dependent gating behavior at potentials between -100 and +100 mV, but exhibits moderate outward rectification in symmetrical Cl- solutions. Fluctuation analysis indicates a unitary chord conductance of approximately 2 pS at -80 mV in the presence of symmetrical 160 mM Cl-. The relationship of mean current to current variance during the osmotic activation of Cl- current implies that each cell contains on the order of 10(4) activatable Cl- channels, making it the most abundant ion channel in lymphocytes yet described. The current is blocked in a voltage-dependent manner by DIDS and SITS (Ki = 17 and 89 microM, respectively, at +40 mV), the degree of blockade increasing with membrane depolarization. The biophysical and pharmacological properties of this Cl- channel are consistent with a role in triggering volume regulation in lymphocytes exposed to hyposmotic conditions.