Sodium-bicarbonate cotransport occurs in rat kidney cortical membranes but not in rat small intestinal basolateral membranes

Intestinal ion transport in NKCC1-deficient mice

The Na(+)-K(+)-2Cl(-) cotransporter (NKCC1) located on the basolateral membrane of intestinal epithelia has been postulated to be the major basolateral Cl(-) entry pathway. With targeted mutagenesis, mice deficient in the NKCC1 protein were generated. The basal short-circuit current did not differ between normal and NKCC1 -/- jejuna. In the -/- jejuna, the forskolin response (22 microA/cm(2); bumetanide insensitive) was significantly attenuated compared with the bumetanide-sensitive response (52 microA/cm(2)) in normal tissue. Ion-replacement studies demonstrated that the forskolin response in the NKCC1 -/- jejuna was HCO(3)(-) dependent, whereas in the normal jejuna it was independent of the HCO(3)(-) concentration in the buffer. NKCC1 -/- ceca exhibited a forskolin response that did not differ significantly from that of normal ceca, but unlike that of normal ceca, was bumetanide insensitive. Ion-substitution studies suggested that basolateral HCO(3)(-) as well as Cl(-) entry (via non-NKCC1) paths played a role in the NKCC1 -/- secretory response. In contrast to cystic fibrosis mice, which lack both basal and stimulated Cl(-) secretion and exhibit severe intestinal pathology, the absence of intestinal pathology in NKCC1 -/- mice likely reflects the ability of the intestine to secrete HCO(3)(-) and Cl(-) by basolateral entry mechanisms independent of NKCC1.

A chloride-activated Na(+)/HCO(3)(-)-coupled transport activity in corneal endothelial membranes

Investigations of corneal endothelium were made to resolve the apparent contradiction of the presence of sodium/bicarbonate cotransporter (NBC) in fresh and cultured cells and NBC's reported absence in isolated plasma membrane vesicles. Gradient-driven ion fluxes into the vesicles were measured. Short-term incubations (0-30 s) showed the presence of a bicarbonate-dependent inward sodium flux (BDSF), which was active when the insides of the vesicles were preloaded with chloride ions. The BDSF was absent if chloride was present only externally to the vesicles. Chloride at concentrations between 30 and 40 mM inside the vesicle had its maximum effect on BDSF. Other anions (acetate, thiocyanate, or gluconate) inside the vesicles did not mimic the chloride effect. Associated with the net inward sodium flux was a net inward bicarbonate flux. Hill plots of sodium influx with respect to external bicarbonate concentrations indicated that the stoichiometry of the net transfer was 1.7 +/- 0.2 (mean +/- standard error, n = 5) bicarbonate ions for each sodium ion transported. There was no net chloride flux found across the membrane vesicles. The finding of a novel chloride-activated NBC activity fully resolves the apparent contradiction between whole-cell and membrane vesicle preparations.

Regulation of intracellular pH during H+-coupled oligopeptide absorption in enterocytes from guinea-pig ileum

1. The mechanisms for regulating the intracellular pH (pHi) level during oligopeptide absorption were investigated in the enterocytes from guinea-pig ileum by identifying the acid-base transporters responsible for extruding H+ that enters the cell through the H+-oligopeptide cotransporter. 2. The pHi level was measured by microfluorometry in an isolated villus tip loaded with the pH-sensitive fluoroprobe 2'7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). The oligopeptide-induced increment in the short-circuit current (Isc) was determined in a mucosal sheet in Ussing chambers. A CO2/HCO3--buffered solution was used. 3. The superfusion of glycylglycine (Gly-Gly, l0 mM) caused a decrease in pHi level, which returned to the basal level after removing Gly-Gly. This pHi recovery was strongly dependent on extracellular Na+. Amiloride partially inhibited the pHi recovery rate with an IC50 value of 41 microM, the maximum inhibition being approximately 70%. In the presence of amiloride at its maximum concentration (0.3 mM), the addition of 0.6 mM DIDS caused a further decrease, but did not abolish the pHi recovery rate. In the absence of CO2 and HCO3-, the pHi recovery was almost completely abolished by 0.3 mM amiloride. 4. The intracellular H+ accumulation induced by 0.3 mM amiloride or by 0.6 mM DIDS, as estimated from the pHi decrease and buffer capacity, was significantly greater during Gly-Gly superfusion than under resting conditions. 5. The increase in Isc induced by luminal glycylproline was attenuated by either removing serosal Na+ or by adding 0.5 mM amiloride or 0.6 mM DIDS to the serosal side. 6. We conclude that both Na+-dependent, amiloride-sensitive acid extrusion, probably by the Na+-H+ exchanger, and Na+- and HCO3--dependent, DIDS-sensitive acid extrusion, possibly by the Na+-HCO3- cotransporter, are involved in extruding H+ that enters cells by the H+-oligopeptide cotransport. It is proposed that these acid extrusion (or base loading) mechanisms are present in the basolateral membrane and are important for maintaining oligopeptide absorption, as well as the acid extrusion mechanism in the apical membrane.

Na+-HCO3(-) cotransporter and intracellular pH regulation in chicken enterocytes

The current studies examine the presence of the Na+-HCO3(-) cotransporter in chicken enterocytes and its role in cytosolic pH (pHi) regulation. The pH-sensitive dye 2',7'-bis(carboxyethyl)-5,6-carboxy-fluorescein (BCECF) was used to monitor pHi. Under resting conditions, pHi was 7.25 in solutions buffered with bis(2-hydroxyethyl)-1-piperazine ethanesulphonic acid (HEPES) and 7.17 in those buffered with HCO3(-). Removal of external Na+ decreased pHi and readdition of Na+ rapidly increased pHi towards the control values. These Na+-dependent changes were greater in HCO3(-)- than in HEPES-buffered solutions. In HCO3- - free solutions the Na+-dependent changes in pHi were prevented by 5-(N-ethyl-N-isopropyl)-amiloride (EIPA) and unaffected by 4,4'-diisothiocyanatostilbene disulphonic acid (H2-DIDS). In the presence of HCO3-, the Na+-induced changes in pHi were sensitive to both EIPA and H2-DIDS. In the presence of EIPA, cells partially recovered from a moderate acid load only when both Na+ and HCO3- were present. This pHi recovery, which was EIPA resistant, and dependent on Na+ and HCO3-, was inhibited by H2-DIDS and occurred at equal rates in both Cl--containing and Cl--free solutions. Kinetic analysis of the rate of HCO3- and Na+-dependent pHi recovery from an acid load as a function of the Na+ concentration revealed first-order kinetics with a Michaelis constant, Km, of 11 mmol/l Na+. It is concluded that in HCO3(-) buffered solutions both the Na+/H+ exchanger and the Na+-HCO3(-) cotransporter participate in setting the resting pHi in isolated chicken enterocytes and help the recovery from acid loads.

Bicarbonate secretion in rabbit ileum: electrogenicity, ion dependence, and effects of cyclic nucleotides

BACKGROUND

Ileal HCO3- secretion is not well understood. The aim of this study was to examine its Na+ and Cl- dependencies, electrogenicity, and responses to amiloride, 4-acetamido-4'-isothiocyano-stilbene-2,2'-disulfonate (SITS), and cyclic nucleotides.

METHODS

The serosa to mucosa HCO3- flux (Jsm) across rabbit ileal mucosa mounted between HCO(3-)-free mucosal solution and HCO(3-)-containing serosal solutions was determined by titration.

RESULTS

In SO4(2-)-containing Ringer's solution, Jsm varied with [Na+] in two phases, one with a high and one with a low affinity for Na+; amiloride inhibited the high- and SITS inhibited the low-affinity phase. Switching from SO4(2-)- to Cl(-)-containing Ringer's solution caused a SITS-inhibitable 42% increase in Jsm. Changes in Jsm were coupled 3:2 with changes in short-circuit current. Cyclic nucleotide effects on Jsm were as follows. In SO4(2-)-containing Ringer's solution at 141 (but not 80) mmol/L Na+, theophylline caused equal increases in Jsm and short-circuit current that equaled the combined effects of 8-Br-5'-cyclic guanosine monophosphate (cGMP) and 8-Br-5'-cyclic adenosine monophosphate (cAMP). Serosal SITS blocked these effects, but amiloride did not. In Cl(-)-containing Ringer's solution, theophylline and bumetanide together (but not separately) increased Jsm.

CONCLUSIONS

(1) Basolateral HCO3- entry occurs via Na+/H exchange and a SITS-inhibitable process (Na(+)-HCO3- cotransport?). (2) Most HCO3- exit across the brush border occurs by a Cl(-)-independent process and some by Cl-/HCO3- exchange. (3) At low cellular [Cl-], HCO3- can be secreted via anion channels activated by cAMP and cGMP. (4) Ileal HCO3- secretion is electrogenic.

Basolateral Cl/HCO3 exchange in rat jejunum: the effect of sodium

Basolateral membrane vesicles isolated from rat jejunum were used to characterize a Cl/HCO3 exchange mechanism previously evidenced. Cl uptake experiments provided no evidence for Cl/OH countertransport, confirming anyhow the presence of Cl/HCO3 antiport, which was inhibited by 2 mM furosemide and unaffected by 2 mM amiloride. An outwardly directed Na gradient stimulated Cl uptake and this effect was increased if Na was present at both vesicle surfaces. To investigate the mechanism of coupling between Na and the transport protein, we performed Na uptake experiments. Na uptake was unaffected by cis-bicarbonate and trans-Cl gradients; the reversal of anion gradients was still ineffective. Similar results were obtained when a pH difference across the membrane vesicles was imposed. This study seems to suggest that Na is not transported by the Cl/HCO3 exchanger and that another mode of Na dependence must be taken into account.

Sodium transport in human intestinal basolateral membrane vesicles

The current investigation was aimed at characterizing transport pathways for Na+ in basolateral membrane vesicles (BLMV) isolated from organ donor jejunum and ileum. An outward proton gradient [pH inside, 5.5; pH outside, 7.5] led to a 4-5-fold stimulation of transport rates compared with the absence of proton-gradient conditions in both human jejunal and ileal BLMV. Voltage-clamping the vesicles (K+ inside = K+ outside + valinomycin) reduced the uptake of 22Na by 20%, indicating a minor conductive component of Na+ transport. Uptake of 22Na (1 mmol/L) in voltage-clamped BLMV was inhibited 70% by 2 mmol/L amiloride. Li+ and NH4+ inhibited transport of 22Na into voltage-clamped BLMV. Transport of Na+ exhibited saturation kinetics, and the Michaelis constant (Km) and Vmax values for jejunum and ileum were similar [Km, 27 +/- 3 mmol/L (jejunum) and 18 +/- 2 mmol/L (ileum); Vmax, 19 +/- 2 nmol.mg protein-1.min-1 (jejunum) and 16 +/- 1 nmol.mg protein-1.min-1 (ileum)]. Vmax values were < 15% of those reported for brush border membrane, whereas Km values were comparable. The results show that Na+ transport in human jejunal and ileal BLMV occurs via an Na+/H+ exchanger and a minor conductive pathway.

Basolateral Cl-/HCO3- exchange in rat jejunum: evidence from H14CO3- uptake in membrane vesicles

Bicarbonate transport across basolateral membrane vesicles from rat jejunal enterocyte was studied at 28 degrees C and pH 8.2. These experimental conditions make possible the determination of [14C]bicarbonate uptake. Inward gradients of Na+, K+, and Li+ did not stimulate HCO3- uptake, suggesting that a cotransport mechanism with these cations does not occur. On the contrary a countertransport of bicarbonate driven by a Cl- gradient was evidenced. The ability of other inorganic anions to exchange with HCO3- was examined and results indicate that Cl- can be substituted by NO3-, Br- and SCN-. The Cl(-)-dependent HCO3- uptake was strongly inhibited by SITS and DIDS, whereas acetazolamide was ineffective: thus transfer of labelled CO2 is eliminated as a possible mode of HCO3- permeation. HCO3- uptake was also affected by the presence of superimposed membrane potentials, suggesting that a HCO3- conductive pathway is present in the jejunal basolateral membrane. These results show that there are no fundamental differences between data obtained performing H14CO3- and 36Cl- (previously reported) uptake experiments.

Cl/HCO3 exchange in the basolateral membrane domain of rat jejunal enterocyte

Basolateral membrane vesicles isolated from rat jejunal enterocyte and well purified from brush border contamination were tested to examine Cl and HCO3 movements. Uptake experiments provided no evidence for a coupling between Na and HCO3 fluxes; K-HCO3 and K-Cl cotransports also could be excluded. Transport studies revealed the presence of a Cl/HCO3 exchanger accepting other anions and inhibitable by the disulfonic stilbenes SITS and DIDS. We can exclude that the evidenced HCO3-dependent Cl uptake is due to brush border contamination, since in jejunal brush border membranes this mechanism, if present, has a very low transport rate. Besides the Cl/HCO3 antiporter, a Cl-conductive pathway seems to exist in jejunal basolateral membranes.

Sodium uptake across basolateral membrane of rat distal colon. Evidence for Na-H exchange and Na-anion cotransport

This study sought to characterize the mechanism of Na transport across basolateral membrane vesicles of rat distal colon. Both an outward proton gradient and an inward bicarbonate gradient stimulated 22Na uptake. Proton gradient-stimulated 22Na uptake was activated severalfold by the additional presence of an inward bicarbonate gradient, and bicarbonate gradient-stimulated 22Na uptake was significantly enhanced by an imposed intravesicular membrane positive potential. 0.1 mM amiloride inhibited both proton gradient- and bicarbonate gradient-stimulated 22Na uptake by 80 and 95%, respectively, while 1 mM 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) inhibited both proton gradient- and bicarbonate gradient-stimulated 22Na uptake by 40 and 80%, respectively. Both proton gradient- and bicarbonate gradient-stimulated 22Na uptake saturated as a function of increasing Na concentration: the apparent kinetic constants (Km) for Na for the DIDS-insensitive component of proton gradient-stimulated 22Na uptake was 46.4 mM, while the DIDS-sensitive component of proton gradient- and bicarbonate gradient-stimulated 22Na uptake had Km for Na of 8.1 and 6.4 mM, respectively. Amiloride inhibited both DIDS-insensitive proton gradient- and bicarbonate gradient-stimulated 22Na uptake with an inhibitory constant (Ki) of approximately 35 and 1 microM, respectively. We conclude from these results that proton gradient-stimulated 22Na uptake represents both DIDS-insensitive Na-H exchange and DIDS-sensitive electrogenic Na-OH cotransport, and that the DIDS-sensitive component of proton gradient-stimulated 22Na uptake and bicarbonate gradient-stimulated 22Na uptake may represent the same electrogenic Na-anion cotransport process.

Characterization of basolateral membrane Na/H antiport in rat jejunum

Na uptake studies were performed in order to examine the activity of a Na/H exchanger in basolateral membrane vesicles isolated from rat jejunum. Experiments were carried out under voltage-clamped conditions in order to avoid electrodiffusional ionic movements. 1 mM Na uptake was found to be enhanced by an outward proton gradient and its initial rate was further increased by the presence of monensin or nigericin. The pH gradient-driven Na uptake was inhibited by 2 mM amiloride and unaffected by 0.1 mM 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid. The initial rate of the proton gradient-induced Na uptake was saturable with respect to external Na, with a Km of 13.6 +/- 1.4 mM and a Vmax of 35.4 +/- 2.2 nmol/mg protein per min. Li competed with Na for the exchange process, whereas K, Rb, Cs, tetramethylammonium had no effect. We conclude that rat jejunal basolateral membrane contains a Na/H exchanger whose properties are similar to those of the antiporter identified in the brush-border membrane.

Na+/H+ exchange mechanism in the basolateral membrane of the rat enterocyte

Basolateral membrane vesicles from rat jejunal enterocytes, especially purified of brush-border contamination, were used for Na+ uptake. The basolateral membrane vesicles are osmotically active and under our experimental conditions Na+ binding is much lower than transport. An outwardly directed proton gradient stimulates Na+ uptake at both 5 microM and 5 mM concentrations. The proton gradient effect can be inhibited completely by 2 mM amiloride and partially by either FCCP or NH4Cl (NH3 diffusion). Membrane potential effects can be excluded by having valinomycin plus K+ on both sides of the vesicles. These results suggest that there is an Na+/H+ exchanger in the basolateral membrane of rat enterocytes.