The colon carcinoma cell line Caco-2 contains an H+/K(+)-ATPase that contributes to intracellular pH regulation

The electroneutral Na+-HCO3- cotransporter NBCn1 (SLC4A7) modulates colonic enterocyte pHi, proliferation, and migration

NBCn1 (SLC4A7) is one of the two major Na+-HCO3- cotransporters in the human colonic epithelium, expressed predominantly in the highly proliferating colonocytes at the cryptal base. Increased NBCn1 expression levels are reported in tumors, including colorectal cancer. The study explores its importance for maintenance of the intracellular pH (pHi), as well as the proliferative, adhesive, and migratory behavior of the self-differentiating Caco2BBe colonic tumor cell line. In the self-differentiating Caco2BBe cells, NBCn1 mRNA was highly expressed from the proliferative stage until full differentiation. The downregulation of NBCn1 expression by RNA interference affected proliferation and differentiation and decreased intracellular pH (pHi) of the cells in correlation with the degree of knockdown. In addition, a disturbed cell adhesion and reduced migratory speed were associated with NBCn1 knockdown. Murine colonic Nbcn1-/- enteroids also displayed reduced proliferative activity. In the migrating Caco2BBe cells, NBCn1 was found at the leading edge and in colocalization with the focal adhesion markers vinculin and paxillin, which suggests that NBCn1 is involved in the establishment of cell-matrix adhesion. Our data highlight the physiological significance of NBCn1 in modulating epithelial pH homeostasis and cell-matrix interactions in the proliferative region of the colonic epithelium and unravel the molecular mechanism behind pathological overexpression of this transporter in human colorectal cancers.NEW & NOTEWORTHY The transporter NBCn1 plays a central role in maintaining homeostasis within Caco2BBe colonic epithelial cells through its regulation of intracellular pH, matrix adhesion, migration, and proliferation. These observations yield valuable insights into the molecular mechanism of the aberrant upregulation of this transporter in human colorectal cancers.

Colonic Potassium Absorption and Secretion in Health and Disease

The colon has large capacities for K⁺ absorption and K⁺ secretion, but its role in maintaining K⁺ homeostasis is often overlooked. For many years, passive diffusion and/or solvent drag were thought to be the primary mechanisms for K⁺ absorption in human and animal colon. However, it is now clear that apical H⁺ ,K⁺ -ATPase, in coordination with basolateral K⁺ -Cl^- cotransport and/or K⁺ and Cl^- channels operating in parallel, mediate electroneutral K⁺ absorption in animal colon. We now know that K⁺ absorption in rat colon reflects ouabain-sensitive and ouabain-insensitive apical H⁺ ,K⁺ -ATPase activities. Ouabain-insensitive and ouabain-sensitive H⁺ ,K⁺ -ATPases are localized in surface and crypt cells, respectively. Colonic H⁺ ,K⁺ -ATPase consists of α- (HKC_α ) and β- (HKC_β ) subunits which, when coexpressed, exhibit ouabain-insensitive H⁺ ,K⁺ -ATPase activity in HEK293 cells, while HKC_α coexpressed with the gastric β-subunit exhibits ouabain-sensitive H⁺ ,K⁺ -ATPase activity in Xenopus oocytes. Aldosterone enhances apical H⁺ ,K⁺ -ATPase activity, HKC_α specific mRNA and protein expression, and K⁺ absorption. Active K⁺ secretion, on the other hand, is mediated by apical K⁺ channels operating in a coordinated way with the basolateral Na⁺ -K⁺ -2Cl^- cotransporter. Both Ca²⁺ -activated intermediate conductance K⁺ (IK) and large conductance K⁺ (BK) channels are located in the apical membrane of colonic epithelia. IK channel-mediated K⁺ efflux provides the driving force for Cl^- secretion, while BK channels mediate active (e.g., cAMP-activated) K⁺ secretion. BK channel expression and activity are increased in patients with end-stage renal disease and ulcerative colitis. This review summarizes the role of apical H⁺ ,K⁺ -ATPase in K⁺ absorption, and apical BK channel function in K⁺ secretion in health and disease. © 2018 American Physiological Society. Compr Physiol 8:1513-1536, 2018.

Relationship between intracellular pH, metabolic co-factors and caspase-3 activation in cancer cells during apoptosis

A complex cascade of molecular events occurs in apoptotic cells but cell-to-cell variability significantly complicates determination of the order and interconnections between different processes. For better understanding of the mechanisms of programmed cell death, dynamic simultaneous registration of several parameters is required. In this paper we used multiparameter fluorescence microscopy to analyze energy metabolism, intracellular pH and caspase-3 activation in living cancer cells in vitro during staurosporine-induced apoptosis. We performed metabolic imaging of two co-factors, NAD(P)H and FAD, and used the genetically encoded pH-indicator SypHer1 and the FRET-based sensor for caspase-3 activity, mKate2-DEVD-iRFP, to visualize these parameters by confocal fluorescence microscopy and two-photon fluorescence lifetime imaging microscopy. The correlation between energy metabolism, intracellular pH and caspase-3 activation and their dynamic changes were studied in CT26 cancer cells during apoptosis. Induction of apoptosis was accompanied by a switch to oxidative phosphorylation, cytosol acidification and caspase-3 activation. We showed that alterations in cytosolic pH and the activation of oxidative phosphorylation are relatively early events associated with the induction of apoptosis.

Apical acidity decreases inhibitory effect of omeprazole on Mg(2+) absorption and claudin-7 and -12 expression in Caco-2 monolayers

Clinical studies reported hypomagnesaemia in long-term omeprazole usage that was probably due to intestinal Mg(2+) wasting. Our previous report demonstrated the inhibitory effect of omeprazole on passive Mg(2+) transport across Caco-2 monolayers. The present study aimed to identify the underlying mechanism of omeprazole suppression of passive Mg(2+) absorption. By using Caco-2 monolayers, we demonstrated a potent inhibitory effect of omeprazole on passive Mg(2+), but not Ca(2+), transport across Caco-2 monolayers. Omeprazole shifted the %maximum passive Mg(2+) transport-Mg(2+) concentration curves to the right, and increased the half maximal effective concentration of those dose-response curves, indicating a lower Mg(2+) affinity of the paracellular channel. By continually monitoring the apical pH, we showed that omeprazole suppressed apical acid accumulation. Neomycin and spermine had no effect on passive Mg(2+) transport of either control or omeprazole treated monolayers, indicating that omeprazole suppressed passive Mg(2+) transport in a calcium sensing receptor (CaSR)-independent manner. The results of western blot analysis showed that omeprazole significantly suppressed claudin (Cldn)-7 and -12, but not Cldn-2, expression in Caco-2 cells. By using apical solution of pH 5.5, 6.0, 6.5, and 7.0, we found that apical acidity markedly increased passive Mg(2+) transport, Mg(2+) affinity of the paracellular channel, and Cldn-7 and -12 expression in Caco-2 monolayers. Apical acidity abolished the inhibitory effect of omeprazole on passive Mg(2+) transport and Cldn-7 and -12 expression. Our results provided the evidence for the regulation of intestinal passive Mg(2+) absorption by luminal acidity-induced increase in Cldn-7 and -12 expression.

Lansoprazole Novel Effector Sites Revealed by Autoradiography: Relation to Helicobacter pylori, Colon, Esophagus and Others

Lansoprazole uptake sites by two kinds of autoradiographic procedures were compared with recent literature. The uptake sites have been seen in the Helicobacter pylori, colonic epithelial cells, inflammatory cells, peripheral autonomic nerves and enterochromaffinlike cells as well as gastric parietal cells. Each uptake sites corresponded to the reported localization of P-type ATPase or acidic compartment.

Omeprazole decreases magnesium transport across Caco-2 monolayers

AIM

To elucidate the effect and underlying mechanisms of omeprazole action on Mg(2+) transport across the intestinal epithelium.

METHODS

Caco-2 monolayers were cultured in various dose omeprazole-containing media for 14 or 21 d before being inserted into a modified Ussing chamber apparatus to investigate the bi-directional Mg(2+) transport and electrical parameters. Paracellular permeability of the monolayer was also observed by the dilution potential technique and a cation permeability study. An Arrhenius plot was performed to elucidate the activation energy of passive Mg(2+) transport across the Caco-2 monolayers.

RESULTS

Both apical to basolateral and basolateral to apical passive Mg(2+) fluxes of omeprazole-treated epithelium were decreased in a dose- and time-dependent manner. Omeprazole also decreased the paracellular cation selectivity and changed the paracellular selective permeability profile of Caco-2 epithelium to Li(+), Na(+), K(+), Rb(+), and Cs(+) from series VII to series VI of the Eisenman sequence. The Arrhenius plot revealed the higher activation energy for passive Mg(2+) transport in omeprazole-treated epithelium than that of control epithelium, indicating that omeprazole affected the paracellular channel of Caco-2 epithelium in such a way that Mg(2+) movement was impeded.

CONCLUSION

Omeprazole decreased paracellular cation permeability and increased the activation energy for passive Mg(2+) transport of Caco-2 monolayers that led to the suppression of passive Mg(2+) absorption.

Membrane mechanisms for electrogenic Na(+)-independent L-alanine transport in the lizard duodenal mucosa

The active Na(+)-independent transport of L-alanine across the duodenal mucosa of the lizard Gallotia galloti was studied in Ussing-type chambers using a computer-controlled voltage clamp. Addition of L-alanine to the Na(+)-free bathing solutions resulted in a significant L-alanine absorption (J(net)) that was paralleled by an increase in transepithelial short-circuit current (I(sc)) and potential difference (PD) without apparent changes in the tissue conductance. The concentration dependence of J(net), PD, and I(sc) displayed Michaelis-Menten kinetics. L-alanine-induced electrical changes were completely inhibited by external alkaline pH or by the H(+)-ionophore carbonyl cyanide m-chlorophenyl-hydrazone in the bathing solution. The alanine-induced electrogenicity was dependent on the presence of extracellular K(+) and could be blocked by serosal Ba(2+) or mucosal orthovanadate. These results suggest the existence of an H(+)-coupled L-alanine cotransport at the apical membrane of enterocytes. The favorable H(+) driving force is likely to be maintained by an apical vanadate-sensitive H(+)-K(+)-ATPase, allowing the extrusion of H(+) in an exchange with K(+). Potassium exit through a basolateral barium-sensitive conductance provides the key step for the electrogenicity of L-alanine absorption.

Maintenance and regulation of the pH microclimate at the luminal surface of the distal colon of guinea-pig

1. The fluorescent dye 5-N-hexadecanoyl-aminofluorescein (HAF) was used to study the mechanisms involved in maintaining a relatively constant luminal surface pH (pHs) in the distal colon of the guinea-pig. The fatty acyl chain of the HAF molecule inserts into the apical membrane of epithelial cells. This allows a continuous measurement of the surface pH for several hours. 2. The localization of HAF was confirmed by confocal laser-scanning microscopy and by using monoclonal antibodies against fluorescein. The insertion of HAF into the apical membrane of the colonocytes did not change the transepithelial conductance or the short-circuit current of the epithelium. 3. With the HAF method a pH microclimate was confirmed at the colonic surface. Although the pH of the bulk luminal solution was decreased in bicarbonate-containing solution from 7.4 to 6.4 the pHs changed only in the range 7.54-6.98. 4. In the absence of bicarbonate pHs almost followed changes of bulk luminal pH. In the presence of bicarbonate there was a decrease in pHs after removal of chloride from the luminal side and an increase in pHs after addition of butyrate to the luminal solution. This suggests the involvement of a bicarbonate-anion exchange in bicarbonate secretion: a Cl--HCO3- as well as a short-chain fatty acid--HCO3- exchange. 5. The apical K+-H+-ATPase in the distal colon of guinea-pig has little influence on pHs in the presence of physiological buffer concentrations. 6. Our findings indicate that bicarbonate plays a major role in maintaining the pH microclimate at the colonic surface.

HCO3(-)-dependent ion transport systems and intracellular pH regulation in colonocytes from the chick

The current study examines the presence of the Na+/HCO3- cotransporter and of the Cl-/HCO3- exchanger in chicken colonocytes and their role in cytosolic pH (pHi) homeostasis. pHi was measured with 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein (BCECF) at 25 degreesC. Basal pHi was 7.16 in HEPES-buffered solutions and 7.06 in those buffered with HCO3-. Removal of external Cl- increased pHi and Cl- reinstatement brought the pHi towards resting values. These Cl--induced pHi changes were Na+-independent, inhibited by H2-DIDS and faster in the presence than in the absence of HCO3-. Cells recovered from alkaline loads by a mechanism that was Cl--dependent, Na+-independent and inhibited by H2-DIDS. This rate of Cl--dependent cell acidification decreased as the pHi decreased, with a Hill coefficient value close to 4. Removal of external Na+ decreased pHi and readdition of Na+ brought pHi towards the control values. The rate of the Na+-induced changes was not modified by the presence of HCO3- and was prevented by EIPA and unaffected by H2-DIDS. In the presence of EIPA cells partially recovered from a moderate acid load only when both Na+ and HCO3- were present. The EIPA resistant Na+- and bicarbonate-dependent pHi recovery was inhibited by H2-DIDS and occurred at equal rates in both Cl--containing and Cl--free solutions. It is concluded that in chicken colonocytes bathed in HCO3--buffered solutions, both the Na+/H+ exchanger and the Cl-/HCO3- exchanger participate in setting the resting pHi value. The latter transporter helps the cells to recover from alkaline loads and the first transporter, together with the Na+/HCO3- cotransporter, is involved in pHi recovery from an acid load.

Anion exchange proteins and regulation of intracellular pH in cultured rat astrocytes and neurones

We used primary cultures of rat hippocampal tissue to estimate the contribution of anion exchange (AE) proteins to the regulation of intracellular pH in neurones and astrocytes. After induction of acidosis, neonatal rat astrocytes were able to restore the intracellular pH in the absence of extracellular bicarbonate. Neonatal neurones, however, were able to recover from acidosis only when bicarbonate was present in the extracellular medium. This recovery was inhibited by inhibition of anion exchange and was independent of the presence of sodium ions. Antibodies against AE proteins reacted predominantly with neurones. These data suggest that neurones in particular are dependent on functional AE proteins for the maintenance of their intracellular pH.

Excitation-contraction coupling of cultured human skeletal muscle cells and the relation between basal cytosolic Ca2+ and excitability

Cultured human skeletal muscle cells are frequently used as a model to study muscle pathology, in which Ca2+ homeostasis might be affected. However, their excitation-contraction (E-C) coupling has been poorly investigated. In order to elucidate E-C coupling of cultured muscle cells, we activated the acetylcholine receptors, voltage-dependent Na+ channels, dihydropyridine receptors or ryanodine receptors both in the presence and absence of external Ca2+, as well as after specific inhibition, and measured the effects on the cytosolic Ca2+ concentration ([Ca2+]i) using Fura-2. Furthermore, we examined the excitability of these cells during iterative high (125 mM) K+ stimulation with various repolarisation intervals. The resting [Ca2+]i in muscle cells of controls is about 130 nM. Acetylcholine, veratridine, high K+ and caffeine elicit dose-dependent Ca2+ transients, which are independent of extracellular Ca2+ and can be inhibited by alpha-bungarotoxin, tetrodotoxin, nifedipine or ryanodine. During repetitive K+ stimulation, the excitability of the muscle cells depends on the repolarisation interval between successive stimulations. Upon shortening the repolarisation time the Ca2+ transients become smaller and slower. Thereby, the basal [Ca2+]i rises, the Ca2+ response amplitude declines and both the half-increase and half-decay time increase. However, if the basal [Ca2+]i equals the resting [Ca2+]i the initial Ca2+ response can be recovered. The intracellular pH of 7.23, measured by BCECF, is unaffected by repeated K+ stimulation, whatever the repolarisation interval was. In conclusion, cultured human skeletal muscle cells possess a 'skeletal muscle type' of E-C coupling and their excitability at iterative stimulation is set by their basal [Ca2+]i.

Uptake characteristics of loracarbef and cephalexin in the Caco-2 cell culture model: effects of the proton gradient and possible presence of a distinctive second component

The mechanisms of apical (AP) uptake of cephalexin (CEPH) and loracarbef (LOR) in the absence or presence of an (extemally imposed) proton gradient were determined using well-stirred diffusion chambers that minimize the effects of the unstirred water layer. The results indicated that, compared to AP uptake in the presence of an imposed proton gradient, AP uptake in the absence of an imposed proton gradient had higher K(m) values and lower Jmax values. Furthermore, when inhibition studies were performed in the absence of a proton gradient, only natural peptides were effective, whereas the peptide analogs (e.g., enalapril) were not. In addition to the effects of concentration and competitive inhibitors, the results also indicated that (1) the AP uptake of both drugs was decreased more than 60% by FCCP, regardless of whether the proton gradient was present or absent; (2) effects of protein kinase C promoter were dependent upon the presence of a proton gradient; and (3) AP uptake in the presence of an imposed proton gradient was not affected by feeding restriction, whereas AP uptake in the absence of an imposed proton gradient was. These results showed for the first time that two substrates with similar AP uptake characteristics in the presence of an imposed proton gradient may not share those characteristics in the absence of an imposed proton gradient. Taken together, these results suggest that the AP uptake component that functions in the absence of an imposed proton gradient is distinctly different from the one that functions in the presence of an imposed proton gradient. Data generated from the present study and those in the literature lend support to the hypothesis that this distinctive component represents the second binding site on the AP peptide transporter. However, an alternative hypothesis that there are two AP peptide transporters remains to be disapproved.

Immunocytochemical localization of H(+)-K(+)-ATPase in the rat colon

The presence and distribution of gastric-type H(+)-K(+)-ATPase were investigated in the rat colon using a monoclonal antibody raised against hog gastric H(+)-K(+)-ATPase. Rat stomach was used as positive control. Rat kidney and ileum, in both of which H(+)-K(+)-ATPase has been reported in the past, were also studied. In stomach, very strong staining was found confined to the parietal cells, and a strong band at M(r) approximately 94 kDa on the immunoblots. In colon a moderate staining was found in the supranuclear region of the epithelial cells, with similar intensity and distribution of staining of the surface and deep mucosa of the crypts, throughout the length of the colon. Another monoclonal antibody, specific to the 31 kDa subunit of H(+)-ATPase, used as a negative control, or omission of the primary antibody, resulted in lack of any staining in either colon or stomach. On immunoblots of homogenates of colonic mucosa, no specific band could be identified, either due to very low expression of the H(+)-K(+)-ATPase or loss of antigenicity of the epitope during the processing steps. No positive staining was observed in rat kidney and ileum, suggesting that they contain isoforms that are structurally different.

The role of the proton electrochemical gradient in the transepithelial absorption of amino acids by human intestinal Caco-2 cell monolayers

We determined the extent of Na(+)-independent, proton-driven amino acid transport in human intestinal epithelia (Caco-2). In Na(+)-free conditions, acidification of the apical medium (apical pH 6.0, basolateral pH 7.4) is associated with a saturable net absorption of glycine. With Na(+)-free media and apical pH set at 6.0, (basolateral pH 7.4), competition studies with glycine indicate that proline, hydroxyproline, sarcosine, betaine, taurine, beta-alanine, alpha-aminoisobutyric acid (AIB), alpha-methylaminoisobutyric acid (MeAIB), tau-amino-n-butyric acid and L-alanine are likely substrates for pH-dependent transport in the brush border of Caco-2 cells. Both D-serine and D-alanine were also substrates. In contrast leucine, isoleucine, valine, phenylalanine, methionine, threonine, cysteine, asparagine, glutamine, histidine, arginine, lysine, glutamate and D-aspartate were not effective substrates. Perfusion of those amino acids capable of inhibition of acid-stimulated net glycine transport at the brush-border surface of Caco-2 cell monolayers loaded with the pH-sensitive dye 2',7'-bis(2-carboxyethyl-5(6)-carboxyfluorescein) (BCECF) caused cytosolic acidification consistent with proton/amino acid symport. In addition, these amino acids stimulate an inward short-circuit current (Isc) in voltage-clamped Caco-2 cell monolayers in Na(+)-free media (pH 6.0). Other amino acids such as leucine, isoleucine, phenylalanine, tryptophan, methionine, valine, serine, glutamine, asparagine, D-aspartic acid, glutamic acid, cysteine, lysine, arginine and histidine were without effect on both pHi and inward Isc. In conclusion, Caco-2 cells express a Na(+)-independent, H(+)-coupled, rheogenic amino acid transporter at the apical brush-border membrane which plays an important role in the transepithelial transport of a range of amino acids across this human intestinal epithelium.

Mechanism and kinetics of transcellular transport of a new beta-lactam antibiotic loracarbef across an intestinal epithelial membrane model system (Caco-2)

Various processes involved in the transcellular transport (TT) of loracarbef (LOR) were studied in the Caco-2 cell monolayer, a cell culture model of the small intestinal epithelium. The results provide support for presence of two AP to BL peptide TT pathways in the intestinal epithelial cell monolayer (Caco-2). The H+ gradient-dependent pathway (Km = 0.789 mM, and Jmax = 163 pmol/min per cm2) is relatively "high affinity" and "low capacity" compared to H+ gradient-independent pathway (Km = 8.28 mM, and Jmax = 316 pmol/min per cm2). In addition, TT of LOR in the presence of a H+ gradient was inhibited 77% to 88% (p < 0.05) by 10 mM of cephalexin, enalapril, Gly-Pro and Phe-Pro, while TT of LOR in the absence of a H+ gradient was only inhibited 42% to 48% (p < 0.05) by 10 mM of Gly-Pro and Phe-Pro. Since AP uptake is H+ gradient-dependent and saturable while the BL efflux is mostly nonsaturable and not driven by a H+ gradient, these two transmembrane transport processes must be different, which could be the result of two different peptide carriers. In vivo, these two transport processes must have worked in concert to produce transcellular flux of loracarbef. To explain the differences between kinetic characteristics of AP uptake and TT transport, a cellular pharmacokinetic (PK) model was developed and the results indicate that the PK model appropriately described the kinetics of LOR TT. The use of this PK model may provide an additional advantage to the use of the cell culture model because kinetic parameters at both sides of the intestinal epithelial membrane may be obtained using the same preparation. Taken together, the Caco-2 model system represents an excellent model system for the study of carrier-mediated processes involved in the TT of peptides and peptide-like drugs.

Cell association and invasion of Caco-2 cells by Campylobacter jejuni

Adherence and invasion studies were conducted in monolayers of Caco-2 cells. Three-day-old monolayers were inoculated with Campylobacter jejuni 81-176 at a bacterium/cell ratio of 1,000:1. Saturation studies demonstrated time- and dose-dependent saturation curves for C. jejuni cell association and invasion into Caco-2 cells. Electron microscopy revealed intracellular C. jejuni located within membrane-bound vacuoles. Cell association and invasion were inhibited by 0.3 and 0.5 M concentrations of various sugars, including D-glucose, D-mannose, and D-fucose. However, there was no inhibition with the corresponding L-sugars, indicating physiological specificity. The inhibition of cell association with phloridzin was less pronounced. There was no inhibition of bacterial entry with monodansylcadaverine or g-strophanthin, indicating that it was unlikely that coated-pit formation is important in the invasion of C. jejuni into Caco-2 cells. Furthermore, there was no inhibition with cytochalasin D, vincristine, or vinblastine. Inhibition of cell association was demonstrated at 4 degrees C. Significantly decreased cell association and invasion were seen in potassium-depleted cells. Treatment of cells with bromelain also caused reduction in the number of C. jejuni binding to cells. A nonmotile aflagellate variant of C. jejuni also showed reduced invasion. The results of this study are consistent with energy-dependent invasion mechanisms. The results do not support an endocytic method of invasion for C. jejuni into Caco-2 cells.

pH regulation in HT29 colon carcinoma cells

The pH regulation in HT29 colon carcinoma cells has been investigated using the pH-sensitive fluorescent indicator 2',7'-biscarboxyethyl-5(6)-carboxyfluorescein (BCECF). Under control conditions, intracellular pH (pHi) was 7.21 +/- 0.07 (n = 22) in HCO3(-)-containing and 7.21 +/- 0.09 (n = 12) in HCO3(-)-free solution. HOE-694 (10 mumol/l), a potent inhibitor of the Na+/H+ exchanger, did not affect control pHi. As a means to acidify cells we used the NH4+/NH3 (20 mmol/l) prepulse technique. The mean peak acidification was 0.37 +/- 0.07 pH units (n = 6). In HCO3(-)-free solutions recovery from acid load was completely blocked by HOE-694 (1 mumol/l), whereas in HCO3(-)-containing solutions a combination of HOE-694 and 4,4'-diisothiocyanatostilbene-2,2'-disulphonate (DIDS, 0.5 mmol/l) was necessary to show the same effect. Recovery from acid load was Na(+)-dependent in HCO3(-)-containing and HCO3(-)-free solutions. Removal of external Cl- caused a rapid, DIDS-blockable alkalinization of 0.33 +/- 0.03 pH units (n = 15) and of 0.20 +/- 0.006 pH units (n = 5), when external Na+ was removed together with Cl-. This alkalinization was faster in HCO3(-)-containing than in HCO3(-)-free solutions. The present observations demonstrate three distinct mechanisms of pHi regulation in HT29 cells: (a) a Na+/H+ exchanger, (b) a HCO3-/Cl- exchanger and (c) a Na(+)-dependent HCO3- transporter, probably the Na(+)-HCO3-/Cl- antiporter.(ABSTRACT TRUNCATED AT 250 WORDS)

L-alanine absorption in human intestinal Caco-2 cells driven by the proton electrochemical gradient

In human Caco-2 intestinal epithelial layers, L-alanine absorption can be energized by a proton gradient across the brush-border membrane. Acidification of the apical medium, even in Na(+)-free media, is associated with a saturable net transepithelial absorption of L-alanine. L-Alanine transport causes cytosolic acidification consistent with proton/amino acid symport. L-Alanine transport in Na(+)-free media is rheogenic, stimulating an inward short-circuit current in voltage-clamped epithelial monolayers. By measurement of rapid L-alanine influx across the apical membrane, L-alanine-stimulated inward short-circuit current and intracellular acidification in the same cell batch, we estimate L-alanine/proton stoichiometry to be 1:0.62 +/- 0.25 (SD) (short-circuit current) or 1:0.73 +/- 0.19 (intracellular acidification). From competition studies, it is likely that L-proline, alpha-aminoisobutyric acid, and beta-alanine, but not L-valine and L-serine, are substrates for proton-linked, substrate transport in the brush border of Caco-2 cells.

H(+)-coupled (Na(+)-independent) proline transport in human intestinal (Caco-2) epithelial cell monolayers

Previously, absorption of L-proline across the apical membrane of the intestinal enterocyte has been attributed to transport via the Na(+)-dependent Imino system. However, net (absorptive) transport of proline across intact Caco-2 cell monolayers was enhanced by acidification of the apical environment, under both Na(+)-containing and Na(+)-free conditions. This Na(+)-independent pH-dependent proline flux was associated with H+ flow across the apical membrane as determined by continuous measurement of intracellular pH. H+/proline symport was associated with an inward Isc in voltage-clamped Caco-2 epithelial layers demonstrating the electrogenic nature of this transport process. In conclusion Caco-2 cells possess an apically-localised, Na(+)-independent, electrogenic H+/imino acid transporter which may play an important role in intestinal proline absorption.

Transepithelial dipeptide (glycylsarcosine) transport across epithelial monolayers of human Caco-2 cells is rheogenic

Net transepithelial transport (and cellular accumulation) of the dipeptide glycylsarcosine (Gly-Sar), across the apical membrane of human intestinal Caco-2 epithelia, is driven by a proton gradient (Na(+)-free conditions) and displays saturation kinetics (Km 17.4 +/- 5.1 mM, Vmax of 92.8 +/- 15.6 nmol.cm-2.h-1). Net Gly-Sar transport is associated with the stimulation of an inward short-circuit current (Isc). This dipeptide-stimulated Isc is observed in both Na(+)-containing and Na(+)-free conditions, is stimulated by apical acidity, and displays saturation kinetics (in Na(+)-free media at apical pH 6.0, Km of 13.6 +/- 4.5 mM and a Vmax of 284.1 +/- 39.3 nmol.cm-2.h-1). The maximal capacities of Gly-Sar transport and Isc suggest a dipeptide/proton stoichiometry greater than unity (1:3).