Adenosine and adenosine analogues stimulate adenosine cyclic 3', 5'-monophosphate-dependent chloride secretion in the mammalian ileum

Adenosine in Intestinal Epithelial Barrier Function

At the intestinal front, several lines of defense are in place to resist infection and injury, the mucus layer, gut microbiome and strong epithelial junctions, to name a few. Their collaboration creates a resilient barrier. In intestinal disorders, such as inflammatory bowel disease (IBD), barrier function is compromised, which results in rampant inflammation and tissue injury. In response to the destruction, the intestinal epithelium releases adenosine, a small but powerful nucleoside that functions as an alarm signal. Amidst the chaos of inflammation, adenosine aims to restore order. Within the scope of its effects is the ability to regulate intestinal epithelial barrier integrity. This review aims to define the contributions of adenosine to mucus production, microbiome-dependent barrier protection, tight junction dynamics, chloride secretion and acid-base balance to reinforce its importance in the intestinal epithelial barrier.

Endogenous luminal surface adenosine signaling regulates duodenal bicarbonate secretion in rats

Luminal ATP increases duodenal bicarbonate secretion (DBS) via brush border P2Y receptors. Because ATP is sequentially dephosphorylated to adenosine (ADO) and the brush border highly expresses adenosine deaminase (ADA), we hypothesized that luminal [ADO] regulators and sensors, including P1 receptors, ADA, and nucleoside transporters (NTs) regulate DBS. We measured DBS with pH and CO(2) electrodes, perfusing ADO ± adenosine receptor agonists or antagonists or the cystic fibrosis transmembrane conductance regulator (CFTR) inhibitor CFTR(inh)-172 on DBS. Furthermore, we examined the effect of inhibitors of ADA or NT on DBS. Perfusion of AMP or ADO (0.1 mM) uniformly increased DBS, whereas inosine had no effect. The A(1/2) receptor agonist 5'-(N-ethylcarboxamido)-adenosine (0.1 mM) increased DBS, whereas ADO-augmented DBS was inhibited by the potent A(2B) receptor antagonist N-(4-cyanophenyl)-2-[4-(2,3,6,7-tetrahydro-2,6-dioxo-1,3-dipropyl-1H-purin-8-yl)phenoxy]-acetamide (MRS1754) (10 μM). Other selective adenosine receptor agonists or antagonists had no effect. The A(2B) receptor was immunolocalized to the brush border membrane of duodenal villi, whereas the A(2A) receptor was immunolocalized primarily to the vascular endothelium. Furthermore, ADO-induced DBS was enhanced by 2'-deoxycoformycin (1 μM) and formycin B (0.1 mM), but not by S-(4-nitrobenzyl)-6-thioinosine (0.1 mM), and it was abolished by CFTR(inh)-172 pretreatment (1 mg/kg i.p). Moreover, ATP (0.1 mM)-induced DBS was partially reduced by (1R,2S,4S,5S)-4-2-iodo-6-(methylamino)-9H-purin-9-yl]-2-(phosphonooxy)bicyclo[3.1.0]hexane-1-methanol dihydrogen phosphate ester tetraammonium salt (MRS2500) or 8-[4-[4-(4-chlorophenzyl)piperazide-1-sulfonyl)phenyl]]-1-propylxanthine (PSB603) and abolished by both, suggesting that ATP is sequentially degraded to ADO. Luminal ADO stimulates DBS via A(2B) receptors and CFTR. ATP release, ecto-phosphohydrolases, ADA, and concentrative NT may coordinately regulate luminal surface ADO concentration to modulate ADO-P1 receptor signaling in rat duodenum.

A plant-derived hydrolysable tannin inhibits CFTR chloride channel: a potential treatment of diarrhea

PURPOSE

The present study examined the effects and mechanisms of actions of penta-m-digalloyl-glucose (PDG), a hydrolysable tannin extracted from Chinese gallnut, on cystic fibrosis transmembrane conductance regulator protein (CFTR).

MATERIALS AND METHODS

Fisher rat thyroid cells stably expressing human CFTR (FRT cells) and human intestinal T84 cells were used as cell models to investigate the effects of PDG on chloride secretion using short-circuit current analysis. The mechanisms by which PDG affected chloride secretion were also examined. Finally, in vivo antidiarrheal efficacy and effects of PDG on intestinal fluid absorption were evaluated in mouse closed-loop models.

RESULTS

In FRT cells, apical chloride current induced by forskolin, CPT-cAMP and apigenin were reversibly inhibited by PDG (IC50 approximately 10microM) without effects on intracellular cAMP content and cell viability. Similarly, in T84 cells, PDG effectively inhibited chloride secretion induced by forskolin and cholera toxin. However, it had no effect on calcium-induced chloride secretion. In mice, a single intraluminal injection of PDG (0.6 mg/kg) reduced cholera toxin-induced intestinal fluid secretion by 75% with no effect on intestinal fluid absorption.

CONCLUSIONS

PDG represents a new class of CFTR inhibitors. Further development of this class of compounds may provide a new therapeutic intervention for diarrhea.

Regulation of enteric functions by adenosine: pathophysiological and pharmacological implications

The wide distribution of ATP and adenosine receptors as well as enzymes for purine metabolism in different gut regions suggests a complex role for these mediators in the regulation of gastrointestinal functions. Studies in rodents have shown a significant involvement of adenosine in the control of intestinal secretion, motility and sensation, via activation of A1, A2A, A2B or A3 purinergic receptors, as well as the participation of ATP in the regulation of enteric functions, through the recruitment of P2X and P2Y receptors. Increasing interest is being focused on the involvement of ATP and adenosine in the pathophysiology of intestinal disorders, with particular regard for inflammatory bowel diseases (IBDs), intestinal ischemia, post-operative ileus and related dysfunctions, such as gut dysmotility, diarrhoea and abdominal discomfort/pain. Current knowledge suggests that adenosine contributes to the modulation of enteric immune and inflammatory responses, leading to anti-inflammatory actions. There is evidence supporting a role of adenosine in the alterations of enteric motor and secretory activity associated with bowel inflammation. In particular, several studies have highlighted the importance of adenosine in diarrhoea, since this nucleoside participates actively in the cross-talk between immune and epithelial cells in the presence of diarrhoeogenic stimuli. In addition, adenosine exerts complex regulatory actions on pain transmission at peripheral and spinal sites. The present review illustrates current information on the role played by adenosine in the regulation of enteric functions, under normal or pathological conditions, and discusses pharmacological interventions on adenosine pathways as novel therapeutic options for the management of gut disorders and related abdominal symptoms.

A2B adenosine receptor gene deletion attenuates murine colitis

BACKGROUND & AIMS

The A(2B) adenosine receptor (A(2B)AR) is the predominant adenosine receptor expressed in the colonic epithelia. We have previously shown that A(2B)AR mRNA and protein levels are up-regulated during colitis. In this study, we addressed the role of the A(2B)AR in the development of murine colitis and the potential mechanism underlying its effects.

METHODS

Dextran sodium sulfate (DSS), 2,4,6-trinitrobenzene sulfonic acid (TNBS), and Salmonella typhimurium were used to induce colitis in A(2B)AR-null mice (A(2B)AR(-/-)). Colitis was determined using established clinical and histologic scoring. Keratinocyte-derived chemokine (KC) measurements were performed using an enzyme-linked immunosorbent assay.

RESULTS

Colonic inflammation induced by DSS, TNBS, or S typhimurium was attenuated in A(2B)AR(-/-) compared with their wild-type counterparts. Clinical features, histologic score, and myeloperoxidase activity were significantly decreased in A(2B)AR(-/-) mice. However, A(2B)AR(-/-) showed increased susceptibility to systemic Salmonella infection. Tissue levels of the neutrophil chemokine, KC was decreased in colitic A(2B)AR(-/-) mice. In addition, flagellin-induced KC levels were attenuated in A(2B)AR(-/-) mice. Neutrophil chemotaxis in response to exogenous interleukin-8 was preserved in A(2B)AR(-/-) mice, suggesting intact neutrophil migration in response to appropriate stimuli.

CONCLUSIONS

These data demonstrate, for the first time, that the A(2B)AR plays a proinflammatory role in colitis. A(2B) receptor antagonism may be an effective treatment for acute inflammatory intestinal diseases such as acute flare of inflammatory bowel disease.

Purinergic receptors in gastrointestinal inflammation

Purinergic receptors comprise a family of transmembrane receptors that are activated by extracellular nucleosides and nucleotides. The two major classes of purinergic receptors, P1 and P2, are expressed widely in the gastrointestinal tract as well as immune cells. The purinergic receptors serve a variety of functions from acting as neurotransmitters, to autocoid and paracrine signaling, to cell activation and immune response. Nucleosides and nucleotide agonist of purinergic receptors are released by many cell types in response to specific physiological signals, and their levels are increased during inflammation. In the past decade, the advent of genetic knockout mice and the development of highly potent and selective agonists and antagonists for the purinergic receptors have significantly advanced the understanding of purinergic receptor signaling in health and inflammation. In fact, agonist/antagonists of purinergic receptors are emerging as therapeutic modalities to treat intestinal inflammation. In this article, the distribution of the purinergic receptors in the gastrointestinal tract and their physiological and pathophysiological role in intestinal inflammation will be reviewed.

Lysophosphatidic acid inhibits cholera toxin-induced secretory diarrhea through CFTR-dependent protein interactions

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-regulated chloride channel localized primarily at the apical or luminal surfaces of epithelial cells that line the airway, gut, and exocrine glands; it is well established that CFTR plays a pivotal role in cholera toxin (CTX)-induced secretory diarrhea. Lysophosphatidic acid (LPA), a naturally occurring phospholipid present in blood and foods, has been reported to play a vital role in a variety of conditions involving gastrointestinal wound repair, apoptosis, inflammatory bowel disease, and diarrhea. Here we show, for the first time, that type 2 LPA receptors (LPA₂) are expressed at the apical surface of intestinal epithelial cells, where they form a macromolecular complex with Na⁺/H⁺ exchanger regulatory factor–2 and CFTR through a PSD95/Dlg/ZO-1–based interaction. LPA inhibited CFTR-dependent iodide efflux through LPA₂-mediated G_i pathway, and LPA inhibited CFTR-mediated short-circuit currents in a compartmentalized fashion. CFTR-dependent intestinal fluid secretion induced by CTX in mice was reduced substantially by LPA administration; disruption of this complex using a cell-permeant LPA₂-specific peptide reversed LPA₂-mediated inhibition. Thus, LPA-rich foods may represent an alternative method of treating certain forms of diarrhea.

Purinergic regulation of epithelial transport

Purinergic receptors are a family of ubiquitous transmembrane receptors comprising two classes, P1 and P2 receptors, which are activated by adenosine and extracellular nucleotides (i.e. ATP, ADP, UTP and UDP), respectively. These receptors play a significant role in regulating ion transport in epithelial tissues through a variety of intracellular signalling pathways. Activation of these receptors is partially dependent on ATP (or UTP) release from cells and its subsequent metabolism, and this release can be triggered by a number of stimuli, often in the setting of cellular damage. The function of P2Y receptor stimulation is primarily via signalling through the G(q)/PLC-beta pathway and subsequent activation of Ca(2+)-dependent ion channels. P1 signalling is complex, with each of the four P1 receptors A(1), A(2A), A(2B), and A(3) having a unique role in different epithelial tissue types. In colonic epithelium the A(2B) receptor plays a prominent role in regulating Cl(-) and water secretion. In airway epithelium, A(2B) and A(1) receptors are implicated in the control of Cl(-) and other currents. In the renal tubular epithelium, A(1), A(2A), and A(3) receptors have all been identified as playing a role in controlling the ionic composition of the lumenal fluid. Here we discuss the intracellular signalling pathways for each of these receptors in various epithelial tissues and their roles in pathophysiological conditions such as cystic fibrosis.

Stimulatory effects on Na+ transport in renal epithelia induced by extracts of Nigella arvensis are caused by adenosine

Effects of the extract of Nigella arvensis (NA) seeds on transepithelial Na+ transport were studied in cultured A6 toad kidney cells by recording short-circuit current (Isc),transepithelial conductance (GT), transepithelial capacitance (CT) and fluctuation in Isc. Apical application of NA extract had merely a small stimulatory effect on Na+ transport, whereas basolateral administration markedly increased Isc, GT and CT. A maximal effect was obtained at 500 μl l-1 of lyophilized NA extract. The increase in CT suggests that the activation of Isc occurs through the insertion of transport sites in the apical membrane. In experiments performed in the absence of Na+transport [apical Na+ was replaced by N-methyl-D-glucamine(NMDG+)], basolateral NA extract did not affect Isc and GT, indicating that Cl- conductance was not influenced. Noise analysis of Isc using 6-chloro-3,5-diaminopyrazine-2-carboxamide(CDPC) showed that NA extract reduced single-channel current(iNa) and decreased channel open probability(Po) but evoked a threefold increase in channel density(NT), which confirms the insertion of Na+channels. The separation of the compounds in the crude extract of NAwas performed by fast protein liquid chromatography (FPLC) on a Superdex 200 gel-filtration column and by reverse-phase high-pressure liquid chromatography(RPHPLC) on an μRPC C2/C18 SC2.1/10 column connected to a SMART system. Analysis of the purified active fraction by mass spectrometry demonstrated the presence of adenosine as the single organic compound in the extract that had a stimulatory effect on Na+ transport. In a separate series of experiments, we confirmed that 1 μmol l-1 adenosine had similar effects on the parameters of Na+ transport as did the NAextract. The action of adenosine was further identified by experiments in which NA extract was added after adenosine. In these experiments, NA extract did not affect Isc, GT or CT. These results clearly demonstrate an essential role of adenosine in the stimulatory action of NA extract.

Immunologically mediated transport of ions and macromolecules

There is increasing evidence supporting the involvement of immune cells and mediators in the control of intestinal physiology. Cell coculture systems and epithelial cell lines have provided convenient model systems for the study of immunomodulation of epithelial function. Abundant cytokines and immune mediators have been shown to directly or indirectly alter epithelial transport of ions and macromolecules. Animal models of hypersensitivity have shown that luminal antigen challenge in the intestine of sensitized rats induces a rapid ion secretory response due to enhanced transepithelial transport of antigen. Transport of ions and macromolecules is highly regulated and an important component of host defense. Dysregulation of epithelial function may play a role in several intestinal disorders, such as inflammatory bowel diseases and food allergy.

Adenosine induces cyclic-AMP formation and inhibits endothelin-1 production/secretion in guinea-pig tracheal epithelial cells through A(2B) adenosine receptors

1. The adenosine receptor subtype mediating adenosine 3' : 5'-cyclic monophosphate (cyclic AMP) formation and the effect of its activation on endothelin-1 (ET-1) secretion were studied in primary cultures of tracheal epithelial cells. 2. Adenosine analogues showed the following rank order of potency (pD(2) value) and intrinsic activity on the generation of cyclic AMP by tracheal epithelial cells: 5'-N-ethylcarboxyamidoadenosine (NECA, A(1)/A(2A)/A(2B), pD(2): 5.44+/-0.16)>adenosine (ADO, non selective, pD(2): 4.99+/-0. 09; 71+/-9% of NECA response) >/=2-Cl-adenosine (2CADO, non selective, pD(2): 4.72+/-0.14; 65+/-9% of NECA response)>>>CGS21680 (A(2A); inactive at up to 100 microM). 3. Cyclic AMP formation stimulated by NECA in guinea-pig tracheal epithelial cells was inhibited by adenosine receptor antagonist with the following order of apparent affinity (pA(2) value): Xanthine amine congeners (XAC, A(2A)/A(2B), 7.89+/-0.22)>CGS15943 (A(2A)/A(2B), 7.24+/-0. 26)>ZM241385 (A(2A), 6.69+/-0.14)>DPCPX (A(1), 6.51+/-0. 14)>3n-propylxanthine (weak A(2B), 4.30+/-0.10). This rank order of potency is typical for A(2B)-adenosine receptor. 4. Adenosine decreased basal and LPS-stimulated irET production in a concentration-dependent manner. Moreover, NECA but not CGS21680 inhibited LPS-induced irET production. 5. The inhibitory effect of NECA on LPS-induced irET production was reversed by XAC (pA(2)=8.84+/-0. 12) and DPCPX (pA(2)=8.10+/-0.22). 6. These results suggested that adenosine increased cyclic AMP formation and inhibited irET production/secretion by guinea-pig tracheal epithelial cells through the activation of a functional adenosine receptor that is most likely the A(2B) subtype. This adenosine receptor may be involved in the regulation of the level of ET-1 production/secretion by guinea-pig tracheal epithelial cells in physiological as well as in pathophysiological conditions.

Regulation of cAMP-dependent chloride channels in DC1 immortalized rabbit distal tubule cells in culture

Cl- conductances were studied in an immortalized cell line (DC1) derived from rabbit distal bright convoluted tubule (DCTb). The DC1 clone was obtained after transfection of primary cultures of DCTb with pSV3 neo. RT-PCR experiments showed the presence of cystic fibrosis transmembrane conductance regulator (CFTR) mRNA in the DC1 cell line. Using the whole cell patch-clamp technique, we recorded a linear Cl- conductance activated by forskolin (FK). This conductance was insensitive to DIDS and corresponded to a CFTR-like channel conductance. Fluorescence experiments with 6-methoxy-1-(3-sulfonatopropyl)quinolinium (SPQ) showed that FK induced an increase in Cl- efflux and influx in DC1 cells similar to that observed in cultured DCTb cells. 125I- efflux experiments performed on DC1 cells grown on collagen-coated filters showed that exposure of the monolayer to FK led to an increased 125I- loss through the apical membrane only. The addition of 10 microM adenosine activated a linear conductance identical to that recorded with FK and corresponding to the CFTR-like conductance. This conductance was also activated by 5'-(N-ethylcarboxamido)adenosine and CGS-21680 and inhibited in the presence of 8-cyclopentyl-1, 3-diproxylxanthine (DPCPX). This Cl- conductance could also be activated by guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS). The addition of protein kinase A (PKA) inhibitor to the pipette solution inhibited the development of the current activated by CGS-21680. Finally, 125I- efflux showed that adenosine induced an apical efflux mediated through basolateral A2 receptors. Overall, the data show that the DC1 cell line expressed an apical CFTR Cl- conductance that could be activated by adenosine via A2A receptors located in the basolateral membrane and involving G protein and PKA pathways.

A2 adenosine receptors in Mongolian gerbil middle ear epithelium and their regulation of Cl- secretion

The present study investigates the effects of adenosine and its analogues on Cl- secretion in primary cultures of gerbil middle ear epithelium. Short-circuit current (Isc), an index of transepithelial active transport, was measured on the same cells cultured on porous filters. Baseline Isc and transepithelial resistance were 27.0 +/- 0.7 microA cm-2 and 275 +/- 7 omega cm2, respectively (n = 178). Extracellular adenosine and its analogues elicited a sustained increase in Isc when added to apical or basolateral surfaces. Both the A2A selective agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamido adenosine and the A2A/A2B nonselective agonist 5'-(N-ethyl-carboxamido)adenosine (NECA) increased Isc, but NECA was more effective than CGS21680. A1 selective antagonist 8-cyclopentyl-1,3-dipropylxanthine did not reduce NECA-induced Isc. These results suggest the presence of both A2A and A2B receptors. NECA did not stimulate a rise in the intracellular Ca2+ concentration ([Ca2+]i) in single middle ear epithelial cells cultured on glass coverslips. Dibutyryl cAMP (dbcAMP) induced an initial transient increase in Isc followed by the sustained plateau. Addition of dbcAMP also caused a transient increase in [Ca2+]i. The protein kinase A inhibitor, N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide, greatly reduced the increase in the Isc responses to NECA. 1,2-Bis-(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid-acetoxymethyl ester influenced neither the NECA-induced increase in Isc nor the dbcAMP-induced sustained phase of Isc, but greatly inhibited the dbcAMP-induced transient increase in Isc. Glibenclamide, a cystic fibrosis transmembrane conductance regulator (CFTR) channel inhibitor, reduced the NECA-induced Isc. These results indicate that extracellular adenosine and its analogues activate the cAMP-protein kinase A system, but not intracellular Ca(2+)-dependent mechanisms, leading to Cl- secretion, possibly through the CFTR Cl- channels in the cultured gerbil middle ear epithelium.

Characterization and regulation of adenosine transport in T84 intestinal epithelial cells

Adenosine release from mucosal sources during inflammation and ischemia activates intestinal epithelial Cl- secretion. Previous data suggest that A2b receptor-mediated Cl- secretory responses may be dampened by epithelial cell nucleoside scavenging. The present study utilizes isotopic flux analysis and nucleoside analog binding assays to directly characterize the nucleoside transport system of cultured T84 human intestinal epithelial cells and to explore whether adenosine transport is regulated by secretory agonists, metabolic inhibition, or phorbol ester. Uptake of adenosine across the apical membrane displayed characteristics of simple diffusion. Kinetic analysis of basolateral uptake revealed a Na(+)-independent, nitrobenzylthioinosine (NBTI)-sensitive facilitated-diffusion system with low affinity but high capacity for adenosine. NBTI binding studies indicated a single population of high-affinity binding sites basolaterally. Neither forskolin, 5'-(N-ethylcarboxamido)-adenosine, nor metabolic inhibition significantly altered adenosine transport. However, phorbol 12-myristate 13-acetate significantly reduced both adenosine transport and the number of specific NBTI binding sites, suggesting that transporter number may be decreased through activation of protein kinase C. This basolateral facilitated adenosine transporter may serve a conventional function in nucleoside salvage and a novel function as a regulator of adenosine-dependent Cl- secretory responses and hence diarrheal disorders.

Modulation of chloride secretion in the rat ileum by intracellular bicarbonate

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

Apical adenosine activates an amiloride-sensitive conductance in human intestinal cell line HT29cl.19A

We studied the effects of stimulation of the apical adenosine receptor on ion transport by HT29cl.19A cells with the conventional microelectrode technique. Adenosine (100 microM) caused an increase in the transepithelial potential (3.6 +/- 0.4 mV) and equivalent short-circuit current (I(sc), 21 +/- 3 microA/cm2), a transient depolarization of the apical membrane potential (14 +/- 2 mV), and a decrease in the apical membrane resistance. The increase in I(sc) was additive to the effect of forskolin or basolateral addition of a maximal concentration of adenosine. Bumetanide, applied after adenosine, caused a further depolarization (7 +/- 2 mV) concomitant with a decrease in I(sc) (-13 +/- 2 microA/cm2) and an increase in the basolateral membrane resistance. Substitution of Cl- with gluconate or Na+ with N-methylglucamine reduced the response to adenosine by >60%. The response was also reduced by a low concentration of amiloride. We conclude that stimulation of the apical adenosine receptor activated a cation conductance in the apical membrane.

CPX, a selective A1-adenosine-receptor antagonist, regulates intracellular pH in cystic fibrosis cells

The selective A1-adenosine-receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (CPX), has been reported to activate Cl- efflux from cystic fibrosis cells, such as pancreatic CFPAC-1 and lung IB3 cells bearing the cystic fibrosis transmembrane regulator(delta F508) mutation, but has little effect on the same process in cells repaired by transfection with wild-type cystic fibrosis transmembrane regulator (O. Eidelman, C. Guay-Broder, P. J. M. van Galen, K. A. Jacobson, C. Fox, R. J. Turner, Z. I. Cabantchik, and H. B. Pollard. Proc. Natl. Acad. Sci. USA 89: 5562-5566, 1992). We report here that CPX downregulates Na+/H+ exchange activity in CFPAC-1 cells but has a much smaller effect on cells repaired with the wild-type gene. CPX also mildly decreases resting intracellular pH. In CFPAC-1 cells, this downregulation is dependent on the presence of adenosine, since pretreatment of the cells with adenosine deaminase blocks the CPX effect. We also show that, by contrast, CPX action on these cells does not lead to alterations in intracellular free Ca2+ concentration. We conclude that CPX affects pH regulation in CFPAC-1 cells, probably by antagonizing the tonic action of endogenous adenosine.

Adenosine stimulation of Na+ transport is mediated by an A1 receptor and a [Ca2+]i-dependent mechanism

Studies were performed to determine the primary signal transduction mechanism that mediates adenosine stimulation of electrogenic sodium transport in renal epithelial cells. Experiments were performed on cultured amphibian A6 cells with an adenosine analogue that preferentially binds to the A1 receptor, cyclohexyladenosine (CHA). Sodium transport was assessed by the equivalent short circuit current (Ieq). CHA was found to stimulate Ieq via activation of an A1 receptor because (1) the threshold concentration was 1 nM compared to that of 10 microM for the specific A2 agonist CGS21680, (2) CHA inhibited vasopressin (AVP)-stimulated cAMP production by a pertussis toxin-sensitive mechanism, and (3) the action of CHA was inhibited by the A1 antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX). CHA increased intracellular Ca2+ ([Ca2+]i) and stimulated phosphoinositide turnover at concentrations that increased Ieq and in a time course that paralleled the increase in Ieq. Ion transport was stimulated by a Ca(2+)-dependent mechanism because the CHA induced increase in Ieq was inhibited by chelating [Ca2+]i with 5,5'dimethyl BAPTA in a dose-dependent manner, with a Ki of approximately 10 microM. The increase in Ieq was also dose-dependently inhibited by the specific PKC inhibitors dihydroxychlorpromazine and chelerythrine, and by trifluoperazine which inhibits PKC and calmodulin. Further studies indicated that CHA-stimulated Ieq was independent of cAMP generation because CHA did not induce an increase in cAMP accumulation parallel to the increase in Ieq in a dose-response analysis, and the adenylate cyclase inhibitor 2',5' dideoxy-adenosine (DDA) did not affect the CHA-induced increase in Ieq.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification and function of A1 adenosine receptors in normal and cystic fibrosis human airway epithelial cells

A role for adenosine in the regulation of ion transport in pulmonary epithelial cells has recently been proposed. Although evidence exists documenting the presence and function of adenosine A2 receptors in airway epithelia, the presence of adenosine A1 receptors remains controversial. The present study used reverse transcriptase-polymerase chain reaction (PCR) and whole cell patch-clamp analysis to investigate A1 receptor presence and function in normal and cystic fibrosis (CF) human airway epithelial cells. Oligonucleotide primers complementary to the human brain A1 receptor sequence generated a PCR product of the predicted size (311 bp) in normal tracheal (9HTEo-) and CF submucosal (2CFSMEo-) airway cell lines and in primary cultures of CF nasal polyp epithelial cells. An oligonucleotide probe internal to the PCR primers hybridized with the 311-bp cDNAs by Southern blot analysis. cDNA sequencing demonstrated that the normal and CF airway cell PCR products are 100% identical to the corresponding sequence of the human brain adenosine A1 receptor. Northern blot analysis of 9HTEo-and 2CFSMEo- poly(A)+ RNA revealed the presence of two bands of approximately 3.0 and approximately 5.5 kb corresponding to the A1 receptor. Whole cell patch-clamp analyses demonstrated that 8-cyclopentyl-1,3-dipropylxanthine, a specific A1 receptor antagonist, increases adenosine 3',5'-cyclic monophosphate (cAMP)-activated Cl- conductance in 9HTEo-airway cells and allows cAMP to increase Cl- conductance in 2CFSMEo- CF airway cells and CF nasal polyp epithelial cells in primary culture. These results provide evidence for the presence and function of A1 receptors in normal and CF airway epithelial cells and provide support for a role of adenosine A1 receptors in modulating airway epithelial cell Cl- transport.

The A2b adenosine receptor mediates cAMP responses to adenosine receptor agonists in human intestinal epithelia

Adenosine is thought to be a major effector in immunological stimulation of Cl- secretion in intestinal epithelia. Previous studies indicate that both apical and basolateral domains of intestinal epithelial cells possess functionally defined adenosine receptors. However, it is unclear whether the same receptor subclass is expressed, what the receptor subclass(es) is, or how the receptors signal the Cl- secretory response. We now characterize the intestinal epithelial adenosine receptor subtype using the model epithelium, T84. Both apical and basolateral adenosine receptor agonist response profiles revealed a hierarchy (ED50) of 5'-(N-ethylcarboxamido)adenosine > adenosine > CGS-21680. Similarly inhibition studies revealed identical ID50 hierarchies for apical and basolateral antagonism by xanthine amine congener > 1,3-diethyl-8-phenylxanthine > aminophylline. Analyses of both agonist and antagonist pharmacological hierarchies in Chinese hamster ovary cells stably expressing the A2b receptor revealed these same hierarchies. Northern blots performed on RNA extracted from polarized T84 monolayers demonstrated no detectable message for A1 or A2a adenosine receptor, but strong hybridization was detected for the A2b adenosine receptor. Subsequent Northern blots of RNA prepared from human alimentary tract revealed that A2b adenosine receptor message was heavily expressed throughout the colon, in the appendix, and more modestly expressed in the small intestine (ileum). Analyses of cAMP generation in T84 cells in response to adenosine indicated that the basolateral A2b receptor elicits Cl- secretion through this signaling pathway. Stimulation of Cl- secretion through the apical A2b receptor exhibited relatively small but significant increases in cAMP compared with basolateral stimulation. The protein kinase A inhibitor H-89, used at concentrations that did not affect short circuit current responses to the Ca(2+)-mediated agonist carbachol, effectively inhibited short circuit current elicited by either apical or basolateral adenosine. These data suggest that the major intestinal epithelial adenosine receptor is the A2b subclass, which is positively coupled to adenylate cyclase. Such observations have potentially important implications for the treatment of diarrheal diseases.

The adenosine agonist NECA inhibits intestinal secretion and peristalsis

This study aimed to determine whether the antidiarrhoeal effect of the mixed A1/A2 adenosine agonist NECA (5'-N-ethylcarboxamido adenosine) is due to inhibition of intestinal fluid transport or to contractility. Intestinal secretion was stimulated in anaesthetized rats by intra-arterial infusions of PGE2 (4 micrograms min-1) or vasoactive intestinal peptide (0.8 micrograms min-1). NECA reversed PGE2-induced secretion in the jejunum (ED50 16 micrograms kg-1) and ileum (ED50 21 micrograms kg-1, i.v.) and inhibited VIP-induced secretion in the jejunum (ED50 21.5 micrograms kg-1). NECA inhibited twitch responses (0.1 Hz, 1 ms, IC50 11.2 nM) but not tetanic contractions at 10 Hz of the transmurally stimulated guinea-pig ileum. Likewise, NECA (10 microM) did not inhibit frequency-related contractions over the range of 2.5 to 40 Hz of rat jejunum or ileum. However, NECA was shown to be a potent inhibitor (30 nM) of the peristaltic reflex in the rat ileum. The results indicate that adenosine receptors are involved in modulating peristalsis as well as the secretory activity of the mucosa in the rat small intestine.

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.

Intestinal epithelial function: the case for immunophysiological regulation. Implications for disease (2)

Substantial amounts of data have been reported showing a role for immunomodulation of epithelial function (particularly ion secretion and permeability) using animal models of anaphylactic reactions. In part one of this review we outlined the main immune cell types and mediators/cytokines that are currently known to influence epithelial physiology either directly, or indirectly via an intermediate cell type. Here we will expand on the significance of these studies and show how antigenic activation of the mucosal immune system can evoke changes in epithelial function that may be beneficial to the host by mediating loss/inactivation of the antigen. However, a continued and inappropriate immune stimulation can lead to pathophysiological reactions and disease. Thus, we will present data on immune regulation of epithelial function with direct applicability to understanding the mechanism underlying human intestinal inflammatory and secretory disease. Finally, we highlight key strategic points in the cascade of immune events that can control epithelial function and thus may be of relevance in the formulation of new therapeutic approaches to intestinal inflammation.

Adenosine receptors on human airway epithelia and their relationship to chloride secretion

1. We have characterized an adenosine receptor subtype present in human airway epithelial cells by measuring the changes in the intracellular levels of adenosine 3':5'-cyclic monophosphate (cyclic AMP) and the rate of transepithelial Cl- secretion. 2. Primary cultures of human nasal epithelium obtained from excised surgical airway epithelial tissues and the cell lines BEAS39 and CF/T43 derived from human airway epithelium were grown on plastic dishes and labelled with [3H]-adenine for measurement of intracellular cyclic AMP accumulation. Primary cultures were loaded with the calcium indicator fura-2 to measure [Ca2+]i and studied as polarized, ion transporting epithelia on collagen matrix supports for measurement of Cl- secretion. 3. Adenosine analogues stimulated cyclic AMP accumulation with a rank order of potency characteristic of an A2-receptor: 5-N-ethyl-carboxamidoadenosine (NECA) greater than adenosine greater than R-phenylisopropyladenosine (R-PIA), 6-N-cyclopentyladenosine (CPA) greater than S-PIA. NECA increased cyclic AMP accumulation in normal and cystic fibrosis (CF) primary cells as well as in the CF/T43 and BEAS39 cell lines with K0.5 values ranging from 0.3 to 3 microM. Preincubation with NECA resulted in the homologous desensitization of airway epithelial cells. The effect of NECA was specifically inhibited by the adenosine receptor antagonist, aminophylline, in a competitive manner. 4. The A1-adenosine receptor agonists CPA and R-PIA did not inhibit isoprenaline-stimulated cyclic AMP accumulation in CF/T43 cells, and potentiating effects of the adenosine analogues were observed on forskolin-stimulated cyclic AMP accumulation. Adenosine analogues did not cause significant changes in intracellular Ca2+ ([Ca2+]i) in airway epithelium.5. Adenosine analogues, applied to either the serosal or mucosal side of the polarized amiloride pretreated primary cultures, induced changes in I,, with a rank order of potency of agonists similar to that observed for stimulation of cyclic AMP accumulation. Intracellular microelectrode studies indicated that the locus of action was the apical membrane Cl- conductance. Adenosine failed to stimulate C1- secretion in CF airway epithelium.6. These results provide evidence for the existence of an A2-adenosine receptor that modulates intracellular levels of cyclic AMP in human airway epithelium. Activation of this receptor might lead to stimulation of Cl- secretion in amiloride pretreated normal but not CF cells.

A1 adenosine-receptor antagonists activate chloride efflux from cystic fibrosis cells

A1 adenosine-receptor-antagonist drugs such as 8-cyclopentyl-1,3-dipropylxanthine (CPX) and xanthine amine congener (XAC) are found to activate the efflux of 36Cl- from CFPAC cells. These cells are a pancreatic adenocarcinoma cell line derived from a cystic fibrosis (CF) patient homozygous for the common mutation, deletion of Phe-508. The active concentrations for these compounds are in the low nanomolar range, consistent with action on A1 adenosine receptors. In addition, drug action can be blocked by exogenous agonists such as 2-chloroadenosine and also can be antagonized by removal of endogenous agonists by treatment with adenosine deaminase. Cells lacking the CF genotype and phenotype, such as HT-29 and T84 colon carcinoma cell lines, appear to be resistant to activation of chloride efflux by either drug. CFPAC cells transfected with the CF transmembrane regulator gene, CFTR, are also resistant to activation by CPX. We conclude that, since these antagonists are of relatively low toxicity and appear to act somewhat selectively, they might be considered as promising therapeutic candidates for CF.

Adenosine regulates a chloride channel via protein kinase C and a G protein in a rabbit cortical collecting duct cell line

We examined the regulation by adenosine of a 305-pS chloride (Cl-) channel in the apical membrane of a continuous cell line derived from rabbit cortical collecting duct (RCCT-28A) using the patch clamp technique. Stimulation of A1 adenosine receptors by N6-cyclohexyladenosine (CHA) activated the channel in cell-attached patches. Phorbol 12,13-didecanoate and 1-oleoyl 2-acetylglycerol, activators of protein kinase C (PKC), mimicked the effect of CHA, whereas the PKC inhibitor H7 blocked the action of CHA. Stimulation of A1 adenosine receptors also increased the production of diacylglycerol, an activator of PKC. Exogenous PKC added to the cytoplasmic face of inside-out patches also stimulated the Cl- channel. Alkaline phosphatase reversed PKC activation. These results show that stimulation of A1 adenosine receptors activates a 305-pS Cl-channel in the apical membrane by a phosphorylation-dependent pathway involving PKC. In previous studies, we showed that the protein G alpha i-3 activated the 305-pS Cl- channel (Schwiebert et al. 1990. J. Biol. Chem. 265:7725-7728). We, therefore, tested the hypothesis that PKC activates the channel by a G protein-dependent pathway. In inside-out patches, pertussis toxin blocked PKC activation of the channel. In contrast, H7 did not prevent G protein activation of the channel. We conclude that adenosine activates a 305-pS Cl- channel in the apical membrane of RCCT-28A cells by a membrane-delimited pathway involving an A1 adenosine receptor, phospholipase C, diacylglycerol, PKC, and a G protein. Because we have shown, in previous studies, that this Cl- channel participates in the regulatory volume decrease subsequent to cell swelling, adenosine release during ischemic cell swelling may activate the Cl-channel and restore cell volume.

Effects of adenosine receptor agonists on volume-activated ion transport in pig red cells

Swelling of pig red cells leads to an increase in a chloride-dependent K flux which can be potentiated by cAMP, whereas cell shrinking causes a selective increase in Na movement which is mediated by a Na/H exchanger. We examined the influence of adenosine and adenosine receptor agonists on the volume-sensitive, ouabain-resistant, chloride-dependent K flux, referred to as Rb flux and volume-activated Na/H exchange pathway. It was found that adenosine and adenosine receptor agonists inhibited the Rb flux. N6-cyclohexyl adenosine (CHA) has been found to be the most potent inhibitor with EC50 of approximately 4.5 microM followed by 2-chloroadenosine (Cl-ado) with EC50 of approximately 27 microM and 5'-(N-ethyl)-carboxamido-adenosine (NECA) with EC50 of approximately 185 microM. CHA also inhibits the cAMP-stimulated Rb flux. However, CHA does not alter the basal intracellular cAMP level nor the intracellular cAMP content raised by exogenously added cAMP. In contrast to the adenosine agonist action on the Rb flux, Na/H exchange, which is activated upon cell shrinkage, exhibits a slight stimulation in response to CHA. These findings suggest that the presence of A1 adenosine receptors on the surface of red cells influences the regulation of volume-activated ion transport.

A1 adenosine receptors inhibit chloride transport in the shark rectal gland. Dissociation of inhibition and cyclic AMP

In the in vitro perfused rectal gland of the dogfish shark (Squalus acanthias), the adenosine analogue 2-chloroadenosine (2Clado) completely and reversibly inhibited forskolin-stimulated chloride secretion with an IC50 of 5 nM. Other A1 receptor agonists including cyclohexyladenosine (CHA), N-ethylcarboxamideadenosine (NECA) and R-phenylisopropyl-adenosine (R-PIA) also completely inhibited forskolin stimulated chloride secretion. The "S" stereoisomer of PIA (S-PIA) was a less potent inhibitor of forskolin stimulated chloride secretion, consistent with the affinity profile of PIA stereoisomers for an A1 receptor. The adenosine receptor antagonists 8-phenyltheophylline and 8-cyclopentyltheophylline completely blocked the effect of 2Clado to inhibit forskolin-stimulated chloride secretion. When chloride secretion and tissue cyclic (c)AMP content were determined simultaneously in perfused glands, 2Clado completely inhibited secretion but only inhibited forskolin stimulated cAMP accumulation by 34-40%, indicating that the mechanism of inhibition of secretion by 2Clado is at least partially cAMP independent. Consistent with these results, A1 receptor agonists only modestly inhibited (9-15%) forskolin stimulated adenylate cyclase activity and 2Clado markedly inhibited chloride secretion stimulated by a permeant cAMP analogue, 8-chlorophenylthio cAMP (8CPT cAMP). These findings provide the first evidence for a high affinity A1 adenosine receptor that inhibits hormone stimulated ion transport in a model epithelia. A major portion of this inhibition occurs by a mechanism that is independent of the cAMP messenger system.

Immune-related intestinal chloride secretion. II. Effect of adenosine on T84 cell line

The inflammatory mediator adenosine caused sustained Cl- secretion across monolayers of T84 cells. The effect was promptly reversed by the adenosine receptor antagonist 8-phenyltheophylline and appeared to be mediated through an adenosine A2-receptor [rank order of potency: 5'-(N-ethyl)-carboxamido-adenosine (NECA) greater than adenosine greater than (-)-N6-(phenylisopropyl)adenosine (PIA) greater than or equal to (+)-PIA]. High doses of adenosine and its analogues increased cellular adenosine 3',5'-cyclic monophosphate (cAMP) but not guanosine 3',5'-cyclic monophosphate (cGMP) or free cytosolic Ca2+. However, lower concentrations of adenosine had maximal effects on Cl- secretion with little or no effect on cAMP. In other respects, Cl- secretion resembled that induced by cAMP-mediated secretagogues such as vasoactive intestinal peptide (VIP). Addition of both low and high doses of NECA activated basolateral K+ and apical Cl- channels, exhibited synergism with Ca2(+)-mediated secretagogues, did not produce additive effects with VIP or Escherichia coli heat-stable enterotoxin, and was associated with cAMP-dependent protein kinase-mediated protein phosphorylation. The results suggest that either adenosine mobilizes an intracellular pool of cAMP that is extremely efficiently coupled to the cAMP-dependent protein kinase and is thereafter rapidly destroyed or that second messenger(s) other than cAMP, cGMP, or Ca2+ are able to activate Cl- secretion in the T84 cell line. In the latter case, such messenger(s), as yet unidentified, might represent a final common pathway for cyclic nucleotide-activated Cl- secretion.

Effect of adenosine and adenosine analogs on [14C]aminopyrine accumulation by rabbit parietal cells

Adenosine receptors that modulate adenylate cyclase activity have been identified recently in a number of tissues. Adenosine A2 receptor is stimulatory to adenylate cyclase, whereas adenosine A1 receptor is inhibitory to adenylate cyclase. We investigated the effect of adenosine and its analogs on [14C]aminopyrine accumulation by rabbit parietal cells. Rabbit gastric mucosal cells were isolated by enzyme digestion. Parietal cells were enriched by nonlinear percoll gradients. [14C]Aminopyrine accumulation was used as an indicator of acid secretion. The effect of 2-chloroadenosine on histamine-stimulated [14C]aminopyrine accumulation was studied. The effects of N-ethylcarboxamideadenosine, 2-chloroadenosine, stable analogs of adenosine, and adenosine on [14C]aminopyrine accumulation were assessed. Cyclic AMP content of parietal cells was determined by radioimmunoassay. Histamine and carbachol, known secretagogues, stimulated [14C]aminopyrine accumulation. 2-Chloroadenosine did not suppress histamine-stimulated [14C]aminopyrine accumulation. 2-Chloroadenosine, N-ethylcarboxamideadenosine, and adenosine dose dependently increased [14C]aminopyrine accumulation. The order of potency was N-ethylcarboxamideadenosine greater than 2-chloroadenosine greater than adenosine. 8-Phenyltheophylline and theophylline, adenosine-receptor antagonists, or cimetidine did not have significant effects on the increase of AP uptake induced by 2-chloroadenosine. Coadministration of dipyridamole, and adenosine uptake inhibitor, augmented the effect of adenosine on [14C]aminopyrine accumulation. 2-Chloroadenosine, N-ethylcarboxamideadenosine, and adenosine each induced a significant increase in cellular cyclic AMP. We conclude that there may be adenosine A2 receptors on rabbit parietal cells which modulate gastric acid secretion.

Differing effects of apical and basolateral adenosine on colonic epithelial cell line T84

Adenosine and its agonists, 5'-(N-ethylcarboxamido)adenosine and N6-(D-2-phenylisopropyl)-adenosine, induced a sustained increase in chloride secretion when added to either the apical or basolateral aspect of monolayers of the human colonic epithelial cell line T84. Secretion was induced with identical kinetics by addition to both sides, but apical addition was less potent. The rank order of potency of the agonists on either side was consistent with the presence of an adenosine A2-receptor, but the apical and basolateral receptors differed in both their ability to stimulate increases in adenosine 3',5'-cyclic monophosphate, and their susceptibility to down-modulation by chronic exposure to 5'-(N-ethylcarboxamido) adenosine in culture.

Influence of 2-chloroadenosine on the nucleotide content of isolated rat hepatocytes

2-Chloroadenosine is presumably a non-metabolizable analogue of adenosine; however, this compound induced an increase in the enzymatically measured nucleotide content of isolated rat hepatocytes. HPLC separation and spectral analysis of the peaks showed that this increase may be related to the formation of 2-chloro nucleotides and that the 2-chloro nucleotides appeared in the first minutes of the incubation period. These results demonstrate that 2-chloroadenosine may be metabolized by phosphorylation in rat liver cells.

Effects of isoproterenol on Cl transport in rat submandibular salivary-gland acini

In the absence of the beta-adrenoceptor agonist, isoproterenol, acini accumulated 36Cl in a time-dependent manner until a steady-state content of 7.4 +/- 0.2 nmol/mg protein was attained after 5 min of incubation. In the presence of 10 microM isoproterenol the tracer content increased significantly in the first few minutes (40 per cent after 2 min, 33 per cent after 5 min) but then decreased and was essentially the same as in the absence of agonist after 15 min. This effect was inhibited by 1 mM furosemide but was still observed when the cells were incubated in K-free medium. Exposure of tracer-preloaded cells with steady-state isotope content to 10 microM isoproterenol likewise caused a transient but significant increase in tracer content (32 per cent after 6 min); exposure to 1 microM acetylcholine caused a significant and sustained decrease in tracer content (30 per cent), which was not prevented by pre-incubating the cells for 5 min with isoproterenol. Similar transient increases in tracer content of preloaded cells were observed upon exposure to 10 microM concentrations of forskolin or dibutyryl cAMP. Stimulation of beta-receptors does not therefore cause a net Cl efflux in salivary acini and does not prevent the net efflux induced by stimulation of cholinergic receptors. As this efflux may be via a conductance pathway or channels in the apical membrane, these channels do not seem to be regulated by a beta-receptor-coupled, cAMP-mediated pathway in salivary cells.(ABSTRACT TRUNCATED AT 250 WORDS)

The mechanism by which adenosine decreases gluconeogenesis from lactate in isolated rat hepatocytes

Incubation of hepatocytes from 24 h-starved rats in the presence of 0.5 mM-adenosine decreased gluconeogenesis from lactate, but not from alanine. The inhibition of gluconeogenesis was associated with a stimulation of ketone-body production and an inhibition of pyruvate oxidation. These metabolic changes were suppressed in the presence of iodotubercidin (an inhibitor of adenosine kinase), but were reinforced in the presence of deoxycoformycin (an inhibitor of adenosine deaminase); 2-chloroadenosine induced no change in gluconeogenesis from lactate. These data indicate that the inhibition of gluconeogenesis by adenosine probably results from its conversion into adenine nucleotides. In the presence of lactate or pyruvate, but not with alanine or asparagine, this conversion resulted in a decrease in the [ATP]/[ADP] ratio in both mitochondrial and cytosolic compartments. Adenosine decreased the Pi concentration with all gluconeogenic substrates.

Intestinal arteriolar responses to mucosal and serosal applications of adenosine analogues

Adenosine or its synthetic analogues were topically applied to the intestinal jejunum while steady-state blood flow was calculated in submucosal arterioles using video microscopy. Blood flow increased (220 or 130% of control) with the serosal application of 10(-6) M N-ethyl carboxamido adenosine (NECA, A2-selective agonist) or 2-chloro adenosine (2CA, nonselective agonist) but not with cyclohexyl adenosine (CHA, A1-selective agonist). The nonselective competitive antagonist, 8-phenyl theophylline, attenuated the response evoked by NECA. The mucosal application of 10(-6) M CHA caused blood flow decreases (81% of control), but neither NECA nor 2CA evoked a response. These observations suggest a mucosal diffusion barrier, so the concentrations of the analogues were raised one hundredfold. Serosal 10(-4) M CHA or NECA caused blood flow increases, but the effects were negligible with mucosal application, suggesting that the mucosa was indeed impermeable to these compounds. The responses evoked by 10(-4) M 2CA were similar on the serosa or mucosa (200-220% of control), submaximal (maximum = 400% of control at 10(-3) M), and not antagonized by 8-phenyl theophylline or by the cellular uptake inhibitor, nitrobenzyl-6-thio guanosine. In context with earlier studies, greater than 10(-6) M 2CA probably evokes vasodilation that is not entirely mediated by extracellular receptors. Serosal adenosine (10(-4) M) caused submaximal blood flow increases (200% of control) that were not potentiated by nitrobenzyl-6-thio guanosine or another transport inhibitor, dipyridamole.(ABSTRACT TRUNCATED AT 250 WORDS)

Exogenous ATP-stimulated calcium uptake in isolated rat intestinal epithelial cells

ATP in the extracellular medium is known to stimulate Ca uptake into avian intestinal epithelial cells. We have now demonstrated a similar effect of ATP in mammalian intestinal epithelial cells and have further characterized this effect. Exogenous ATP increased 45Ca uptake 2-6 fold in isolated rat small intestinal epithelial cells, with a maximal effect at 1 mM and an ED50 of 290 microM. A strict structural requirement for nucleotide-stimulated 45Ca uptake was observed. ADP was much less effective than ATP and gamma-thio-ATP, and 5'-AMP, cyclic AMP, adenosine, non-adenine nucleotides, non-hydrolyzable ATP analogs and ATP analogs with ring substitutions at the 8 position were inactive. Prenylamine (100 microM) completely inhibited ATP-stimulated 45Ca uptake, while verapamil (100 microM) had only a small effect. In the intact intestine, ATP increased short-circuit current (Isc) when added to the mucosal side of the tissue. This effect was reduced by 10 microM and abolished by 100 microM prenylamine. The effect of ATP on Isc was markedly reduced in Cl-free solutions and in reduced-Ca solutions. Serosal and mucosal addition of the nonhydrolyzable ATP analog, beta, gamma-methylene-ATP, and serosal addition of ATP had little or no effect on Isc. The similarities between the effects of ATP in isolated cells and in the intact intestine suggest that the effect of ATP on Isc may be at least partially mediated through stimulation of Ca uptake into the epithelial cells.

A1 and A2 adenosine receptors in rabbit cortical collecting tubule cells. Modulation of hormone-stimulated cAMP

Adenosine analogs were used to investigate the cellular mechanisms by which adenosine may alter renal tubular function. Cultured rabbit cortical collecting tubule (RCCT) cells, isolated by immunodissection, were treated with 5'-N-ethylcarboxamideadenosine (NECA), N6-cyclohexyladenosine (CHA), and R-N6-phenylisopropyladenosine (PIA). All three analogs produced both dose-dependent inhibition and stimulation of RCCT cell cyclic AMP (cAMP) production. Stimulation of cAMP accumulation occurred at analog concentrations of 0.1 microM to 100 microM with the rank order of potency NECA greater than PIA greater than CHA. Inhibition occurred at concentrations of 1 nM to 1 microM with the rank order of potency CHA greater than PIA greater than NECA. These effects on cAMP production were inhibited by 1,3-diethyl-8-phenylxanthine and isobutylmethylxanthine. CHA (50 nM) blunted AVP- and isoproterenol-stimulated cAMP accumulation. This modulation of hormone-induced cAMP production was abolished by pretreatment of RCCT cells with pertussis toxin. Prostaglandin E2 production was unaffected by 0.1 mM CHA. These findings indicate the presence of both inhibitory (A1) and stimulatory (A2) receptors for adenosine in RCCT cells. Moreover, occupancy of the A1 receptor causes inhibition of both basal and hormone-stimulated cAMP formation through an action on the inhibitory guanine nucleotide-binding regulatory component, Ni, of the adenylate cyclase system.

Mucosal adenosine stimulates chloride secretion in canine tracheal epithelium

Adenosine is a local regulator of a variety of physiological functions in many tissues and has been observed to stimulate secretion in several Cl-secreting epithelia. In canine tracheal epithelium we found that adenosine stimulates Cl secretion from both the mucosal and submucosal surfaces. Addition of adenosine, or its analogue 2-chloroadenosine, to the mucosal surface potently stimulated Cl secretion with no effect on the rate of Na absorption. Stimulation resulted from an interaction of adenosine with adenosine receptors, because it was blocked by the adenosine receptor blocker, 8-phenyltheophylline. The adenosine receptor was a stimulatory receptor as judged by the rank-order potency of adenosine and its analogues and by the increase in cellular adenosine 3',5'-cyclic monophosphate levels produced by 2-chloroadenosine. Adenosine also stimulated Cl secretion when it was added to the submucosal surface, although the maximal increase in secretion was less and it was much less potent. Part, but not all, of the lower potency of submucosal adenosine resulted from submucosal uptake and metabolism of the drug. The observation that mucosal 8-phenyltheophylline blocked the effect of submucosal 2-chloroadenosine, whereas submucosal 8-phenyltheophylline did not prevent a response to mucosal or submucosal 2-chloroadenosine, suggests that adenosine receptors are located on the mucosal surface. Thus submucosal adenosine may stimulate secretion by crossing the epithelium and interacting with receptors located on the mucosal surface. Because adenosine can be released from mast cells located in the airway lumen in response to inhaled material, and because adenosine stimulated secretion from the mucosal surface, it may be in a unique position to control the epithelium on a regional level.

The effect of adenosine and chloroadenosine on sex differences in the energy metabolism of rat hepatocytes

The intensity and regulation of metabolic pathways are different depending on the sex of the source animal for hepatocytes isolated from mature rats. In cells from fed animals incubated without exogenous substrate, ATP level and ketone body production are higher in males (+25% and +100%) and lactate production is higher (+64%) in females; oleate enhances mitochondrial pyruvate oxidation in hepatocytes from fed male rats but not from fed females; in cells from starved animals oleate increases gluconeogenesis in both sexes at saturating levels of gluconeogenic substrates. However, at physiological levels (1 mM lactate and 0.1 mM pyruvate), this activation can only be detected in cells from males. In both sexes, oleate activation is abolished by adenosine which reduces in parallel the mitochondrial oxidation of pyruvate; chloroadenosine, an A2-receptor agonist, increases glycogenolysis strongly in hepatocytes from male animals (+80%) but only very slightly in female cells (+12%).

Purinergic regulation of basal and arginine vasopressin-stimulated hydraulic conductivity in rabbit cortical collecting tubule

An extracellular adenosine responsive site that stimulates adenylate cyclase activity has been identified in several tissues. There is limited information on the presence and physiologic significance of adenosine receptors in well-defined segments of the mammalian nephron. We therefore examined the effect of adenosine and selected analogues on basal hydraulic conductivity in rabbit cortical collecting tubules (CCT) perfused in vitro. Adenosine and analogues with an intact ribose moiety produced a significant, sustained increase in hydraulic conductivity. No increase in hydraulic conductivity was seen in either time control CCT's or CCT's exposed to an adenosine analogue with an altered ribose moiety. These experiments are compatible with the presence of a functional adenosine receptor which requires an intact ribose moiety and acts to increase hydraulic conductivity in the mammalian CCT. An intracellular adenosine responsive site, termed the "P site," which inhibits adenylate cyclase activity, has also been described in several tissues. We therefore examined the effect of a P site agonist on hydraulic conductivity responses to arginine vasopressin, forskolin and cAMP. P site stimulation with 2'5' dideoxyadenosine inhibited the effect of AVP and of forskolin but not of cAMP to increase hydraulic conductivity. These results are compatible with a functional P site in the rabbit CCT which acts at the catalytic subunit of adenylate cyclase to inhibit hydraulic conductivity. Together, these results demonstrate purinergic modulation of basal and arginine vasopressin-stimulated water flux in the mammalian collecting tubule.

Adenosine stimulates sodium transport in kidney A6 epithelia in culture

The effects of adenosine receptor agonists and antagonists were examined in epithelia formed in culture by A6 cells, a continuous cell line derived from Xenopus laevis kidney. A6 epithelia have a high electrical resistance and a short-circuit current that is equal to net sodium flux from mucosal to serosal surface. Adenosine, 2-chloroadenosine, 5'-(N-ethyl)carboxamidoadenosine, and N6-(L-2-phenylisopropyl) adenosine produced concentration-dependent increases in short-circuit current. Stimulation of short-circuit current by 2-chloroadenosine occurred at concentrations of 0.05 microM and above, with half-maximal stimulation occurring at 0.3 microM. 5'-(N-ethyl)carboxamidoadenosine was more potent than N6-(L-2-phenylisopropyl)adenosine, the usual order of potency for activation of stimulatory adenosine receptors. Theophylline (100 microM), an adenosine receptor antagonist, reduced the short-circuit current response to adenosine and 2-chloroadenosine by 85-90%. Amiloride, an agent that inhibits both basal and adenosine 3',5'-cyclic monophosphate (cAMP)-stimulated short-circuit current in A6 epithelia, completely and reversibly inhibited short-circuit current stimulated by 2-chloroadenosine. Adenosine and 2-chloroadenosine stimulated adenylate cyclase activity in a crude membrane preparation from A6 cells. Stimulation by adenosine was blocked by adenosine deaminase. 2-Chloroadenosine increased cell cAMP accumulation in intact epithelia. The results provide evidence that adenosine and adenosine receptor agonists stimulate adenylate cyclase and active sodium transport in an epithelial cell line of renal origin.

Indirect effects of adenosine triphosphate on chloride secretion in mammalian colon

The effects of adenosine triphosphate (ATP) on short-circuit current (SCC) in rat colonic epithelium are described. ATP caused a large increase in inward-going current and was considerably more potent in this respect than ADP, AMP or adenosine. The response to ATP was sided, there being only minor effects when the nucleotide was added to the apical side of the tissue. The effects of ATP were not modified by the cyclooxygenase inhibitor, indomethacin, eliminating eicosanoid formation as a mechanism. The effects of ATP were potentiated by theophylline and not blocked by alpha, beta-methylene ATP. The data are consistent with the effect being dependent on the activation of adenylate cyclase, but it has not been possible to classify the receptors into P1 or P2 categories. Using inhibitors of NaCl cotransport (piretanide), carbonic anhydrase (acetazolamide), and chloride channels (diphenylamine-2-carboxylate), it was concluded that the SCC response to ATP was due to chloride secretion with, perhaps, a minor contribution from bicarbonate. Flux measurements with 22Na and 36Cl confirmed this view, there being approximate equivalence of chloride secretion with the SCC responses. Additionally, flux measurements revealed an inhibition of electroneutral NaCl absorption in response to ATP. The effects of ATP were antagonized by tetrodotoxin (TTX), greater than 50% inhibition being achieved with 10 nM TTX. This result suggests that ATP does not act directly on receptors in the epithelial cells but rather on neuronal elements in the lamina propria. It will be necessary to re-examine other secretagogues for indirect effects of this kind and to search for the final effector neurotransmitter which evokes secretion.