Extracellular UTP stimulates electrogenic bicarbonate secretion across CFTR knockout gallbladder epithelium

Mechanism of luminal ATP activated chloride secretion in a polarized epithelium

There are both secretory and absorptive pathways working in tandem to support ionic movement driving fluid secretion across epithelia. The mechanisms exerting control of fluid secretion in the oviduct is yet to be fully determined. This study explored the role of apical or luminal extracellular ATP (ATPe)-stimulated ion transport in an oviduct epithelium model, using the Ussing chamber short-circuit current (Isc) technique. Basal Isc in oviduct epithelium in response to apical ATPe comprises both chloride secretion and sodium absorption and has distinct temporal phases. A rapid transient peak followed by a sustained small increase above baseline. Both phases of the apical ATPe Isc response are sensitive to anion (HCO3-, Cl-) and cation (Na+) replacement. Additionally, the role of apical chloride channels, basolateral potassium channels and intracellular calcium in supporting the peak Isc current was confirmed. The role of ATP breakdown to adenosine resulting in the activation of P2 receptors was supported by examining the effects of non-hydrolyzable forms of ATP. A P2YR2 potency profile of ATP = UTP > ADP was generated for the apical membrane, suggesting the involvement of the P2YR2 subtype of purinoceptor. A P2X potency profile of ATP = 2MeSATP > alpha,beta-meATP > BzATP was also generated for the apical membrane. In conclusion, these results provide strong evidence that purinergic activation of apical P2YR2 promotes chloride secretion and is thus an important factor in fluid formation by the oviduct.

Purinergic signalling in the gastrointestinal tract and related organs in health and disease

Purinergic signalling plays major roles in the physiology and pathophysiology of digestive organs. Adenosine 5'-triphosphate (ATP), together with nitric oxide and vasoactive intestinal peptide, is a cotransmitter in non-adrenergic, non-cholinergic inhibitory neuromuscular transmission. P2X and P2Y receptors are widely expressed in myenteric and submucous enteric plexuses and participate in sympathetic transmission and neuromodulation involved in enteric reflex activities, as well as influencing gastric and intestinal epithelial secretion and vascular activities. Involvement of purinergic signalling has been identified in a variety of diseases, including inflammatory bowel disease, ischaemia, diabetes and cancer. Purinergic mechanosensory transduction forms the basis of enteric nociception, where ATP released from mucosal epithelial cells by distension activates nociceptive subepithelial primary afferent sensory fibres expressing P2X3 receptors to send messages to the pain centres in the central nervous system via interneurons in the spinal cord. Purinergic signalling is also involved in salivary gland and bile duct secretion.

Purinergic signalling in epithelial ion transport: regulation of secretion and absorption

Intracellular ATP, the energy source for many reactions, is crucial for the activity of plasma membrane pumps and, thus, for the maintenance of transmembrane ion gradients. Nevertheless, ATP and other nucleotides/nucleosides are also extracellular molecules that regulate diverse cellular functions, including ion transport. In this review, I will first introduce the main components of the extracellular ATP signalling, which have become known as the purinergic signalling system. With more than 50 components or processes, just at cell membranes, it ranks as one of the most versatile signalling systems. This multitude of system components may enable differentiated regulation of diverse epithelial functions. As epithelia probably face the widest variety of potential ATP-releasing stimuli, a special attention will be given to stimuli and mechanisms of ATP release with a focus on exocytosis. Subsequently, I will consider membrane transport of major ions (Cl(-) , HCO(3)(-) , K(+) and Na(+) ) and integrate possible regulatory functions of P2Y2, P2Y4, P2Y6, P2Y11, P2X4, P2X7 and adenosine receptors in some selected epithelia at the cellular level. Some purinergic receptors have noteworthy roles. For example, many studies to date indicate that the P2Y2 receptor is one common denominator in regulating ion channels on both the luminal and basolateral membranes of both secretory and absorptive epithelia. In exocrine glands though, P2X4 and P2X7 receptors act as cation channels and, possibly, as co-regulators of secretion. On an organ level, both receptor types can exert physiological functions and together with other partners in the purinergic signalling, integrated models for epithelial secretion and absorption are emerging.

Identification and functional characterization of TMEM16A, a Ca2+-activated Cl- channel activated by extracellular nucleotides, in biliary epithelium

Cl(-) channels in the apical membrane of biliary epithelial cells (BECs) provide the driving force for ductular bile formation. Although a cystic fibrosis transmembrane conductance regulator has been identified in BECs and contributes to secretion via secretin binding basolateral receptors and increasing [cAMP](i), an alternate Cl(-) secretory pathway has been identified that is activated via nucleotides (ATP, UTP) binding apical P2 receptors and increasing [Ca(2+)](i). The molecular identity of this Ca(2+)-activated Cl(-) channel is unknown. The present studies in human, mouse, and rat BECs provide evidence that TMEM16A is the operative channel and contributes to Ca(2+)-activated Cl(-) secretion in response to extracellular nucleotides. Furthermore, Cl(-) currents measured from BECs isolated from distinct areas of intrahepatic bile ducts revealed important functional differences. Large BECs, but not small BECs, exhibit cAMP-stimulated Cl(-) currents. However, both large and small BECs express TMEM16A and exhibit Ca(2+)-activated Cl(-) efflux in response to extracellular nucleotides. Incubation of polarized BEC monolayers with IL-4 increased TMEM16A protein expression, membrane localization, and transepithelial secretion (I(sc)). These studies represent the first molecular identification of an alternate, noncystic fibrosis transmembrane conductance regulator, Cl(-) channel in BECs and suggest that TMEM16A may be a potential target to modulate bile formation in the treatment of cholestatic liver disorders.

P2Y receptors mediate Ca2+ signaling in duodenocytes and contribute to duodenal mucosal bicarbonate secretion

Since little is known about the role of P2Y receptors (purinoceptors) in duodenal mucosal bicarbonate secretion (DMBS), we sought to investigate the expression and function of these receptors in duodenal epithelium. Expression of P2Y(2) receptors was detected by RT-PCR in mouse duodenal epithelium and SCBN cells, a duodenal epithelial cell line. UTP, a P2Y(2)-receptor agonist, but not ADP (10 microM), significantly induced murine duodenal short-circuit current and DMBS in vitro; these responses were abolished by suramin (300 microM), a P2Y-receptor antagonist, or 2-aminoethoxydiphenyl borate (2-APB; 100 microM), a store-operated channel blocker. Mucosal or serosal addition of UTP induced a comparable DMBS in wild-type mice, but markedly impaired response occurred in P2Y(2) knockout mice. Acid-stimulated DMBS in vivo was significantly inhibited by suramin (1 mM) or PPADS (30 microM). Both ATP and UTP, but not ADP (1 microM), raised cytoplasmic-free Ca(2+) concentrations ([Ca(2+)](cyt)) with similar potencies in SCBN cells. ATP-induced [Ca(2+)](cyt) was attenuated by U-73122 (10 microM), La(3+) (30 microM), or 2-APB (10 microM), but was not significantly affected by nifedipine (10 microM). UTP (1 microM) induced a [Ca(2+)](cyt) transient in Ca(2+)-free solutions, and restoration of external Ca(2+) (2 mM) raised [Ca(2+)](cyt) due to capacitative Ca(2+) entry. La(3+) (30 microM), SK&F96365 (30 microM), and 2-APB (10 microM) inhibited UTP-induced Ca(2+) entry by 92, 87, and 94%, respectively. Taken together, our results imply that activation of P2Y(2) receptors enhances DMBS via elevation of [Ca(2+)](cyt) that likely results from an initial increase in intracellular Ca(2+) release followed by extracellular Ca(2+) entry via store-operated channel.

Purinergic receptors and gastrointestinal secretomotor function

Secretomotor reflexes in the gastrointestinal (GI) tract are important in the lubrication and movement of digested products, absorption of nutrients, or the diarrhea that occurs in diseases to flush out unwanted microbes. Mechanical or chemical stimulation of mucosal sensory enterochromaffin (EC) cells triggers release of serotonin (5-HT) (among other mediators) and initiates local reflexes by activating intrinsic primary afferent neurons of the submucous plexus. Signals are conveyed to interneurons or secretomotor neurons to stimulate chloride and fluid secretion. Inputs from myenteric neurons modulate secretory rates and reflexes, and special neural circuits exist to coordinate secretion with motility. Cellular components of secretomotor reflexes variably express purinergic receptors for adenosine (A1, A2a, A2b, or A3 receptors) or the nucleotides adenosine 5'-triphosphate (ATP), adenosine diphosphate (ADP), uridine 5'-triphosphate (UTP), or uridine diphosphate (UDP) (P2X(1-7), P2Y(2), P2Y(4), P2Y(6), P2Y(12) receptors). This review focuses on the emerging concepts in our understanding of purinergic regulation at these receptors, and in particular of mechanosensory reflexes. Purinergic inhibitory (A(1), A(3), P2Y(12)) or excitatory (A(2), P2Y(1)) receptors modulate mechanosensitive 5-HT release. Excitatory (P2Y(1), other P2Y, P2X) or inhibitory (A(1), A(3)) receptors are involved in mechanically evoked secretory reflexes or "neurogenic diarrhea." Distinct neural (pre- or postsynaptic) and non-neural distribution profiles of P2X(2), P2X(3), P2X(5), P2Y(1), P2Y(2), P2Y(4), P2Y(6), or P2Y(12) receptors, and for some their effects on neurotransmission, suggests their role in GI secretomotor function. Luminal A(2b), P2Y(2), P2Y(4), and P2Y(6) receptors are involved in fluid and Cl(-), HCO(3) (-), K(+), or mucin secretion. Abnormal receptor expression in GI diseases may be of clinical relevance. Adenosine A(2a) or A(3) receptors are emerging as therapeutic targets in inflammatory bowel diseases (IBD) and gastroprotection; they can also prevent purinergic receptor abnormalities and diarrhea. Purines are emerging as fundamental regulators of enteric secretomotor reflexes in health and disease.

Agonists of protease-activated receptors 1 and 2 stimulate electrolyte secretion from mouse gallbladder

Cholecystitis is one of the most common gastrointestinal diseases. Inflammation induces the activation of proteases that can signal to cells by cleaving protease-activated receptors (PARs) to induce hemostasis, inflammation, pain, and repair. However, the distribution of PARs in the gallbladder is unknown, and their effects on gallbladder function have not been fully investigated. We localized immunoreactive PAR(1) and PAR(2) to the epithelium, muscle, and serosa of mouse gallbladder. mRNA transcripts corresponding to PAR(1) and PAR(2), but not PAR(4), were detected by RT-PCR and sequencing. Addition of thrombin and a PAR(1)-selective activating peptide (TFLLRN-NH(2)) to the serosal surface of mouse gallbladder mounted in an Ussing chamber stimulated an increase in short-circuit current in wild-type but not PAR(1) knockout mice. Similarly, serosally applied trypsin and PAR(2) activating peptide (SLIGRL-NH(2)) increased short-circuit current in wild-type but not PAR(2) knockout mice. Proteases and activating peptides strongly inhibited electrogenic responses to subsequent stimulation with the same agonist, indicating homologous desensitization. Removal of HCO(3)(-) ions from the serosal buffer reduced responses to thrombin and trypsin by >80%. Agonists of PAR(1) and PAR(2) increase intracellular Ca(2+) concentration in isolated and cultured gallbladder epithelial cells. The COX-2 inhibitor meloxicam and an inhibitor of CFTR prevented the stimulatory effect of PAR(1) but not PAR(2). Thus PAR(1) and PAR(2) are expressed in the epithelium of the mouse gallbladder, and serosally applied proteases cause a HCO(3)(-) secretion. The effects of PAR(1) but not PAR(2) depend on generation of prostaglandins and activation of CFTR. These mechanisms may markedly influence fluid and electrolyte secretion of the inflamed gallbladder when multiple proteases are generated.

Cholangiocyte anion exchange and biliary bicarbonate excretion

Primary canalicular bile undergoes a process of fluidization and alkalinization along the biliary tract that is influenced by several factors including hormones, innervation/neuropeptides, and biliary constituents. The excretion of bicarbonate at both the canaliculi and the bile ducts is an important contributor to the generation of the so-called bile-salt independent flow. Bicarbonate is secreted from hepatocytes and cholangiocytes through parallel mechanisms which involve chloride efflux through activation of Cl^- channels, and further bicarbonate secretion via AE2/SLC4A2-mediated Cl^-/HCO₃^- exchange. Glucagon and secretin are two relevant hormones which seem to act very similarly in their target cells (hepatocytes for the former and cholangiocytes for the latter). These hormones interact with their specific G protein-coupled receptors, causing increases in intracellular levels of cAMP and activation of cAMP-dependent Cl^- and HCO₃^- secretory mechanisms. Both hepatocytes and cholangiocytes appear to have cAMP-responsive intracellular vesicles in which AE2/SLC4A2 colocalizes with cell specific Cl^- channels (CFTR in cholangiocytes and not yet determined in hepatocytes) and aquaporins (AQP8 in hepatocytes and AQP1 in cholangiocytes). cAMP-induced coordinated trafficking of these vesicles to either canalicular or cholangiocyte lumenal membranes and further exocytosis results in increased osmotic forces and passive movement of water with net bicarbonate-rich hydrocholeresis.

P2 receptors: intracellular signaling

P2 receptors for extracellular nucleotides are divided into two categories: the ion channel receptors (P2X) and the G-protein-coupled receptors (P2Y). For the P2X receptors, signal transduction appears to be relatively simple. Upon activation by extracellular ATP, a channel comprised of P2X receptor subunits opens and allows cations to move across the plasma membrane, resulting in changes in the electrical potential of the cell that, in turn, propagates a signal. This regulated flux of ions across the plasma membrane has important signaling functions, especially in impulse propagation in the nervous system and in muscle contractility. In addition, P2X receptor activation causes the accumulation of calcium ions in the cytoplasm, which is responsible for activating numerous signaling molecules. For the P2Y receptors, signal transduction is more complex. Intracellular signaling cascades are the main routes of communication between G-protein-coupled receptors and regulatory targets within the cell. These signaling cascades operate mainly by the sequential activation or deactivation of heterotrimeric and monomeric G proteins, phospholipases, protein kinases, adenylyl and guanylyl cyclases, and phosphodiesterases that regulate many cellular processes, including proliferation, differentiation, apoptosis, metabolism, secretion, and cell migration. In addition, there are numerous ion channels, cell adhesion molecules and receptor tyrosine kinases that are modulated by P2Y receptors and operate to transmit an extracellular signal to an intracellular response. These intracellular signaling pathways and their regulation by P2 receptors are discussed in this review.

Mechanisms of lipid malabsorption in Cystic Fibrosis: the impact of essential fatty acids deficiency

Transport mechanisms, whereby alimentary lipids are digested and packaged into small emulsion particles that enter intestinal cells to be translocated to the plasma in the form of chylomicrons, are impaired in cystic fibrosis. The purpose of this paper is to focus on defects that are related to intraluminal and intracellular events in this life-limiting genetic disorder. Specific evidence is presented to highlight the relationship between fat malabsorption and essential fatty acid deficiency commonly found in patients with cystic fibrosis that are often related to the genotype. Given the interdependency of pulmonary disease, pancreatic insufficiency and nutritional status, greater attention should be paid to the optimal correction of fat malabsorption and essential fatty acid deficiency in order to improve the quality of life and extend the life span of patients with cystic fibrosis.

K+ secretion activated by luminal P2Y2 and P2Y4 receptors in mouse colon

Extracellular nucleotides are important regulators of epithelial ion transport, frequently exerting their action from the luminal side. Luminal P2Y receptors have previously been identified in rat distal colonic mucosa. Their activation by UTP and ATP stimulates K+ secretion. The aim of this study was to clarify which of the P2Y receptor subtypes are responsible for the stimulated K+ secretion. To this end P2Y2 and P2Y4 knock-out mice were used to measure distal colonic ion transport in an Ussing chamber. In mouse (NMRI) distal colonic mucosa, luminal UTP and ATP with similar potency induced a rapid and transient increase of the transepithelial voltage (V(te)) (UTP: from -0.81 +/- 0.23 to 3.11 +/- 0.61 mV, n = 24), an increase of equivalent short circuit current (I(sc)) by 166.9 +/- 22.8 microA cm(-2) and a decrease of transepithelial resistance (R(te)) from 29.4 +/- 2.4 to 23.5 +/- 2.0 Omega cm2. This effect was completely inhibited by luminal Ba2+ (5 mm, n = 5) and iberiotoxin (240 nm, n = 6), indicating UTP/ATP-stimulated K+ secretion. RT-PCR analysis of isolated colonic crypts revealed P2Y2, P2Y4 and P2Y6 specific transcripts. The luminal UTP-stimulated K+ secretion was still present in P2Y2 receptor knock-out mice, but significantly reduced (DeltaV(te): 0.83 +/- 0.26 mV) compared to wild-type littermates (DeltaV(te): 2.08 +/- 0.52 mV, n = 9). In P2Y4 receptor knock-out mice the UTP-induced K+ secretion was similarly reduced. Luminal UTP-stimulated K+ secretion was completely absent in P2Y2/P2Y4 double receptor KO mice. Basolateral UTP showed no effect. In summary, these results indicate that both the P2Y2 and P2Y4 receptors are present in the luminal membrane of mouse distal colonic mucosa, and stimulation of these receptors leads to K+ secretion.

P2Y2 receptors activate neuroprotective mechanisms in astrocytic cells

Mechanical or ischemic trauma to the CNS causes the release of nucleotides and other neurotransmitters into the extracellular space. Nucleotides can activate nucleotide receptors that modulate the expression of genes implicated in cellular adaptive responses. In this investigation, we used human 1321N1 astrocytoma cells expressing a recombinant P2Y2 receptor to assess the role of this receptor in the regulation of anti-apoptotic (bcl-2 and bcl-xl) and pro-apoptotic (bax) gene expression. Acute treatment with the P2Y2 receptor agonist UTP up-regulated bcl-2 and bcl-xl, and down-regulated bax, gene expression. Activation of P2Y2 receptors was also coupled to the phosphorylation of cyclic AMP responsive element binding protein that positively regulates bcl-2 and bcl-xl gene expression. Cyclic AMP responsive element decoy oligonucleotides markedly attenuated the UTP-induced increase in bcl-2 and bcl-xl mRNA levels. Activation of P2Y2 receptors induced the phosphorylation of the pro-apoptotic factor Bad and caused a reduction in bax/bcl-2 mRNA expression ratio. All these signaling pathways are known to be involved in cell survival mechanisms. Using cDNA microarray analysis and RT-PCR, P2Y2 receptors were found to up-regulate the expression of genes for neurotrophins, neuropeptides and growth factors including nerve growth factor 2; neurotrophin 3; glia-derived neurite-promoting factor, as well as extracellular matrix proteins CD44 and fibronectin precursor--genes known to regulate neuroprotection. Consistent with this observation, conditioned media from UTP-treated 1321N1 cells expressing P2Y2 receptors stimulated the outgrowth of neurites in PC-12 cells. Taken together, our results suggest an important novel role for the P2Y2 receptor in survival and neuroprotective mechanisms under pathological conditions.

Control of epithelial transport via luminal P2 receptors

P2 membrane receptors are specifically activated by extracellular nucleotides like ATP, ADP, UTP, and UDP. P2 receptors are subdivided into metabotropic P2Y and ionotropic P2X receptors. They are expressed in all tissues and induce a variety of biological effects. In epithelia, they are found in both the basolateral and the luminal membranes. Their widespread luminal expression in nearly all transporting epithelia and their effect on transport are summarized. The P2Y(2) receptor is a prominent luminal receptor in many epithelia. Other luminal P2 receptors include the P2X(7), P2Y(4), and P2Y(6) receptors. Functionally, luminal P2Y(2) receptor activation elicits differential effects on ion transport. In nearly all secretory epithelia, intracellular Ca(2+) concentration-activated ion conductances are stimulated by luminal nucleotides to induce Cl(-), K(+), or HCO(3)(-) secretion. This encompasses respiratory and various gastrointestinal epithelia or tissues like the conjunctiva of the eye and the epithelium of sweat glands. In the distal nephron, all active transport processes appear to be inhibited by luminal nucleotides. P2Y(2) receptors inhibit Ca(2+) and Na(+) absorption and K(+) secretion. Commonly, in all steroid-sensitive epithelia (lung, distal nephron, and distal colon), luminal ATP/UTP inhibits epithelial Na(+) channel-meditated Na(+) absorption. ATP is readily released from epithelial cells onto their luminal aspect, where ecto-nucleotidases promote their metabolism. Adenosine generated by the action of 5'-nucleotidase may elicit further effects on ion transport, often opposite those of ATP. ATP release from epithelia continues to be poorly understood. Integrated functional concepts for luminal P2 receptors are suggested: 1) luminal P2 receptors are part of an epithelial "secretory" defense mechanism; 2) they may be involved in the regulation of cell volume when transcellular solute transport is out of balance; 3) ATP and adenosine may be important autocrine/paracrine regulators mediating cellular protection and regeneration after ischemic cell damage; and 4) ATP and adenosine have been suggested to mediate renal cyst growth and enlargement in polycystic kidney disease.

Lung infections associated with cystic fibrosis

While originally characterized as a collection of related syndromes, cystic fibrosis (CF) is now recognized as a single disease whose diverse symptoms stem from the wide tissue distribution of the gene product that is defective in CF, the ion channel and regulator, cystic fibrosis transmembrane conductance regulator (CFTR). Defective CFTR protein impacts the function of the pancreas and alters the consistency of mucosal secretions. The latter of these effects probably plays an important role in the defective resistance of CF patients to many pathogens. As the modalities of CF research have changed over the decades from empirical histological studies to include biophysical measurements of CFTR function, the clinical management of this disease has similarly evolved to effectively address the ever-changing spectrum of CF-related infectious diseases. These factors have led to the successful management of many CF-related infections with the notable exception of chronic lung infection with the gram-negative bacterium Pseudomonas aeruginosa. The virulence of P. aeruginosa stems from multiple bacterial attributes, including antibiotic resistance, the ability to utilize quorum-sensing signals to form biofilms, the destructive potential of a multitude of its microbial toxins, and the ability to acquire a mucoid phenotype, which renders this microbe resistant to both the innate and acquired immunologic defenses of the host.

Chloride dependent intracellular pH effects of external ATP in cultured human non-pigmented ciliary body epithelium

PURPOSE

To examine the effects of extracellular adenosine 5-triphosphate (ATP) on intracellular pH ([pH](i)) in cultured human non-pigmented ciliary body epithelium (HNPE).

METHODS

Intracellular pH was measured using spectrofluorescence video microscopy in isolated HNPE cells loaded with the cell-permeable acetoxymethyl ester form of the fluorescent probe BCECF.

RESULTS

In 5%CO(2)/HCO(3)(-) buffered Ringer's the resting [pH](i) was 7.25 +/- 0.006 (mean +/- SEM). Application of 10 microM ATP significantly decreased [pH](i) to 7.00 +/- 0.007 (P < 10(-5), n = 14). In the presence of 1 mM suramin, a P(2) receptor inhibitor, this process was significantly blocked. This [pH](i) effect required the presence of Cl(-) and was significantly inhibited by 0.1 mM diisothiocyanatostilbene-2-2'-disulfonic acid or acetazolamide (500 microM), indicating the involvement of a Cl(-)/HCO(3)( +) exchange mechanism. This response exhibited little dependence on external Na(+) and remained unaffected by the addition of the Na(+)/H( +) exchanger inhibitor amiloride (1 mM). Clamping intracellular calcium levels by incubation in the cell permeable calcium chelator, the acetoxymethyl ester form of BAPTA (100 microM) in low extracellular calcium solution (pCa9) did not affect the ATP-induced [pH](i) signal. In addition, the vacuolar H(+)-ATPase (V-ATPase) inhibitor, bafilomycin A(1) (1 microM), failed to alter the [pH](i) transient.

CONCLUSION

We have demonstrated that extracellular ATP leads to a sustained increase in [H(+)](i) in HNPE cells via a purinergic receptor activated pathway which is independent of the intracellular calcium signaling system. This study demonstrates that the ATP induced [pH]( i) transient is mediated through an upregulation in Cl(-)/HCO( 3)(-) exchange across the plasmamembrane in HNPE cells.

Polarized expression and function of P2Y ATP receptors in rat bile duct epithelia

Extracellular nucleotides may be important regulators of bile ductular secretion, because cholangiocytes express P2Y ATP receptors and nucleotides are found in bile. However, the expression, distribution, and function of specific P2Y receptor subtypes in cholangiocytes are unknown. Thus our aim was to determine the subtypes, distribution, and role in secretion of P2Y receptors expressed by cholangiocytes. The molecular subtypes of P2Y receptors were determined by RT-PCR. Functional studies measuring cytosolic Ca2+ (Ca) signals and bile ductular pH were performed in isolated, microperfused intrahepatic bile duct units (IBDUs). PCR products corresponding to P2Y1, P2Y2, P2Y4, P2Y6, and P2X4 receptor subtypes were identified. Luminal perfusion of ATP into IBDUs induced increases in Ca that were inhibited by apyrase and suramin. Luminal ATP, ADP, 2-methylthioadenosine 5'-triphosphate, UTP, and UDP each increased Ca. Basolateral addition of adenosine 5'-O-(3-thiotriphosphate) (ATP-gamma-S), but not ATP, to the perifusing bath increased Ca. IBDU perfusion with ATP-gamma-S induced net bile ductular alkalization. Cholangiocytes express multiple P2Y receptor subtypes that are expressed at the apical plasma membrane domain. P2Y receptors are also expressed on the basolateral domain, but their activation is attenuated by nucleotide hydrolysis. Activation of ductular P2Y receptors induces net ductular alkalization, suggesting that nucleotide signaling may be an important regulator of bile secretion by the liver.

Alterations in airway ion transport in NKCC1-deficient mice

Airways of Na(+)-K(+)-2Cl(-) (NKCC1)-deficient mice (-/-) were studied in Ussing chambers to determine the role of the basolateral NKCC1 in transepithelial anion secretion. The basal short-circuit current (I(sc)) of tracheae and bronchi from adult mice did not differ between NKCC1-/- and normal mice, whereas NKCC1-/- tracheae from neonatal mice exhibited a significantly reduced basal I(sc). In normal mouse tracheae, sensitivity to the NKCC1 inhibitor bumetanide correlated inversely with the age of the mouse. In contrast, tracheae from NKCC1-/- mice at all ages were insensitive to bumetanide. The anion secretory response to forskolin did not differ between normal and NKCC1-/- tissues. However, when larger anion secretory responses were induced with UTP, airways from the NKCC1-/- mice exhibited an attenuated response. Ion substitution and drug treatment protocols suggested that HCO secretion compensated for reduced Cl(-) secretion in NKCC1-/- airway epithelia. The absence of spontaneous airway disease or pathology in airways from the NKCC1-/- mice suggests that the NKCC1 mutant mice are able to compensate adequately for absence of the NKCC1 protein.

Polarized signaling via purinoceptors in normal and cystic fibrosis airway epithelia

Airway epithelia are confronted with distinct signals emanating from the luminal and/or serosal environments. This study tested whether airway epithelia exhibit polarized intracellular free calcium (Ca(2+)(i)) and anion secretory responses to 5' triphosphate nucleotides (ATP/UTP), which may be released across both barriers of these epithelia. In both normal and cystic fibrosis (CF) airway epithelia, mucosal exposure to ATP/UTP increased Ca(2+)(i) and anion secretion, but both responses were greater in magnitude for CF epithelia. In CF epithelia, the mucosal nucleotide-induced response was mediated exclusively via Ca(2+)(i) interacting with a Ca(2+)-activated Cl(-) channel (CaCC). In normal airway epithelia (but not CF), nucleotides stimulated a component of anion secretion via a chelerythrine-sensitive, Ca(2+)-independent PKC activation of cystic fibrosis transmembrane conductance regulator. In normal and CF airway epithelia, serosally applied ATP or UTP were equally effective in mobilizing Ca(2+)(i). However, serosally applied nucleotides failed to induce anion transport in CF epithelia, whereas a PKC-regulated anion secretory response was detected in normal airway epithelia. We conclude that (1) in normal nasal epithelium, apical/basolateral purinergic receptor activation by ATP/UTP regulates separate Ca(2+)-sensitive and Ca(2+)-insensitive (PKC-mediated) anion conductances; (2) in CF airway epithelia, the mucosal ATP/UTP-dependent anion secretory response is mediated exclusively via Ca(2+)(i); and (3) Ca(2+)(i) regulation of the Ca(2+)-sensitive anion conductance (via CaCC) is compartmentalized in both CF and normal airway epithelia, with basolaterally released Ca(2+)(i) failing to activate CaCC in both epithelia.

Studies on the expression of mRNA for anion transport related proteins in corneal endothelial cells

PURPOSE

Chloride and bicarbonate are necessary for maintenance of fluid transport by the corneal endothelium, however there is little information on the identity of anion transport proteins that could serve as anion efflux mechanisms in endothelial cells. Therefore, we ask whether mRNA for the anion transport related proteins, CFTR, CLC-2, ClC-3, ClC-5 and AE2, are expressed in human, bovine or rabbit corneal endothelium.

METHODS

RT-PCR was performed for CFTR, CLC-2, ClC-3, ClC-5 and AE2 using total RNA from fresh human, bovine and rabbit corneal endothelium as well as cultured bovine corneal endothelial cells (CBCEC). Specificity of PCR products was confirmed by sequencing.

RESULTS

RT-PCR analysis gave positive bands at the predicted size for CLC-3 and CLC-5 from fresh human, rabbit and bovine as well as CBCEC. However, for CLC-2, no band was apparent around the predicted size from fresh and cultured corneal endothelium. A band at the predicted size was obtained for CFTR from fresh human, rabbit and bovine endothelium, as well as from CBCEC. RT-PCR analysis for AE2 produced specific bands from fresh human, rabbit and bovine corneal endothelium, but no positive band was obtained from CBCEC. Sequencing analysis further confirmed the identities of CLC-3, CLC-5, CFTR and AE2 in corneal endothelium.

CONCLUSIONS

CFTR, CLC-3 and ClC-5 are expressed in fresh and cultured corneal endothelial cells. However, consistent with previous immunoblots studies, AE2 is only expressed in fresh corneal endothelium. These results have implications for modeling possible apical anion efflux mechanisms in corneal endothelium.