Characterisation of volume-activated ion transport across epithelial monolayers of human intestinal T84 cells

Functional and pharmacological characterization of volume-regulated anion channels in human normal and cystic fibrosis bronchial and nasal epithelial cells

Volume-regulated anion channels (VRACs) are widely present in various cell types and have important functions ranging from regulatory volume decrease to control of cell proliferation and apoptosis. Here we aimed to compare the biophysical features and pharmacological profiles of VRAC currents in healthy and cystic fibrosis (CF) respiratory epithelial cells in order to characterize these currents both functionally and pharmacologically. Whole-cell electrophysiology was used to characterize the VRAC current in normal (16HBE14o-; HBE) and CF cell lines (CFBE14o-; CFBE), as well as in native human nasal epithelial cells. Application of hypotonic solution produced current responses of similar sizes in both HBE and CFBE cells. Biophysical properties of VRACs, such as instantaneous activation and deactivation upon voltage step, some inactivation at potentials positive to 40 mV and outwardly-rectifying I-V curves, were indistinguishable in both cell types. Extensive pharmacological analysis of the currents revealed a similar pharmacological profile in response to three blockers--NPPB, DCPIB and DIDS. Native primary human nasal epithelial cells from both healthy and CF volunteers also showed typical VRAC responses of comparable sizes. VRACs in these cells were more sensitive to external solution hypotonicity compared to HBE and CFBE cells. In all cell types studied robust VRAC currents could be induced at constant cell volume by G-protein activation with GTPγS infusion. This study provides the first extensive comparative functional and pharmacological analysis of VRAC currents in normal and CF airway epithelial cells and shows that VRACs are unimpaired molecularly or functionally in CF.

Hypotonic stress activates an intermediate conductance K+ channel in human colonic crypt cells

AIM

To investigate the effect of hypotonic stress on human colonic crypts cells in terms of ion channel activity and intracellular Ca2+ concentration.

METHODS

Single crypts were isolated from biopsies taken during colonoscopy. The patch clamp technique was used (in the cell-attached mode) to observe the activity of ion channels during hypotonic stress. Calcium measurements were made using the fluophores Fluo 3 or 4.

RESULTS

The intermediate conductance (29 pS), Ca2+ -sensitive, K+ channel (also known as KCNN4) previously described (Sandle et al. 1994) was seen in 54 of 149 patches (36%) when the crypts were bathed in normal extracellular solution (290 mOsm kg(-1)). Forty-one patches could be used for further analysis. Activation of one or several 29 pS channels was seen in 15 of 41 patches (39%) after 30 s to 4 min of exposure to hypotonic solution (160 mOsm kg(-1)). The open probability increased from 0.0043 in control solution to 0.44 at 5 min of hypotonic stress. When the crypts were exposed to hypotonic solution, an increase in intracellular Ca2+ could be seen. The increase in intracellular Ca2+ emanates mainly from intracellular stores.

CONCLUSION

The 29 pS K+ channel takes part in volume regulation in human colonic crypt cells. The activation of this channel is mediated through an increase in intracellular Ca2+.

ClC-2 contributes to native chloride secretion by a human intestinal cell line, Caco-2

It has been previously determined that ClC-2, a member of the ClC chloride channel superfamily, is expressed in certain epithelial tissues. These findings fueled speculation that ClC-2 can compensate for impaired chloride transport in epithelial tissues affected by cystic fibrosis and lacking the cystic fibrosis transmembrane conductance regulator. However, direct evidence linking ClC-2 channel expression to epithelial chloride secretion was lacking. In the present studies, we show that ClC-2 transcripts and protein are present endogenously in the Caco-2 cell line, a cell line that models the human small intestine. Using an antisense strategy we show that ClC-2 contributes to native chloride currents in Caco-2 cells measured by patch clamp electrophysiology. Antisense ClC-2-transfected monolayers of Caco-2 cells exhibited less chloride secretion (monitored as iodide efflux) than did mock transfected monolayers, providing the first direct molecular evidence that ClC-2 can contribute to chloride secretion by the human intestinal epithelium. Further, examination of ClC-2 localization by confocal microscopy revealed that ClC-2 contributes to secretion from a unique location in this epithelium, from the apical aspect of the tight junction complex. Hence, these studies provide the necessary rationale for considering ClC-2 as a possible therapeutic target for diseases affecting intestinal chloride secretion such as cystic fibrosis.

Oleuropein, an antioxidant polyphenol from olive oil, is poorly absorbed from isolated perfused rat intestine

Epidemiological studies have shown that the incidence of heart disease and certain cancers is lower in the Mediterranean region. This has been attributed to the high consumption of olive oil in the Mediterranean diet, which contains polyphenolic compounds with antioxidant activity. Although many in vitro studies have been performed to elucidate mechanisms by which these compounds may act, there are virtually no data relating to their fate after ingestion. Therefore, we decided to investigate the intestinal absorption of one of the major olive oil polyphenolics, oleuropein. To do this, a novel in situ intestinal perfusion technique was developed, and the absorption of oleuropein was studied under both iso-osmotic and hypotonic luminal conditions. Oleuropein was absorbed, with an apparent permeability coefficient (P:(app)) of 1.47 +/- 0.13 x 10(-6) cm/s (+/-SE) observed under iso-osmotic conditions. The mechanism of absorption is unclear but may involve transcellular transport (SGLT1) or paracellular movement. Under hypotonic conditions, the permeability of oleuropein was significantly greater (5.92 +/- 0.49 x 10(-6) cm/s, P: < 0.001). This increase is thought to be due to an increase in paracellular movement facilitated by the opening of paracellular junctions in response to hypotonicity. Overall, we determined that the olive oil polyphenolic oleuropein can be absorbed, albeit poorly, from isolated perfused rat intestine. Therefore, it is possible that it or its metabolites may confer a positive health benefit after the consumption of olive oil, most likely via an antioxidant mechanism.

Osmosensitive C1- currents and their relevance to regulatory volume decrease in human intestinal T84 cells: outwardly vs. inwardly rectifying currents

1. The swelling-activated outwardly rectifying Cl- current (ICl(swell)) recorded in T84 human intestinal cells was completely blocked by 10 microM tamoxifen, while 300 microM Cd2+ had no effect. 2. A ClC-2-like, inwardly rectifying Cl- current was activated after strong hyperpolarization in T84 cells. This current was completely inhibited by 300 microM Cd2+, unaffected by 10 microM tamoxifen, and its magnitude increased slightly in response to cell swelling under hyposmotic conditions. However, the swelling-dependent modulation occurred only after prior activation by hyperpolarizing voltages. 3. T84 cells behaved initially close to perfect osmometers in response to changes in external osmolalities between +20 and -30 %. The cells underwent full regulatory volume decrease (RVD) within 16 min when exposed to 30 or 10 % hyposmotic shocks. 4. Pharmacological tools were used to determine the anionic pathway(s) involved in RVD in T84 cells. Tamoxifen (10 microM), 1,9-dideoxyforskolin (DDFSK; 100 microM) and 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS; 100 microM) blocked RVD while 300 microM Cd2+ had no effect upon RVD following a 30 % hyposmotic shock. The RVD response was similarly unaffected by Cd2+ when cells were exposed to a smaller (10 %) hyposmotic shock. 5. In conclusion, these data show that the anionic pathway primarily activated by cell swelling and relevant to RVD in T84 cells is the tamoxifen-, DDFSK- and DIDS-sensitive ICl(swell) and not the hyperpolarization-activated, Cd2+-sensitive Cl- current associated with the ClC-2 Cl- channel.

Osmotic regulation of intestinal epithelial Na(+)-K(+)-Cl- cotransport: role of Cl- and F-actin

Previous data indicate that adenosine 3',5'-cyclic monophosphate activates the epithelial basolateral Na(+)-K(+)-Cl- cotransporter in microfilament-dependent fashion in part by direct action but also in response to apical Cl- loss (due to cell shrinkage or decreased intracellular Cl-). To further address the actin dependence of Na(+)-K(+)-Cl- cotransport, human epithelial T84 monolayers were exposed to anisotonicity, and isotopic flux analysis was performed. Na(+)-K(+)-Cl- cotransport was activated by hypertonicity induced by added mannitol but not added NaCl. Cotransport was also markedly activated by hypotonic stress, a response that appeared to be due in part to reduction of extracellular Cl- concentration and also to activation of K+ and Cl- efflux pathways. Stabilization of actin with phalloidin blunted cotransporter activation by hypotonicity and abolished hypotonic activation of K+ and Cl- efflux. However, phalloidin did not prevent activation of cotransport by hypertonicity or isosmotic reduction of extracellular Cl-. Conversely, hypertonic but not hypotonic activation was attenuated by the microfilament disassembler cytochalasin D. The results emphasize the complex interrelationship among intracellular Cl- activity, cell volume, and the actin cytoskeleton in the regulation of epithelial Cl- transport.

Modulation of the hyperpolarization-activated Cl- current in human intestinal T84 epithelial cells by phosphorylation

1. Hyperpolarization-activated Cl- currents (ICl,hyp) were investigated in the T84 human adenocarcinoma cell line, using the patch-clamp whole-cell configuration. 2. During whole-cell recording with high-chloride and ATP-containing internal solutions, hyperpolarizing jumps from a holding potential of 0 mV elicited slow inward current relaxations, carried by Cl- and detected at membrane potentials more negative than -40 mV. Analysis of the relative permeabilities to monovalent anions gave the following sequence: Cl- > Br- > I- > glutamate. 3. ICl,hyp was partially inhibited by 1 mM diphenylamine-2-carboxylic acid or 0.1 mM 5-nitro-2-(3-phenylpropylamino)-benzoate, and was completely blocked by Cd2+ (> 300 microM). It was insensitive to 1 mM external 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid or 1 mM Ba2+. 4. ICl,hyp was inhibited by external application of 500 microM cptcAMP (8-(4-chlorophenylthio)-adenosine 3':5'-cyclic monophosphate) or 500 nM of the protein kinase C activator, phorbol 12-myristate, 13-acetate. 5. (i) Omission of ATP from the pipette solution, (ii) ATP replacement by the non-hydrolysable ATP analogue 5'-adenylylimidodiphosphate, and (iii) inhibition of protein kinase C by staurosporine or calphostin C accelerated the activation kinetics of the current and increased its amplitude, but did not alter its pharmacological properties. 6. We conclude that hyperpolarization-activated Cl- channels similar to those of ClC-2 channels (mammalian homologue of Torpedo chloride channel ClC-0) are present in T84 cells, and that their gating properties are modulated by phosphorylation.

Carbachol stimulates Cl- secretion via activation of two distinct apical Cl- pathways in cultured human T84 intestinal epithelial monolayers

The mode of action of carbachol in stimulation of transepithelial Cl- secretion in intact human intestinal T84 epithelial monolayers has been investigated in order to determine whether a DIDS-insensitive exit pathway (via CFTR) coexists with a DIDS-sensitive exit pathway at the apical membrane. Carbachol stimulates a transient inward Isc due to Cl- secretion whose magnitude is related to the basal level of inward Isc. The inward current responses to both carbachol and hypo-osmotic media are abolished in nominally Ca(2+)-free media. The action of apical DIDS (100 microM) upon carbachol-stimulated Isc depends on the initial value of the basal Isc. At basal Isc levels < 10 microA cm-2, 100 microM DIDS applied to the apical cell border abolishes the inward Isc following exposure to both carbachol and hypo-osmotic media. In contrast a VIP-stimulated inward Isc is observed in the presence of 100 microM DIDS. After VIP stimulation of inward Isc, or if spontaneous basal values of Isc were > 10 microA cm-2, the carbachol stimulation of inward Isc was largely insensitive to 100 microM DIDS. The data are consistent with the participation of both DIDS-sensitive and DIDS insensitive pathways for Cl- at the apical membrane of human intestinal T84 epithelial cells.