Carbachol induces K+, Cl-, and nonselective cation conductances in T84 cells: a perforated patch-clamp study

Role of Ca2+-activated K+ channels in duodenal mucosal ion transport and bicarbonate secretion

Stimulation of muscarinic receptors in the duodenal mucosa raises cytosolic free Ca(2+) concentration ([Ca(2+)](cyt)), thereby regulating duodenal epithelial ion transport. However, little is known about the downstream molecular targets that account for this Ca(2+)-mediated biological action. Ca(2+)-activated K(+) (K(Ca)) channels are candidates, but the expression and function of duodenal K(Ca) channels are poorly understood. Therefore, we determined whether K(Ca) channels are expressed in the duodenal mucosa and investigated their involvement in Ca(2+)-mediated duodenal epithelial ion transport. Two selective blockers of intermediate-conductance Ca(2+)-activated K(+) (IK(Ca)) channels, clotrimazole (30 muM) and 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (TRAM-34; 10 muM), significantly inhibited carbachol (CCh)-induced duodenal short-circuit current (I(sc)) and duodenal mucosal bicarbonate secretion (DMBS) in mice but did not affect responses to forskolin and heat-stable enterotoxin of Escherichia coli. Tetraethylammonium, 4-aminopyridine, and BaCl(2) failed to inhibit CCh-induced I(sc) and DMBS. A-23187 (10 muM), a Ca(2+) ionophore, and 1-ethyl-2-benzimidazolinone (1-EBIO; 1 mM), a selective opener of K(Ca) channels, increased both I(sc) and DMBS. The effect of 1-EBIO was more pronounced with serosal than mucosal addition. Again, both clotrimazole and TRAM-34 significantly reduced A23187- or 1-EBIO-induced I(sc) and DMBS. Moreover, clotrimazole (20 mg/kg ip) significantly attenuated acid-stimulated DMBS of mice in vivo. Finally, the molecular identity of IK(Ca) channels was verified as KCNN4 (SK4) in freshly isolated murine duodenal mucosae by RT-PCR and Western blotting. Together, our results suggest that the IK(Ca) channel is one of the downstream molecular targets for [Ca(2+)](cyt) to mediate duodenal epithelial ion transport.

Regulatory volume increase after secretory volume decrease in colonic epithelial cells under muscarinic stimulation

To address the question of whether colonic secretory cells change their volume in response to carbachol (CCh) stimulation and, if so, the mechanisms involved therein, we used two-photon laser scanning microscopy to measure the volume of individual epithelial cells in the fundus region of crypts isolated from the guinea-pig distal colon. We also measured the volume of human colonic epithelial T84 cells using an electronic sizing technique. Both types of colonocytes responded to stimulation by CCh with shrinkage and then underwent a regulatory volume increase (RVI), even during continued stimulation by CCh. The secretory volume decrease (SVD) induced by CCh was antagonized by atropine, BAPTA loading and niflumic acid, a blocker of Ca(2+)-activated Cl(-) channels. An increase in the intracellular free [Ca(2+)] was observed with fura-2 during these volume responses to CCh. Removal of all Na(+) or K(+) or of most of the Cl(-) from the extracellular solution abolished the RVI, but not the preceding SVD. The RVI, but not the preceding SVD, was abolished by bumetanide, a blocker of the Na(+)-K(+)-2Cl(-) cotransporter. We conclude that guinea-pig crypt colonocytes and human T84 cells exhibit a cytosolic Ca(2+)-dependent SVD and undergo a subsequent RVI that is dependent on the operation of Na(+)-K(+)-2Cl(-) cotransporters.

Muscle tissue changes with aging

PURPOSE OF REVIEW

This review article focuses on the changes that occur in muscle with age, specifically the involuntary loss of muscle mass, strength and function, termed sarcopenia. Particular emphasis is given to the metabolic alterations that characterize sarcopenia, and to the potentially treatable causes of this condition, including age-related endocrine and nutritional changes, and inactivity.

RECENT FINDINGS

Recent data reported include those regarding the potential role of insulin resistance in the development of sarcopenia, the potential role of androgens and growth hormone in the treatment of this condition, the usefulness of exercise including both resistance and aerobic training to improve muscle growth and function, and, finally, the possible use of nutritional manipulations to improve muscle mass.

SUMMARY

Sarcopenia is likely a multifactorial condition that impairs physical function and predisposes to disability. It may be prevented or treated with lifestyle interventions and pharmacological treatment. Further long-term investigations are needed, however, to ascertain what type and combinations of interventions are the most efficacious in improving muscle mass and function in older people.

Diarrhea-associated HIV-1 APIs potentiate muscarinic activation of Cl- secretion by T84 cells via prolongation of cytosolic Ca2+ signaling

Aspartyl protease inhibitors (APIs) effectively extend the length and quality of life in human immunodeficiency virus (HIV)-infected patients, but dose-limiting side effects such as lipodystrophy, insulin resistance, and diarrhea have limited their clinical utility. Here, we show that the API nelfinavir induces a secretory form of diarrhea in HIV-infected patients. In vitro studies demonstrate that nelfinavir potentiates muscarinic stimulation of Cl(-) secretion by T84 human intestinal cell monolayers through amplification and prolongation of an apical membrane Ca(2+)-dependent Cl(-) conductance. This stimulated ion secretion is associated with increased magnitude and duration of muscarinically induced intracellular Ca(2+) transients via activation of a long-lived, store-operated Ca(2+) entry pathway. The enhanced intracellular Ca(2+) signal is associated with uncoupling of the Cl(-) conductance from downregulatory intracellular mediators generated normally by muscarinic activation. These data show that APIs modulate Ca(2+) signaling in secretory epithelial cells and identify a novel target for treatment of clinically important API side effects.

Paracellular ion channel at the tight junction

The tight junction of epithelial cells excludes macromolecules but allows permeation of ions. However, it is not clear whether this ion-conducting property is mediated by aqueous pores or by ion channels. To investigate the permeability properties of the tight junction, we have developed paracellular ion flux assays for four major extracellular ions, Na(+), Cl(-), Ca(2+), and Mg(2+). We found that the tight junction shares biophysical properties with conventional ion channels, including size and charge selectivity, dependency of permeability on ion concentration, competition between permeant molecules, anomalous mole-fraction effects, and sensitivity to pH. Our results support the hypothesis that discrete ion channels are present at the tight junction. Unlike conventional ion channels, which mediate ion transport across lipid bilayers, the tight junction channels must orient parallel to the plane of the plasma membranes to support paracellular ion movements. This new class of paracellular-tight junction channels (PTJC) facilitates the transport of ions between separate extracellular compartments.

Transactivation of the epidermal growth factor receptor in colonic epithelial cells by carbachol requires extracellular release of transforming growth factor-alpha

We have shown previously that the muscarinic agonist, carbachol (CCh), transactivates the epidermal growth factor receptor (EGFr) via calmodulin, Pyk-2, and Src kinase activation. EGFr phosphorylation causes extracellular signal-regulated kinase (ERK) activation and inhibits CCh-stimulated chloride secretion across intestinal epithelial cells. Here we investigated whether CCh-stimulated EGFr transactivation involves EGFr ligand release. Pre-incubation of T(84) cell monolayers with a neutralizing antibody to the EGFr ligand binding domain decreased CCh-induced phosphorylation of EGFr and ERK. CCh-stimulated efflux of (86)Rb+ from T(84) cell monolayers, which parallels changes in chloride secretion, was potentiated by anti-EGFr pre-incubation. Anti-EGFr did not reduce CCh-stimulated Pyk-2 phosphorylation. Co-incubation with the Src kinase inhibitor PP2 and anti-EGFr had an additive inhibitory effect on CCh-induced ERK phosphorylation greater than either inhibitor alone. CCh caused the basolateral release of transforming growth factor alpha (TGF-alpha) into T(84) cell bathing media. A metalloproteinase inhibitor, WAY171318, reduced CCh-induced phosphorylation of ERK and completely blocked EGFr phosphorylation and TGF-alpha release. We conclude that CCh-stimulated EGFr transactivation and subsequent ERK activation, a pathway that limits CCh-induced chloride secretion, is mediated by metalloproteinase-dependent extracellular release of TGF-alpha and intracellular Src activation. These findings have important implications for our understanding of the role of growth factors in regulating epithelial ion secretion.

Splice variants of a ClC-2 chloride channel with differing functional characteristics

We identified two ClC-2 clones in a guinea pig intestinal epithelial cDNA library, one of which carries a 30-bp deletion in the NH(2) terminus. PCR using primers encompassing the deletion gave two products that furthermore were amplified with specific primers confirming their authenticity. The corresponding genomic DNA sequence gave a structure of three exons and two introns. An internal donor site occurring within one of the exons accounts for the deletion, consistent with alternative splicing. Expression of the variants gpClC-2 and gpClC-2Delta77-86 in HEK-293 cells generated inwardly rectifying chloride currents with similar activation characteristics. Deactivation, however, occurred with faster kinetics in gpClC-2Delta77-86. Site-directed mutagenesis suggests that a protein kinase C-mediated phosphorylation consensus site lost in gpClC-2Delta77-86 is not responsible for the observed change. The deletion-carrying variant is found in most tissues examined, and it appears more abundant in proximal colon, kidney, and testis. The presence of a splice variant of ClC-2 modified in its NH(2)-terminal domain could have functional consequences in tissues where their relative expression levels are different.

Chloride secretion by the intestinal epithelium: molecular basis and regulatory aspects

Chloride secretion is the major determinant of mucosal hydration throughout the gastrointestinal tract, and chloride transport is also pivotal in the regulation of fluid secretion by organs that drain into the intestine. Moreover, there are pathological consequences if chloride secretion is either reduced or increased such as in cystic fibrosis and secretory diarrhea, respectively. With the molecular cloning of many of the proteins and regulatory factors that make up the chloride secretory mechanism, there have been significant advances in our understanding of this process at the cellular level. Similarly, emerging data have clarified the intercellular relationships that govern the extent of chloride secretion. The goal of our article is to review this area of investigation, with an emphasis on recent developments and their implications for the physiology and pathophysiology of chloride transport.

Inhibition of Ca(2+)-dependent Cl- secretion in T84 cells: membrane target(s) of inhibition is agonist specific

Previous studies have indicated that Ca(2+)-dependent Cl- secretion across monolayers of T84 epithelial cells is subject to a variety of negative influences that serve to limit the overall extent of secretion. However, the downstream membrane target(s) of these inhibitory influences had not been elucidated. In this study, nuclide efflux techniques were used to determine whether inhibition of Ca(2+)-dependent Cl- secretion induced by carbachol, inositol 3,4,5,6-tetrakisphosphate, epidermal growth factor, or insulin reflected actions at an apical Cl- conductance, a basolateral K+ conductance, or both. Pretreatment of T84 cell monolayers with carbachol or a cell-permeant analog of inositol 3,4,5,6-tetrakisphosphate reduced the ability of subsequently added thapsigargin to stimulate apical 125I-, but not basolateral 86Rb+, efflux. These data suggested an effect on an apical Cl- channel. Conversely, epidermal growth factor reduced Ca(2+)-stimulated 86Rb+ but not 125I- efflux, suggesting an effect of the growth factor on a K+ channel. Finally, insulin inhibited 125I- and 86Rb+ effluxes. Thus effects of agents that inhibit transepithelial Cl- secretion are also manifest at the level of transmembrane transport pathways. However, the precise nature of the membrane conductances targeted are agonist specific.

Modulation of K+ channels by arachidonic acid in T84 cells. I. Inhibition of the Ca(2+)-dependent K+ channel

The Cl- secretory response of colonic cells to Ca(2+)-mediated agonists is transient despite a sustained elevation of intracellular Ca2+. We evaluated the effects of second messengers proposed to limit Ca(2+)-mediated Cl- secretion on the basolateral membrane, Ca(2+)-dependent K+ channel (Kca) in colonic secretory cells, T84. Neither protein kinase C (PKC) nor inositol tetrakisphosphate (1,3,4,5 or 3,4,5,6 form) affected Kca in excised inside-out patches. In contrast, arachidonic acid (AA; 3 microM) potently inhibited Kca, reducing NP0, the product of number of channels and channel open probability, by 95%. The apparent inhibition constant for this AA effect was 425 nM. AA inhibited Kca in the presence of both indomethacin and nordihydroguaiaretic acid, blockers of the cyclooxygenase and lipoxygenase pathways. In the presence of albumin, the effect of AA on Kca was reversed. A similar effect of AA was observed on Kca during outside-out recording. We determined also the effect of the cis-unsaturated fatty acid linoleate, the trans-unsaturated fatty acid elaidate, and the saturated fatty acid myristate. At 3 microM, all of these fatty acids inhibited Kca, reducing NP0 by 72-86%. Finally, the effect of the cytosolic phospholipase A2 inhibitor arachidonyltrifluoromethyl ketone (AACOCF3) on the carbachol-induced short-circuit current (Isc) response was determined. In the presence of AACOCF3, the peak carbachol-induced Isc response was increased approximately 2.5-fold. Our results suggest that AA generation induced by Ca(2+)-mediated agonists may contribute to the dissociation observed between the rise in intracellular Ca2+ evoked by these agonists and the associated Cl- secretory response.

The antifungal antibiotic, clotrimazole, inhibits chloride secretion by human intestinal T84 cells via blockade of distinct basolateral K+ conductances. Demonstration of efficacy in intact rabbit colon and in an in vivo mouse model of cholera

The antifungal antibiotic clotrimazole (CLT) blocks directly and with high potency the Ca2+-activated K+ channels of human erythrocytes, erythroleukemia cells, and ferret vascular smooth muscle cells. We recently reported that CLT inhibits Cl- secretion in human intestinal T84 cells, likely by affecting K+ transport (Rufo, P.A., L. Jiang, S.J. Moe, C. Brugnara, S.L. Alper, and W.I. Lencer. 1996. J. Clin. Invest. 98:2066-2075). To determine if CLT had direct effects on K+ conductances in T84 cells, we selectively permeabilized apical membranes of confluent T84 cell monolayers using the ionophore amphotericin B. This technique permits direct measurement of basolateral K+ transport. We found that CLT and a stable des-imidazolyl derivative inhibited directly two pharmacologically distinct basolateral membrane K+conductances, but had no effect on apical membrane Cl- conductances. The effects of CLT on Cl- secretion were also examined in intact tissue. CLT inhibited forskolin-induced Cl- secretion in rabbit colonic mucosal sheets mounted in Ussing chambers by 91%. CLT also inhibited cholera toxin-induced intestinal Cl- secretion in intact mice by 94%. These data provide direct evidence that CLT blocks Cl- secretion in intestinal T84 cells by inhibition of basolateral K+ conductances, and show that CLT inhibits salt and water secretion from intact tissue in vitro and in vivo. The results further support the suggestion that CLT and its metabolites may show clinical efficacy in the treatment of secretory diarrheas of diverse etiologies.

Carbachol induces oscillations in membrane potential and intracellular calcium in a colonic tumor cell line, HT-29

The patch-clamp technique was used to study the effects of carbachol (CCh) on HT-29 cells. During CCh exposure, the cells (n = 23) depolarized close to the equilibrium potential for Cl- (E(Cl-); -48 mV) and the membrane potential then started to oscillate (16/23 cells). In voltage-clamp experiments, similar oscillations in whole cell currents could be demonstrated. The whole cell conductance increased from 225 +/- 25 pS in control solution to 6,728 +/- 1,165 pS (means +/- SE, n = 17). In substitution experiments (22 mM Cl- in bath solution, E(Cl-) = 0 mV), the reversal potential changed from -41.6 +/- 2.2 mV (means +/- SE, n = 9) to -3.2 +/- 2.0 mV (means +/- SE, n = 7). When the cells were loaded with the calcium-sensitive fluorescent dye, fluo 3, and simultaneously patch clamped, CCh caused a synchronous oscillating pattern of fluorescence and membrane potential. In cell-attached patches, the CCh-activated currents reversed at a relative membrane potential of 1.9 +/- 3.7 mV (means +/- SE, n = 11) with control solution in the pipette and at 46.2 +/- 5.3 mV (means +/- SE, n = 10) with a 15 mM Cl- solution in the pipette. High K+ (144 mM) did not change the reversal potential significantly (P < or = 0.05, n = 8). In inside-out patches, calcium-dependent Cl- channels could be demonstrated with a conductance of 19 pS (n = 7). It is concluded that CCh causes oscillations in membrane potential that involve calcium-dependent Cl- channels and a K+ permeability.

Modulation of chloride secretion in the rat ileum by intracellular bicarbonate

Increasing intracellular bicarbonate concentration ([HCO3-]i) inhibits calcium-mediated Cl- secretion in rat distal colon and T84 cells. We investigated the effect of [HCO3-]i on Cl- secretion in rat ileum. Segments of intact ileum from Sprague-Dawley rats were studied in Ussing chambers and villus and crypt intracellular pH and [HCO3-]i were determined using BCECF. A range of crypt and villus [HCO3-]i from 0 to 31 mM was obtained by varying Ringer's composition. Basal serosal-to-mucosal Cl- flux (JsmCl) averaged 8.5 +/- 0.2 mu eq.h-1.cm-2 and was unaffected by changing [HCO3-]i or serosal bumetanide. Carbachol increased JsmCl by 3.9 +/- 0.5 mu eq.h-1.cm-2 at [HCO3-]i = 0 mM but only by 1.0 +/- 0.3 mu eq.h-1.cm-2 at high crypt and villus [HCO3-]i. Dibutyryl-cAMP increased JsmCl by 2.5 +/- 0.2 mu eq.h-1.cm-2 at all [HCO3-]i. Carbachol and db-cAMP showed mutual antagonism at low [HCO3-]i and near-additivity at high [HCO3-]i. We conclude that like rat colon and T84 cells, calcium-mediated but not cAMP-mediated Cl- secretion in the ileum is inhibited by increasing [HCO3-]i. Mutual antagonism between carbachol and db-cAMP at low [HCO3-]i was present in ileum and distal colon but not in T84 cells.

Identification and function of type-2 and type-3 ryanodine receptors in gut epithelial cells

Reverse transcription-PCR (RT-PCR) techniques were used to identify the expression of ryanodine receptor (RyR) isoforms in gut epithelial cells. Restriction digest and sequence analysis of the PCR product showed the presence of RyR 2 and RyR 3. [3H]Ry binding studies on a microsome preparation, in a high-salt buffer, showed specific binding with an EC50 of 15 microM. In order to determine a potential functional role for these RyRs, we first characterized the response of the cells to acetylcholine. At all concentrations used acetylcholine induced sinusoidal cytosolic Ca2+ concentration ([Ca2+]i) oscillations. In response to 10(-4) M acetylcholine, levels of inositol 1,4,5-trisphosphate (InsP3) showed a peak of six times the basal level, at 30 s after stimulation. Application of caffeine alone failed to elicit a rise in cytosolic Ca2+. However, caffeine (5-50 mM) did rapidly and reversibly inhibit the acetylcholine-induced [Ca2+]i oscillations. The effects of Ry were more complex. Applied alone, Ry had no effect on the [Ca2+]i signal. When applied during agonist-evoked [Ca2+]i oscillations, Ry (10 microM) slowly blocked the response. In the continuous presence of Ry (10 microM) a short application of acetylcholine elicited a [Ca2+]i response that continued as oscillations even when the agonist was removed. The oscillations, in the presence of Ry (10 microM) but absence of agonist, were blocked either by removal of extracellular Ca2+ or by an application of a higher concentration of Ry (100 microM). These effects are consistent with the known use-dependence and dose-dependence for Ry action at the RyR. We conclude that the RyR 2 and RyR 3, identified by RT-PCR, play a central role in [Ca2+]i oscillations in gut epithelial cells.

A non-selective cation current activated via the multifunctional Ca(2+)-calmodulin-dependent protein kinase in human epithelial cells

1. Activation of macroscopic membrane currents by intracellular calcium ([Ca2+]i) signalling pathways was examined in human T84 epithelial cells, a model secretory cell line. 2. Elevation of [Ca2+]i by either the calcium ionophore A23187 (1 microM) or the cholinergic agonist carbachol, led to the transient activation of both a chloride and cation current in single voltage clamped cells. The channels underlying the cation conductance were found to be equally permeable to external Na+, K+ and Cs+, but impermeable to the large organic cations tetraethylammonium and N-methyl-D-glucamine (NMDG). These observations indicate that the cation channels are non-selective with respect to monovalent cations. 3. Persistent activation of both the chloride and non-selective cation currents by [Ca2+]i was observed following inhibition of cellular phosphatase activity by the phosphatase inhibitor microcystin LR or the ATP analogue ATP gamma S. This finding strongly suggests the presence of a phosphorylation event in the calcium-dependent activation pathway for both currents. 4. Intracellular dialysis with peptide inhibitors of the multifunctional Ca(2+)-calmodulin-dependent protein kinase (CaM kinase) blocked the activation of both the chloride and cation conductances by elevated [Ca2+]i. Dialysis with an inactive control peptide had no effect on the activation of either current. CaM kinase thus appears to be critically involved in the calcium-dependent activation of both the chloride and cation currents in these cells. 5. Associated with the whole-cell cation conductance were macroscopic tail currents observed at the chloride reversal potential. The distinct kinetic properties of these tail currents were used as a biophysical 'signature' of the whole-cell conductance. 6. In excised, inside-out membrane patches, [Ca2+]i activated single cation channel activity. These channels had a mean conductance of 20 pS, were impermeable to NMDG, and their mean open probability increased at positive membrane potentials. The properties of these single channel events thus closely resemble those reported previously for calcium-activated cation channels in epithelia. 7. Using a novel 'tail current' voltage clamp protocol in excised membrane patches, we observed that ensemble averages of single cation channel events reproduced the behaviour and kinetic properties of the macroscopic tail currents of the calcium-activated cation conductance. This finding provides evidence that the observed single channel events probably underlie the macroscopic cation current recorded from intact cells. 8. The results from this study demonstrate that CaM kinase mediates the calcium-dependent activation of both a chloride and a non-selective cation current in human T84 epithelial cells. Using single channel recordings, we believe we have identified the corresponding whole-cell current for the 20-40 pS calcium-activated cation channel activity reported previously in epithelia and other cell preparations. Physiologically, a calcium-activated inward cation current would allow sodium influx in association with calcium-dependent electrolyte and protein secretion. Thus CaM kinase-dependent activation of cation channels may serve as a co-ordinated influx pathway to balance the efflux and influx of osmotically active solutes as part of an overall cell volume regulatory mechanism.

Determinants of insulin-stimulated skeletal muscle glycogen metabolism in man

To examine factors which influence skeletal muscle glycogen synthesis in man, we related insulin sensitivity measured by euglycaemic insulin clamp in 43 healthy males to muscle glycogen synthase (GS) activity, GS protein content (Western blot), glycogen concentrations and fibre composition. Insulin increased muscle glycogen content (P < 0.05) and the change in glycogen content correlated with the GS protein content (r = 0.90, P = 0.01). GS protein concentration correlated inversely with age (r = -0.69, P = 0.04). Non-oxidative glucose disposal was inversely related to per cent type 2B fibres (r = -0.52, P < 0.05). The influence of age on these relationships was separately studied in young (n = 12, age = 26 +/- 2 years) and elderly (n = 15, age = 56 +/- 2 years) males. Insulin increased GS activity significantly only in young subjects (from 17.8 +/- 3.0 to 25.3 +/- 3.2 nmol mg protein-1 min-1; P = 0.015). GS activity and non-oxidative glucose disposal correlated in young (r = 0.69, P = 0.01) but not in the elderly (r = 0.064, P = 0.82) males, and this relationship was not influenced by the degree of obesity. In conclusion, muscle fibre type and GS activity are both determinants of muscle glycogen metabolism in healthy, normoglycaemic males. The close relationship between non-oxidative glucose metabolism and GS activity in young males is altered in ageing.

Aspects of calcium-activated chloride currents: a neuronal perspective

Ca(2+)-activated Cl- channels are expressed in a variety of cell types, including central and peripheral neurones. These channels are activated by a rise in intracellular Ca2+ close to the cell membrane. This can be evoked by cellular events such as Ca2+ entry through voltage- and ligandgated channels or release of Ca2+ from intracellular stores. Additionally, these Ca(2+)-activated Cl currents (ICl(Ca)) can be activated by raising intracellular Ca2+ through artificial experimental procedures such as intracellular photorelease of Ca2+ from "caged" photolabile compounds (e.g. DM-nitrophen) or by treating cells with Ca2+ ionophores. The potential changes that result from activation of Ca(2+)-activated Cl- channels are dependent on resting membrane potential and the equilibrium potential for Cl-. Ca2+ entry during a single action potential is sufficient to produce substantial after potentials, suggesting that the activity of these Cl- channels can have profound effects on cell excitability. The whole cell ICl(Ca) can be identified by sensitivity to increased Ca2+ buffering capacity of the cell, anion substitution studies and reversal potential measurements, as well as by the actions of Cl- channel blockers. In cultured sensory neurones, there is evidence that the ICl(Ca) deactivates as Ca2+ is buffered or removed from the intracellular environment. To date, there is no evidence in mammalian neurones to suggest these Ca(2+)-sensitive Cl- channels undergo a process of inactivation. Therefore, ICl(Ca) can be used as a physiological index of intracellular Ca2+ close to the cell membrane. The ICl(Ca) has been shown to be activated or prolonged as a result of metabolic stress, as well as by drugs that disturb intracellular Ca2+ homeostatic mechanisms or release Ca2+ from intracellular stores. In addition to sensitivity to classic Cl- channel blockers such as niflumic acid, derivatives of stilbene (4,4'diisothiocyanostilbene-2,2'-disulphonic acid, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid) and benzoic acid (5-nitro 2-(3-phenylpropylamino) benzoic acid), ICl(Ca) are also sensitive to polyamine spider toxins and some of their analogues, particularly those containing the amino acid residue arginine. The physiological role of Ca(2+)-activated Cl- channels in neurones remains to be fully determined. The wide distribution of these channels in the nervous system, and their capacity to underlie a variety of events such as sustained or transient depolarization or hyperpolarizations in response to changes in intracellular Ca2+ and variations in intracellular Cl- concentration, suggest the roles may be subtle, but important.

Flufenamate and Gd3+ inhibit stimulated Ca2+ influx in the epithelial cell line CFPAC-1

The relevant influx pathway for stimulated Ca2+ entry into epithelial cells is largely unknown. Using flufenamate (Flu) and Gd3+, both known pharmacological blockers of non-selective cation currents in other epithelial preparations, we tested whether the stimulated Ca2+ entry in CFPAC-1 cells was inhibited by these agents. Transmembraneous Ca2+ influx into CFPAC-1 cells was stimulated by either ATP (10(-4) and 10(-5) mol/l), carbachol (CCH, 10(-4) mol/l) or thapsigargin (TG, 10(-8) mol/l). Three different experimental approaches were used. (1) Because the plateau phase of an agonist-induced [Ca2+]i transient reflects Ca2+ influx into these cells, we investigated the influence of Flu and Gd3+ on the level of the stimulated [Ca2+]i plateau. (2) The fura-2 Mn(2+)-quenching technique was used to visualise divalent cation entry and monitor its inhibition. (3) During the "refilling period" after agonist-induced discharge of the intracellular pools the putative influx inhibitors Flu and Gd3+ were given and subsequently the filling state of the agonist-sensitive intracellular stores tested. The results from the first experimental approach showed that both Flu and Gd3+ were potent inhibitors of the stimulated Ca2+ entry in CFPAC-1 cells. Flu reversibly decreased the ATP-induced [Ca2+]i plateau in a concentration dependent manner, with an IC50 value of 33 mumol/l (n = 6). Similar results were obtained for the CCH- (n = 5) and the TG-induced (n = 5) [Ca2+]i plateau. Gd3+ concentration dependently inhibited the stimulated Ca2+ plateau. A complete block of the ATP-induced [Ca2+]i plateau was seen at 0.5 mumol/l (ATP 10(-5) mol/l, n = 8).(ABSTRACT TRUNCATED AT 250 WORDS)

Concomitant activation of Cl- and K+ currents by secretory stimulation in human epithelial cells

1. Whole-cell currents were investigated in the model salt-secreting epithelium, human T84 cell line, by means of the perforated patch-clamp technique. In the control extracellular medium containing Cl-, depolarizing voltage ramps evoked current responses which peaked at 5.43 +/- 0.81 pA pF-1 at +60 mV and had a reversal potential (Erev) of -38.4 +/- 2.5 mV (n = 23). 2. Activation of the cAMP pathway with forskolin increased the current at +60 mV from 3.81 +/- 0.61 to 20.79 +/- 5.08 pA pF-1 (n = 18). In thirteen cells, Erev was initially shifted towards positive potentials (Erev of the cAMP-activated initial current was -18.2 +/- 1.2 mV) and subsequently shifted towards more negative potentials, consistent with the activation of both Cl- and K+ currents during cAMP stimulation. 3. Increasing the intracellular Ca2+ concentration, [Ca2+]i, with ionomycin (1 microM) or with acetylcholine (1 microM), increased the current at +60 mV from 7.79 +/- 1.57 to 57.50 +/- 12.10 pA pF-1 (n = 6) and from 6.36 to 34.13 pA pF-1 (n = 4), respectively. With both agonists, Erev was shifted either towards the reversal potential for potassium, EK, or towards the reversal potential for chloride, ECl, depending on the cell. 4. In the absence of chloride ions (gluconate substituted), stimulation of the Ca2+ pathway activated a time-independent outward current of large amplitude. This current exhibited inward rectification at positive voltages, reverted at -89.5 +/- 0.2 mV and was markedly reduced by charybdotoxin (10 nM), a specific blocker of Ca(2+)-activated K+ channels. When a voltage step protocol was used, increased [Ca2+]i also activated an outward current at potentials more positive than -40 mV which slowly relaxed during depolarizing steps. 5. The activation of both (i) a time-dependent inwardly rectifying charybdotoxin-sensitive K+ current, and (ii) a time-dependent slowly inactivating current was also produced by cAMP stimulation. 6. We concluded that (i) in the T84 epithelial cells, both Cl- and K+ currents are concomitantly increased by secretagogue stimulation, and (ii) two different types of K+ conductances are activated by either the cAMP or the intracellular Ca2+ secreting pathways.

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.

Muscarinic-receptor activation stimulates oscillations in K+ and Cl- currents which are acutely dependent on extracellular Ca2+ in avian salt gland cells

By utilizing the perforated-patch variant of the whole-cell patch-clamp recording technique, in order to maintain the integrity of the normal cellular buffering systems, we demonstrate that carbachol (CCh) stimulates simultaneous oscillations in a Ca(2+)- and voltage-activated K+ current and a linear Ca(2+)-activated Cl- current in an exocrine avian salt gland cell preparation. Similar conductance changes, although sustained rather than oscillatory, are stimulated by the Ca2+ ionophore A23187. The outward K+ current can be inhibited by tetraethylammonium chloride (TEA) whereas the Cl- current is inhibited by the Cl- channel blockers 5-nitro-2-(3-phenylpropylamino) (NPPB) and N-phenylanthranilic acid (DPC). The oscillations in current stimulated by CCh are acutely dependent on extracellular Ca2+ and are not affected by the application of low doses of caffeine. In addition, the application of caffeine at all doses fails to mimic the current transients stimulated by CCh. As both caffeine and A23187 are unable to stimulate oscillations under the perforated-patch conditions we suggest that in avian salt gland cells the primary oscillatory mechanism probably involves a one-pool mechanism of Ca2+ release which is intimately related to the activation of a Ca2+ influx pathway.

Taurodeoxycholate activates potassium and chloride conductances via an IP3-mediated release of calcium from intracellular stores in a colonic cell line (T84)

Whole-cell patch-clamp techniques and fluorescence measurements of intracellular Ca2+ concentration, (Ca2+)i, were used to investigate the mechanism of taurodeoxycholate (TDC) stimulation of Cl- secretion in the T84 colonic cell line. During perforated whole-cell recordings, the cell membrane voltage was alternately clamped to EK and ECl. Initially, TDC (0.75 mM) stimulated inward nonselective cation currents that were composed of discrete large conductance single-channel events. This initial response was followed by activation of K+ and Cl- currents with peak values of 385 +/- 41 pA and 98 +/- 28 pA, respectively (n = 12). The K+ and Cl- currents oscillated while TDC was present and returned to baseline levels upon its removal. The threshold for activation of the oscillatory currents was 0.1 mM TDC. Taurocholate, a bile acid that does not stimulate colonic Cl- secretion, induced no current response. The TDC-induced currents could be activated in Ca(2+)-free bathing solutions. Preincubation of cells with the Ca2+ chelator, bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid, tetra(acetoxymethy)-ester (20 microM), (BAPTA-AM), eliminated the K+ and Cl- current responses, although the nonselective cation channel events were still present. Replacement of bath Na+ with NMDG+ inhibited the TDC-induced nonselective cation current but did not affect the K+ or Cl- currents. TDC induced a transient (Ca2+)i rise of 575 +/- 70 nM from a baseline of 71 +/- 5 nM (n = 15); thereafter, (Ca2+)i either plateaued or oscillated. TDC-induced (Ca2+)i oscillations were observed in the absence of bath Ca2+; however, removal of bath Ca2+ during the TDC response caused (Ca2+)i to return to near baseline values. Simultaneous K+ current and (Ca2+)i measurements confirmed that the initial nonselective cation current was independent of (Ca2+)i, while K+ current oscillations were in phase with the (Ca2+)i oscillations. TDC induced inositol monophosphate (IP) accumulation, reflecting production of inositol 1,4,5-trisphosphate (IP3) during TDC stimulation. The response to TDC during standard whole-cell patch-clamp was similar to that observed with perforated whole-cell recordings, except the nonselective cation current was prolonged. When heparin (1 mg/ml) was added to the pipette under these conditions, the Ca(2+)-activated currents were inhibited, but the nonselective cation currents were unaffected. These data suggest that TDC induces a Ca(2+)-independent nonselective cation conductance, perhaps by directly permeabilizing the plasma membrane. TDC stimulates Cl- secretion by activating K+ and Cl- conductances via an IP3-mediated release of Ca2+ from intracellular stores.