pH regulation in HT29 colon carcinoma cells

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.

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.

Calcium dynamics during physiological acidification in Xenopus oocyte

Interplays between intracellular pH (pHi) and calcium ([Ca(2+)](i)) variations remain unclear, though both proton and calcium homeostasis changes accompany physiological events such as Xenopus laevis oocyte maturation. In this report, we used NH(4)Cl and changes of extracellular pH (pHe) to acidify the cytosol in a physiological range. In oocytes voltage-clamped at -80 mV, NH(4)Cl triggered an inward current, the main component of which is a Ca(2+)-dependent chloride current. Calcium imaging confirmed that NH(4)Cl provoked a [Ca(2+)](i) increase. The mobilized sources of calcium were discriminated using the triple-step protocol as a means to follow both the calcium-activated chloride currents (ICl-Ca) and the hyperpolarization- and acid-activated nonselective cation current (I(In)). These currents were stimulated during external addition of NH(4)Cl. This upregulation was abolished by BAPTA-AM, caffeine and heparin. By both buffering pHi changes with MOPS and by inhibiting calcium influx with lanthanum, intracellular acidification, initiated by NH(4)Cl and extracellular acidic medium, was shown to trigger a [Ca(2+)](i) increase through both calcium release and calcium influx. The calcium pathways triggered by pHe changes are similar to those activated by NH(4)Cl, thus suggesting that there is a robust signaling mechanism allowing the cell to adjust to variable environmental conditions.

Action of ANG II and ANP on colon epithelial cells

The interaction of angiotensin II (ANG II) and atrial natriuretic peptide (ANP) on intracellular pH (pH(i)) and calcium ([Ca2+](i)) was investigated in T84 cells (a permanent cell line derived from human colon epithelium) using the fluorescent stains BCECF/AM and Fluo 4/AM, respectively. pH(i) recovery rate mediated by the Na(+)/H+ exchanger (NHE) was examined following an NH4Cl pulse. Under control conditions pH(i) recovered at 0.114+/-0.005 pH units/min (n=35). ANG II (10(-12) or 10(-9) M) increased this value, whilst ANG II (10(-7) M) decreased it. These effects of ANG II were impaired by simultaneous addition of 1 microM or 25 microM HOE-694, indicating that the stimulatory and inhibitory effects of ANG II on pH(i) recovery are mediated in part via the NHE1 and NHE2 isoforms. ANG II increased [Ca2+]i concentration-dependently. ANP (10(-6) M) or dimethyl-BAPTA/AM (50 microM) blocked the effects of ANG II on [Ca2+]i and on the rate of pH(i) recovery. Thapsigargin (10(-5) M) enhanced the effect of ANG II on [Ca2+]i and reversed its stimulatory effect on the rate of pH(i) recovery to an inhibitory one. External Ca(2+)-free solution did not affect the effects of ANG II on these parameters. These data suggest that the [Ca2+]i increase induced by ANG II is dependent on intracellular calcium stores. They are compatible with the demonstration of two sites on the C-terminal of the Na(+)/H+ exchanger, one stimulating Na(+)/H+ activity by increases of [Ca2+]i in the lower range (at 10(-12) or 10(-9) M ANG II) and the other inhibiting this activity at high [Ca2+]i levels (at 10(-7) M ANG II). ANP or dimethyl-BAPTA/AM, by impairing the pathway mediating the increase in [Ca2+]i, block both the stimulatory and inhibitory effects of ANG II.

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.

Intracellular pH regulation in colonocytes of rat proximal colon

The regulation of intracellular pH (pH(i)) in colonocytes of the rat proximal colon has been investigated using the pH-sensitive dye BCECF and compared with the regulation of pH(i) in the colonocytes of the distal colon. The proximal colonocytes in a HEPES-buffered solution had pH(i)=7.24+/-0.04 and removal of extracellular Na(+) lowered pH(i) by 0.24 pH units. Acid-loaded colonocytes by an NH(3)/NH(4)(+) prepulse exhibited a spontaneous recovery that was partially Na(+)-dependent and could be inhibited by ethylisopropylamiloride (EIPA). The Na(+)-dependent recovery rate was enhanced by increasing the extracellular Na(+) concentration and was further stimulated by aldosterone. In an Na(+)- and K(+)-free HEPES-buffered solution, the recovery rate from the acid load was significantly stimulated by addition of K(+) and this K(+)-dependent recovery was partially blocked by ouabain. The intrinsic buffer capacity of proximal colonocytes at physiological pH(i) exhibited a nearly 2-fold higher value than in distal colonocytes. Butyrate induced immediate colonocyte acidification that was smaller in proximal than in distal colonocytes. This acidification was followed by a recovery phase that was both EIPA-sensitive and -insensitive and was similar in both groups of colonocytes. In a HCO(3)(-)/CO(2)-containing solution, pH(i) of the proximal colonocytes was 7.20+/-0.04. Removal of external Cl(-) caused alkalinization that was inhibited by DIDS. The recovery from an alkaline load induced by removal of HCO(3)(-)/CO(2) from the medium was Cl(-)-dependent, Na(+)-independent and blocked by DIDS. Recovery from an acid load in EIPA-containing Na(+)-free HCO(3)(-)/CO(2)-containing solution was accelerated by addition of Na(+). Removal of Cl(-) inhibited the effect of Na(+). In summary, the freshly isolated proximal colonocytes of rats express Na(+)/H(+) exchanger, H(+)/K(+) exchanger ((H(+)-K(+))-ATPase) and Na(+)-dependent Cl(-)/HCO(3)(-) exchanger that contribute to acid extrusion and Na(+)-independent Cl(-)/HCO(3)(-) exchanger contributing to alkali extrusion. All of these are likely involved in the regulation of pH(i) in vivo. Proximal colonocytes are able to maintain a more stable pH(i) than distal cells, which seems to be facilitated by their higher intrinsic buffer capacity.

Cloning, characterization, and chromosomal mapping of a human electroneutral Na(+)-driven Cl-HCO3 exchanger

The electroneutral Na(+)-driven Cl-HCO3 exchanger is a key mechanism for regulating intracellular pH (pH(i)) in neurons, glia, and other cells. Here we report the cloning, tissue distribution, chromosomal location, and functional characterization of the cDNA of such a transporter (NDCBE1) from human brain (GenBank accession number AF069512). NDCBE1, which encodes 1044 amino acids, is 34% identical to the mammalian anion exchanger (AE2); approximately 50% to the electrogenic Na/HCO3 cotransporter (NBCe1) from salamander, rat, and humans; approximately 73% to mammalian electroneutral Na/HCO3 cotransporters (NBCn1); 71% to mouse NCBE; and 47% to a Na(+)-driven anion exchanger (NDAE1) from Drosophila. Northern blot analysis of NDCBE1 shows a robust approximately 12-kilobase signal in all major regions of human brain and in testis, and weaker signals in kidney and ovary. This human gene (SLC4A8) maps to chromosome 12q13. When expressed in Xenopus oocytes and running in the forward direction, NDCBE1 is electroneutral and mediates increases in both pH(i) and [Na(+)](i) (monitored with microelectrodes) that require HCO3(-) and are blocked by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS). The pH(i) increase also requires extracellular Na(+). The Na(+):HCO3(-) stoichiometry is 1:2. Forward-running NDCBE1 mediates a 36Cl efflux that requires extracellular Na(+) and HCO3(-) and is blocked by DIDS. Running in reverse, NDCBE1 requires extracellular Cl(-). Thus, NDCBE1 encodes a human, electroneutral Na(+)-driven Cl-HCO3 exchanger.

Role of CFTR in the colon

In contrast to the airways, the defects in colonic function in cystic fibrosis (CF) patients are closely related to the defect in CFTR. The gastrointestinal phenotype of CF transgenic mice closely resembles the phenotype in CF patients, which clearly indicates the crucial role of CFTR in colonic Cl- secretion and the absence of an effective compensation. In the colon, stimulation of CFTR Cl- channels involves cAMP- or cGMP-dependent phosphorylation. Exocytosis is not involved. Activation of CFTR leads to coactivation of basolateral KVLQT1-type K+ channels and inhibition of luminal Na+ channels (ENaC). In contrast to cultured cells, Ca2+ does not activate luminal Cl- channels in intact enterocytes. It activates basolateral SK4-type K+ channels and luminal K+ channels, which provide additional driving force for Cl- exit. The magnitude of Cl- secretion, however, completely depends on the presence of at least a residual CFTR function in the luminal membrane. These findings have been clearly demonstrated by Ussing chamber experiments in colon epithelium biopsies of CF and normal individuals: Colonic Cl- secretion in CF patients is variable and reflects the genotype; a complete defect of CFTR is paralleled by the absence of Cl- secretion and unmasks Ca(2+)-regulated K+ channels in the luminal membrane; overabsorption of Na+ in CF reflects the absence of ENaC inhibition by CFTR; and the functional status of CF colon can be mimicked by the complete suppression of cAMP stimulation in enterocytes of healthy individuals.

Simultaneous measurements of cytosolic and mitochondrial Ca2+ transients in HT29 cells

Loading of HT29 cells with the Ca2+ dye fura-2/AM resulted in an nonhomogeneous intracellular distribution of the dye. Cellular compartments with high fura-2 concentrations were identified by correlation with mitochondrial markers, cellular autofluorescence induced by UV, and dynamic measurement of autofluorescence after inhibition of oxidative phosphorylation. Stimulation with carbachol (10(-4) mol/liter) increased cytosolic, nuclear, and mitochondrial Ca2+ activity ([Ca2+]c, [Ca2+]n, and [Ca2+]m, respectively) measured by UV confocal and conventional imaging. Similar results were obtained with a prototype two-photon microscope (Zeiss, Jena, Germany) allowing for fura-2 excitation. The increase of [Ca2+]m lagged behind that of [Ca2+]c and [Ca2+]n by 10-20 s, and after removing the agonist, [Ca2+]m also decreased with a delay. A strong increase of [Ca2+]m occurred only when a certain threshold of [Ca2+]c (around 1 micromol/liter) was exceeded. In a very similar way, ATP, neurotensin, and thapsigargin increased [Ca2+]c and [Ca2+]m. Carbonyl cyanide p-trifluoromethoxyphenylhyrdrazone reversibly reduced the increase of [Ca2+]m. The source of the mitochondrial Ca2+ increase had intra- and extracellular components, as revealed by experiments in low extracellular Ca2+. We conclude that agonist-induced Ca2+ signals are transduced into mitochondria. 1) Mitochondria could serve as a Ca2+ sink, 2) mitochondria could allow the modulation of [Ca2+]c and [Ca2+]n signals, and 3) [Ca2+]m may serve as a stimulatory metabolic signal when a cell is highly stimulated.

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.

Apoptosis induced by hyperthermia and verapamil in vitro in a human colon cancer cell line

The aim of this study was to determine the mechanisms responsible for the growth inhibitory effect of hyperthermia and verapamil in human colon cancer cell line HT-29. Apoptotic cell death was verified by flow cytometry analysis. The effect of treatment with hyperthermia and verapamil on the expression of apoptosis-associated proteins including Bcl-2, p53, bax, and c-Myc was studied by Western blot analysis. Changes in intracellular calcium homeostasis was analysed by fluorescence microscopy. The combination of 42 degrees C hyperthermia and verapamil caused a significant delay of human colon cancer cell proliferation as a result of apoptosis. Administration of these agents alone did not cause any cell inhibitory effect. Our experiments have shown that HT-29 cells constitutively express apoptosis-promoting proteins, such as Bax and c-Myc, while they fail to produce Bcl-2. Therefore, we hypothesize that HT-29 cells must have Bcl-2 independent pathways to protect cells against death-inducing signals. Also, apoptosis of HT-29 cells produced by hyperthermia in the presence of verapamil is a p53-independent process. Verapamil, when it did not act as a calcium channel blocker or inhibitor of release from intracellular storages under hyperthermic conditions, accelerated the increase of [Ca2+]i in HT-29 cells which resulted in programmed cell death (apoptosis).

The membrane transporters regulating epithelial NaCl secretion

Ten years ago, the basic principles operating in one specific, albeit non-mammalian, exocrine gland, the rectal gland of Squalus acanthias, were described in detail. The concept emerging from these studies appeared applicable to almost any other exocrine gland, because it involved membrane transporters which are also present in mammalian epithelial cells. Meanwhile, it has become clear that the mechanisms of NaCl secretion are diverse: the mechanisms of NaCl uptake; the ion channels involved; and also the mechanisms of hormonal control. Nevertheless, several steps in NaCl secretion still appear to be uniform: (1) several signalling pathways converge and act cooperatively, (2) one primary regulatory step is the upregulation of the luminal Cl- conductance, (3) secondarily active NaCl uptake mechanisms are upregulated, (4) increasing evidence links NaCl secretion to membrane trafficking and (5) the entire machinery seems to be primed to secure cellular homeostasis in terms of cytosolic ion concentrations. This brief review summarizes the mechanisms of control of NaCl secretion. The major issues addressed are the NaCl uptake mechanisms, the ion channels involved and the cellular mechanisms coordinating secretion. The major NaCl secreting cells discussed here will be the respiratory epithelial cells, the exocrine cells of pancreatic acini and the cells of colonic crypts.

Whole-cell conductive properties of rat pancreatic acini

Acetylcholine-controlled exocrine secretion by pancreatic acini has been explained by two hypotheses. One suggests that NaCl secretion occurs by secondary active secretion as has been originally described for the rectal gland of Squalus acanthias. The other is based on a "push-pull" model whereby Cl- is extruded luminally and sequentially taken up basolaterally. In the former model Cl- uptake is coupled to Na+ and basolateral K+ conductances play a crucial role, in the latter model, Na+ uptake supposedly occurs via basolateral non-selective cation channels. The present whole-cell patch-clamp studies were designed to further explore the conductive properties of rat pancreatic acini. Pilot studies in approximately 300 cells revealed that viable cells usually had a membrane voltage (Vm) more hyperpolarized than -30 mV. In all further studies Vm had to meet this criterion. Under control conditions Vm was -49 +/- 1 mV (n = 149). The fractional K+ conductance (fK) was 0.13 +/- 0.1 (n = 49). Carbachol (CCH, 0.5 micromol/l) depolarized to -19 +/- 1.1 mV (n = 63) and increased the membrane conductance (Gm) by a factor of 2-3. In the seeming absence of Na+ [replacement by N-methyl-D-glucamine (NMDG+)] Vm hyperpolarized slowly to -59 +/- 2 mV (n = 90) and CCH still induced depolarizations to -24 +/- 2 mV (n = 34). The hyperpolarization induced by NMDG+ was accompanied by a fall in cytosolic pH by 0.4 units, and a very slow and slight increase in cytosolic Ca2+. fK increased to 0.34. The effect of NMDG+ on Vm was mimicked by the acidifying agents propionate and acetate (10 mmol/l) added to the bath. The present study suggests that fK makes a substantial contribution to Gm under control conditions. The NMDG+ experiments indicate that the non- selective cation conductance contributes little to Vm in the presence of CCH. Hence the present data in rat pancreatic acinar cells do not support the push-pull model.

Effect of alkalinization of cytosolic pH by amines on intracellular Ca2+ activity in HT29 cells

The effect of secondary, tertiary and quaternary methyl- and ethylamines on intracellular pH (pHi) and intracellular Ca2+ activity ([Ca2+]i) of HT29 cells was investigated microspectrofluorimetrically using pH- and Ca2+- sensitive fluorescent indicators, [i.e. 2', 7'-biscarboxyethyl-5(6)-carboxyfluorescein (BCECF) and fura-2 respectively]. Membrane voltage (Vm) was studied by the patch-clamp technique. Secondary and tertiary amines led to a rapid and stable concentration-dependent alkalinization which was independent of their pKa value. Trimethylamine (20 mmol/l) increased pHi by 0.78 +/- 0.03 pH units (n = 9) and pH remained stable for the application time. Removal led to an undershoot of pHi and a slow and incomplete recovery: pHi stayed 0.26 +/- 0.06 pH units more acid than the resting value. The quaternary amines, tetramethyl- and tetraethylamine were without influence on pHi. All tested secondary and tertiary amines (dimethyl-, diethyl-, trimethyl-, and triethyl-amine) induced a [Ca2+]i transient which reached a peak value within 10-25 s and then slowly declined to a [Ca2+]i plateau. The initial Delta[Ca2+]i induced by trimethylamine (20 mmol/l) was 160 +/- 15 nmol/l (n = 17). The [Ca2+]i peak was independent of the Ca2+ activity in the bath solution, but the [Ca2+]i plateau was significantly lower under Ca2+-free conditions and could be immediately interrupted by application of CO2 (10%; n = 6), a manoeuvre to acidify pHi in HT29 cells. Emptying of the carbachol- or neurotensin-sensitive intracellular Ca2+ stores completely abolished this [Ca2+]i transient. Tetramethylamine led to higher [Ca2+]i changes than the other amines tested and only this transient could be completely blocked by atropine (10(-6) mol/l). Trimethylamine (20 mmol/l) hyperpolarized Vm by 22.5 +/- 3.7 mV (n = 16) and increased the whole-cell conductance by 2.3 +/- 0.5 nS (n = 16). We conclude that secondary and tertiary amines induce stable alkaline pHi changes, release Ca2+ from intracellular, inositol-1,4, 5-trisphosphate-sensitive Ca2+ stores and increase Ca2+ influx into HT29 cells. The latter may be related to both the store depletion and the hyperpolarization.

8-(N,N-diethylamino)octyl 3,4,5-trimethoxybenzoate (TMB-8) acts as a muscarinic receptor antagonist in the epithelial cell line HT29

8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8) is a widely used pharmacological tool to investigate the involvement of intracellular Ca2+ stores in cellular responses. In this study we investigate the effect of TMB-8 as a putative inhibitor of "Ca2+ signalling" in single fura-2 loaded HT29 colonic epithelial cells stimulated by ATP, carbachol (CCH) and neurotensin (NT). TMB-8 effectively inhibited the CCH-induced (100 mumol/l intracellular Ca2+ ([Ca2+]i) transient with an IC50 of 20 mumol/l. However, [Ca2+]i transients induced by other phospholipase C coupled agonists ATP (10 mumol/l, n = 4) and NT (10 nmol/l, n = 4) remained unaffected by TMB-8 (50 mumol/l). The agonist-induced [Ca2+]i transients remained equally unaffected by 100 mumol/l TMB-8 when the stimulatory concentration was reduced to 0.5 mumol/l for ATP (n = 4) or 1 nmol/l for NT (n = 4). The competitive nature of the TMB-8-induced inhibition of the CCH-induced [Ca2+]i transient was demonstrated by examining the agonist at various concentrations in absence and presence of the antagonist. High TMB-8 concentrations (100 mumol/l) alone induced a small [Ca2+]i increase (delta[Ca2+]i: 40 +/- 5 nmol/l, n = 7). We assume that this increase is a consequence of a TMB-8 induced intracellular alkalinization (delta pH: 0.1 +/- 0.02, n = 7) occurring simultaneously with the increase in [Ca2+]i. From these results we draw the following conclusions: (1) In sharp contrast to a large number of other studies, but in agreement with studies in other types of cells, these results substantially challenge the value of the "tool" TMB-8 as an "intracellular Ca2+ antagonist"; (2) TMB-8 acts a muscarinic receptor antagonist at the M3 receptor; (3) TMB-8 does not influence the release of Ca2+ from intracellular stores when IP3 signal transduction is activated by ATP or NT; (4) TMB-8 as a weak organic base alkalinizes the cytosol at high concentrations; and (5) TMB-8 induces small [Ca2+]i transients at higher concentrations.

N-Acetyl-L-cysteine and its derivatives activate a Cl- conductance in epithelial cells

N-Acetyl-L-cysteine (NAC) is a widely used mucolytic drug in patients with a variety of respiratory disorders including cystic fibrosis (CF). The beneficial effects of NAC are empirical and the exact mechanism of action in the airways remains obscure. In the present study we examined the effects on whole-cell (wc) conductance (Gm) and voltage (Vm) of NAC and the congeners S-carboxymethyl-L-cysteine (CMC) and S-carbamyl-L-cysteine (CAC) and L-cysteine in normal and CF airway epithelial cells. L-Cysteine (1 mmol/l) had no detectable effect. The increase in Gm (delta Gm) by the other compounds was concentration dependent and was (all substances at 1 mmol/l) 3.8 +/- 1.4 nS (NAC; n = 11), 4.2 +/- 1.0 nS (CMC; n = 16) and 3.8 +/- 1.6 nS (CAC; n = 18), respectively. The changes in Gm were paralleled by an increased depolarization (delta Vm) when extracellular Cl- concentration was reduced to 34 mmol/l: under control conditions = -4.1 +/- 2.1 versus 10.2 +/- 2.1 mV in the presence of NAC, CMC, CAC (n = 36). In the presence of NAC, CMC and CAC, the reduction in Cl- concentration was paralleled by a reduction of Gm by 2.1 +/- 0.4 nS (n = 35), indicating that all substances acted by increasing the Cl- conductance. Analysis of intracellular pH did not reveal any changes by any of the compounds (1 mmol/l). A Cl- conductance was also activated in HT29 colonic carcinoma and CF tracheal epithelial (CFDE) cells but not in CFPAC-1 cells, which do not express detectable levels of delta F508-CFTR, suggesting that the presence of CFTR may be a prerequisite for the induction of Cl- currents. Next we examined the ion currents in Xenopus oocytes microinjected with CFTR-cRNA. Water-injected oocytes did not respond to activation by forskolin and 3-isobutyl-1-methylxanthine (IBMX) (delta Gm = 0.08 +/- 0.04 microS; n = 10) and no current was activated when these oocytes were exposed to NAC or CMC. In contrast, in CFTR-cRNA-injected oocytes Gm was enhanced when intracellular adenosine 3',5'-cyclic monophosphate (cAMP) was increased by forskolin and IBMX (Gm = 4.5 +/- 1.3 microS; n = 8). Gm was significantly increased by 0.74 +/- 0.2 microS (n = 11) and 0.46 +/- 0.1 microS (n = 10) when oocytes were exposed to NAC and CMC, respectively (both 1 mmol/l). In conclusion, NAC and its congeners activate Cl- conductances in normal and CF airway epithelial cells and hence induce electrolyte secretion which may be beneficial in CF patients. CFTR appears to be required for this response in an as yet unknown fashion.

pH dependence of K+ conductances of rat cortical collecting duct principal cells

The K+ channels of the principal cells of rat cortical collecting duct (CCD) are pH sensitive in excised membranes. K+ secretion is decreased with increased H+ secretion during acidosis. We examined whether the pH sensitivity of these K+ channels is present also in the intact cell and thus could explain the coupling between K+ and H+ secretion. Membrane voltages (Vm), whole-cell conductances (gc), and single-channel currents of K+ channels were recorded from freshly isolated CCD cells or isolated CCD segments with the patch-clamp method. Intracellular pH (pHi) was measured using the pH-sensitive fluorescent dye 2'-7'-bis(carboxyethyl)-5-6-carboxyfluorescein (BCECF). Acetate (20 mmol/l) had no effect on Vm, gc, or the activity of the K+ channels in these cells. Acetate, however, acidified pHi slightly by 0.17 +/- 0.04 pH units (n = 19). Vm depolarized by 12 +/- 3 mV (n = 26) and by 23 +/- 2 mV (n = 66) and gc decreased by 26 +/- 5% (n = 13) and by 55 +/- 5% (n = 12) with 3-5 or 8-10% CO2, respectively. The same CO2 concentrations decreased pHi by 0.49 +/- 0.07 (n = 15) and 0.73 +/- 0.11 pH units (n = 12), respectively. Open probability (Po) of all four K+ channels in the intact rat CCD cells was reversibly inhibited by 8-10% CO2. pHi increased with the addition of 20 mmol/l NH4+/NH3 by a maximum of 0.64 +/- 0.08 pH units (n = 33) and acidified transiently by 0.37 +/- 0.05 pH units (n = 33) upon NH4+/NH3 removal. In the presence of NH4+/NH3 Vm depolarized by 16 +/- 2 mV (n = 66) and gc decreased by 26 +/- 7% (n = 16). The activity of all four K+ channels was also strongly inhibited in the presence of NH4+/NH3. The effect of NH4+/NH3 on Vm and gc was markedly increased when the pH of the NH4+/NH3-containing solution was set to 8.5 or 9.2. From these data we conclude that cellular acidification in rat CCD principal cells down-regulates K+ conductances, thus reduces K+ secretion by direct inhibition of K+ channel activity. This pH dependence is present in all four K+ channels of the rat CCD. The inhibition of K+ channels by NH4+/NH3 is independent of changes in pHi and rather involves an effect of NH3.