Effects of thyromimetric drugs on aldosterone-dependent sodium transport in the toad bladder

Summary Aldosterone increases transepithelial Na⁺ transport in the urinary bladder ofBufo marinus. The response is characterized by 3 distinct phases: 1) a lag period of about 60 min, ii) an initial phase (early response) of about 2 hr during which Na⁺ transport increases rapidly and transepithelial electrical resistance falls, and iii) a late phase (late response) of about 4 to 6 hr during which Na⁺ transport still increases significantly but with very little change in resistance. Triiodothyronine (T₃, 6nm) added either 2 or 18 hr before aldosterone selectively antagonizes the late response. T₃ per se (up to 6nm) has no effect on base-line Na⁺ transport. The antagonist activity of T₃ is only apparent after a latent period of about 6 to 8 hr. It is not rapidly reversible after a 4-hr washout of the hormone. The effects appear to be selective for thyromimetic drugs since reverse T₃ (rT₃) is inactive and isopropyldiiodothyronine (isoT₂) is more active than T₃. The relative activity of these analogs corresponds to their relative affinity for T₃ nuclear binding sites which we have previously described. Our data suggest that T₃ might control the expression of aldosterone by regulating gene expression, e.g. by the induction of specific proteins, which in turn will inhibit the late mineralocorticoid response, without interaction with the early response.     

Effects of thyroid hormones and aldosterone on mineralocorticoid binding sites in the toad bladder

Summary In the urinary bladder of the toadBufo marinus triiodothyronine selectively inhibits the late effect of aldosterone on Na⁺ transport. We have investigated whether T₃ might mediate its antimineralocorticoid action by controlling: i) the level of aldosterone binding sites in the soluble (cytosolic) pool isolated from tissues treated with T₃ (60nm) for up to 20 hr of incubation; ii) the kinetics of uptake of³H-aldosterone into cytoplasmic and nuclear fractions after 2 or 20 hr of exposure to T₃. The number and the affinity of Type I (high affinity, low capacity) and Type II (low affinity, high capacity) cytosolic binding sites (measured at 0°C) did not vary significantly after 18 hr of exposure to T₃, while aldosterone-dependent Na⁺ transport was significantly inhibited. In addition, T₃ did not modify the kinetics of uptake (90 min) of³H-aldosterone into cytoplasmic and nuclear fractions of toad bladder incubatedin vitro at 25°C. By contrast, aldosterone itself was able to down-regulate its cytosolic and nuclear binding sites after an 18-hr exposure to the steroid hormone (10 or 80nm). T₃ slightly (20%) but significantly potentiated the down regulation of nuclear binding sites. In conclusion, T₃ does not appear to have major effects on the regulation of the aldosterone receptor, which could explain in a simple manner its antimineralocorticoid action.     

Kinetics of sodiumd-glucose cotransport in bovine intestinal brush border vesicles

Summary Brush border membrane vesicles (BBMV) purified from steer jejunum were used to study the kinetics of sodiumd-glucose cotransport under voltage clamped, zero-trans conditions. When the initial rate of glucose transport (J _gluc) was measured over a wide range of glucose concentrations ([S]=0.01–20mm), curvature of the Woolf-Augustinsson-Hofstee plots was seen, compatible with a diffusional and one major, high capacity (maximal transport rateJ _max=5.8–8.8 nmol/mg·min) saturable system. Further studies indicated that changes incis [Na] altered theK _t , but not theJ _max, suggesting the presence of a rapid-equilibrium, ordered bireactant system with sodium adding first.Trans sodium inhibitedJ _gluc hyperbolically. KCl-valinomycin diffusion potentials, inner membrane face positive, loweredJ _gluc, while potentials of the opposite polarity raiseJ _gluc. At low glucose concentrations ([S]<0.05mm), a second, minor, high affinity transport system was indicated. Further evidence for this second saturable system was provided by sodium activation curves, which were hyperbolic when [S]=0.5mm, but were sigmoidal when [S]=.0.01mm. Simultaneous fluxes of²²Na and [³H]glucose at 1mm glucose and 30mm NaCl yielded a cotransport-dependent flux ratio of 21 sodium/glucose, suggestive of 11 (Na/glucose) high capacity, low affinity system and a 31 (Na/glucose) high affinity, low capacity system. Kinetic experiments with rabbit jejunal brush borders revealed two major Na-dependent saturable systems. Extravesicular (cis) Na changed theK _t , but not theJ _max of the major system.     

Sodium-alanine cotransport in oocytes ofXenopus laevis: Correlation of alanine and sodium fluxes with potential and current changes

Summary The sodium-dependentl-alanine transport across the plasma membrane of oocytes ofXenopus laevis was studied by means of [¹⁴C]-l-alanine,²²Na⁺ and electrophysiological measurements. At fixed sodium concentrations, the dependence of alanine transport on alanine concentration follows Michaelis-Menten kinetics; at fixed alanine concentrations, the transport varies with sodium concentration with a Hill coefficient of 2. In the presence of sodium the uptake of alanine is accompanied by a depolarization of the membrane. Under voltage-clamp conditions this depolarization can be compensated by an inward-directed current. Assuming that this current is carried by sodium we arrive at a 21 stoichiometry for the sodium-alanine cotransport. The assumption was confirmed by direct measurements of both sodium and alanine fluxes at saturating concentrations of the two substrates, which also yielded a stoichiometry close to 21. The sodium-l-alanine cotransport is neither inhibited by furosemide (0.5 mmol/liter) nor by N-methyl amino isobutyric acid (5 mmol/liter). A 20-fold excess ofd-alanine overl-alanine caused about 60% inhibition.     

Aldosterone regulation of active sodium chloride transport in the lizard colon (Gallotia galloti)

Summary Bioelectrical parameters and unidirectional sodium and chloride fluxes were measured under voltageclamp conditions in groups of lizards submitted to single or chronic aldosterone treatment. Both acute (AT) and chronic (CT) treatment induced significant increases in the short-circuit current (I _sc), as well as in the mucosa-to-serosa (J _m-s ^Na ) and net sodium flux (J _net ^Na ). In AT tissues, aldosterone did not change net chloride flux (J _net ^Cl ) but did so in CT tissues. Amiloride reduced the aldosterone-increased I _sc in AT and CT tissues, inhibited J _net ^Na in AT tissues and abolished it in CT colons. J _net ^Cl was also reduced by the diuretic in the group of AT colons, whereas no changes were observed in the CT tissues. Addition of luminal DIDS reduced Na⁺ absorption and totally inhibited Cl^- absorption in the AT tissues, but did not change I _sc. However, in CT tissues neither Na⁺ nor Cl^- transport were affected by DIDS. A good relationship between I _sc and J _m-s ^Na was apparent after DIDS treatment in AT tissues. In this group, simultaneous addition of DIDS and amiloride totally abolished J _net ^Na and reduced I _sc to untreated control values. Addition of serosal ouabain abolished I _sc and Na⁺ absorption in AT and CT colons, but Cl^- absorption was only altered in AT tissues. These results support the hypothesis that aldosterone induces an electrogenic, amiloride-sensitive sodium absorption, and in a dose-dependent fashion suppresses electroneutral NaCl absorption in the lizard colon.Abbreviations AT acutely treated - CT chronically treated animals - DIDS 4-4-diisothiocyanatostibene-2-2-disulfonic acid - DMSO dimethylsulphoxide - G _t tissue conductance - I _sc short circuit current - PD transepithelial potential difference - SITS 4-acetamido-4-isothiocyanatostilbene-2-2-disulfonic acid - UC untreated controls Preliminary results of this paper were presented at the X ^th meeting of the European Intestinal Transport Group (EITG), Askov Hojskole, Denmark, 16–19 September 1990     

Amino acid metabolism in the thermophilic phototroph,Chloroflexus aurantiacus: properties and metabolic role of two l-threonine (l-serine) dehydratases

Two l-threonine (l-serine) dehydratases (EC 4.2.1.16) of the thermophilic phototrophic bacterium Chloroflexus aurantiacus Ok-70-fl were purified to electrophoretic homogeneity by procedures involving anion exchange and hydrophobic interaction chromatography. Only one of the two enzymes was sensitive to inhibition by l-isoleucine (K _i=2 M) and activation by l-valine. The isoleucine-insensitive dehydratase was active with l-threonine (K _m=20 mM) as well as with l-serine (K _m=10 mM) whereas the other enzyme, which displayed much higher affinity to l-threonine (K _m=1.3 mM), was inactivated when acting on l-serine. Both dehydratases contained pyridoxal-5-phosphate as cofactor. When assayed by gel filtration techniques at 20 to 25° C, the molecular weights of both enzymes were found to be 106,000±6,000. In sodium dodecylsulfate-polyacrylamide gel electrophoresis, the two dehydratases yielded only one type of subunit with a molecular weight of 55,000±3,000. The isoleucine-insensitive enzyme was subject to a glucose-mediated catabolite repression.Abbreviations A absorbance - ile isoleucine - PLP pyridoxal-5-phosphate - SDS sodium dodecyl sulfate - TDH threonine dehydratase - U unit     

Transepithelial transport in cell culture: Stoichiometry of Na/phlorizin binding and Na/d-glucose cotransport. A two-step,two-sodium model of binding and translocation

Summary The renal cell line LLC-PK₁ cultured on a membrane filter forms a functional epithelial tissue. This homogeneous cell population exhibits rheogenic Na-dependentd-glucose coupled transport. The short-circuit current (I _sc) was acccounted for by net apical-to-basolaterald-glucose coupled Na flux, which was 0.53±0.09(8) eq cm^–2hr^–1, andI _sc, 0.50±0.50(8) eq cm^–2hr^–1. A linear plot of concurrent net Na vs. netd-glucose apical-to-basolateral fluxes gave a regression coefficient of 2.08. As support for a 21 transepithelial stoichiometry, sodium was added in the presence ofd-glucose and the response ofI _sc analyzed by a Hill plot. A slope of 2.08±0.06(5) was obtained confirming a requirement of 2 Na for 1d-glucose coupled transport. A Hill plot ofI _sc increase to addedd-glucose in the presence of Na gave a slope of 1.02±0.02(5). A direct determination of the initial rates of Na andd-glucose translocation across the apical membrane using phlorizin, a nontransported glycoside competitive inhibitor to identify the specific coupled uptake, gave a stoichiometry of 2.2 A coupling ratio of 2 for Na,d-glucose uptake, doubles the potential energy available for Na-gradient coupledd-glucose transport. In contrast to coupled uptake, the stoichiometry for Na-dependentphlorizin binding was 1.1±0.1(8) from Hill plot analyses of Na-dependent-phlorizin binding as a function of [Na]. Although occurring at the same site the process of Na-dependent binding of phlorizin differs from the binding and translocation ofd-glucose. Our results support a two-step, two-sodium model for Na-dependentd-glucose cotransport; the initial binding to the cotransporter requires a single Na andd-glucose, a second Na then binds to the ternary complex resulting in translocation.     

Sodium currents in toad cardiac pacemaker cells

Cells in the pacemaker region of toad (Bufo marinus) sinus venosus had spontaneous rhythmic action potentials. The rate of firing of action potentials, the rate of diastolic depolarization and the maximum rate of rise of action potentials were reduced by TTX (10 nm to 1 m). Currents were recorded with the whole cell, tight seal technique from cells enzymatically dissociated from this region. Cells studied were identified as pacemaker cells by their characteristic morphology, spontaneous rhythmic action potential activity that could be blocked by cobalt but not by TTX and lack of inward rectification. When calcium, potassium and nonselective cation currents (I_f) activated by hyperpolarization were blocked, depolarization was seen to generate transient and persistent inward currents. Both were sodium currents: they were abolished by tetrodotoxin (10 to 100 nm), their reversal potential was close to the sodium equilibrium potential and their amplitude and reversal potential were influenced as expected for sodium currents when extracellular sodium ions were replaced with choline ions. The transient sodium current was activated at potentials more positive than –40 mV while the persistent sodium current was obvious at more negative potentials. It was concluded that, in toad pacemaker cells, TTX-sensitive sodium currents contributing both to the upstroke of action potentials and to diastolic depolarization may play an important role in setting heart rate.We thank the Australian National Heart Foundation for their support. D.A.S. is an NHMRC Senior Research Officer.     

Phenamil: An irreversible inhibitor of sodium channels in the toad urinary bladder

Summary Several new amiloride analogues and two reported photoaffinity analogues were tested for irreversible inhibition of short-circuit current,I _sc, in toad bladder. Bromoamiloride, a photoaffinity analogue, induced 40% irreversible inhibition at 500 m after irradiation with ultraviolet light 320 nm. Iodoamiloride caused no irreversible inhibition. Of the new analogues tested, only 3,5-diamino-6-chloro-N-[(phenylamino) aminomethylene] pyrazinecarboxamide,phenamil, irreversibly inhibitedI _sc at concentrations of 0.05 to 5 m when added to the mucosal solution. Irreversible inhibition ofI _sc by phenamil may be attributed to specific blockage of the mucosal sodium channels, which depended on: 1) time of exposure; 2) mucosal pH: 3) mucosal sodium concentration. For example, 5 m phenamil irreversibly inhibitedI _sc by 38% in 103mm Na at pH 8.6 and nearly 75% in 30mm Na at pH 6.4 after a 40-min exposure. Irreversible inhibition occurred in two phases with time constants of 10 min and approximately 140 min. Due to its irreversible nature, phenamil may be used to measure channel density.     

Lymphocyte subsets and T(h)1/T(h)2 immune responses in patients with adenocarcinoma of the oesophagus or oesophagogastric junction: relation to pTNM stage and clinical outcome

Introduction Recent studies have indicated that the cytokines produced by CD4⁺ T helper type 1 (T_h1) and type 2 (T_h2) cells are critically important in antitumour immunity and perhaps clinical outcome. From this perspective, we investigated the immunocompetence of patients with previously untreated cancer of the oesophagus or oesophagogastric junction (OGJ) in relation to stage of disease and postoperative survival.Methods Blood samples were taken prior to surgery from 32 patients with adenocarcinoma of the oesophagus or OGJ. Ten healthy volunteers served as normal controls. T-cell and monocyte subpopulations were determined using flow cytometry. Monocyte as well as T_h1- and T_h2-lymphocyte cytokine levels were assessed in stimulated whole blood cultures.Results Absolute T-cell and monocyte (subset) counts as well as monocyte cytokine levels were similar among patients and controls. Production of T_h1-type cytokines was higher in patients than in controls (IFN-, p=0.01; IL-2, p=0.05), whereas T_h2-type cytokine levels were comparable (IL-4, p=0.5; IL-13, p=0.3). T-cell CD4⁺/CD8⁺ ratios decreased as pTNM stage worsened (stage I/II vs stage III/IV, p=0.009). Of all measured immunological parameters, only IL-2 production significantly affected both overall survival (p=0.015) and disease-free survival (p=0.0062). High IL-2 levels corresponded with a favourable prognosis.Conclusions Patients awaiting surgery for adenocarcinoma of the oesophagus or oesophagogastric junction demonstrated a shift in the T_h1/T_h2 balance—in favour of T_h1—compared with healthy volunteers. The ability of T cells to produce IL-2 was related to survival indicating a crucial role of T_h1-type cells in antitumour immunosurveillance.     

Purification and characterization of extracellular β-glucosidase from Myceliophthora thermophila

The extracellular -glucosidase has been purified from culture broth of Myceliophthora thermophila ATCC 48104 grown on crystalline cellulose. The enzyme was purified approximately 30-fold by (NH₄)₂SO₄ precipitation and column chromatography on DEAE-Sephadex A-50, Sephadex G-200 and DEAE-Sephadex A-50. The molecular mass of the enzyme was estimated to be about 120 kD by both sodium dodecyl sulphate gel electrophoresis and gel filtration chromatography. It displayed optimal activity at pH 4.8 and 60°C. The purified enzyme in the absence of substrate was stable up to 60°C and pH between 4.5 and 5.5. The enzyme hydrolysed p-nitrophenyl--d-glucoside, cellobiose and salicin but not carboxymethyl cellulose or crystalline cellulose. The K _m of the enzyme was 1.6mm for p-nitrophenyl--d-glucoside and 8.0mm for cellobiose. d-Glucose was a competitive inhibitor of the enzyme with a K of 22.5mm. Enzyme K activity was inhibited by HgCl₂, FeSO₄, CuSO₄, EDTA, sodium dodecyl sulphate, p-chloromercurobenzoate and iodoacetamide and was stimulated by 2-mercaptoethanol, dithiothreitol and glutathione. Ethanol up to 1.7 m had no effect on the enzyme activity.The authors are with the Department of Microbiology, Bose Institute, 93/1, A.P.C. Road, Calcutta 700 009, India. S.K. Raha is presently with the Department of Medicine, University of Saskatchewan, Saskatoon, Canada S7N OXO.     

Purification and characterization of a sialidase fromClostridium chauvoei NC08596

The sialidase secreted byClostridium chauvoei NC08596 was purified to apparent homogeneity by ion-exchange chromatography, gel filtration, hydrophobic interaction-chromatography, FPLC ion-exchange chromatography, and FPLC gel filtration. The enzyme was enriched about 10 200-fold, reaching a final specific activity of 24.4 U mg^–1. It has a relatively high molecular mass of 300 kDa and consists of two subunits each of 150 kDa. The cations Mn²⁺, Mg²⁺, and Ca²⁺ and bovine serum albumin have a positive effect on the sialidase activity, while Hg²⁺, Cu²⁺, and Zn²⁺, chelating agents and salt decrease enzyme activity. The substrate specificity, kinetic data, and pH optimum of the enzyme are similar to those of other bacterial sialidases.Abbreviations FPLC fast protein liquid chromatography - NCTC National Collection of Type Cultures - ATCC American Type Culture Collection - MU-Neu5Ac 4-methylumbelliferyl--d-N-acetylneuraminic acid - buffer A 0.02m piperazine, 0.01m CaCl₂, pH 5.5 - buffer B 0.02m piperazine, 0.01m CaCl₂, 1.0m NaCl, pH 5.5 - buffer C 0.1m sodium acetate, 0.01m CaCl₂, pH 5.5 - SDS sodium dodecyl sulfate - PAGE polyacrylamide gel electrophoresis - Neu5Ac N-acetylneuraminic acid - BSM bovine submandibular gland mucin - GD1a IV³Neu5Ac, II³Neu5Ac-GgOse₄Cer - GM1 II³Neu5Ac-GgOse₄Cer - MU-Neu4,5Ac₂ 4-methylumbelliferyl--d-N-acetyl-4-O-acetylneuraminic acid - TLC thin-layer chromatography - HPTLC high performance thin-layer chromatography - EDTA ethylenediamine tetraacetic acid - EGTA ethylene glycol bis(2-aminoethyl-ethen)-N,N,N,N-tetraacetic acid - BSA bovine serum albumin - Neu5Ac2en 2-deoxy-2,3-didehydro-N-acetylneuraminic acid - IEF isoelectric focusing - IEP isoelectric point     

Enzymic synthesis of lacto-N-triose II and its positional analogues

N-acetylhexosaminidase fromNocardia orientalis catalysed the synthesis of lacto-N-triose II glycoside (-d-GlcNAc-(1-3)--d-Gal-(1-4)--d-Glc-OMe,3) with its isomers -d-GlcNAc-(1-6)--d-Gal-(1-4)--d-Glc-OMe (4) and -d-Gal-(1-4)-[-d-GlcNAc-(1-6)]--d-Glc-OMe (5) throughN-acetylglucosaminyl transfer fromN,N-diacetylchitobiose (GlcNAc₂) to methyl -lactoside. The enzyme formed the mixture of trisac-charides3, 4 and5 in 17% overall yield based on GlcNAc₂, in a ratio of 20:21:59. Withp-nitrophenyl -lactoside as an acceptor, the enzyme also producedp-nitrophenyl -lacto-N-trioside II (-d-GlcNAc-(1-3)--d-Gal-(1-4)--d-Glc-OC₆H₄NO₂-p,6) with its isomers -d-GlcNAc-(1-6)--d-Gal-(1-4)--d-Glc-OC₆H₄NO₂-p (7) and -d-Gal-(1-4)-[-d-GlcNAc-(1-6)]--d-Glc-OC₆H₄NO₂-p (8). In this case, when an inclusion complex ofp-nitrophenyl lactoside acceptor with -cyclodextrin was used, the regioselectivity of glycosidase-catalysed formation of trisaccharide glycoside was substantially changed. It resulted not only in a significant increase of the overall yield of transfer products, but also in the proportion of the desired compound6.Abbreviations GlcNAc₂ 2-acetamido-2-deoxy--d-glucopyranosyl-(1-4)-2-acetamido-2-deoxy-d-glucose - NAHase N-acetylhexosaminidase - -CD -cyclodextrin     

Characterization of sodium and pyruvate interactions of the two carrier systems specific of mono- and di- or tricarboxylic acids by renal brush-border membrane vesicles

Summary The experiments reported in this paper aim at characterizing the carboxylic acid transport, the interactions of pyruvate and citrate with their transport sites and specificity. The study of these carriers was performed using isotopic solutes for the influx measurements in brush-border membrane vesicles under zerotrans conditions where the membrane potential was abolished with KCl preloading with valinomycin or equilibrium exchange conditions and =0.Under zerotrans condition and =0, the influence of pyruvate concentrations on its initial rates of transport revealed the existence of two families of pyruvate transport sites, one with a high affinity for pyruvate (K _t =88 m) and a low affinity for sodium (K _t =57.7mm) (site I), the second one with a low affinity for pyruvate (K _t =6.1mm) and a high affinity for sodium (K _t =23.9mm) (site II). The coupling factor [Na]/[pyruvate] stoichiometry were determined at 0.25mm and 8mm pyruvate and estimated at 1.8 for site I, and 3 when the first and the second sites transport simultaneously.Under chemical equilibrium (0) single isotopic labeling, transport kinetics of pyruvate carrier systems have shown a double interaction of pyruvate with the transporter; the sodium/pyruvate stoichiometry also expressed according to a Hill plot representation wasn=1.7. The direct method of measuring Na⁺/pyruvate stoichiometry from double labeling kinetics and isotopic exchange, for a time course, gives an=1.67.Studies of transport specificity, indicate that the absence of inhibition of lactate transport by citrate and the existence of competitive inhibition of lactate and citrate transports by pyruvate leads to the conclusion that the low pyruvate affinity site can be attributed to the citrate carrier (tricarboxylate) and the high pyruvate affinity site to the lactate carrier (monocarboxylate).     

Primary structure of the xylose-containingN-linked carbohydrate moiety from ascorbic acid oxidase ofCucurbita pepo medullosa

Ascorbic acid oxidase (E.C.1.10.3.3) from the green zucchini squash (Cucurbita pepo medullosa) is a copper-containing glycoprotein which catalyzes the reaction:l-ascorbic acid +1/2 O₂l-dehydroascorbic acid + H₂O. The carbohydrate content of the purified plant glycoprotein amounted to 3% (w/w), and monosaccharide analysis revealed the carbohydrate moiety to be of theN-glycosidic type. The carbohydrate chains were released from the apoenzyme by digestion with PNGase-F immobilized on Sepharose 4B. After fractionation on Bio-Gel P-2 and purification on Mono-Q, the neutral oligosaccharide was investigated by 500-MHz¹H-NMR spectroscopy. The primary structure of theN-linked carbohydrate chain was established to be: Abbreviations AAO ascorbic acid oxidase - PNGase-F peptide-N ⁴-(N-acetyl--glucosaminyl)asparagine amidase-F - GalNAc N-acetylgalactosamine - GlcNAc N-acetylglucosamine - Man mannose - Xyl xylose - GLC gas-liquid chromatography - FPLC fast protein liquid chromatography - NMR nuclear magnetic resonance - SDS-PAGE sodium dodecyl sulfate-polyacrylamide gel electrophoresis     

Transport of an anionic substrate by the H+/monosaccharide symport inRhodotorula gracilis: Only the protonated form of the carrier is catalytically active

Summary The yeastRhodotorula gracilis accumulated glucuronate by an H⁺/symport. The transport was electroneutral, driven by the chemical gradient of protons pH. The observed stoichiometry amounted to 1 proton per molecule glucuronate. At pH 4, the half-saturation constantK _T was at its lowest value (K _T =8mm), whereas the maximal velocityV _T reached a maximum (V _T =15 nmol/min×mg dry wt). Monosaccharides competitively inhibited the uptake of glucuronate and vice versa. Hence, the two substrates share the same transport system. The steady-state accumulation of glucuronate reflected the course of the pH gradient. It is concluded that glucuronate is transported as an anionic substrate by the protonated carrier, the driving force being the chemical gradient of the H⁺ (pH). The ternary carrier/H⁺/glc-COOO-complex is electroneutral and independent of the membrane potential. Simultaneous uptake of organic acids (acetic or propionic acid) which is also energized by the pH gradient led to a noncompetitive inhibition of glucuronate transport. Thus, manipulation of the driving force, pH, reducedV _T without affectingK _T . Kinetic and energetic arguments are presented which stronly suggest that only the protonated carrier is catalytically active inR. gracilis.     

Characterization of a Na+/glucose cotransporter cloned from rabbit small intestine

Summary The Na⁺/glucose cotransporter from rabbit intestinal brush border membranes has been cloned, sequenced, and expressed inXenopus oocytes. Injection of cloned RNA into oocytes increased Na⁺/sugar cotransport by three orders of magnitude. In this study, we have compared and contrasted the transport properties of this cloned protein expressed inXenopus oocytes with the native transporter present in rabbit intestinal brush borders. Initial rates of¹⁴C--methyl-d-glucopyranoside uptake into brush border membrane vesicles andXenopus oocytes were measured as a function of the external sodium, sugar, and phlorizin concentrations. Sugar uptake into oocytes and brush borders was Na⁺-dependent (Hill coefficient 1.5 and 1.7), phlorizin inhibitable (K _i 6 and 9 m), and saturable (-methyl-d-glucopyranosideK _m 110 and 570 m). The sugar specificity was examined by competition experiments, and in both cases the selectivity wasd-glucose>-methyl-d-glucopyranoside>d-galactose>3-O-methyl-d-glucoside. In view of the close similarity between the properties of the cloned protein expressed in oocytes and the native brush border transporter, we conclude that we have cloned the classical Na⁺/glucose cotransporter.     

Effects of the anion transport inhibitor,SITS, on the proximal straight tubule of the rabbit perfusedin vitro

Summary Conventional microelectrodes were used to study the effects of SITS (4-acetamido-4-isothiocyanostilbene-2,2-disulfonate) on the basolateral membrane potentialVbl of the superficial proximal straight tubule (PST) of the rabbit kidney perfusedin vitro. Addition of 0.1mm SITS to the bathing solution resulted in a slow and irreversible hyperpolarization ofVbl from –42.5±1.17 (37) mV to –77.3±0.83 (52) mV. The new steady-state potential was reached in 10 to 15 min and was accompanied by visible cell swelling. Associated with thisVbl hyperpolarization was: 1) an increased steady-state depolarization (from 6.2±0.77 (17) mV to 25.7±0.83 (29) mV) in response to increasing bath potassium concentration from 5 to 16.7mm (HK); 2) a decreased transient depolarization (from 19.8±1.88 (8) mV to 0.43±0.37 (8) mV) in response to decreasing bath bicarbonate concentration from 22 to 6.6mm at constant bath pH (L-HCO₃); and 3) inhibition of a depolarizing overshoot and a decreased steady-state depolarization (from 35.9±1.84 (12) mV to 4.7±1.37 (13) mV) in response to reducing bath sodium concentration from 144 to zero (0-Na). Sodium, chloride and NMDG (N-methyl-d-glucamine) were used as the substituting ions, respectively. These results are consistent with the presence of a coupled sodium-bicarbonate carrier in the basolateral membrane which is electrogenic and SITS inhibitable. Comparison of the time course of SITS effects on these ion-substitution responses suggests that the inhibition of the bicarbonate exit pathway(s) is the primary event and that the changes inVbl and in the steady-stateVbl responses to HK and 0-Na are secondary events which may be related to changes in intracellular composition and/or basolateral membrane properties.     

Isoforms of Na,K-ATPase inArtemia salina: II. Tissue distribution and kinetic characterization

Summary To characterize the molecular properties conveyed by the isoforms of the subunit of Na,K-ATPase, the two major transepithelial transporting organs in the brine shrimp (Artemia salina), the salt glands and intestines, were isolated in pure form. The isoforms were quantified by ATP-sensitive fluorescein isothiocyanate (FITC) labeling. The salt gland enzyme exhibits only the 1 isoform, whereas the intestinal enzyme exhibits both the 1 and the 2 isoforms. After 32 hours of development, Na,K-ATPase activity [in mol P_i/mg protein/hr (1u)] in whole homogenates was 32±6 in the salt glands and 12±3 in the intestinal preparations (mean±sem). The apparent half-maximal activation constants (K _1/2) of the salt gland enzyme as compared to the intestinal enzyme were 3.7±0.6mm vs. 23.5±4mm (P<0.01) for Na⁺, 16.6±2.2mm vs. 8.29±1.5mm for K⁺ (P<0.01), and 0.87±0.8mm vs. 0.79±1.1mm for ATP (NS). The apparentK _i's for ouabain inhibition were 1.1×10^–4 m vs. 2×10^–5 m, respectively. Treatment of whole homogenates with deoxycholic acid (DOC) produced a maximal Na,K-ATPase activation of 46% in the salt gland as compared to 23% in the intestinal enzyme. Similar differences were found with sodium dodecyl sulfate (SDS). The two distinct forms of Na,K-ATPase isolated from the brine shrimp differed markedly in three kinetic parameters as well as in detergent sensitivity. The differences inK _1/2 for Na⁺ and K⁺ are more marked than those reported for the mammalian Na,K-ATPase isoforms. These differences may be attributed to the relative abundances of the subunit isoforms; other potential determinants (e.g. differences in membrane lipids), however, have not been investigated.During the tenure of an Educational Commission For Foreign Medical Graduates Visiting Associate Professorship.