Regulation of Cl- channels in normal and cystic fibrosis airway epithelial cells by extracellular ATP

Role of CFTR in airway disease

Role of CFTR in Airway Disease. Physiol. Rev. 79, Suppl.: S215-S255, 1999. - Cystic fibrosis (CF) is caused by mutations in the gene encoding the CF transmembrane conductance regulator (CFTR), which accounts for the cAMP-regulated chloride conductance of airway epithelial cells. Lung disease is the chief cause of morbidity and mortality in CF patients. This review focuses on mechanisms whereby the deletion or impairment of CFTR chloride channel function produces lung disease. It examines the major themes of the channel hypothesis of CF, which involve impaired regulation of airway surface fluid volume or composition. Available evidence indicates that the effect of CFTR deletion alters physiological functions of both surface and submucosal gland epithelia. At the airway surface, deletion of CFTR causes hyperabsorption of sodium chloride and a reduction in the periciliary salt and water content, which impairs mucociliary clearance. In submucosal glands, loss of CFTR-mediated salt and water secretion compromises the clearance of mucins and a variety of defense substances onto the airway surface. Impaired mucociliary clearance, together with CFTR-related changes in the airway surface microenvironment, leads to a progressive cycle of infection, inflammation, and declining lung function. Here, we provide the details of this pathophysiological cascade in the hope that its understanding will promote the development of new therapies for CF.

CFTR is a conductance regulator as well as a chloride channel

CFTR Is a Conductance Regulator as well as a Chloride Channel. Physiol. Rev. 79, Suppl.: S145-S166, 1999. - Cystic fibrosis transmembrane conductance regulator (CFTR) is a member of the ATP-binding cassette (ABC) transporter gene family. Although CFTR has the structure of a transporter that transports substrates across the membrane in a nonconductive manner, CFTR also has the intrinsic ability to conduct Cl- at much higher rates, a function unique to CFTR among this family of ABC transporters. Because Cl- transport was shown to be lost in cystic fibrosis (CF) epithelia long before the cloning of the CF gene and CFTR, CFTR Cl- channel function was considered to be paramount. Another equally valid perspective of CFTR, however, derives from its membership in a family of transporters that transports a multitude of different substances from chemotherapeutic drugs, to amino acids, to glutathione conjugates, to small peptides in a nonconductive manner. Moreover, at least two members of this ABC transporter family (mdr-1, SUR) can regulate other ion channels in the membrane. More simply, ABC transporters can regulate somehow the function of other cellular proteins or cellular functions. This review focuses on a plethora of studies showing that CFTR also regulates other ion channel proteins. It is the hope of the authors that the reader will take with him or her the message that CFTR is a conductance regulator as well as a Cl- channel.

Membrane-limited expression and regulation of Na+-H+ exchanger isoforms by P2 receptors in the rat submandibular gland duct

1. Cell-specific reverse transcriptase-polymerase chain reaction (RT-PCR), immunolocalization and microspectrofluorometry were used to identify and localize the Na+-H+ exchanger (NHE) isoforms expressed in the submandibular gland (SMG) acinar and duct cells and their regulation by basolateral and luminal P2 receptors in the duct. 2. The molecular and immunofluorescence analysis showed that SMG acinar and duct cells expressed NHE1 in the basolateral membrane (BLM). Duct cells also expressed NHE2 and NHE3 in the luminal membrane (LM). 3. Expression of NHE3 was unequivocally established by the absence of staining in SMG from NHE3 knockout mice. NHE3 was expressed in the LM and in subluminal regions of the duct. 4. Measurement of the inhibition of NHE activity by the amiloride analogue HOE 694 (HOE) suggested expression of NHE1-like activity in the BLM and NHE2-like activity in the LM of the SMG duct. Several acute and chronic treatments tested failed to activate NHE activity with low affinity for HOE as expected for NHE3. Hence, the physiological function and role of NHE3 in the SMG duct is not clear at present. 5. Activation of P2 receptors resulted in activation of an NHE-independent, luminal H+ transport pathway that markedly and rapidly acidified the cells. This pathway could be blocked by luminal but not basolateral Ba2+. 6. Stimulation of P2U receptors expressed in the BLM activated largely NHE1-like activity, and stimulation of P2Z receptors expressed in the LM activated largely NHE2-like activity. 7. The interrelation between basolateral and luminal NHE activities and their respective regulation by P2U and P2Z receptors can be used to co-ordinate membrane transport events in the LM and BLM during active Na+ reabsorption by the SMG duct.

Bioluminescence detection of ATP release mechanisms in epithelia

Autocrine and paracrine release of and extracellular signaling by ATP is a ubiquitous cell biological and physiological process. Despite this knowledge, the mechanisms and physiological roles of cellular ATP release are unknown. We tested the hypothesis that epithelia release ATP under basal and stimulated conditions by using a newly designed and highly sensitive assay for bioluminescence detection of ATP released from polarized epithelial monolayers. This bioluminescence assay measures ATP released from cystic fibrosis (CF) and non-CF human epithelial monolayers in a reduced serum medium through catalysis of the luciferase-luciferin reaction, yielding a photon of light collected by a luminometer. This novel assay measures ATP released into the apical or basolateral medium surrounding epithelia. Of relevance to CF, CF epithelia fail to release ATP across the apical membrane under basal conditions. Moreover, hypotonicity is an extracellular signal that stimulates ATP release into both compartments of non-CF epithelia in a reversible manner; the response to hypotonicity is also lost in CF epithelia. The bioluminescence detection assay for ATP released from epithelia and other cells will be useful in the study of extracellular nucleotide signaling in physiological and pathophysiological paradigms. Taken together, these results suggest that extracellular ATP may be a constant regulator of epithelial cell function under basal conditions and an autocrine regulator of cell volume under hypotonic conditions, two functions that may be lost in CF and contribute to CF pathophysiology.

What we know and what we do not know about cystic fibrosis transmembrane conductance regulator

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-regulated chloride channel that resides in the apical membrane of many epithelial cells. Channel opening requires phosophorylation of serine residues in an intracellular regulatory domain by protein kinase A and as the binding and hydrolysis of ATP by intracellular nucleotide binding domains. Besides conducting the chloride ion, CFTR also regulates the function of other membrane proteins, directly or indirectly, notably the outwardly rectifying chloride channel and the epithelial sodium channel. The disease cystic fibrosis is caused by mutations in CFTR, which can result in defective protein production, defective processing and degradation in the endoplasmic reticulum, or defective channel pore properties or gating properties.

Purinoceptor activation of chloride transport in cystic fibrosis and CFTR-transfected pancreatic cell lines

The regulation of chloride efflux from cystic fibrosis pancreatic adenocarcinoma cells (CFPAC-1) and wild-type CFTR-transfected CFPAC-1 cells (TPAC) was compared. Forskolin (10 microM) stimulated chloride efflux from the corrected TPAC cells but not from CFPAC-1 cells. Chloride efflux from both cell types was activated by thapsigargin (0.5 microM). The nucleotides ATP and UTP and the non-hydrolyzable ATP analogue, adenosine 5'-O-(3-thio) triphosphate (ATPgammaS), stimulated chloride efflux from both cell types. None of the other P2 purinoceptor agonists investigated elicited a response. The order of potency was ATP > or = UTP > or = ATPgammaS. Adenosine (10-100 microM) activated choride efflux from the TPAC but not the CFPAC cell line with no increase in intracellular cyclic AMP. Small but statistically significant inhibitions of the adenosine-(50 microM)-stimulated increase in chloride efflux were elicited by the A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (CPX, 100 nM) and the A2 receptor antagonist 3,7-dimethyl-1-propylargylxanthine (DMPX, 10 microM). The A2A receptor antagonist 8-(3-chlorostyryl)caffeine (CSC, 100 nM) had no significant effect. These results provide evidence for the regulation of chloride efflux by P2Y2 purinoceptors in genetically-corrected and CF pancreatic cell lines. Studies with adenosine receptor antagonists indicate some possible involvement of A1 and A2 (but not A2A) receptors in the adenosine stimulation of chloride efflux, but the relatively small effects of the inhibitors coupled with lack of increase in cyclic AMP and a response only in the CFTR-transfected cells also suggests a possible direct effect of adenosine on CFTR.

Extracellular ATP raises cytosolic calcium and activates basolateral chloride conductance in Necturus proximal tubule

1. Extracellular nucleotides modulate ionic transport mechanisms in various epithelia. In the present study, we investigated the effects of extracellular ATP on the intracellular free Ca+2 concentration ([Ca2+]i) and electrophysiological properties of Necturus maculosus proximal convoluted tubule (PCT). 2. ATP raised [Ca2+]i in microdissected fura-2-loaded PCTs (half-maximal effect, approximately mumol 1(-1) ATP). The initial ATP-induced changes in [Ca2+]i were not blunted by the removal of external Ca2+ nor by the presence of Ca2+ channel blockers, but were abolished by thapsigargin and suramin. The sequence for the potency of various agonists on [Ca2+]i was 2-methylthioATP (2MeSATP) = ADP = ATP >> UTP, 2',3',-O-(4-benzoilbenzoil) ATP (BzATP), alpha, beta-methylene ATP (AMPCPP), adenosine. 3. In vivo electrophysiological measurements showed that 100 mumol 1(-1) peritubular ATP added to a Ringer solution reduced the basolateral cell membrane potential (Vm) and increased the cell membrane input conductance. In a low Cl- solution, this ATP-induced depolarization was enhanced. These effects were inhibited by 1 mmol l-1 SITS, consistent with the activation of a basolateral Cl- conductance. 4. The ATP-induced change in Vm was reproduced by ADP but not by UTP or adenosine, and was prevented by suramin. 5. The ATP-induced membrane depolarization was not influenced by thapsigargin, BAPTA AM, or staurosporine and was not reproduced by manoeuvres increasing [Ca2+]i or intracellular cAMP content. 6. We conclude that, in Necturus PCT, a P2y receptor mobilizes Ca2+ mainly from intracellular pools and increases a basolateral Cl- conductance, GCl. The activation of GCl occurs by a mechanism which is not related either to an increase in [Ca2+]i or cAMP content, or to PKC activation.

Roles of K+ and Cl- channels in cAMP-induced pulmonary vasodilation

Increase in intracellular adenosine 3', 5'-cyclic monophosphate (cAMP) is a common pathway for many clinically used drugs to cause pulmonary artery (PA) relaxation. Activity of sarcolemmal K+ and Cl(-)-channels is an important determinant of membrane potential (Em), which, in turn, plays a critical role in regulating pulmonary vascular tone. Whether K+ and Cl- channels were involved in cAMP-induced PA relaxation was tested using isolated rat PA rings. Raising extracellular K+ concentration from 20 to 142.7 mM increased the K(+)-evoked contraction, but significantly decreased the relaxation induced by the adenylate cyclase activator, forskolin (FSK, 2.5 microM), suggesting that FSK-induced PA relaxation depended on transmembrane K+ gradient. Indeed, the FSK-induced relaxation was inhibited by 4-aminopyridine (4-AP, 10 mM), a voltage-gated K+ (Kv) channel blocker. Neither the Ca(2+)-activated K+ channel blocker, charybdotoxin, nor the ATP-sensitive K+ channel blocker, glibenclamide, had this effect. Furthermore, reducing extracellular Cl- concentration from 142.7 to 50 mM significantly decreased the FSK-induced relaxation in PA rings precontracted with 142.7 mM K+ (Ek approximately 0 mV), but negligibly affected the evoked contraction. This indicates that transmembrane Cl- gradient also regulates FSK-induced PA relaxation. Indeed, the Cl- channel blocker, 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB, 10 microM), significantly inhibited the FSK-induced relaxation in PA rings preconstricted by 142.7 mM K+. In summary, the data suggest that the cAMP-induced PA relaxation is attributable, at least partly, to both activation of the 4-AP-sensitive Kv channels and stimulation of the NPPB-sensitive Cl- channels.

Membrane-specific regulation of Cl- channels by purinergic receptors in rat submandibular gland acinar and duct cells

Measurement of [Cl-]i and the Cl- current in the rat salivary submandibular gland (SMG) acinar and duct cells was used to evaluate the role of Cl- channels in the regulation of [Cl-]i during purinergic stimulation. Under resting conditions [Cl-]i averaged 56 +/- 8 and 26 +/- 7 mM in acinar and duct cells, respectively. In both cells, stimulation with 1 mM ATP resulted in Cl- efflux and subsequent influx. Inhibition of NaKCl2 cotransport had no effect on [Cl-]i changes in duct cells and inhibited only about 50% of Cl- uptake in acinar cells. Accordingly, low levels of expression of NaKCl2 cotransporter protein were found in duct cells. Acinar cells expressed high levels of the cotransporter. Measurement of Cl- current under selective conditions revealed that acinar and duct cells express at least five distinct Cl- channels; a ClCO-like, volume-sensitive, inward rectifying, Ca2+-activated and CFTR-like Cl- currents. ATP acting on both cell types activated at least two channels, the Ca2+-activated Cl- channel and a Ca2+-independent glibenclamide-sensitive Cl--current, possibly cystic fibrosis transmembrane regulator (CFTR). Of the many nucleotides tested only 2'-3'-benzoylbenzoyl (Bz)-ATP and UTP activated Cl- channels in SMG cells. Despite their relative potency in increasing [Ca2+]i, BzATP in both SMG cell types largely activated the Ca2+-independent, glibenclamide-sensitive Cl- current, whereas UTP activated only the Ca2+-dependent Cl- current. We interpret this to suggest that BzATP and UTP largely activate Cl- channels residing in the membrane expressing the receptor for the active nucleotide. The present studies reveal a potentially new mechanism for transcellular Cl- transport in a CFTR-expressing tissue, the SMG. Coordinated action of the P2z (luminal) and P2u (basolateral) receptors can mediate part of the transcellular Cl- transport by acinar and duct cells to determine the final electrolyte composition of salivary fluid.

Characterization and localization of P2 receptors in rat submandibular gland acinar and duct cells

[Ca2+]i and the Cl- current were measured in isolated submandibular gland acinar and duct cells to characterize and localize the purinergic receptors expressed in these cells. In both cell types 2'-3'-benzoylbenzoyl (Bz)-ATP and ATP increased [Ca2+]i mainly by activation of Ca2+ influx. UTP had only minimal effect on [Ca2+]i at concentrations between 0.1 and 1 mM. However, a whole cell current recording showed that all nucleotides effectively activated Cl- currents. Inhibition of signal transduction through G proteins by guanyl-5'-beta-thiophosphate revealed that the effect of ATP on Cl- current was mediated in part by activation of a G protein-coupled and in part by a G protein-independent receptor. BzATP activated exclusively the G protein-independent portion, whereas UTP activated only the G protein-dependent portion of the Cl- current. Measurement of [Ca2+]i in the microperfused duct showed that ATP stimulated a [Ca2+]i increase when applied to the luminal or the basolateral sides. BzATP increased [Ca2+]i only when applied to the luminal side, whereas UTP at 100 microM increased -Ca2+-i only when applied to the basolateral side. The combined results suggest that duct and possibly acinar cells express P2z receptors in the luminal and P2u receptors in the basolateral membrane.

Disruption of monolayer integrity enables activation of a cystic fibrosis "bypass" channel in human airway epithelia

Cystic fibrosis (CF) is a genetic disease characterized by marked reduction in Cl- conductance across many epithelia. Two kinds of Cl- channels have been associated with CF. One channel, termed the cystic fibrosis transmembrane conductance regulator (CFTR), is directly coded by the CF gene. The other channel is an outwardly rectifying depolarization induced Cl- channel (ORDIC) that is distinguished from other outwardly rectifying chloride channels (ORCCs) because its activity is induced most reliably by patch excision and depolarization. An issue in current CF research is whether ORDIC channels are indirectly activated by CFTR to contribute a significant portion of apical membrane Cl- conductance in airway cells. We now show that ORDIC channels are readily activated in patches excised and depolarized from isolated cells, but are rarer or refractory to activation in patches from the apical membranes of confluent human airway epithelia. These findings have important implications for proposed therapies that would bypass the CFTR conductance by activating ORDIC channels.

Fluid transport across cultures of human tracheal glands is altered in cystic fibrosis

1. There is evidence that defective submucosal gland secretion contributes to the airway pathology of cystic fibrosis (CF). Using a capacitance probe technique, we have compared fluid transport across submucosal gland cultures from individuals with and without CF. 2. Under baseline conditions, approximately 60% of non-CF cultures secreted fluid; the rest absorbed. In secreting tissues, amiloride increased secretion, whereas in absorbing tissues it reduced or reversed absorption. 5-Nitro-2(3-phenylpropylamino)-benzoate (NPPB) a blocker of the CF transmembrane conductance regulator (CFTR), converted secretion to absorption. Thus, the direction and magnitude of baseline fluid movement depended on a balance between active absorption of Na+ and cAMP-dependent secretion of Cl-. 3. 8-(4-Chlorophenylthio)-adenosine 3',5'-cyclic monophosphate (CPT-cAMP), methacholine and luminal uridine 5'-triphosphate (UTP) all induced or increased fluid secretion across non-CF cultures. Results with NPPB and with 4,4'-diisothiocyanatostilbene-2,2'-disulphonate (DIDS), a blocker of Ca(2+)-activated Cl- channels, suggested that fluid secretion induced by CPT-cAMP was mediated primarily by CFTR; UTP acted entirely via Ca(2+)-activated Cl- channels, and methacholine activated both pathways. 4. All CF cultures showed baseline fluid absorption, which was abolished by amiloride. 5. CF cultures showed a normal secretory response to UTP, a reduced response to methacholine, and no response to CPT-cAMP. 6. Thus, the absorptive processes of airway glands are retained in CF, but the cAMP-dependent secretory process is lost. This would markedly reduce the water content of gland secretions. The resulting change in viscosity would contribute to the accumulation of airway mucus which is characteristic of this disease.

Human monocyte interleukin-1beta posttranslational processing. Evidence of a volume-regulated response

Interleukin (IL)-1beta produced by monocytes and macrophages is not released via the normal secretory apparatus, and prior to its release, this cytokine must be proteolytically processed to generate a mature biologically active species. Biochemical mechanisms that regulate these posttranslational steps are not well understood. Lipopolysaccharide (LPS) is a poor activator of IL-1 posttranslational processing despite serving as a potent inducer of IL-1 synthesis. For example, freshly isolated human monocytes treated with LPS released <30% of their newly synthesized IL-1beta as the mature 17-kDa cytokine species, and monocytes that were aged overnight in culture prior to LPS treatment released no 17-kDa cytokine. In contrast, addition of extracellular ATP promoted IL-1beta posttranslational processing from both monocyte populations. Previous studies indicated that ATP, acting via surface P2Z-type receptors, promoted major intracellular ionic changes. To explore whether these ionic changes were required for cytokine posttranslational processing, LPS-stimulated human monocytes were maintained in ionically altered media. Hypotonic conditions promoted an efficient and selective release of mature 17-kDa IL-1beta from LPS-activated monocytes in the absence of ATP. In contrast, hypertonic conditions blocked the ATP-induced posttranslational processing reactions. Both hypotonic stress- and ATP-induced processing were blocked when NaI was substituted for NaCl within the medium; substitution with NaSCN or NaNO3 also blocked the ATP response, but these salts were less inhibitory against the hypotonic stimulus. Sodium glucuronate substitution did not inhibit cytokine processing induced by either stimulus. Removal of divalent cations from the medium did not affect the ATP response, but pretreatment of monocytes with the phosphatase inhibitor okadaic acid dose-dependently suppressed ATP-induced IL-1beta posttranslational processing. A volume-induced change to the intracellular ionic environment, therefore, may represent a key element of the mechanism by which IL-1beta posttranslational processing is initiated. The strong dependence of this cytokine release mechanism on chloride anions suggests that selective anion transporters function as important components of this response.

Expression of Drosophila melanogaster P-glycoproteins is associated with ATP channel activity

Two distinct Drosophila melanogaster P-glycoprotein (Pgp) gene homologues of different chromosomal origin, MDR49 and MDR65, have been previously identified (38). Most Pgps are implicated in the development of the multidrug-resistance phenotype. Despite intense efforts to identify the molecular mechanism(s) associated with Pgp function, the endogenous substrate(s) of these transport molecules is largely unknown. Recent studies from our laboratory indicate that a murine Pgp homologue (E. H. Abraham, A. G. Prat, L. Gerweck, T. Seneveratne, R. J. Arceci, R. Kramer, G. Guidotti, and H. F. Cantiello. Proc. Natl. Acad. Sci. USA 90: 312-316, 1993) and a related protein, the cystic fibrosis transmembrane conductance regulator (CFTR; I. L. Reisin, A. Prat, E. H. Abraham, J. F. Amara, R. J. Gregory, D. A. Ausiello, and H. F. Cantiello. J. Biol. Chem. 269: 20584-20591, 1994), are novel ATP-permeable ion channels. The common feature of these two proteins is the conserved ATP-binding cassettes (ABC); thus molecules structurally linked to the ABC transporter family may be also functionally associated with ATP channel activity. In this study, MDR65 and MDR49 Pgps were functionally expressed in Sf9 cells, and patch-clamp techniques were applied to assess the role of these proteins in the electrodiffusional movement of ATP. In the presence of intracellular ATP and external NaCl, expression of MDR65 was associated with a linear electrodiffusional pathway that was permeable to both ATP and Cl-. Under symmetrical ATP conditions, only voltage depolarization activated a MDR65-mediated ATP-conductive pathway. Expression of MDR49 was also associated with a voltage-activated ATP conductance in symmetrical ATP, but no apparent permeability to either Cl- or ATP was observed under asymmetrical conditions. The different functional properties of MDR65 and MDR49 may be indicative of distinct physiological roles in this organism. The study indicates, however, that the two Drosophila Pgp homologues share strong functional similarities with their mammalian relatives Pgp and CFTR.

Purinergic regulation of anion secretion by cystic fibrosis pancreatic duct cells

The present study explored regulation of anion secretion across cystic fibrosis pancreatic ductal epithelium by extracellular ATP with the short-circuit current (Isc) technique. CFPAC-1 cells grown on Millipore filters formed polarized monolayers with junctional complexes as revealed by light and electron microscopy. The cultured monolayers exhibited an increase in Isc in response to apical application of ATP in a concentration-dependent manner (concentration eliciting 50% of maximal response = 3 microM). Replacement of Cl- in the bathing solution or treatment of the cells with a Cl- channel blocker, 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), markedly reduced Isc, indicating that a substantial portion of ATP-activated Isc was Cl- dependent. The effects of different adenosine nucleosides and/or nucleotides on Isc were also studied to identify the type of purinoceptors involved. The order of potency, ATP = UTP > ADP > adenosine, was consistent with that for P2 purinoceptors. Reactive blue 2 (100 microM), a P2 antagonist, was found to inhibit 86% of ATP-induced Isc. ATP-induced Isc was also inhibited by pretreatment of the cells with a Ca2+ chelator, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester (50 microM). Confocal microscopic study also demonstrated a rise in intracellular Ca2+ with stimulation by extracellular ATP, indicating a role of intracellular Ca2+ in mediating the ATP response. ATP-induced Isc was observed in monolayers whose basolateral membranes had been permeabilized by nystatin, which was also sensitive to apical addition of DIDS, suggesting that Isc was mediated by apical Cl- channels. The results of the present study demonstrate the presence of a purinergic regulatory mechanism involving P2U receptor and Ca2+ mobilization in pancreatic duct anion secretion.

Recruitment of purinergically stimulated Cl- channels from granule membrane to plasma membrane

HT29-Cl.16E and HT29-Cl.19A are two different subcloned cell lines derived from the human adenocarcinoma cell line HT-29. They are similar in their ability to grow and differentiate to polarized epithelial cells but differ in that HT29-Cl.16E is goblet cell-like with many mucin granules, whereas HT29-Cl.19A lacks mucin granules. Extracellular ATP stimulates Cl- secretion in both cell lines through luminal purinergic P20 receptors and, in HT29-Cl.16E, also mucin secretion release. To evaluate whether fusion of mucin granules is associated with an increase in Cl- conductance of the plasma membrane, the effects of two fusion inhibitors on luminal Cl- conductance were measured. Blockage of actin depolymerization with phalloidin (1 microM) inhibited purinergically stimulated but not adenosine 3',5'-cyclic monophosphate (cAMP)-stimulated luminal Cl- efflux by 50% in HT29-Cl.16E. The same treatment was without effect in HT29-Cl.19A. The fungal metabolite wortmannin, which is an inhibitor of regulated exocytosis in leukocytes, at 100 nM inhibited Cl- secretion by 70% in HT29-Cl.16E. This inhibition was not a direct effect on purinergically stimulated Cl- channels because wortmannin concentrations of up to 1 microM did not affect the secretory response in HT29-Cl.19A. The wortmannin inhibition of Cl- secretion is associated with an inhibition of granule fusion as judged by electron microscopy. The differential inhibition of Cl- secretion in the related HT-29 clones that differ with respect to the presence of mucin granules indicates that 1) the granule fusion inhibitors, phalloidin and wortmannin, have no direct inhibitory effects on purinergically and cAMP-activated Cl- channels, 2) a major portion of purinergically but not cAMP-activated Cl- channels is associated with granule fusion in HT29-Cl.16E, and 3) the signaling pathways for Cl- secretion and granule fusion are not completely identical.

Activation of endogenous deltaF508 cystic fibrosis transmembrane conductance regulator by phosphodiesterase inhibition

Many heterologously expressed mutants of the cystic fibrosis transmembrane conductance regulator (CFTR) exhibit residual chloride channel activity that can be stimulated by agonists of the adenylate cyclase/protein kinase A pathway. Because of clinical implications for cystic fibrosis of activating mutants in vivo, we are investigating whether deltaF508, the most common disease-associated CFTR mutation, can be activated in airway epithelial cells. We have found that, 36Cl- efflux can be stimulated 19-61% above baseline by beta-adrenoreceptor agonists and cGI-phosphodiesterase inhibitors in transformed nasal polyp (CF-T43) cells homozygous for the deltaF508 mutation. The increase in 36Cl- permeability is diminished by protein kinase A inhibitors and is not mediated by an increase in intracellular calcium concentrations. Preincubation of CF-T43 cells with CFTR anti-sense oligonucleotides prevented an increase in 36Cl- efflux in response to beta-agonist and phosphodiesterase inhibitor. Primary cells isolated from CF nasal polyps gave similar results. These data indicate that endogenous levels of deltaF508 protein can be stimulated to increase 36Cl- permeability in airway epithelial cells.

Immortalization and characterization of proximal tubule cells derived from kidneys of spontaneously hypertensive and normotensive rats

Epithelial cell lines from the proximal tubule of SHR and WKY rats were generated by microdissection, cell growth on 3T3 cell feeder layers, and transduction of the SV40 large T-antigen gene. The cell lines that formed confluent, electrically-resistive monolayers (basal conductance 1 to 20 mS/cm2) were selected for further study. Of these, cell lines generated from one rat did not show evidence of T-antigen expression or integration, and apparently immortalized spontaneously. Cell lines from three other rats expressed high levels of T-antigen, and showed evidence of integration of one or more copies of T-antigen. All cell lines formed polarized monolayers with apical microvilli, tight junctional complexes, and convolutions of the basolateral plasma membrane. Most cell lines grew in the absence of extracellular glucose indicating a capacity for gluconeogenesis. Sodium succinate cotransport and P2-purinergic receptor mediated signaling were demonstrated in all lines tested. The cell lines also showed that Na/H exchanger activity is regulated by angiotensin II. The results indicate that these cell lines express a proximal tubular phenotype, and are morphologically and functionally similar to primary cultures. These rat cell lines represent a new, potentially useful cell model for elucidating the cellular and molecular mechanisms of genetic differences in proximal tubule Na+ reabsorption.

cAMP- but not Ca(2+)-regulated Cl- conductance is lacking in cystic fibrosis mice epididymides and seminal vesicles

Cystic fibrosis (CF) reflects the loss of adenosine 3',5'-cyclic monophosphate (cAMP)-regulated Cl- secretion consequent to mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. In humans, but not mice, with CF, the disease is associated with male infertility. The present study investigated the relative magnitudes of the cAMP pathways and an alternative Ca(2+)-regulated Cl- secretory pathway in primary cultures of the epididymides and the seminal vesicles of normal and CF mice. The basal equivalent short-circuit currents (Ieq) of cultures derived from the epididymides and the seminal vesicles from the CF mice were lower (6.0 +/- 0.6 and 4.0 +/- 1.0 muA/cm2, respectively) than those from normal mice (11.1 +/- 1.0 and 6.6 +/- 0.6 muA/cm2, respectively). Forskolin induced significant Ieq responses in both the epididymis (8.0 +/- 0.7 muA/cm2) and seminal vesicles (4.0 +/- 0.5 muA/cm2) from normal mice, whereas forskolin-induced changes in Ieq in CF mouse epididymis and seminal vesicles were absent, consistent with defective cAMP-CFTR-mediated Cl- secretion in CF mice. Ieq responses to agonists (ionomycin, ATP) that raise intracellular Ca2+ (Ca2+i) were larger than forskolin responses in normal animals (6.6 +/- 0.9 and 13.4 +/- 1.8 muA/cm2, respectively) and were preserved in CF (6.5 +/- 0.9 and 17.1 +/- 1.0 muA/cm2, respectively). We speculate that the fertility of male CF mice is maintained by persistent expression of the predominant alternative Ca(2+)-mediated Cl- transport system in the epididymides and seminal vesicles.

Apical and basolateral ATP stimulates tracheal epithelial chloride secretion via multiple purinergic receptors

Stimulation of Cl- secretion across the airway epithelium by ATP or UTP as agonists has therapeutic implications for cystic fibrosis. Our results demonstrate that ATP stimulates Cl- secretion in rat tracheal epithelial cell monolayers in primary culture from the apical or basolateral side of the monolayer. Multiple types of ATP-sensitive Cl- conductances in intact monolayers were elucidated through inhibition by Cl- channel-blocking drugs. Multiple Cl- conductances stimulated by ATP and adenosine 3',5'-cyclic monophosphate (cAMP) (tested for comparison) were also deciphered more specifically by nystatin permeabilization of the basolateral membrane, subsequent imposition of symmetrical Cl-, I-, or Br- solutions to test halide permselectivity, inhibition by Cl- channel-blocking drugs, and construction of current-voltage plots to study time and voltage dependence of the currents. Apical ATP stimulates Cl- secretion through P2U (or P2Y2) purinergic receptors via both intracellular Ca2+ (Ca(2+)i)-dependent and Cai(2+)-independent signaling pathways by opening outwardly rectifying Cl- channels (ORCCs), cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channels, and Cai(2+)-dependent Cl- channels. Basolateral ATP stimulates Cl- secretion via a combination of receptor subtypes (P2T and P2U) or a novel type of receptor (P2Y3), independent of Cai2+ or cAMP signaling by opening only CFTR channels. cAMP also stimulated multiple types of Cl- conductances, consistent with simultaneous activation of CFTR and ORCCs. Together, these results suggest that ATP as an agonist stimulates Cl- secretion via multiple purinergic receptors and multiple signal transduction pathways activated in different membrane domains of tracheal epithelia.

Cellular ATP release by the cystic fibrosis transmembrane conductance regulator

Recent studies from our laboratory indicate that members of the ATP-binding cassette (ABC) family of transporters, including P-glycoprotein and cystic fibrosis transmembrane conductance regulator (CFTR), are ATP-permeable channels. The physiological relevance of this novel transport mechanism is largely unknown. In the present study, intra- and extracellular ATP content, cellular ATP release, and O2 consumption before and after adenosine 3',5'-cyclic monophosphate (cAMP) stimulation were determined to assess the role of CFTR in the transport of ATP under physiological conditions. The functional expression of CFTR by the stable transfection of mouse mammary carcinoma cells, C1271, with human epithelial CFTR cDNA resulted in a stimulated metabolism, since both basal and cAMP-inducible O2 consumption were increased compared with mock-transfected cells. The stimulated (but not basal) O2 consumption was inhibited by diphenyl-2-carboxylic acid (DPC), a known inhibitor of CFTR. CFTR expression was also associated with the cAMP-activated and DPC-inhibitable release of intracellular ATP. The recovery of intracellular ATP from complete depletion after metabolic poisoning was also assessed under basal and cAMP-stimulated conditions. The various maneuvers indicate that CFTR may be an important contributor to the release of cellular ATP, which may help modify signal transduction pathways associated with secretory Cl- movement or other related processes. Changes in the CFTR-mediated delivery of nucleotides to the extracellular compartment may play an important role in the onset and reversal of the cystic fibrosis phenotype.

Stimulation of Cl- secretion by extracellular ATP does not depend on increased cytosolic Ca2+ in HT-29.cl16E

Extracellular ATP and elevated cytosolic Ca2+ concentration ([Ca2+]i) are major secretagogues for Cl- in the goblet cell-like clone cl.16E derived from colonic HT-29 cells. The involvement of [Ca2+]i as a messenger for the purinergically stimulated Cl- secretion was investigated using whole cell patch-clamp and Ussing chamber techniques, as well as [Ca2+]i measurements using fura 2-loaded cells. Under voltage-clamp conditions, the whole cell current at +50 mV was 3 +/- 1 pA/pF under unstimulated conditions. Stimulation of purinergic receptors with 200 microM extracellular ATP increased the current at +50 mV to 41 +/- 10 pA/pF, with a half-maximal effective dose (ED50) of approximately 3 microM. The current was transient, usually lasting 1-2 min, and the current-voltage relationship was approximately linear between -70 and +50 mV. Evidence that the ATP-stimulated current was carried by Cl- included 1) the reversal potential of the current closely followed the Cl- equilibrium potential, and 2) the stimulated current was absent when Cl- was removed from both bath and pipette solutions. Exposure to ATP also increased [Ca2+]i, with an ED50 of approximately 1 microM and maximal changes (at 200 microM) from baseline (71 +/- 3 nM) to 459 +/- 50 nM. The ATP-dependent Cl- conductance increase was not diminished when [Ca2+]i was clamped at 100 nM using a Ca(2+)-1,2-bis(2- aminophenoxy)ethane-N,N,N',N'-tetraacetic acid or Ca(2+)-ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid buffering system. However, the ATP effect did require some basal level of Ca2+ because clamping [Ca2+]i at < 10 nM abolished activation of the Cl- conductance. The presence of the protein kinase A inhibitor H-89 or the protein kinase C inhibitor staurosprine did not change the ATP-activated Cl-conductance. These data demonstrate that the ATP-stimulated increase in Cl- current does not require an increase in [Ca2+]i, suggesting the involvement of either another signaling pathway or direct activation of Cl- channels by purinergic receptors.

Calcium- and CaMKII-dependent chloride secretion induced by the microsomal Ca(2+)-ATPase inhibitor 2,5-di-(tert-butyl)-1,4-hydroquinone in cystic fibrosis pancreatic epithelial cells

Microsomal Ca(2+)-ATPase inhibitors such as thapsigargin (THG), cyclopiazonic acid (CPA) and 2,5-di-(tert-butyl)-1,4-hydroquinone (DBHQ) have been shown to inhibit Ca2+ reuptake by the intracellular stores and increase cytosolic free Ca2+ ([Ca2+]i). DBHQ is a commercially available non-toxic synthetic compound chemically unrelated to THG and CPA. In this study, we tested the feasibility of utilizing DBHQ to improve Cl- secretion via the Ca(2+)-dependent pathway, in the cystic fibrosis (CF)-derived pancreatic epithelial cell line CFPAC-1. DBHQ stimulated 125I efflux and mobilized intracellular free Ca2+ in a dose-dependent manner. The maximal effects were seen at concentrations of 25-50 microM. DBHQ (25 microM) caused a short-term rise in [Ca2+]i in the absence of ambient Ca2+, and a sustained elevation of [Ca2+]i in cell monolayers bathed in the efflux solution (1.2 mM Ca2+), which was largely attenuated by Ni2+ (5 mM). Bath-application of DBHQ induced an outwardly-rectifying whole-cell Cl- current, which was abolished by pipette addition of BAPTA (5 mM) or CaMK [273-302] (20 microM), an inhibitory peptide of multifunctional Ca2+/calmodulin-dependent protein kinase (CaMKII). Pretreatment of monolayers of CFPAC-1 cells with DBHQ for 4-5 min significantly increased the Ca(2+)-independent or autonomous activity of CaMKII assayed in the cell homogenates. Thus, DBHQ appears to enhance Cl- channel activity via a Ca(2+)-dependent mechanism involving CaMKII. Pretreatment of CFPAC-1 cells with up to 50 microM DBHQ for 6 h did not cause any detectable change in cell viability and did not significantly affect the cell proliferation rate. These results suggest that appropriate selective microsomal Ca(2+)-ATPase inhibitors may be therapeutically useful in improving Cl- secretion in CF epithelial cells.

Extracellular ATP activates both Ca(2+)- and cAMP-dependent Cl- conductances in rat epididymal cells

Activation of Ca2+ and cAMP-dependent Cl- conductances by extracellular ATP was studied using the whole-cell patch clamp technique. Immediately after addition of extracellular ATP (10 microM), activation of whole-cell Cl- current exhibiting delayed inactivation and activation kinetics at hyperpolarizing and depolarizing voltages, respectively, was observed. After prolonged activation, the kinetic characteristics of the ATP-induced Cl- current became time- and voltage-independent. When applied to the later phase of the ATP-activated whole-cell current, the disulfonic acid stilbene DIDS (200 microM) could only inhibit 64% of the current while diphenylamine-dicarboxylic acid (DPC, 1 mM) completely inhibited it. Inclusion of a peptide inhibitor for protein kinase A (PKI, 10 nM) in the pipette solution blocked ATP-induced time- and voltage-independent current activation but did not affect the delayed activating and inactivating current activation but did not affect the delayed activating and inactivating current which could be totally blocked by DIDS. Anion selectivity sequence was determined in the presence of either PKI or DIDS and found to be significantly different. Increased pipette EGTA (10 mM) or treatment of the cells with trifluoperazine (40 microM), an inhibitor of calmodulin, suppressed both types of ATP-induced Cl- currents. No current activation by ATP was observed when cells were dialyzed with the IP3 receptor blocker, heparin (10 ng/ml). These results suggest that extracellular ATP activates IP3-linked Ca(2+)-dependent regulatory pathway, which in turn activates cAMP-dependent pathway, leading to activation of both Ca2+ and cAMP-dependent Cl- conductances in epididymal cells.

Different regulatory pathways involved in ATP-stimulated chloride secretion in rat epididymal epithelium

The regulatory pathways involved in the ATP-stimulated Cl- secretion across rat epididymal epithelium were investigated by the short-circuit current (ISC) technique. Biphasic characteristic was observed in the ISC responded to ATP (0.01-10 microM). Inhibitor of P1 receptor, 8-phenyltheophylline (up to 100 microM), did not have any effect on both phases of the ATP-stimulated ISC. The order of potency for stimulation of the two phases in ISC was ATP > ADP >> AMP, adenosine, consistent with the presence of P2-purinoceptors. Cl- channel blocker, disulfonic acid stilbene (DIDS, 300 microM), only inhibited the first peak of the ATP-stimulated ISC while diphenylamine-dicarboxylic acid (DPC, 1 mM) reduced both, indicating the involvement of different conductance pathways. DIDS was found to have an inhibitory effect on Ca(2+)-activated ISC (induced by ionomycin, 10 microM) but not cAMP-activated ISC (induced by forskolin, 1 microM) which could only be blocked by DPC. Both peaks of the ATP-activated ISC could be significantly inhibited by pretreatment with a Ca(2+)-chelating agent, BAPTA-AM (50 microM). An increase in cellular cAMP content upon stimulation of ATP was measured by radioimmunoassay. No significant increase in cAMP production was observed in cells stimulated with adenosine. The ATP-induced cAMP increase was prevented by pretreatment with BAPTA-AM (100 microM) indicating that cAMP production upon ATP stimulation was secondary to an increase in intracellular Ca2+ concentration. These results indicate that the ATP-stimulated Cl- secretion could be mediated by Ca2+ and cAMP-dependent regulatory pathways giving rise to the biphasic nature of the ATP-induced ISC.

The delayed basolateral membrane hyperpolarization of the bovine retinal pigment epithelium: mechanism of generation

1. Conventional and ion-selective double-barrelled microelectrodes were used in an in vitro preparation of bovine retinal pigment epithelium (RPE)-choroid to measure the changes in membrane voltage, resistance and intracellular Cl- activity (aCli) produced by small, physiological changes in extracellular potassium concentration ([K+]o). These apical [K+]o changes approximate those produced in the extracellular (subretinal) space between the photoreceptors and the RPE following transitions between light and dark. 2. Changing apical [K+]o from 5 to 2 mM in vitro elicited membrane voltage responses with three distinct phases. The first phase was generated by an apical membrane hyperpolarization, followed by a (delayed) basolateral membrane hyperpolarization (DBMH); the third phase was an apical membrane depolarization. The present experiments focus on the membrane and cellular mechanisms that generate phase 2 of the response, the DBMH. 3. The DBMH was abolished in the presence of apical bumetanide (100 microM); this response was completely restored after bumetanide removal. 4. Reducing apical [K+]o, adding apical bumetanide (500 mM), or removing apical Cl- decreased aCli by 25 +/- 6 (n = 8), 28 +/- 1 (n = 2) and 26 +/- 5 mM (n = 3), respectively; adding 100 microM apical bumetanide decreased aCli by 12 +/- 2 mM (n = 3). Adding apical bumetanide or removing apical bath Cl- hyperpolarized the basolateral membrane and decreased the apparent basolateral membrane conductance (GB). 5. DIDS (4,4'-diisothiocyanostilbene-2,2'-disulphonic acid) blocked the RPE basolateral membrane Cl- conductance and inhibited the DBMH and the basolateral membrane hyperpolarization produced by apical bumetanide addition or by removal of apical Cl-o. The present results show that the DBMH is caused by delta[K]o-induced inhibition of the apical membrane Na(+)-K(+)-2Cl- cotransporter; the subsequent decrease in aCli generated a hyperpolarization at the basolateral membrane Cl- channel.

Pharmacologic modulation of salt and water in the airway epithelium in cystic fibrosis

Cystic fibrosis (CF) is a recessive genetic disease with thickened airway secretions that result from abnormal airway epithelial ion transport, including defective cyclic AMP-mediated Cl- (liquid) secretion and excessive Na+ (liquid) absorption. These abnormalities reflect mutations in the gene coding for the cystic fibrosis transmembrane conductance regulator (CFTR) protein, which normally functions as a cyclic AMP-regulated Cl- channel. Aerosolized pharmacologic agents are being tested as novel treatment for these genetic ion transport defects. Amiloride aerosol inhibits excessive Na+ absorption, and pilot studies in adult patients with CF show improved biorheology and mucociliary clearance of airway secretions, as well as slowing of the decline in lung function. Phase III studies of amiloride in adults and adolescents are underway, and short-term safety studies in children are under way. Aerosolized uridine triphosphate (UTP) induces Cl- secretion in CF airway epithelia via non-CFTR Cl- channels. Initial safety studies suggest that acute aerosolized UTP is well tolerated, and acute studies of the effect on mucociliary clearance are underway. Pharmacotherapy that targets abnormal ion transport holds promise for the treatment of CF airway disease.

Unitary chloride channels activated by protein kinase C in guinea pig ventricular myocytes

Recent evidence suggests that protein kinase A (PKA)-activated Cl- channels in heart are encoded by an isoform of the epithelial cystic fibrosis transmembrane conductance regulator gene (CFTR). Macroscopic current measurements indicate that a similar time-independent Cl- conductance can be activated through a protein kinase C (PKC)-dependent pathway in guinea pig and feline ventricle. However, it is presently not clear whether PKC is activating the same population of channels as PKA or a separate class of Cl- channels. even though the regulatory (R) domain of CFTR is known to contain consensus phosphorylation sites for both PKA and PKC. In the present study we directly compare the properties of single Cl- channels activated by PKC and PKA in cell-attached patches of guinea pig ventricular myocytes. Pipette and bath solutions contained N-methyl-D-glucamine and Cs+ or tetraethylammonium as substitutes for Na+ and K+, respectively, and Cl- concentration in the patch pipette was either 150 mmol/L or 40 mmol/L. Bath application of phorbol 12,13-dibutyrate or phorbol 12-myristate 13-acetate (PDBu or PMA; 50 nmol/L), activators of PKC, resulted in the appearance of unitary Cl- channels with a mean conductance of 9.31 +/- 0.94 pS (n = 8) and 8.8 pS (n = 2), respectively, and reversal potentials were close to predicted ECl. Addition of staurosporine (500 nmol/L) reduced open probability (Po) of channels activated by PDBu. Bath application of the phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine (IBMX, 500 mumol/L) resulted in the activation of Cl- channels with a conductance (mean 8.76 +/- 0.67 pS, n = 3) similar to those activated by PDBu.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of CFTR Cl- conductance in polarized T84 cells by luminal extracellular ATP

Luminal extracellular ATP evoked a bumetanide-sensitive short-circuit current in cultured T84 cell epithelia (90.2 +/- 18.2 microA/cm2 at 100 microM ATP, apparent 50% effective concentration, 11.5 microM). ATP appeared to increase the Cl- conductance of the apical membrane but not the driving force for Cl- secretion determined by basolateral membrane K+ conductance. Specifically, the magnitude of Cl- secretion stimulated by ATP was independent of basal current, and forskolin pretreatment abolished subsequent stimulation of Cl- secretion by ATP. Whereas ATP stimulated modest production of adenosine 3',5'-cyclic monophosphate (cAMP) by T84 cells, ATP caused smaller increases in intracellular Ca2+ and inositol phosphate activities than the Ca(2+)-signaling Cl- secretagogue carbachol. An inhibitor of 5'-nucleotidase, alpha,beta-methyleneadenosine 5'-diphosphate, blocked most of the response to luminal ATP. The adenosine receptor antagonist 8-(p-sulfophenyl)theophylline blocked both the luminal ATP-dependent generation of cAMP and Cl- secretion when administered to the luminal but not submucosal bath. These results demonstrate that the Cl- secretion stimulated by luminal ATP is mediated by a A2-adenosine receptor located on the apical cell membrane. Thus metabolism of extracellular ATP to adenosine regulates the activity of cystic fibrosis transmembrane conductor regulator (CFTR) in the apical membrane of polarized T84 cells.

Cystic fibrosis transmembrane conductance regulator is required for protein kinase A activation of an outwardly rectified anion channel purified from bovine tracheal epithelia

Our laboratory has developed a protocol for the isolation of a 140-kDa protein that forms an anion-selective channel when reconstituted into planar lipid bilayers. Polyclonal antibodies have been raised against the 38-kDa component of this purified protein. This channel has a linear current-voltage relationship and is not activated by protein kinase A (PKA) plus ATP. Using the same antibody and a modified purification protocol (eliminating the ion exchange chromatography steps), we isolated and reconstituted two other anion channels from tracheal membrane vesicles. In vitro phosphorylation of these isolated proteins by PKA and ATP revealed four bands migrating at 52, 85, 120, and 174 kDa. Immunoprecipitation experiments with anti-CFTR antibodies indicate that the 174-kDa phosphoprotein was CFTR. Upon incorporation of these isolated proteins into planar bilayers, an anion channel that exhibited a marked outward rectification in symmetrical Cl- solutions with a slope conductance of 82 pS at depolarizing voltages was observed. PKA and ATP increased channel activity but only from one side of the bilayer. However, channel activity was unaffected by addition of ATP alone from either side of the membrane. DIDS (100 microM) applied to the opposite side of the bilayer to which PKA and ATP act, blocked channel activity. A linear anion-selective channel with a conductance of 16 pS could be also resolved after inhibition of the outwardly rectified anion channel by DIDS in the presence of PKA and ATP. This small conductance channel was inhibited by 300 microM diphenylamine-2-carboxylic acid. Immunodepletion of the 174-kDa phosphoprotein from the preparation prevented activation of the 82-pS outwardly rectified anion channel by PKA and ATP. However, the PKA-dependent in vitro phosphorylation of the 52-, 85-, and 120-kDa phosphoproteins was unaffected by the absence of CFTR. Our results suggest a direct regulatory relationship between an outwardly rectified anion channel and CFTR.

Evidence for Purinergic Receptors in Vestibular Dark Cell and Strial Marginal Cell Epithelia of Gerbil

Purinergic receptors have been found to modulate ion transport in several types of epithelial cells as well as excitable cells. It was of interest to determine whether vestibular dark cells and strial marginal cells contain purinergic receptors in either the apicalor basolateral membrane which modulate transepithelial ion transport. Vestibular dark cell and strial marginal cell epithelia were mounted in a micro-Ussing chamber for the measurement of the transepithelial voltage and resistance from which the equivalent short circuit current (I(sc)) was obtained. The apical and basolateral sides were independently perfused with adenosine and adenosine 5'-triphosphate (ATP). Adenosine (10(-5) M) had no effect on I(sc) at either the apical or basolateral side of vestibular dark cells and strial marginal cells, suggesting either the absence of P(1) receptors or the absence of coupling of P(1) receptors to vectorial ion transport by these epithelia. Apical perfusion of ATP (10(-8) to 10(-4) M) caused a decrease in I(sc) of both vestibular dark cells and strial marginal cells. Apical perfusion of the nucleotides uridine 5'-triphosphate (UTP), 2-methylthioadenosine triphosphate (2-meS-ATP), adenosine 5'-O-(3-thiotriphosphate) (ATPγS) and α,β-methylene adenosine 5'-triphosphate (α,β-meth-ATP) caused qualitatively similar responses with different magnitudes of response. The sequence of the magnitude of response of each compound at 10(-6) or 10(-5) M was assessed from the fractional change of I(sc). The sequence for vestibular dark cells was UTP = ATP = ATPγS ≫ 2-meS-ATP > α,β-meth-ATP, and for strial marginal cells it was UTP = ATP ≫ 2-meS-ATP, corresponding to the sequence for the P(2U) receptor. The effect of agonist on the apical membrane was reduced by the antagonist 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) but not cibacron blue or suramin. DIDS in the absence of exogenous purinergic agonist caused a sustained increase in I(sc). The effect of ATP on the apical membrane was greater in the absence of divalent cations. Basolateral perfusion of ATP led to a biphasic response of I(sc) in vestibular dark cell and strial marginal cell epithelia, consisting of an initial rapid increase followed by a slower decrease. Perfusion of the perilymphatic surface of the stria vascularis (basal cell layer) with ATP had no acute effect on I(sc). The initial increase of I(sc) in vestibular dark cell epithelium during basolateral perfusion had a sequence of 2-meS-ATP > ATP ≫ UTP = α,β-meth-ATP = ATPγS, corresponding to the sequence for the P(2Y) receptor. Subsequently, the agonists caused a sustained decrease in I(sc) with a sequence of ATPγS > 2-meS-ATP > ATP > UTP >α,β-meth-ATP. This sequence is most simply interpreted as the result of the coexistence of P(2U) and P(2Y) receptors in the basolateral membrane. Both the increase and decrease of I(sc) by ATP at the basolateral membrane were reduced by the antagonist suramin. These findings provide evidence for the regulation of transepithelial ion transport by P(2U) receptors in the apical membrane and by coexisting P(2U) and P(2Y) receptors in the basolateral membrane of K(+)-secretory epithelial cells in the inner ear and are consistent with the hypothesis that the apical receptors are part of an autocrine negative feedback system in these cells.

Ca(2+)- and swelling-induced activation of ion conductances in bronchial epithelial cells

The present study was performed to examine Ca(2+)-dependent and cell-swelling-induced ion conductances in a polarized bronchial epithelial cell line (16HBE14o-). Whole-cell currents were measured in fast and slow whole-cell patch-clamp experiments in cells grown either on filters or on coated plastic dishes. In addition the transepithelial voltage (Vte) and resistance (Rte) were measured in confluent monolayers. Resting cells had a membrane voltage (Vm) of -36 +/- 1.1 mV (n = 137) which was mainly caused by K+ and Cl- conductances and to a lesser extent by a Na+ conductance. Vte was apical-side-negative after stimulation. Equivalent short-circuit (Isc = Vte/Rte) was increased by the secretagogues histamine (0.1 mmol/l), bradykinin (0.1 - 10 mumol/l). and ATP (0.1 - mumol/l). The histamine-induced Is was blocked by either basolateral diphenhydramine (0.1 mmol/l, n = 4) or apical cimetidine (0.1 mmol/l, n = 4). In fast and slow whole-cell recordings ATP and bradykinin primarily activated a transient K+ conductance and hyperpolarized Vm. This effect was mimicked by the Ca2+ ionophore ionomycin (1 mumol/l, n = 11). Inhibition of the bradykinin-induced Isc by the blocker HOE140 (1 mumol/l, n = 3) suggested the presence of a BK2 receptor. The potency sequence of different nucleotide agonists on the purinergic receptor was UTP approximately ATP > ITP > GTP approximately CTP approximately [beta, gamma-methylene] ATP approximately 2-methylthio-ATP = 0 and was obtained in Isc measurements and patch-clamp recordings. This suggests the presence of a P2u receptor. Hypotonic cell swelling activated both Cl- and K+ conductances.(ABSTRACT TRUNCATED AT 250 WORDS)

Properties of single outwardly rectifying Cl- channels in heart

A variety of potentially important macroscopic Cl- currents have been described in the heart. Although the single-channel properties of the cAMP-dependent current (ICl.cAMP) have been well described, the single-channel equivalents of the other forms of cardiac Cl- current remain unknown. Unlike ICl.cAMP, many of these currents show prominent outward rectification in the presence of symmetrical transmembrane Cl- gradients and sensitivity to disulfonic stilbene Cl- transport blockers. We used the patch-clamp technique to search for single Cl- channels in inside-out patches from rabbit atrial cell membranes, under conditions minimizing the chances of observing channels carrying Na+, Ca2+, or K+. Under symmetrical Cl- conditions, single-channel activity was seen in 14 (9%) of 155 patches. Channels showed strong outward rectification and a unitary conductance of 60 +/- 3 picosiemens (mean +/- SEM) at positive voltages. The current-voltage relation was not altered by replacement of cations by the impermeant cation N'-methyl-D-glucamine (NMDG) and shifted as expected for a Cl(-)-selective channel when methanesulfonate was substituted for Cl-. The Cl- transport blockers DIDS (diisothiocyanatostilbene-2,2'-disulfonic acid, 100 mumol/L) and SITS (4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid, 1 mmol/L) strongly and reversibly inhibited channel activity when added to the bath and caused channel flickering suggesting open-channel block. Ensemble-average currents showed no time dependence, and the form of the ensemble-average current-voltage relation was similar to that of macroscopic background Cl- current.(ABSTRACT TRUNCATED AT 250 WORDS)

Dissociation of depolarization-activated and swelling-activated Cl- channels

In many cells, patch excision and depolarization induce outwardly rectifying Cl- channels (ORDIC channels) whose function and normal mode of regulation are unknown. One possible function is the mediation of swelling-activated Cl- conductance, because in many cells rectifying Cl- currents are activated by cell swelling. However, swelling-activated Cl- channels in some epithelia have larger conductances than ORDIC channels and inactivate more rapidly, although both have similar anion selectivity and are blocked by stilbenes. Thus it has not been possible to determine whether the two types of channel current arise from distinct proteins or alternate states of a single protein. We studied 14 cell lines and found 2 lines, C127 mouse mammary epithelial cells and IEC-6 rat intestinal crypt cells, with very low levels of ORDIC channels. However, despite the near absence of ORDIC channels in these rodent cells, a large swelling-activated Cl-conductance was demonstrated by whole cell, efflux, and single-channel methods. Thus it is likely that ORDIC and swelling-activated channel currents arise from different channel proteins.

Blockade by 4,4'-diisothiocyanatostilbene-2,2'-disulphonate (DIDS) of P2X-purinoceptors in rat vas deferens

1. The possibility of an antagonist effect of 4,4'-diisothiocyanatostilbene-2,2'-disulphonate (DIDS) at P2X-purinoceptors was studied in rat vas deferens. 2. DIDS reduced contractions elicited by alpha,beta-methylene ATP 3 microM, IC50 1.6 microM, but did not change contractions elicited by K+ 35 mM. DIDS 3.2 microM slightly shifted the concentration-response curve of alpha,beta-methylene ATP to the right and reduced the maximum. DIDS 10 microM markedly decreased and DIDS 32 microM abolished contractions over the entire range of the alpha, beta-methylene ATP concentration-response curve. DIDS 32 microM also abolished contractions elicited by ATP but did not change contractions elicited by noradrenaline. The antagonist effect of DIDS was only slowly reversible. 3. The presence of either suramin 320 microM or alpha,beta-methylene ATP 10 microM during the exposure to DIDS protected the tissue from the long-lasting blocking effect of DIDS. 4. 4,4'-Diisothiocyanatodihydrostilbene-2,2'-disulphonate (H2DIDS) was equipotent with DIDS whereas several analogues in which one or both of the isothiocyanate residues were replaced were less effective or without effect against alpha,beta-methylene ATP. 5. DIDS attenuated the purinergic component of neurogenic contractions elicited by electrical field stimulation, IC50 3.9 microM, but did not change the adrenergic component. 6. It is concluded that DIDS causes a selective, long-lasting, non-equilibrium blockade of P2X-purinoceptors in rat vas deferens. Due to this effect it also selectively blocks the purinergic component of neurogenic contractions.

Gene therapy for cystic fibrosis using E1-deleted adenovirus: a phase I trial in the nasal cavity. The University of North Carolina at Chapel Hill

Cystic fibrosis (CF) is an autosomal recessive disease that reflects mutations in the CFTR gene. Multiple mutations in this gene have been detected that lead to a protein (CFTR) that is abnormally metabolized, dysfunction, or both. The full spectrum of the activities of the gene product have not been defined, but it is clear that CFTR can act as a cAMP-regulated Cl- channel. This type of defect is consistent with the physiologic characterization of CF epithelia, which has revealed abnormalities in salt and water transport. In the lung, abnormalities in epithelial salt and water metabolism lead to abnormal mucociliary clearance. This defect in clerance represents a major failure of lung defense and leads ultimately to infection of the lung with Staphylococcus aureus, Pseudomonas aeruginosa, and other bacterial organisms. The chronic inflammatory response to this persistent intraluminal bacterial infection leads to protease-induced destruction of airway walls and finally, lung failure. More than 95% of CF patients die of lung disease. The clinical therapy of CF lung disease is limited to agents designed to promote clearance of secretions from the lung and antibiotics to treat the chronic bacterial infection. Recent laboratory demonstrations that introduction of the normal CFTR cDNA into CF cells corrects the ion transport defects of these cells has led to the hypothesis that gene therapy in the lung can be an effective, novel mode of therapy for this lung disease. The classic gene transfer vectors, e.g., retroviruses, appear to be not well suited for therapy of lung disease because of the low proliferation rate of airway epithelia in vivo. Recently, adenoviruses, which have a natural tropism for airway epithelia, have been genetically modified (E1-deleted) in an attempt to reduce potential toxicity of this virus and provide space for the CFTR cDNA. A series of in vitro studies have shown that this vector is highly efficient for transferring CFTR into airway epithelial cells in culture and correcting the CF defect. Further, studies in whole animals appear to indicate that this mode of gene transfer is associated with a low degree of toxicity. The present study is a dose-effect study designed to test for the safety and efficacy of E1-deleted recombinant adenovirus containing the CFTR cDNA under a CMV-beta-actin promoter in CF nasal epithelia.(ABSTRACT TRUNCATED AT 400 WORDS)

Both CFTR and outwardly rectifying chloride channels contribute to cAMP-stimulated whole cell chloride currents

From whole cell patch-clamp recordings at 35 degrees C utilizing either nystatin perforation or conventional methods with 5 mM MgATP in the pipette solution, it was demonstrated that both cystic fibrosis transmembrane conductance regulator (CFTR) chloride (Cl-) channels and outwardly rectifying Cl- channels (ORCC) contribute to adenosine 3',5'-cyclic monophosphate (cAMP)-activated whole cell Cl- currents in cultured human airway epithelial cells. These results were similar whether recordings were performed on two normal human cell lines or on two cystic fibrosis (CF) cell lines stably complemented with wild-type CF gene. These results were obtained by exploiting dissimilar biophysical properties of CFTR and ORCC currents such as the degree of rectification of the current-voltage relationship, the difference in sensitivity to Cl- channel-blocking drugs such as 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), calixarenes, and diphenylamine carboxylic acid (DPC), and the opposing Cl- relative to I- permeabilities of the two channels. In normal cells or complemented CF cells, 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate stimulated outwardly rectifying whole cell Cl- currents. Addition of DIDS in the presence of cAMP inhibited the outwardly rectifying portion of the cAMP-activated Cl- current. The remaining cAMP-activated, DIDS-insensitive, linear CFTR Cl- current was inhibited completely by DPC. Additional results showed that not only do ORCC and CFTR Cl- channels contribute to cAMP-activated Cl- currents in airway epithelial cells where wild-type CFTR is expressed but that both channels fail to respond to cAMP in delta F508-CFTR-containing CF airway cells. We conclude that CFTR not only functions as a cAMP-regulated Cl- channel in airway epithelial cells but also controls the regulation of ORCC.

Stimulation of chloride secretion by P1 purinoceptor agonists in cystic fibrosis phenotype airway epithelial cell line CFPEo-

1. P1 purinoceptor agonists like adenosine have been shown to stimulate Cl- transport in secretory epithelia. In the present study, we investigated whether P1 agonist-induced Cl- secretion is preserved in cystic fibrosis airway epithelium and which signalling mechanism is involved. The effects of purinoceptor agonists on Cl- secretion were examined in a transformed cystic fibrosis airway phenotype epithelial cell line, CFPEo-. 2. Addition of adenosine (ADO; 0.1-1 mM) markedly increased 125I efflux rate. The rank order of potency of purinoceptor agonists in stimulating 125I efflux was ADO > AMP > ADP approximately equal to ATP. A similar order of potency was seen in transformed cystic fibrosis nasal polyp cells, CFNPEo- (ADO > ATP > AMP > ADP). These results are consistent with the activation of Cl- secretion via a P1 purinoceptor. 3. The P1 agonists tested (at 0.01 and 0.1 mM) revealed a rank order of potency of 5'-N-ethylcarboxamine adenosine (NECA) > 2-chloro-adenosine (2-Cl-ADO) > R-phenylisopropyl adenosine (R-PIA). 4. The known potent A2 adenosine receptor (A2AR) agonist, 5'-(N-cyclopropyl) carboxamidoadenosine (CPCA, 2 microM) but not the A1 adenosine receptor agonist, N6-phenyl adenosine (N6-phenyl ADO, 10 microM) markedly increased 125I efflux rate (baseline, 5.9 +/- 2.0% min-1, + CPCA, 10.9 +/- 0.6% min-1; P < 0.01). The stimulant effect of CPCA (10 microM) was abolished by addition of the A2AR antagonist 3,7-dimethyl-1-propargylxanthine (DMPX) (100 microM; reported K(i) = 11 +/- 3 microM). These results favour the involvement of A2AR. 5. ADO (0.1-mM) and CPCA (2 microM) both induced a marked increase in intracellular [Ca2+] ([Ca2+]i); the effect of the latter was again abolished by pretreatment of the cells with DMPX. By contrast, N6-phenyl ADO did not affect [Ca2+]i. 6. In patch-clamp experiments, ADO (1 mM) induced an outwardly-rectified whole-cell Cl- current (baseline, 2.5 +/- 0.8 pA pF-1, + ADO, 78.4 +/- 23.8 pA pF-1; P < 0.02), which was largely inhibited in cells internally perfused with a selective inhibitory peptide of the multifunctional Ca2+/calmodulin-dependent protein kinase, CaMK [273-302] (20 microM), as compared to a control peptide, CaMK [284-302]. Addition of BAPTA (10 mM), a Ca2+ chelator, to the perfusion pipette also abolished the ADO-elicited Cl- current. 7. In conclusion, our results suggest that A2AR participates in regulation of airway C1 secretion via aCa2+-dependent signalling pathway, which involves CaMK and appears to be at least partially conserved in cystic fibrosis airway epithelial cells.

Mechanism of sodium hyperabsorption in cultured cystic fibrosis nasal epithelium: a patch-clamp study

Transepithelial Na+ absorption is increased two to three times in cystic fibrosis (CF) compared with normal (NL) airway epithelia. This increase has been associated with a higher Na+ permeability of the apical membrane of airway epithelial cells. Because Na+ absorption is electrogenic and abolished by amiloride, Na+ channels are thought to dominate the apical membrane Na+ permeability. Three Na+ channel-related mechanisms may explain the increase in apical Na+ permeability in CF cells: increased number of channels, increased single-channel conductance, and increased single-channel open probability. We compared the properties of Na(+)-permeable channels in the apical membrane of confluent preparations of human NL and CF nasal epithelial cells cultured on permeable supports. Na(+)-permeable channels were studied using the patch-clamp technique in the excised inside-out and cell-attached configurations. The same types of Na(+)-permeable channels were recorded in CF and NL cells. In excised patches, nonselective (Na+/K+) cation channels were recorded, and no differences between CF and NL were found in the properties, incidence, single-channel conductance, and single-channel open probability. In cell-attached patches, channels with a higher Na+ vs. K+ selectivity dominated. There was no difference between CF and NL cells in the incidence (18.8 vs. 21.4%, respectively) and conductance (17.2 +/- 2.8 vs. 21.4 +/- 1.5 pS, respectively) of Na(+)-permeable channels. However, the open probability was higher in CF cells compared with NL cells (30.0 +/- 3.4%, n = 6, vs. 15.0 +/- 3.9%, n = 13; P < 0.05). We conclude that, in CF nasal epithelial cells, the increase in Na+ permeability of the apical membrane results from an increase in the open probability of Na(+)-permeable channels in the apical membrane.

Effect of ATP, carbachol and other agonists on intracellular calcium activity and membrane voltage of pancreatic ducts

The pancreatic duct has been regarded as a typical cAMP-regulated epithelium, and our knowledge about its Ca2+ homeostasis is limited. Hence, we studied the regulation of intracellular calcium, [Ca2+]i, in perfused rat pancreatic ducts using the Ca(2+)-sensitive probe fura-2. In some experiments we also measured the basolateral membrane voltage, Vbl, of individual cells. The resting basal [Ca2+]i was relatively high, corresponding to 263 +/- 28 nmol/l, and it decreased rapidly to 106 +/- 28 nmol/l after removal of Ca2+ from the bathing medium (n = 31). Carbachol increased [Ca2+]i in a concentration-dependent manner. At 10 mumol/l the fura-2 fluorescence ratio increased by 0.49 +/- 0.06 (n = 24), corresponding to an increase in [Ca2+]i by 111 +/- 15 nmol/l (n = 17). ATP, added to the basolateral side at 0.1 mmol/l and 1 mmol/l, increased the fluorescence ratio by 0.67 +/- 0.06 and 1.01 +/- 14 (n = 46; 12), corresponding to a [Ca2+]i increase of 136 +/- 22 nmol/l and 294 +/- 73 nmol/l respectively (n = 15; 10). Microelectrode measurements showed that ATP (0.1 mmol/l) hyperpolarized Vbl from -62 +/- 3 mV to -70 +/- 3 mV, an effect which was in some cases only transient (n = 7). This effect of ATP was different from that of carbachol, which depolarized Vbl. Applied together with secretin, ATP delayed the secretin-induced depolarization and prolonged the initial hyperpolarization of Vbl (n = 4). Several other putative agonists of pancreatic HCO3- secretion were also tested for their effects on [Ca2+]i.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypotonicity activates a native chloride current in Xenopus oocytes

Xenopus oocytes are frequently utilized for in vivo expression of cellular proteins, especially ion channel proteins. A thorough understanding of the endogenous conductances and their regulation is paramount for proper characterization of expressed channel proteins. Here we detail a novel chloride current (ICl.swell) responsive to hypotonicity in Xenopus oocytes using the two-electrode voltage clamp technique. Reducing the extracellular osmolarity by 50% elicited a calcium-independent chloride current having an anion conductivity sequence identical with swelling-induced chloride currents observed in epithelial cells. The hypotonicity-activated current was blocked by chloride channel blockers, trivalent lanthanides, and nucleotides. G-protein, cAMP-PKA, and arachidonic acid signaling cascades were not involved in ICl.swell activation. ICl.swell is distinct from both stretch-activated nonselective cation channels and the calcium-activated chloride current in oocytes and may play a critical role in volume regulation in Xenopus oocytes.

Modulation of the ionic milieu of the airway in health and disease

Airway surface liquid (ASL) is an integral part of lung defense mechanisms. Ion transport by airway epithelia regulates the volume and composition of this fluid. A better understanding of the mechanisms of ion transport will enable the development of new therapies for airway diseases associated with defects in these mechanisms. A useful model of a disease with abnormal airway epithelial ion transport is cystic fibrosis (CF), a distinct genetic syndrome of altered lung defense mechanisms characterized by chronic bacterial infection and a steady decline in lung function. Traditional therapies for CF include antibacterial drugs and augmentation of clearance of secretions, but investigators are now studying pharmacological approaches to target the more basic defect of the disease, i.e. abnormal sodium and chloride ion transport. Early treatment in childhood, prior to lung damage, might prevent or at least retard the decline in pulmonary function that remains the hallmark of CF. Ion transport dysfunction may also contribute to other airway diseases such as asthma and chronic bronchitis. Pharmacological intervention at this level may prove beneficial in these common lung diseases as well.