Adenosine stimulates sodium transport in kidney A6 epithelia in culture

Adenosine-evoked Na+ transport in human airway epithelial cells

BACKGROUND AND PURPOSE

Absorptive epithelia express apical receptors that allow nucleotides to inhibit Na(+) transport but ATP unexpectedly stimulated this process in an absorptive cell line derived from human bronchiolar epithelium (H441 cells) whilst UTP consistently caused inhibition. We have therefore examined the pharmacological basis of this anomalous effect of ATP.

EXPERIMENTAL APPROACH

H441 cells were grown on membranes and the short circuit current (I(SC)) measured in Ussing chambers. In some experiments, [Ca(2+)](i) was measured fluorimetrically using Fura -2. mRNAs for adenosine receptors were determined by the polymerase chain reaction (PCR).

KEY RESULTS

Cross desensitization experiments showed that the inhibitory response to UTP was abolished by prior exposure to ATP whilst the stimulatory response to ATP persisted in UTP-pre-stimulated cells. Apical adenosine evoked an increase in I(SC) and this response resembled the stimulatory component of the response to ATP, and could be mimicked by adenosine receptor agonists. Pre-stimulation with adenosine abolished the stimulatory component of the response to ATP. mRNA encoding A(1), A(2A) and A(2B) receptor subtypes, but not the A(3) subtype, was detected in H441 cells and adenosine receptor antagonists could abolish the ATP-evoked stimulation of Na(+) absorption.

CONCLUSIONS AND IMPLICATIONS

The ATP-induced stimulation of Na(+) absorption seems to be mediated via A(2A/B) receptors activated by adenosine produced from the extracellular hydrolysis of ATP. The present data thus provide the first description of adenosine-evoked Na(+) transport in airway epithelial cells and reveal a previously undocumented aspect of the control of this physiologically important ion transport process.

Gene expression of adenosine receptors along the nephron

BACKGROUND

In view of the multiple effects of adenosine on kidney function, this study aimed to determine the expression of adenosine receptors (AR) along the rat and mouse nephron.

METHODS

For this purpose, we semiquantified mRNA abundance for adenosine A1-, A2A-, A2B-, and A3 receptors by RNAse protection and by reverse transcription-polymerase chain reaction (RT-PCR) in the kidney zones and in the different nephron segments of mice and rats.

RESULTS

We found very similar expression patterns for rat and mice. For the kidney zones A1-AR mRNA and A2A-AR mRNA abundance displayed a marked difference, with an increase from cortex to the inner medulla. This was not seen for A2B receptors, which showed in general a rather weak expression. Along the nephron, A1-AR was strongly expressed in the thin limbs of Henle and in the collecting duct system and to a lesser extent in the medullary thick ascending limb. A2A-AR mRNA was clearly detected in glomeruli but not in other nephron segments. A2B-AR mRNA was strongly expressed in the cortical thick ascending limb of Henle and in the distal convoluted tubule. A3-AR mRNA was not found in any nephron segment.

CONCLUSION

Our data demonstrate a distinct mutual expression of the AR subtypes along the nephron. A1 receptors are expressed in medullary tubular structures, while A2B receptors are predominant in cortical tubular structures. A2A receptor expression in the kidney appears to be restricted to vascular cells.

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.

Adenosine biosynthesis in the collecting duct

Adenosine regulates tubular transport in collecting ducts (CDs); however, the sources of adenosine that modulate ion transport in CDs are unknown. The extracellular cAMP-adenosine pathway refers to the conversion of cAMP to AMP by ectophosphodiesterase, followed by metabolism of AMP to adenosine by ecto-5'-nucleotidase, with all steps occurring in the extracellular compartment. The goal of this study was to assess whether the extracellular cAMP-adenosine pathway exists in CDs. Studies were conducted in both freshly isolated CDs and in CD cells in culture (first passage) that were derived from isolated CDs. Purity of CDs was confirmed by microscopy, by Western blotting for aquaporin-1, aquaporin-2, bumetanide-sensitive cotransporter type 1, and thiazide-sensitive cotransporter; and by reverse transcription-polymerase chain reaction for adenosine receptors. Both freshly isolated CDs and CD cells in culture converted exogenous cAMP to AMP and adenosine. In both freshly isolated CDs and CD cells in culture, conversion of cAMP to AMP and adenosine was affected by a broad-spectrum phosphodiesterase inhibitor (3-isobutyl-1-methylxanthine), an ectophosphodiesterase inhibitor (1,3-dipropyl-8-p-sulfophenylxanthine), and a blocker of ecto-5'-nucleotidase (alpha,beta-methylene-adenosine-5'-diphosphate) in a manner consistent with exogenous cAMP being processed by the extracellular cAMP-adenosine pathway. In CD cells in culture, stimulation of adenylyl cyclase increased extracellular concentrations of cAMP, AMP, and adenosine, and these changes were also modulated by the aforementioned inhibitors in a manner consistent with the extracellular cAMP-adenosine pathway. In conclusion, the extracellular cAMP-adenosine pathway is an important source of adenosine in CDs.

Basolateral and apical A1 adenosine receptors mediate sodium transport in cultured renal epithelial (A6) cells

There are conflicting reports in the literature regarding the adenosine receptor that mediates the increase in sodium transport in the A6 cell. In this study we used specific A1 and A2 adenosine receptor agonists and antagonists, as well as two different subclones of the A6 cell, to determine which adenosine receptor mediates the increase in sodium transport. In the A6S2 subclone, basolateral and apical N6-cyclohexyladenosine (CHA), a selective A1 receptor agonist, stimulated sodium transport at a threshold concentration <10(-7) M, whereas CGS-21680, a selective A2 receptor agonist, had a threshold concentration that was at least 10(-5) M. The A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) was found to have a nonspecific effect on CHA-stimulated sodium transport, whereas the A2 receptor antagonist 8-(3-chlorostyryl)caffeine (CSC) had no effect. As with the A6S2 subclone, basolateral and apical CHA stimulated sodium transport at a nanomolar concentration in the A6C1 subclone and the threshold concentration for CGS-21680 was in the high micromolar range. Concurrent with the increase in 1 receptor in different subclones of the A6 cell, including a subclone capable of anion secretion.

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.

Cisplatin up-regulates the adenosine A(1) receptor in the rat kidney

Cisplatin, a widely used anticancer drug, produces significant oto- and nephrotoxicity. Previous data from our laboratory, using cultured cell lines, indicated that cisplatin increases the expression of the adenosine A(1) receptor subtype through generation of reactive oxygen species and activation of nuclear factor-kappa B (NF-kappa B). Since the adenosine A(1) receptor plays an important role in normal renal physiology, this study was performed to determine whether cisplatin modulates adenosine A(1) receptor expression in vivo and whether these receptors play a role in the nephrotoxicity. Male Sprague-Dawley rats, treated with cisplatin (8 mg/kg), developed nephrotoxicity within 3 days, as demonstrated by increased serum creatinine and blood urea nitrogen. Cisplatin also produced a significant increase in malondialdehyde, apoptosis and necrosis in the kidney. The above changes were associated with a time-dependent increase in the expression of adenosine A(1) receptor, as determined by radioligand binding assays, Western blotting and immunocytochemistry, and an increase in adenosine A(1) receptor transcripts. Administration of selective and nonselective antagonists of the adenosine A(1) receptor produced either no change or exacerbated the nephrotoxicity produced by cisplatin. These data indicate that cisplatin can regulate the adenosine A(1) receptor in the kidney and suggest a cytoprotective role of this receptor subtype against cisplatin-induced nephrotoxicity.

Adenosine modulates Mg(2+) uptake in distal convoluted tubule cells via A(1) and A(2) purinoceptors

tk;1Adenosine plays a role in the control of water and electrolyte reabsorption in the distal tubule. As the distal convoluted tubule is important in the regulation of renal Mg(2+) balance, we determined the effects of adenosine on cellular Mg(2+) uptake in this segment. The effect of adenosine was studied on immortalized mouse distal convoluted tubule (MDCT) cells, a model of the intact distal convoluted tubule. The rate of Mg(2+) uptake was measured with fluorescence techniques using mag-fura 2. To assess Mg(2+) uptake, MDCT cells were first Mg(2+) depleted to 0.22 +/- 0.01 mM by being cultured in Mg(2+)-free media for 16 h and then placed in 1.5 mM MgCl(2); next, changes in intracellular Mg(2+) concentration ([Mg(2+)](i)) were determined. [Mg(2+)](i) returned to basal levels, 0.53 +/- 0.02 mM, with a mean refill rate, d([Mg(2+)](i))/dt, of 137 +/- 16 nM/s. Adenosine stimulates basal Mg(2+) uptake by 41 +/- 10%. The selective A(1) purinoceptor agonist N(6)-cyclopentyladenosine (CPA) increased intracellular Ca(2+) and decreased parathyroid hormone (PTH)-stimulated cAMP formation and PTH-mediated Mg(2+) uptake. On the other hand, the selective A(2) receptor agonist 2-[p-(2-carbonyl-ethyl)-phenylethylamino]-5'-N-ethylcarboxamidoadenosine (CGS) stimulated Mg(2+) entry in a concentration-dependent fashion. CGS increased cAMP formation and the protein kinase A inhibitor RpcAMPS inhibited CGS-stimulated Mg(2+) uptake. Selective inhibition of phospholipase C, protein kinase C, or mitogen-activated protein kinase enzyme cascades with U-73122, Ro-31-8220, and PD-98059, respectively, diminished A(2) agonist-mediated Mg(2+) entry. Aldosterone potentiated CGS-mediated Mg(2+) entry, and elevation of extracellular Ca(2+) diminished CGS-responsive cAMP formation and Mg(2+) uptake. Accordingly, MDCT cells possess both A(1) and A(2) purinoceptor subtypes with intracellular signaling typical of these respective receptors. We conclude that adenosine has dual effects on Mg(2+) uptake in MDCT cells through separate A(1) and A(2) purinoceptor pathways.

Adenosine inhibits the transfected Na+-H+ exchanger NHE3 in Xenopus laevis renal epithelial cells (A6/C1)

1. Adenosine influences the vectorial transport of Na+ and HCO3- across kidney epithelial cells. However, its action on effector proteins, such as the Na+-H+ exchanger NHE3, an epithelial brush border isoform of the Na+-H+ exchanger (NHE) gene family, is not yet defined. 2. The present study was conducted in Xenopus laevis distal nephron A6 epithelia which express both an apical adenosine receptor of the A1 type (coupled to protein kinase C (PKC)) and a basolateral receptor of the A2 type (coupled to protein kinase A (PKA)). The untransfected A6 cell line expresses a single NHE type (XNHE) which is restricted to the basolateral membrane and which is activated by PKA. 3. A6 cell lines were generated which express exogenous rat NHE3. Measurements of side-specific pHi recovery from acid loads in the presence of HOE694 (an inhibitor with differential potency towards individual NHE isoforms) detected an apical resistant Na+-H+ exchange only in transfected cell lines. The sensitivity of the basolateral NHE to HOE694 was unchanged, suggesting that exogenous NHE3 was restricted to the apical membrane. 4. Stimulation of the apical A1 receptor with N 6-cyclopentyladenosine (CPA) inhibited both apical NHE3 and basolateral XNHE. These effects were mimicked by the addition of the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) and partially prevented by the PKC inhibitor calphostin C which also blocked the effect of PMA. 5. Stimulation of the basolateral A2 receptor with CPA inhibited apical NHE3 and stimulated basolateral XNHE. These effects were mimicked by 8-bromo-cAMP and partially prevented by the PKA inhibitor H89 which entirely blocked the effect of 8-bromo-cAMP. 6. In conclusion, CPA inhibits rat NHE3 expressed apically in A6 epithelia via both the apical PKC-coupled A1 and the basolateral PKA-coupled A2 adenosine receptors.

Hormone-stimulated Ca2+ reabsorption in rabbit kidney cortical collecting system is cAMP-independent and involves a phorbol ester-insensitive PKC isotype

BACKGROUND

Hormones such as parathyroid hormone (PTH), arginine vasopressin (AVP), and prostaglandin E2 (PGE2) are generally believed to act through cAMP to stimulate active Ca2+ reabsorption in the distal part of the nephron.

METHODS

This study investigates the relationship between intracellular cAMP levels and the rate of Ca2+ reabsorption in immunodissected rabbit connecting and cortical collecting tubules cultured to confluence on permeable supports.

RESULTS

Basolateral PTH, AVP, and PGE2 and apical adenosine dose dependently increased Ca2+ reabsorption from 48 to 110 nmol. hr-1. cm-2. Measurement of intracellular cAMP levels revealed that in the case of PTH and AVP, the dose-response curve for the increase in cAMP virtually matched that for transcellular Ca2+ transport. By contrast, with PGE2, this curve was shifted two decades to the right, whereas in the case of adenosine, no increase in cAMP was observed. The results with the latter two hormones disagree with the classic concept that Ca2+ reabsorption is stimulated via a cAMP-dependent mechanism. Furthermore, the potent adenylyl cyclase inhibitor 2', 5'-dideoxyadenosine (DDA; 100 micrometers) suppressed the PTH- and AVP-induced increase in cAMP completely without affecting Ca2+ reabsorption. Similarly, concentrations of PGE2, which maximally stimulated Ca2+ reabsorption without increasing cAMP, were not inhibited by DDA. The specific protein kinase C (PKC) inhibitor chelerythrine (5 micrometers) inhibited PTH-, AVP-, PGE2-, and adenosine-stimulated Ca2+ reabsorption by 77%, 67%, 79%, and 100%, respectively. Down-regulation of phorbol ester-sensitive PKC isotypes by prolonged (120 hr) treatment with 0.1 micrometers 12-O-tetradecanoylphorbol 13-acetate did not interfere with the inhibitory action of chelerythrine on hormone-stimulated Ca2+ transport.

CONCLUSION

PTH, AVP, PGE2, and adenosine stimulate Ca2+ reabsorption via a pathway that is independent of cAMP and that involves a phorbol ester-insensitive PKC isotype.

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.

Adenosine-stimulated Ca2+ reabsorption is mediated by apical A1 receptors in rabbit cortical collecting system

Confluent monolayers of immunodissected rabbit connecting tubule and cortical collecting duct cells, cultured on permeable supports, were used to study the effect of adenosine on net apical-to-basolateral Ca2+ transport. Apical, but not basolateral, adenosine increased this transport dose dependently from 48 +/- 3 to 110 +/- 4 nmol.h-1.cm-2. Although a concomitant increase in cAMP formation suggested the involvement of an A2 receptor, the A2 agonist CGS-21680 did not stimulate Ca2+ transport, while readily increasing cAMP. By contrast, the A1 agonist N6-cyclopentyladenosine (CPA) maximally stimulated Ca2+ transport without significantly affecting cAMP. Adenosine-stimulated transport was effectively inhibited by the A1 antagonist 1,3-dipropyl-8-cyclopenthylxanthine but not the A2 antagonist 3,7-dimethyl-1-propargylxanthine, providing additional evidence for the involvement of an A1 receptor. Both abolishment of the adenosine-induced transient increase in intracellular Ca2+ concentration by 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid and downregulation of protein kinase C (PKC) by prolonged phorbol ester treatment were without effect on adenosine-stimulated Ca2+ transport. The data presented suggest that adenosine interacts with an apical A1 receptor to stimulate Ca2+ transport via a hitherto unknown pathway that does not involve cAMP formation, PKC activation, and/or Ca2+ mobilization.

Cross talk of bumetanide-sensitive and HCO3--dependent transporters activated by IBMX in renal epithelial A6 cells

We studied cAMP-dependent regulation of ion transport in aldosterone-untreated renal epithelial A6 cells by measuring short-circuit current (Isc). Biphasic increases in Isc, a transient phase followed by a sustained one, were elicited in response to 1 mm 3-isobutyl-1-methylxanthine (IBMX, an inhibitor of phosphodiesterase) which increased cytosolic cAMP concentration. IBMX increased the apical Cl- conductance. The sustained phase of Isc induced by IBMX was reduced by 50 microM bumetanide (Na+/K+/2 Cl- cotransporter inhibitor) or 100 microM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS, an inhibitor of Cl-/HCO3- exchanger). Under the normal condition, the inhibitory effect of bumetanide was much larger than that of DIDS. On the other hand, under a low Cl- condition, the effect of DIDS was more effective than that of bumetanide. Further, under a Cl--free condition Na+/HCO3- symporter contributed to the IBMX-generated Isc. Taken together, our observations suggest that in A6 cells (i) IBMX stimulates Cl- secretion associated with an increase in apical Cl- conductances, (ii) the ionic components to generate the IBMX-induced Isc are mainly maintained by bumetanide-sensitive Na+/K+/2 Cl- cotransporter and DIDS-sensitive Cl-/HCO3- exchanger, (iii) Cl-/HCO3- exchanger coupled to Na+/HCO3- symporter under a low-Cl- condition or Na+/HCO3- symporter under a Cl--free condition contributes to the IBMX-induced Isc, compensating for diminishment of the Na+/K+/2Cl- cotransporter-mediated Cl- secretion, (iv) IBMX increases Cl- and HCO3- conductances in the apical membrane.

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)

The effects of adenosine on transepithelial resistance and sodium uptake in the inner medullary collecting duct

It has been previously demonstrated that adenosine induces natriuresis when administered directly into the renal circulation of the rat. It was postulated that the mechanism was inhibition of tubule Na+ reabsorption. In the current study, the hypothesis was tested that adenosine inhibits ion reabsorption across the inner medullary collecting duct (IMCD), a tubule segment which is rich in adenosine receptors. IMCD epithelium from rat kidney was grown in primary culture as a confluent monolayer on Costar filters, allowing selective access to the basolateral and apical surfaces of the cells. Transepithelial resistance was taken as a measure of epithelial permeability and ion conductance. Na+ uptake was studied using 22Na+ and used to determine the permeability of the epithelial monolayer specifically to Na+. Exposure of the basolateral aspect of the monolayer to adenosine (10(-8)-10(-7) M) increased transepithelial resistance in a dose- and time-dependent manner; in parallel, adenosine (10(-7)-10(-6) M) reduced apical Na+ uptake from 20 +/- 5 to 10 +/- 2 nmol/cm2. 1,3-Dipropyl-8-(2-amino-4-chlorophenyl)-xanthine (PACPX, 5 x 10(-9) M), an adenosine antagonist with selectivity for the A1 receptor, inhibited the rise in transepithelial resistance and the decrease in Na+ uptake following the addition of adenosine. The effects of adenosine on transepithelial resistance were reproduced with the A1 receptor selective adenosine analogue N6-cyclohexyladenosine (CHA, 10(-8)-10(-7) M) but not with the A2 selective analogues, 5'-N-ethylcarboxamidoadenosine (NECA) or CGS 21680. CHA (10(-7) M) inhibited apical Na+ uptake by 50%, an effect abolished by PACPX. (ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of brefeldin A on hormonally regulated Na+ transport in a model renal epithelial cell line

Na+ transport in renal epithelia is regulated by a wide variety of endogenous and exogenous cellular factors. Although most natriferic agents have an action on the amiloride-sensitive Na+ channel, the biochemical pathways which precede activation of the channel remain incompletely defined. One approach to dissecting such intricate pathways is to perturb a specific cellular process and determine its importance in the postulated mechanism. The current studies examine the effect of brefeldin A (BFA), an inhibitor of the central vacuolar system, on basal as well as aldosterone-, insulin-, and forskolin-stimulated Na+ transport. In the A6 cell line, BFA had a time-dependent effect on basal transport. Aldosterone-induced Na+ transport was sensitive to BFA while insulin's action was only partially blocked and forskolin-stimulated Na+ transport was relatively resistant to the action of the inhibitor. These studies highlight differences as well as points of convergence in the natriferic pathways.

Cloning of an adenosine A1 receptor-encoding gene from rabbit

A partial cDNA encoding the A1 adenosine receptor (A1AR), which lacks nucleotides coding for the first 74 amino acids (aa), was isolated from a rabbit kidney cDNA library. The missing 5' end sequence was obtained from an overlapping rabbit genomic clone which was found to contain the flanking 5' untranslated region (5'UTR), the first exon and part of the first intron. Together, the cDNA and genomic clones provide the entire open reading frame (ORF) encoding rabbit A1AR. The deduced aa sequence is highly homologous to the canine, rat and bovine A1ARs. These data also indicate that the A1AR gene belongs to the family of intron-containing G-protein-linked receptor genes.

Na+/H(+)-exchange in A6 cells: polarity and vasopressin regulation

We have analyzed the mechanism of Na(+)-dependent pHi recovery from an acid load in A6 cells (an amphibian distal nephron cell line) by using the intracellular pH indicator 2'7'-bis(2-carboxyethyl)5,6 carboxyfluorescein (BCECF) and single cell microspectrofluorometry. A6 cells were found to express Na+/H(+)-exchange activity only on the basolateral membrane: Na+/H(+)-exchange activity follows simple saturation kinetics with an apparent Km for Na+ of approximately 11 mM; it is inhibited in a competitive manner by ethylisopropylamiloride (EIPA). This Na+/H(+)-exchange activity is inhibited by pharmacological activation of protein kinase A (PKA) as well as of protein kinase C (PKC). Addition of arginine vasopressin (AVP) either at low (subnanomolar) or at high (micromolar) concentrations inhibits Na+/H(+)-exchange activity; AVP stimulates IP3 production at low concentrations, whereas much higher concentrations are required to stimulate cAMP formation. These findings suggest that in A6 cells (i) Na+/H(+)-exchange is located in the basolateral membrane and (ii) PKC activation (heralded by IP3 turnover) is likely to be the mediator of AVP action at low AVP concentrations.

Identification of a regulated Na/K/Cl cotransport system in a distal nephron cell line

Lack of an adequate cell model has limited investigation of Na/K/Cl cotransporter regulation in the kidney. Using A6 cells, an amphibian distal renal cell line, we observed that 63% of rubidium uptake in confluent A6 monolayers was ouabain-insensitive. Ouabain-insensitive rubidium uptake was inhibited in a dose-dependent fashion by furosemide (IC50 6.6 microM) or bumetanide (IC50 1.7 microM). Kinetic studies confirmed that furosemide-sensitive rubidium uptake had features consistent with cotransporter activity in other cell lines. Furthermore, specific binding of [3H]bumetanide occurred with a capacity of 8.6 pmol/mg protein and a Kd of 1.6 microM bumetanide. Finally, furosemide-sensitive rubidium uptake was rapidly regulated by a calcium ionophore, the phorbol ester PDBu, forskolin, and adenosine. These data demonstrate an Na/K/Cl cotransport system in the A6 cell which will serve as a useful model for studying cotransporter regulation by endogenous signaling pathways.

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.

Novel therapeutics acting via purine receptors

A recent conference entitled Purines in Cell Signalling: Targets for New Drugs, held in Rockville, Maryland, in September, 1989, was one indication of the increasing interest in developing agonists and antagonists of P₁-(adenosine) and P₂-(ATP) purinoceptors [1] as potential therapeutic agents. Extracellular adenosine, acting at its membrane bound A₁ and A₂ receptors, is a ubiquitous modulator of cellular activity. The purine can arise from several sources including ATP hydrolysis by ectokinase activity in the region of the nerve terminal [2] and from S-adenosylhomocysteine [3] and ATP within the cell. Together with its more stable analogs, adenosine is a potent inhibitor of neurotransmitter release in both the central and peripheral nervous systems, and in cardiac, adipose and other tissues. Adenosine can also affect blood pressure and heart rate as well as modulate the function of the immune, inflammatory, gastrointestinal, renal and pulmonary systems, either via its effects on transmitter release or directly via receptor mechanisms altering intracellular transduction processes.

ATP receptor regulation of adenylate cyclase and protein kinase C activity in cultured renal LLC-PK1 cells

In cultured intact LLC-PK1 renal epithelial cells, a nonhydrolyzable ATP analogue, ATP gamma S, inhibits AVP-stimulated cAMP formation. In LLC-PK1 membranes, several ATP analogues inhibit basal, GTP-, forskolin-, and AVP-stimulated adenylate cyclase activity in a dose-dependent manner. The rank order potency of inhibition by ATP analogues suggests that a P2y type of ATP receptor is involved in this inhibition. The compound ATP gamma S inhibits agonist-stimulated adenylate cyclase activity in solubilized and in isobutylmethylxanthine (IBMX) and quinacrine pretreated membranes, suggesting that ATP gamma S inhibition occurs independent of AVP and A1 adenosine receptors and of phospholipase A2 activity. The ATP gamma S inhibition of AVP-stimulated adenylate cyclase activity is not affected by pertussis toxin but is attenuated by GDP beta S, suggesting a possible role for a pertussis toxin insensitive G protein in the inhibition. Exposure of intact LLC-PK cells to ATP gamma S results in a significant increase in protein kinase C activity. However, neither of two protein kinase C inhibitors (staurosporine and H-7) prevents ATP gamma S inhibition of AVP-stimulated adenylate cyclase activity, suggesting that this inhibition occurs by a protein kinase C independent mechanism. These findings suggest the presence of functional P2y purinoceptors coupled to two signal transduction pathways in cultured renal epithelial cells. The effect of P2y purinoceptors to inhibit AVP-stimulated adenylate cyclase activity may be mediated, at least in part, by a pertussis toxin insensitive G protein.

The stepwise hydrolysis of adenine nucleotides by ectoenzymes of rat renal brush-border membranes

Evidence is presented for the existence of ectoenzymes in rat renal cortical brush-border membrane vesicles that produce adenosine as a final product using either ATP, ADP or AMP as substrate. The enzymes are insensitive to levamisole, ouabain, oligomycin and N-ethylmaleimide, and have absolute requirement for divalent cations with following order of activation Mg2+ greater than Ca2+ greater than Mn2+ greater than Ba2+ greater than Zn2+. At least two separate enzymes can be distinguished. One is capable of hydrolyzing ATP, other nucleoside triphosphates and ADP, but not AMP. The enzyme is insensitive to concanavalin A. The other enzyme hydrolyzes AMP and is strongly inhibited by this lectin. Mg2(+)-stimulated ATP hydrolysis displays saturation kinetics which is not of the simple Michaelis-Menten type, but is biphasic with a high-affinity (K'm = 0.16 mM) and low-affinity site (K'm = 9.0 mM), respectively. The low-affinity site hydrolyzes ATP, ITP and GTP to a similar extent, whereas CTP and UTP with about 40% lower rate. The high-affinity site splits ATP much better than other nucleoside triphosphates. Hydrolysis of ADP follows simple Michaelis-Menten saturation kinetic with apparent Km = 0.38 +/- 0.06 mM. Inhibition, activation and substrate specificity studies indicate that nucleoside triphosphatase and nucleoside diphosphatase may reside on the same protein. Kinetics of the AMP hydrolysis is hyperbolic with apparent Km = 76 +/- 9 microM. The cascade of ectonucleotidases in the brush-border membrane of the proximal tubule may catalyze the degradation of filtered nucleotides into adenosine and phosphate, the compounds which are thereafter probably reabsorbed by separate transport systems.

Amiloride-sensitive Na+ transport across cultured renal (A6) epithelium: evidence for large currents and high Na:K selectivity

Electrical techniques were used to determine the Na:K selectivity of the amiloride-sensitive pathway and to characterize cellular and paracellular properties of A6 epithelium. Under control conditions, the mean transepithelial voltage (VT) was -57 +/- 5 mV, the short-circuit current (Isc) averaged 23 +/- 2 microA/cm2 and the transepithelial resistance (RT) was 2.8 +/- 0.3 k omega cm2 (n = 13). VT and Isc were larger than reported in previous studies and were increased by aldosterone. The conductance of the amiloride-sensitive pathway (Gamil) was assessed before and after replacement of Na+ in the mucosal bath by K+, using two independent measurements: (1) the slope conductance (GT), determined from current-voltage (I-V) relationships for control and amiloride-treated tissues and (2) the maximum amiloride-sensitive conductance (Gmax) calculated from the amiloride dose-response relationship. The ratio of Gamil in mucosal Na+ solutions to Gamil for mucosal K+ solutions was 22 +/- 6 for GT measurements and 15 +/- 2 for Gmax data. Serosal ion replacements in tissues treated with mucosal nystatin indicated a potassium conductance in the basolateral membrane. Equivalent circuit analyses of nystatin and amiloride data were used to resolve the cellular (Ec) and paracellular (Rj) resistances (approximately 5 k omega cm2 and 8-9 k omega cm2, respectively). Analysis of I-V relationships for tissues depolarized with serosal K+ solutions revealed that the amiloride-sensitive pathway could be described as a Na+ conductance with a permeability coefficient (PNa) = 1.5 +/- 0.2 x 10(-6) cm/s and the intracellular Na+ concentration (Nai) = 5 +/- 1 mM (n = 5), similar to values from other tight epithelia. We conclude that A6 epithelia are capable of expressing large amiloride-sensitive currents which are highly Na+ selective.

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.

Stimulation of adenylate cyclase activity via A2-adenosine receptors in isolated tubules of the rabbit renal cortex

Adenylate cyclase activity in a tubular fraction obtained from rabbit renal cortex was stimulated by typical adenosine receptor agonists with a rank order of potency NECA (5'-(N-ethyl-carboxamido)-adenosine) (EC50 = 0.48 mumol/l) greater than R-PIA [(-)N6 (R-phenylisopropyl)-adenosine] (3.22 mumol/l). The stimulatory effect of NECA was competitively antagonized by 8-phenyltheophylline. Contamination of the tubular fraction with glomeruli and microvessels was less than 2%, as verified by tissue renin determination and could, therefore, be ruled out as being responsible for the observed effect. Tubular A2-adenosine receptors are probably involved in the control of renal electrolyte secretion and may represent the site of action of methylxanthines.

Glomeruli and microvessels of the rabbit kidney contain both A1- and A2-adenosine receptors

Rabbit renal cortices were fractionated by collagenase dispersion and glomeruli, microvessels and tubuli purified on a discontinuous sucrose gradient. Binding experiments with (-)[125I]N6-(4-hydroxyphenylisopropyl)-adenosine ([125I]HPIA) provided evidence for the presence of A1-adenosine receptors in the glomerular and microvascular fraction. With glomeruli, saturation isotherms for specific [125I]HPIA binding were mono-phasic with a KD of 1.3 nmol/l and a Bmax of 7.7 fmol/mg protein. In kinetic experiments, an association rate constant of 4.9 X 10(5) (mol/l-1 s-1 and a dissociation rate constant of 4.3 X 10(-4) s-1 were obtained, yielding a KD of 0.9 nmol/l. Adenosine analogs displaced [125I]HPIA binding with a rank order of potency typical of A1-adenosine receptors; furthermore, binding was inhibited by methylxanthines and modulated by GTP. Saturation experiments with the microvessels revealed a KD of 1.9 nmol/l and a Bmax of 13.4 fmol/mg protein. However, no inhibition of glomerular and microvascular adenylate cyclase activity could be demonstrated, but instead both 5'-N-ethylcarboxamido-adenosine (NECA) and N6-(R-phenylisopropyl)-adenosine (R-PIA) stimulated enzyme activity, with EC50 values of 0.14 mumol/l and 1.5 mumol/l, respectively. The concentration-response curve for NECA was shifted to the right (factor 9) by 10 mumol/l 8-phenyltheophylline. On the other hand, computer simulation of biphasic curves (adenylate cyclase inhibition in the presence of activation via a stimulatory receptor) indicates that the failure to observe an A1-adenosine receptor-mediated inhibition of adenylate cyclase activity in the presence of stimulatory adenosine receptors may be attributable to methodological constraints.(ABSTRACT TRUNCATED AT 250 WORDS)

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.