Comparative properties of sensitive to GABAA-ergic ligands, Cl(-), HCO3(-)-activated Mg2+-ATPase from brain plasma membranes of fish and rats

Action of Cl(-) + HCO3(-) ions on Mg2+-ATPase from brain plasma membranes of fish and rats has been studied. Maximal effect of the anions on the Mg2+-ATPase activity is revealed in the presence of 10 mM Cl(-) and 3 mM HCO3(-) at physiological values of pH of incubation medium. The studied Cl(-), HCO3(-)-activated Mg2+-ATPases of both animal species, by their sensitivity to SH-reagents (5,5-dithiobis-nitrobenzoic acid, N-ethylmaleimide), oligomycin, and orthovanadate, are similar to transport ATPase of the P-type, but differ from them by molecular properties and by sensitivity to ligands of GABAA-receptors. It has been established that the sensitive to GABAA-ergic ligands, Cl(-), HCO3(-)-activated Mg2+-ATPase from brain of the both animal species is protein of molecular mass around 300 kDa and of Stocks' radius 5.4 nm. In fish the enzyme is composed of one major unit of molecular mass approximately 56 kDa, while in rats--of three subunits of molecular masses about 57, 53, and 45 kDa. A functional and structural coupling of the ATP-hydrolyzing areas of the studied enzyme to sites of binding of GABAA-receptor ligands is suggested.     

Effect of picrotoxin on the Cl-activated ATPase microsomal fraction of bream brain (Abramis brama L.)

It is found that picrotoxine in range concentrations 0.1-10 microM stimulates the basal Mg(2+)-ATPase from microsomal fraction of fish bream (Abramis brama L.), however decreases activating effect of 10(-5) M GABA on the enzyme. The stimulative effect of picrotoxine dependants on duration of preincubation with microsomes. It was established that basal Mg(2+)-ATP-ase activity was activated by anions (Cl- > Br- > I-). The activated effect of anions on the Mg(2+)-ATP-ase is decreased in the presence 1 microM picrotoxine. It was shown that in the dependence on concentration of the Mg(2+)-ATP (0.2 or 1 mM) in the incubation medium the picrotoxine serves as on activator or inhibitor of the enzyme activity. It is supposed that picrotoxine allosterially influences on the enzyme by the receptor-dependent way.     

Phosphorylation of Cl-, HCO3--stimulated Mg2+-ATPase of plasma membranes of carp (Cyprinus carpio L.) brain sensitive to GABA(A)-ergic ligands

Phosphorylation of the sensitive to GABA(A)-ergic ligands Cl-, HCO3--stimulated Mg2+-ATPase of the plasma membranes from fish brain by [gamma-32P]ATP was investigated in the presence of Mg2+. It was established, that formation of the phosphoprotein at 0-1 degrees C is dependent on time incubation and concentration of Mg2+ in the incubation medium. Hydroxylamine (50 mM) and pH (10) completely inhibited formation of phosphorylated intermediate. Ions of Cl- (10 mM)+HCO3- (2 mM) and also GABA (1-100 microM) dephosphorylated the enzyme. The dephosphorylating effect of GABA on the membrane samples did not appear in the presence of bicuculline. o-Vanadate (10 microM) eliminates the dephosphorylating effect of anions and GABA on the phosphoprotein. It was established by SDS-PAAG electrophoresis and autoradiographia that investigated phosphorylation and GABA(A)-induced dephosphorylation is performed by the protein with molecular weight aproximately 56 kDa. Such molecular weight has a subunit which forms oligomer composition of the sensitive to GABA(A)-ergic ligands Cl-, HCO3--ATPase from fish brain. The obtained data demonstrated that Cl, HCO3- ATPase from fish brain can be directly phosphorylated by [gamma-32P]ATP in the presence of Mg2+ and forms the phosphorylation intermediate.     

Effect of GABAergic compounds on the anion-sensitive Mg2+-ATPase from bream (Abramis brama L.) brain

Preincubation of plasma membranes from bream brain with 10-8-10-4 M gamma-aminobutyric acid (GABA) or muscimol increased the anion-sensitive Mg2+-ATPase activity. The activating effect of neurotransmitters on the Mg2+-ATPase is enhanced with increasing preincubation time of the membranes with the ligands, decreases with increasing Mg2+-ATP concentration in the incubation medium, and is inhibited in the presence of the GABAa-receptor antagonist, bicuculline (90 microgr;M). The anions Cl-, Br-, and I- stimulate the basal Mg2+-ATPase activity, and an effect of 10-4 M GABA in the presence of anions was not found. It is supposed that GABAergic chemicals modify the anion-sensitive Mg2+-ATPase in a receptor-dependent way.     

Effect of eosin Y on Ca2+ -independent, Mg2+ -dependent ATPase activity of smooth muscle cell plasma membranes

The effect of eosin Y (2',4',5',7'-tetrabromofluorescin) on basic kinetic parameters of the reaction of Mg2+ -dependent hydrolysis of ATP catalysed "basal" Mg2+ -ATPase myometrial cells plasma membrane has been studied. The eosin Y (10-100 microM) inhibited initial maximal velocity of the "basal" Mg2+ -ATPase of plasma membrane assayed for Mg2+ and ATP. At the same time the given inhibitor reduces the affinity of Mg2+ -ATPase for ATP. However, the difficult effect of the inhibitor action is observed for Mg ions: eosin Y in concentration of 10-50 microM increases the enzyme affinity for the ion-activator, while in concentration of 100 microM the affinity of Mg2+ -ATPase for Mg2+ is reduced. An analysis of eosin Y effect on catalytic efficiency of "basal" Mg2+ -ATPase of plasma membrane has shown, that at saturating concentrations of ATP (1 mM) the enzyme activity is less sensitive to the action of inhibitor. On this basis the conclusion is made that ATP in high concentrations can compete with eosin Y for active centre of Mg2+ -ATPase of smooth muscle cells plasma membrane.     

Distributions of H+,K+-ATPase and Cl–,HCO3–-ATPase in micromere-derived cells of sea urchin embryos

In cultured cells derived from isolated micromeres of sea urchin eggs, H+,K+-ATPase activity, which became detectable simultaneously with the initiation of spicule formation, was localized in the plasma membrane and the microsome fractions. Activities of marker enzymes for plasma membrane, 5'-nucleotidase, Na+,K+-ATPase, and adenylate cyclase, were found to be high in the plasma membrane fraction. Considerable activity of rotenone-insensitive NADPH-cytochrome c reductase, a marker enzyme for microsome, was detectable in the microsome fraction. These fractions exhibited barely any appreciable activity of markers for the other organellae. H+,K+-ATPase in plasma membrane probably mediates H+ release from the cells, in which H+ is produced in overall reaction to form CaCO3, the main component of spicules, from Ca2+, CO2 and H2O. Cl-,HCO3(-)-ATPase activity was also found in these two fractions before and after the initiation of spicule formation. After initiation, the skeletal vacuole fraction was obtained from subcellular structures containing spicules. Considerable activity of Cl-,HCO3(-)-ATPase was observed in this fraction, which exhibited a weak activity of UDP-galactose: N-acetylglucosamine galactosyltransferase, a marker enzyme for Golgi body. Cl-,HCO3(-)-ATPase in the skeletal vacuole membrane probably mediates HCO3- transport into the vacuoles to supply HCO3- for spicule formation.     

Properties of Na+, K+-ATPase of liver plasma membranes in the hamster

This study was designed to establish the properties of liver plasma membranes (LPM) Na+,K+-ATPase in the hamster and to determine whether a similar assay may be used to measure enzyme activity in the hamster and in the rat. Maximal Na+,K+-ATPase activity was obtained when the assay medium contained 5 mM Mg APT2- with or without 1 mM free Mg2+, 120 mM Na+, 12,5 mM K+. The incubation must be performed at 37 degrees C, pH 7.4. In the absence of free Mg2+, the saturation curve with respect to the substrate Mg ATP2- resulted in biphasic complex kinetics with a maximal activity at a substrate concentration of 5 mM. In the presence of 1 mM free Mg2+ activation of Na+,K+-ATPase and modification of the kinetics were observed: the biphasic curve tended to disappear and to become of the Michaelis-Menten type. The apparent Km for Mg APT2- was 0.36 mM and the Vmax 34.5 mumol.h-1.mg protein-1. In the presence of 10 mM free Mg2+ a decrease in the Vmax was observed without any effect on the apparent Km for Mg APT2-. It is concluded that the same incubation medium may be used to assay LPM N+,K+-ATPase from hamster and rat and that the addition of 1 mM free Mg2+ to the incubation medium is recommended to obtain Michaelis-Menten kinetics in order to eliminate complex kinetics due to the absence of free Mg2+.     

Subcellular localization of prostaglandin E2 binding sites in bovine adrenal medulla

Effect of changing [K+], [Na+] and [Cl-] in nutrient solution on potential difference (PD) and resistance was studied in bullfrog antrum with and without nutrient HCO3(-) but with 95% O2/5% CO2 in both cases. In both cases, changing from 4 to 40 mM K+ gave about the same initial PD maximum (anomalous response) which was followed by a decrease below control level. Latter effect was much less with zero than with 25 mM HCO3(-). Changing from 102 to 8 mM Na+ gave initial normal PD response about the same in both cases. However, 10 min later the change in PD with zero HCO3(-) was insignificant but with 25 mM HCO3(-) the PD decreased (anomalous response of electrogenic NaCl symport). PD maxima due to K+ and Na+ were largely related to (Na+ + K+)-ATPase pump. Changes in nutrient Cl- from 81 to 8.1 mM gave only a decrease in PD (normal response). Initial PD increases are explained by relative increases in resistance of simple conductance pathways and of parallel pathways of (Na+ + K+)-ATPase pump and Na+/Cl- symport. Removal of HCO3(-) and concurrent reduction of pH modify resistance of these pathways.     

Potential difference responses due to K+, Na+ and Cl- changes in bullfrog antrum with and without HCO3-

The effect of changing [K+], [Na+] and [Cl-] in nutrient solution was studied in bullfrog antrum with and without HCO3- in nutrient. In 25 mM HCO3- (95% O2/5% CO2) and in zero HCO3- (100% O2), nutrient pH was maintained at 7.3. Changing from 4 to 40 mM K+ or from 81 to 8.1 mM Cl- gave a decrease 10 min later in transmucosal PD (nutrient became more negative)--a normal response. These responses were less in zero than in 25 mM HCO3-. A decrease from 102 to 8 mM Na+ decreased PD (anomalous response of electrogenic NaCl symport). This effect was attenuated or eliminated in zero HCO3-. In contrast, change from 4 to 40 mM K+ gave initial anomalous PD response and change from 102 to 8 mM Na+, initial normal PD response with either zero or 25 mM HCO3-. Both responses were associated with (Na+ + K+)-ATPase pump and were greater in zero than in 25 mM HCO3-. Initial PD increases in zero HCO3- are explained as due to increase in the resistance of passive conductance and/or NaCl symport pathways. Thus, removal of HCO3- modifies conductance pathways of nutrient membrane.     

Anion-dependent Mg2+ influx and a role for a vacuolar H+-ATPase in sheep ruminal epithelial cells

The K+-insensitive component of Mg2+ influx in primary culture of ruminal epithelial cells (REC) was examined by means of fluorescence techniques. The effects of extracellular anions, ruminal fermentation products, and transport inhibitors on the intracellular free Mg2+ concentration ([Mg2+]i), Mg2+ uptake, and intracellular pH were determined. Under control conditions (HEPES-buffered high-NaCl medium), the [Mg2+]i of REC increased from 0.56 +/- 0.14 to 0.76 +/- 0.06 mM, corresponding to a Mg2+ uptake rate of 15 microM/min. Exposure to butyrate did not affect Mg2+ uptake, but it was stimulated (by 84 +/- 19%) in the presence of CO2/HCO(-)3. In contrast, Mg2+ uptake was strongly diminished if REC were suspended in HCO(-)3-buffered high-KCl medium (22.3 +/- 4 microM/min) rather than in HEPES-buffered KCl medium (37.5 +/- 6 microM/min). After switching from high- to low-Cl- solution, [Mg2+]i was reduced from 0.64 +/- 0.09 to 0.32 +/- 0.16 mM and the CO2/HCO(-)3-stimulated Mg2+ uptake was completely inhibited. Bumetanide and furosemide blocked the rate of Mg2+ uptake by 64 and 40%, respectively. Specific blockers of vacuolar H+-ATPase reduced the [Mg2+]i (36%) and Mg2+ influx (38%) into REC. We interpret this data to mean that the K+-insensitive Mg2+ influx into REC is mediated by a cotransport of Mg2+ and Cl- and is energized by an H+-ATPase. The stimulation of Mg2+ transport by ruminal fermentation products may result from a modulation of the H+-ATPase activity.     

Effect of short-chain fatty acids on cyclic 3',5'-guanosine monophosphate-mediated colonic secretion

Short chain fatty acids (SCFA) prevent and reverse cyclic 3',5'-adenosine monophosphate (cAMP) but not Ca(2+)-mediated Cl- secretion. Mucosal [HCO3-]i has an opposite effect on these secretagogues. We examined whether SCFA and [HCO3-]i affect cyclic 3',5'-guanosine monophosphate (cGMP)-induced secretion. Stripped segments of male Sprague-Dawley rat (Rattus norvegicus) proximal and distal colon, and cultured T84 cells were studied in Using chambers, and pHi and [HCO3-]i were determined. Mucosal [cGMP] was measured in proximal colon. In T84 cells, the increase in Cl- secretion (measured as Isc) induced by mucosal 0.25 microM Escherichia coli heat-stable enterotoxin (STa) was prevented/reversed by bilateral 50 mM Na+ butyrate (71%/73%), acetate (58%/76%), propionate (68%/73%) and (poorly metabolized) isobutyrate (80%/79%). In proximal colon in HCO3- Ringer, basal Cl- secretion was not affected by [HCO3-]i or 25 mM butyrate. Mucosal 0.25 microM STa decreased net Na+ and Cl- absorption. Bilateral but not mucosal 25 mM SCFA reversed STa-induced effects on Na+ absorption and Cl- secretion. Bilateral and mucosal 25 mM SCFA but not [HCO3-]i prevented STa-induced Cl- secretion and increases in mucosal [cGMP]. STa did not produce Cl- secretion in distal colon. It was concluded that SCFA but not [HCO3-]i can prevent and reverse cGMP-induced colonic Cl- secretion.     

Molecular Weight and Subunit Composition of the Sensitive to GABA-Ergic Ligands Cl<Superscript>−</Superscript>/HCO<Stack><Subscript>3</Subscript><Superscript>−</Superscript></Stack>-Stimulated Mg<Superscript>2+</Superscript>-ATPase from Plasma Membrane of Rat Brain

The molecular weight and subunit composition of Cl-,HCO3(-)- and picrotoxin-stimulated Mg2+-ATPase from rat brain plasma membrane solubilized in sodium deoxycholate were studied by gel filtration chromatography. The enzyme activity eluted from a Sephacryl S-300 column in a single peak associated with a protein of molecular weight approximately 300 kD and a Stokes radius of 5.4 nm. The enzyme-enriched fraction, concentrated and denatured by SDS, migrated through a Sephacryl S-200 column as three peaks with molecular weights of approximately 57, 53, and 45 kD. SDS-PAGE also showed three major protein bands with molecular weights of about 57, 53, and 48 kD. The molecular weight and subunit composition of the Cl- and HCO3(-)-stimulated Mg2+-ATPase from neuronal membrane of rat brain are similar with the molecular properties of GABA(A)-benzodiazepine receptor complex from mammalian brain but are different from those of P-type transport ATPases.     

Investigation of adenosinetriphosphatase activity of rat liver and thymus cell nuclei]

The activity of ATPase was studied in highly purified rat liver and thymus cell nuclei, HCO3-, CO3(2-) and SO3(2-) stimulated nuclear ATPase in 1.5--2 times. HSO3- did not affect the enzyme activity, and NO3-, J-, ClO4-,F- and SCN- inhibited it. Bicarbonate increased V and decreased Ka for ATP. SCN- inhibited HCO3--ATPase activity non-competitively with respect to HCO3-. Mg2+-ATPase activity did not depend on pH, and HCO3-component of the activity was decreased under alkaline pH. Mg2+, Mn2+ and Co2+ increased the initial ATPase activity and helped its stimulation with HCO3-. Ba2+, Ni2+ and Zn2+ inhibited the ATPase activity, and Ca2+ did not affect it, Nuclear ATPase is sensitive to 2,4-dinitrophenol and DNAase. It is suggested that cell nuclei have their own H+-ATPase differing for some characteristics from mitochondrial H+-ATPase.     

Cystic fibrosis transmembrane conductance regulator regulates luminal Cl-/HCO3- exchange in mouse submandibular and pancreatic ducts.

We have demonstrated previously the regulation of Cl-/HCO3- exchange activity by the cystic fibrosis transmembrane conductance regulator (CFTR) in model systems of cells stably or transiently transfected with CFTR (Lee, M. G., Wigley, W. C., Zeng, W., Noel, L. E., Marino, C. R., Thomas, P. J., and Muallem, S. (1999) J. Biol. Chem. 274, 3414-3421). In the present work we examine the significance of this regulation in cells naturally expressing CFTR. These include the human colonic T84 cell line and the mouse submandibular gland and pancreatic ducts, tissues that express high levels of CFTR in the luminal membrane. As in heterologous expression systems, stimulation of T84 cells with forskolin increased the Cl-/HCO3- exchange activity independently of CFTR Cl- channel activity. Freshly isolated submandibular gland ducts from wild type mice showed variable Cl-/HCO3- exchange activity. Measurement of [Cl-]i revealed that this was largely the result of variable steady-state [Cl-]i. Membrane depolarization with 5 mM Ba2+ or 100 mM K+ increased and stabilized [Cl-]i. Under depolarized conditions wild type and DeltaF/DeltaF mice had comparable basal Cl-/HCO3- exchange activity. Notably, stimulation with forskolin increased Cl-/HCO3- exchange activity in submandibular gland ducts from wild type but not DeltaF/DeltaF mice. Microperfusion of the main pancreatic duct showed Cl-/HCO3- exchange activity in both the basolateral and luminal membranes. Stimulation of ducts from wild type animals with forskolin had no effect on basolateral but markedly stimulated luminal Cl-/HCO3- exchange activity. By contrast, forskolin had no effect on either basolateral or luminal Cl-/HCO3- exchange activity of ducts from DeltaF/DeltaF animals. We conclude that CFTR regulates luminal Cl-/HCO3- exchange activity in CFTR-expressing cells, and we discuss the possible physiological significance of these findings regarding cystic fibrosis.     

Cold inactivation of vacuolar proton-ATPases

Incubation of the reconstituted H+-ATPase from chromaffin granules on ice resulted in inactivation of the proton-pumping and ATPase activities of the enzyme. Inactivation was dependent on the presence of Mg2+, Cl-, and ATP during the incubation at low temperature. Approximately 1 mM ATP, 1 mM Mg2+, and 200 mM Cl- were required for maximum inactivation. Incubation for about 10 min on ice was required to achieve 50% inactivation. A much smaller decline in activity was observed when the enzyme was incubated at room temperature with the same chemicals. Inactivation in the cold resulted in the release of five polypeptides from the membrane with apparent molecular masses of 72, 57, 41, 34, and 33 kDa on sodium dodecyl sulfate gels. Three of the polypeptides of 72, 57, and 34 kDa were identified as subunits of vacuolar H+-ATPases by antibody cross-reactivity. Similar results were obtained with several other vacuolar H+-ATPases including those from plant sources. It was concluded that the catalytic sector of the enzyme is released from the H+-ATPase complex by cold treatment, resulting in inactivation of the enzyme.     

Salinity adaptation of HCO3--dependent ATPase activity in the gills of blue crab (Callinectes sapidus)

A bicarbonate-dependent ATPase (EC 3.6.1.3) was found in microsomal preparations from blue crab gills. When the crabs were transferred to low salinity (200 mosmolal) from seawater (1000 mosmolal), the HCO3- dependent ATPase increased in all gill pairs, reaching its new steady state in 2 weeks. The greatest increase occurred in the sixth and seventh gill pairs (approx. 2.5-fold). Maximal enzyme activity was observed at an Mg2+ concentration of 2 mM and an optimal pH of 7.8. The apparent Ka for HCO3- was found to be 8.9 mM. Kinetic analysis showed that low-salinity adaptation increased the Vmax without altering the Km for ATP. When the microsomes from high-salinity crab gills were treated with detergent or assayed at different temperatures, the total enzyme activity did not reach the activity levels after adaptation to low salinity. These results suggest that the alteration of HCO3- -ATPase activity may be due to synthesis, rather than modulation of membranes or of the existing enzyme activity.     

Imidazole chloride and tris-chloride substitute for sodium chloride in inducing high-affinity AdoPP[NH]P binding to (Na+ + K+)-ATPase

Optimal binding of [2,8-3H]AdoPP[NH]P to (Na+ + K+)-ATPase requires 25 mM Na+ (Cl-), 50 mM imidazole+ (Cl-) or 50 mM Tris+ (Cl-). Chloride is essential as counterion. We conclude that imidazole+ and Tris+ are able to bind to the Na+ site, and recommend the use of dilute buffers for studying the partial reactions of (Na+ + K+)-ATPase. In NaCl or the substituting buffers the dissociation constant for the enzyme-AdoPP[NH]P complex at 0 degrees C and pH 7.25 is 0.4 microM, whereas in millimolar MgCl2 it is about 2 microM. These distinct levels in affinity with MgCl2 as compared to NaCl, together with the MgCl2-dependence of photolabelling of the enzyme with ATP analogues (Rempeters, G. and Schoner, W. (1981) Eur. J. Biochem. 121, 131-137), suggest significant changes within the substrate site of (Na+ + K+)-ATPase upon binding of Mg2+ (Cl-)2.     

Effects of ATP and monovalent cations on Mg2+ inhibition of (Na,K)-ATPase

The hydrolysis of ATP catalyzed by purified (Na,K)-ATPase from pig kidney was more sensitive to Mg2+ inhibition when measured in the presence of saturating Na+ and K+ concentrations [(Na,K)-ATPase] than in the presence of Na+ alone, either at saturating [(Na,Na)-ATPase] or limiting [(Na,0)-ATPase] Na+ concentrations. This was observed at two extreme concentrations of ATP (3 mM where the low-affinity site is involved and 3 microM where only the catalytic site is relevant), although Mg2+ inhibition was higher at low ATP concentration. In the case of (Na,Na)-ATPase activity, inhibition was barely observed even at 10 mM free Mg2+ when ATP was 3 mM. When (Na,K)-ATPase activity was measured at different fixed K+ concentrations the apparent Ki for Mg2+ inhibition was lower at higher monovalent cation concentration. When K+ was replaced by its congeners (Rb+, NH+4, Li+), Mg2+ inhibition was more pronounced in those cases in which the dephosphorylating cation forms a tighter enzyme-cation complex after dephosphorylation. This effect was independent of the ATP concentration, although inhibition was more marked at lower ATP for all the dephosphorylating cations. The K0.5 for ATP activation at its low-affinity site, when measured in the presence of different dephosphorylating cations, increased following the sequence Rb+ greater than K+ greater than NH+4 greater than Li+ greater than none. The K0.5 values were lower with 0.05 mM than with 10 mM free Mg2+ but the order was not modified. The trypsin inactivation pattern of (Na,K)-ATPase indicated that Mg2+ kept the enzyme in an E1 state. Addition of K+ changed the inactivation into that observed with the E2 enzyme form. On the other hand, K+ kept the enzyme in an E2 state and addition of Mg2+ changed it to an E1 form. The K0.5 for KCl-induced E1-to-E2 transformation (observed by trypsin inactivation profile) in the presence of 3 mM MgCl2 was about 0.9 mM. These results concur with two mechanisms for free Mg2+ inhibition of (Na,K)-ATPase: "product" and dead-end. The first would result from Mg2+ interaction with the enzyme in the E2(K) occluded state whereas the second would be brought about by a Mg2+-enzyme complex with the enzyme in an E1 state.     

The regulation of intracellular pH in monkey kidney epithelial cells (BSC-1). Roles of Na+/H+ antiport, Na+-HCO3(-)-(NaCO3-) symport, and Cl-/HCO3- exchange

Using the pH-sensitive absorbance of 5 (and 6)-carboxy-4',5'- dimethylfluorescein, we investigated the regulation of cytoplasmic pH (pHi) in monkey kidney epithelial cells (BSC-1). In the absence of HCO3-, pHi is 7.15 +/- 0.1, which is not significantly different from pHi in 28 mM HCO3-, 5% CO2 (7.21 +/- 0.07). After an acid load, the cells regulate pHi in the absence of HCO3- by a Na+ (or Li+)-dependent, amiloride-inhibitable mechanism (indicative of Na+/H+ antiport). In 28 mM HCO3-, while still dependent on Na+, this regulation is only blocked in part by 1 mM amiloride. A partial block is also observed with 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) (1 mM). With cells pretreated with DIDS, 1 mM amiloride nearly totally inhibits this regulation. Cl- had no effect on pHi regulation in the acidic range. In HCO3(-)-free saline, Na+ removal leads to an amiloride-insensitive acidification, which is dependent on Ca2+. In 28 mM HCO3-, Na+ (and Ca2+) removal led to a pronounced reversible and DIDS-sensitive acidification. When HCO3- was lowered from 46 to 10 mM at constant pCO2 (5%), pHi dropped by a DIDS-sensitive mechanism. Identical changes in pHo (7.6 to 6.9) in the nominal absence of HCO3- led to smaller changes of pHi. In the presence but not in the absence of HCO3-, removal of Cl- led to a DIDS-sensitive alkalinization. This was also observed in the nominal absence of Na+, which leads to a sustained acidification. It is concluded that in nominally bicarbonate-free saline, the amiloride-sensitive Na+/H+ antiport is the predominant mechanism of pHi regulation at acidic pHi, while being relatively inactive at physiological values of pHi. In bicarbonate saline, two other mechanisms effect pHi regulation: a DIDS-sensitive Na+-HCO3- symport, which contributes to cytoplasmic alkalinization, and a DIDS-sensitive Cl-/HCO3- exchange, which is apparently independent of Na+.     

Independence of apical Cl-/HCO3- exchange and anion conductance in duodenal HCO3- secretion

Reduced gastrointestinal HCO3- secretion contributes to malabsorption and obstructive syndromes in cystic fibrosis. The apical HCO3- transport pathways in these organs have not been defined. We therefore assessed the involvement of apical Cl-/HCO3- exchangers and anion conductances in basal and cAMP-stimulated duodenal HCO3- secretion. Muscle-stripped rat and rabbit proximal duodena were mounted in Ussing chambers, and electrical parameters, HCO3- secretion rates, and 36Cl-, 22Na+, and 3H+ mannitol fluxes were assessed. mRNA expression levels were measured by a quantitative PCR technique. Removal of Cl- from or addition of 1 mM DIDS to the luminal perfusate markedly decreased basal HCO3- secretion but did not influence the HCO3- secretory response to 8-bromo-cAMP, which was inhibited by luminal 5-nitro-2-(3-phenylpropylamino)-benzoate. Bidirectional 22Na+ and 36Cl- flux measurements demonstrated an inhibition rather than a stimulation of apical anion exchange during cAMP-stimulated HCO3- secretion. The ratio of Cl- to HCO3- in the anion secretory response was compatible with both Cl- and HCO3- being secreted via the CFTR anion channel. CFTR expression was very high in the duodenal mucosa of both species. We conclude that in rat and rabbit duodena, an apical Cl-/HCO3- exchanger mediates a significant part of basal HCO3- secretion but is not involved in the HCO3- secretory response to cAMP analogs. The inhibitor profile, the strong predominance of Cl- over HCO3- in the anion secretory response, and the high duodenal CFTR expression levels suggest that a major portion of cAMP-stimulated duodenal HCO3- secretion is directly mediated by CFTR.