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

Adenosine inhibits the basolateral Cl- ClC-K2/b channel in collecting duct intercalated cells

Adenosine plays an important role in various aspects of kidney physiology, but the specific targets and mechanisms of actions are not completely understood. The collecting duct has the highest expression of adenosine receptors, particularly adenosine A₁ receptors (A₁Rs). Interstitial adenosine levels are greatly increased up to a micromolar range in response to dietary salt loading. We have previously shown that the basolateral membrane of principal cells has primarily K⁺ conductance mediated by K_ir4.1/5.1 channels to mediate K⁺ recycling and to set up a favorable driving force for Na⁺/K⁺ exchange (47). Intercalated cells express the Cl^- ClC-K2/b channel mediating transcellular Cl^- reabsorption. Using patch-clamp electrophysiology in freshly isolated mouse collecting ducts, we found that acute application of adenosine reversely inhibits ClC-K2/b open probability from 0.31 ± 0.04 to 0.17 ± 0.06 and to 0.10 ± 0.05 for 1 and 10 µM, respectively. In contrast, adenosine (10 µM) had no measureable effect on K_ir4.1/5.1 channel activity in principal cells. The inhibitory effect of adenosine on ClC-K2/b was abolished in the presence of the A₁R blocker 8-cyclopentyl-1,3-dipropylxanthine (10 µM). Consistently, application of the A₁R agonist N⁶-cyclohexyladenosine (1 µM) recapitulated the inhibitory action of adenosine on ClC-K2/b open probability. The effects of adenosine signaling in the collecting duct were independent from its purinergic counterpartner, ATP, having no measurable actions on ClC-K2/b and K_ir4.1/5.1. Overall, we demonstrated that adenosine selectively inhibits ClC-K2/b activity in intercalated cells by targeting A₁Rs. We propose that inhibition of transcellular Cl^- reabsorption in the collecting duct by adenosine would aid in augmenting NaCl excretion during high salt intake.

Renal Autocrine and Paracrine Signaling: A Story of Self-protection

Autocrine and paracrine signaling in the kidney adds an extra level of diversity and complexity to renal physiology. The extensive scientific production on the topic precludes easy understanding of the fundamental purpose of the vast number of molecules and systems that influence the renal function. This systematic review provides the broader pen strokes for a collected image of renal paracrine signaling. First, we recapitulate the essence of each paracrine system one by one. Thereafter the single components are merged into an overarching physiological concept. The presented survey shows that despite the diversity in the web of paracrine factors, the collected effect on renal function may not be complicated after all. In essence, paracrine activation provides an intelligent system that perceives minor perturbations and reacts with a coordinated and integrated tissue response that relieves the work load from the renal epithelia and favors diuresis and natriuresis. We suggest that the overall function of paracrine signaling is reno-protection and argue that renal paracrine signaling and self-regulation are two sides of the same coin. Thus local paracrine signaling is an intrinsic function of the kidney, and the overall renal effect of changes in blood pressure, volume load, and systemic hormones will always be tinted by its paracrine status.

Intrarenal purinergic signaling in the control of renal tubular transport

Renal tubular epithelial cells receive hormonal input that regulates volume and electrolyte homeostasis. In addition, numerous intrarenal, local signaling agonists have appeared on the stage of renal physiology. One such system is that of intrarenal purinergic signaling. This system involves all the elements necessary for agonist-mediated intercellular communication. ATP is released from epithelial cells, which activates P2 receptors in the apical and basolateral membrane and thereby modulates tubular transport. Termination of the signal is conducted via the breakdown of ATP to adenosine. Recent far-reaching advances indicate that ATP is often used as a local transmitter for classical sensory transduction. This transmission apparently also applies to sensory functions in the kidney. Locally released ATP is involved in sensing of renal tubular flow or in detecting the distal tubular load of NaCl at the macula densa. This review describes the relevant aspects of local, intrarenal purinergic signaling and outlines its integrative concepts.

Ectonucleotidases in the kidney

Members of all four families of ectonucleotidases, namely ectonucleoside triphosphate diphosphohydrolases (NTPDases), ectonucleotide pyrophosphatase/phosphodiesterases (NPPs), ecto-5'-nucleotidase and alkaline phosphatases, have been identified in the renal vasculature and/or tubular structures. In rats and mice, NTPDase1, which hydrolyses ATP through to AMP, is prominent throughout most of the renal vasculature and is also present in the thin ascending limb of Henle and medullary collecting duct. NTPDase2 and NTPDase3, which both prefer ATP over ADP as a substrate, are found in most nephron segments beyond the proximal tubule. NPPs catalyse not only the hydrolysis of ATP and ADP, but also of diadenosine polyphosphates. NPP1 has been identified in proximal and distal tubules of the mouse, while NPP3 is expressed in the rat glomerulus and pars recta, but not in more distal segments. Ecto-5'-nucleotidase, which catalyses the conversion of AMP to adenosine, is found in apical membranes of rat proximal convoluted tubule and intercalated cells of the distal nephron, as well as in the peritubular space. Finally, an alkaline phosphatase, which can theoretically catalyse the entire hydrolysis chain from nucleoside triphosphate to nucleoside, has been identified in apical membranes of rat proximal tubules; however, this enzyme exhibits relatively high K (m) values for adenine nucleotides. Although information on renal ectonucleotidases is still incomplete, the enzymes' varied distribution in the vasculature and along the nephron suggests that they can profoundly influence purinoceptor activity through the hydrolysis, and generation, of agonists of the various purinoceptor subtypes. This review provides an update on renal ectonucleotidases and speculates on the functional significance of these enzymes in terms of glomerular and tubular physiology and pathophysiology.

Adenosine receptors and the kidney

The autacoid, adenosine, is present in the normoxic kidney and generated in the cytosol as well as at extracellular sites. The rate of adenosine formation is enhanced when the rate of ATP hydrolysis prevails over the rate of ATP synthesis during increased tubular transport work or during oxygen deficiency. Extracellular adenosine acts on adenosine receptor subtypes (A(1), A(2A), A(2B), and A(3)) in the cell membranes to affect vascular and tubular functions. Adenosine lowers glomerular filtration rate by constricting afferent arterioles, especially in superficial nephrons, and thus lowers the salt load and transport work of the kidney consistent with the concept of metabolic control of organ function. In contrast, it leads to vasodilation in the deep cortex and the semihypoxic medulla, and exerts differential effects on NaCl transport along the tubular and collecting duct system. These vascular and tubular effects point to a prominent role of adenosine and its receptors in the intrarenal metabolic regulation of kidney function, and, together with its role in inflammatory processes, form the basis for potential therapeutic approaches in radiocontrast media-induced acute renal failure, ischemia reperfusion injury, and in patients with cardiorenal failure.

SLC28 genes and concentrative nucleoside transporter (CNT) proteins

The human concentrative nucleoside transporter (hCNT) protein family has three members, hCNT1, 2, and 3, encoded by SLC28A1, A2, and A3 genes, respectively. hCNT1 and hCNT2 translocate pyrimidine- and purine-nucleosides, respectively, by a sodium-dependent mechanism, whereas hCNT3 shows broad substrate selectivity and the unique ability of translocating nucleosides both in a sodium- and a proton-coupled manner. hCNT proteins are also responsible for the uptake of most nucleoside-derived antiviral and anticancer drugs. Thus, hCNTs are key pharmacological targets. This review focuses on several crucial aspects of hCNT biology and pharmacology: protein structure-function, structural determinants for transportability, pharmacogenetics of hCNT-encoding genes, role of hCNT proteins in nucleoside-based therapeutics, and finally hCNT physiology.

Adenosine and renal tubular function

PURPOSE OF REVIEW

Intrarenal adenosine is present in the cytoplasm of renal epithelial cells and in the extracellular space. Adenosine is generated at high levels in response to imbalance between energy demand and supply (e.g. increased tubular sodium chloride transport or hypoxia) and activates cell membrane adenosine receptors to affect renal vascular and tubular functions. Adenosine regulates renal sodium and water excretion via a myriad of effects on renal hemodynamic, glomerular filtration rate, renin secretion and direct effects on the renal tubule epithelium. This review examines the direct effects of adenosine on renal tubular epithelial transport in light of the most recent evidence and discusses some physiologic and pathophysiologic implications.

RECENT FINDINGS

Intrarenal adenosine affects proximal fluid and solute transport in a biphasic fashion. Under physiological conditions adenosine stimulates proximal tubular re-absorption, thus reducing the load delivered to the distal nephron. A supra-physiologic increase in adenosine such as in ischemia reduces reabsorption in the proximal tubule, thus reducing renal oxygen consumption.

SUMMARY

Intrarenal adenosine and its receptors have important regulatory functions in the renal epithelium. A complete understanding of this autocrine/paracrine system holds great potential for novel therapeutic strategies, such as the use of nucleoside analogues for reno-protection in renal ischemia.

Adenosine and kidney function: potential implications in patients with heart failure

Therapy of heart failure is more difficult when renal function is impaired. Here, we outline the effects on kidney function of the autacoid, adenosine, which forms the basis for adenosine A(1) receptor (A(1)R) antagonists as treatment for decompensated heart failure. A(1)R antagonists induce a eukaliuretic natriuresis and diuresis by blocking A(1)R-mediated NaCl reabsorption in the proximal tubule and the collecting duct. Normally, suppressing proximal reabsorption will lower glomerular filtration rate (GFR) through the tubuloglomerular feedback mechanism (TGF). But the TGF response, itself, is mediated by A(1)R in the preglomerular arteriole, so blocking A(1)R allows natriuresis to proceed while GFR remains constant or increases. The influence of A(1)R over vascular resistance in the kidney is augmented by angiotensin II while A(1)R activation directly suppresses renin secretion. These interactions could modulate the overall impact of A(1)R blockade on kidney function in patients taking angiotensin II blockers. A(1)R blockers may increase the energy utilized for transport in the semi-hypoxic medullary thick ascending limb, an effect that could be prevented with loop diuretics. Finally, while the vasodilatory effect of A(1)R blockade could protect against renal ischaemia, A(1)R blockade may act on non-resident cells to exacerbate reperfusion injury, where ischaemia to occur. Despite these uncertainties, the available data on A(1)R antagonist therapy in patients with decompensated heart failure are promising and warrant confirmation in further studies.

P2 receptors in the regulation of renal transport mechanisms

Extracellular nucleotides (e.g., ATP) regulate physiological and pathophysiological processes through activation of nucleotide P2 receptors in the plasma membrane. Examples include such diverse processes as communication from taste buds to gustatory nerves, platelet aggregation, nociception, or neutrophil chemotaxis. Over approximately the last 15 years, evidence has also accumulated that cells in renal epithelia release nucleotides in response to physiological stimuli and that these nucleotides act in a paracrine and autocrine way to activate P2 receptors and play a significant role in the regulation of transport mechanisms and cell volume regulation. This review discusses potential stimuli and mechanisms involved in nucleotide release in renal epithelia and summarizes the available data on the expression and function of nucleotide P2 receptors along the native mammalian tubular and collecting duct system. Using established agonist profiles for P2 receptor subtypes, significant insights have been gained particularly into a potential role for P2Y(2)-like receptors in the regulation of transport mechanisms in the collecting duct. Due to the lack of receptor subtype-specific antagonists, however, the in vivo relevance of P2 receptor subtypes is unclear. Studies in gene knockout mice provided first insights including an antihypertensive activity of P2Y(2) receptors that is linked to an inhibitory influence on renal Na(+) and water reabsorption. We are only beginning to unravel the important roles of extracellular nucleotides and P2 receptors in the regulation of the diverse transport mechanisms of the kidney.

Adenosine and Kidney Function

In this review we outline the unique effects of the autacoid adenosine in the kidney. Adenosine is present in the cytosol of renal cells and in the extracellular space of normoxic kidneys. Extracellular adenosine can derive from cellular adenosine release or extracellular breakdown of ATP, AMP, or cAMP. It is generated at enhanced rates when tubular NaCl reabsorption and thus transport work increase or when hypoxia is induced. Extracellular adenosine acts on adenosine receptor subtypes in the cell membranes to affect vascular and tubular functions. Adenosine lowers glomerular filtration rate (GFR) by constricting afferent arterioles, especially in superficial nephrons, and acts as a mediator of the tubuloglomerular feedback, i.e., a mechanism that coordinates GFR and tubular transport. In contrast, it leads to vasodilation in deep cortex and medulla. Moreover, adenosine tonically inhibits the renal release of renin and stimulates NaCl transport in the cortical proximal tubule but inhibits it in medullary segments including the medullary thick ascending limb. These differential effects of adenosine are subsequently analyzed in a more integrative way in the context of intrarenal metabolic regulation of kidney function, and potential pathophysiological consequences are outlined.

Immunolocalization of ectonucleotidases along the rat nephron

Evidence is accumulating that extracellular nucleotides act as autocrine/paracrine agents in most tissues, including the kidneys. Several families of surface-located enzymes, collectively known as ectonucleotidases, can degrade nucleotides. Using immunohistochemistry, we have examined the segmental distribution of five ectonucleotidases along the rat nephron. Perfusion-fixed kidneys were obtained from anesthetized male Sprague-Dawley rats. Cryostat sections of cortical and medullary regions were incubated with antibodies specific to the following enzymes: ectonucleoside triphosphate diphosphohydrolase (NTPDase) 1, NTPDase2, NTPDase3, ectonucleotide pyrophosphatase phosphodiesterase 3 (NPP3), and ecto-5'-nucleotidase. Sections were then costained with Phaseolus vulgaris erythroagglutinin (for identification of proximal tubules) and antibodies against Tamm-Horsfall protein (for identification of thick ascending limb), calbindin-D(28k) (for identification of distal tubule), and aquaporin-2 (for identification of collecting duct). The tyramide signal amplification method was used when the ectonucleotidase and marker antibody were raised in the same species. The glomerulus expressed NTPDase1 and NPP3. The proximal tubule showed prominent expression of NPP3 and ecto-5'-nucleotidase in most, but not all, segments. NTPDase2 and NTPDase3, but not NPP3 or ecto-5'-nucleotidase, were expressed in the thick ascending limb and distal tubule. NTPDase3, with some low-level expression of ecto-5'-nucleotidase, was also found in cortical and outer medullary collecting ducts. Inner medullary collecting ducts displayed low-level staining for NTPDase1, NTPDase2, NTPDase3, and ecto-5'-nucleotidase. We conclude that these ectonucleotidases are differentially expressed along the nephron and may play a key role in activation of purinoceptors by nucleotides and nucleosides.

Expression of concentrative nucleoside transporters SLC28 (CNT1, CNT2, and CNT3) along the rat nephron: Effect of diabetes

BACKGROUND

The renal reabsorption of natural nucleosides and a variety of nucleoside-derived drugs relies on the function of the apically located, Na(+)-dependent, concentrative nucleoside transporters CNT1, CNT2, and CNT3 (SLC28A1, SLC28A2, and SLC28A3). The aims of this study were to determine the segmental localization of the three SLC28 family members and to establish whether streptozotocin-induced diabetes alters their expression.

METHODS

SLC28 expression was measured by real-time polymerase chain reaction (PCR) on microdissected sections of rat nephrons. Diabetes was induced by streptozotocin treatment and the biochemical profiles of control, diabetic, and insulin-treated rats were established. The effect of diabetes on SLC28 expression was assessed in those segments that significantly express SLC28 genes.

RESULTS

CNT1-3 mRNAs were expressed in the proximal tubule and glomerulus. In addition, CNT2 and CNT3 mRNAs were expressed in the outer medullary and cortical collecting duct, respectively. Diabetes reduced expression of the three CNTs in almost all nephron segments, and this effect was not prevented by an insulin treatment that normalized all blood and urine parameters. Diabetes increased CNT1 and CNT3 expression in the glomerulus and insulin treatment decreased it.

CONCLUSION

The relative distribution of SLC28 gene expression suggests a role for the proximal tubule in renal nucleoside clearance and an accessory role for CNT2 and CNT3, in adenosine-mediated regulation of collecting duct functions. Diabetes probably may impair nucleoside clearance independently of insulin.

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.

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.

Adenosine type 1 receptor antagonists in fluid retaining disorders

Adenosine is a vasoactive hormone whose action is mediated through at least four receptors. The most prevalent receptors are type 1, which promote vasoconstriction, and type 2, comprised of 2 subtypes (a,b) that promote vasodilation. In the kidney, type 1 receptors located on preglomerular vessels and in the tubule are involved in the regulation of glomerular filtration. Whole body fluid balance is strongly dependent on the ability of the kidney to maintain stable glomerular filtration. Several antagonists to adenosine type 1 receptors have been developed. These agents generate excess fluid (diuresis) and sodium (natriuresis) excretion in control animals and animal models of fluid retention, as well as in normal and oedematous humans. In both animals and humans, these effects are generally achieved without major changes in glomerular filtration. Animal studies have confirmed the location of adenosine type 1 receptors in relevant tissue sites in the kidney. More highly selective antagonists for adenosine type 1 receptors are regularly developed, improving their use in fluid retaining disorders. Clinical trials with these agents have commenced for the treatment of hypertension, renal failure and congestive heart failure, all disorders that include varying levels of fluid retention. The clinical trial results have been mixed. The early results with congestive heart failure suggest great promise for these agents, whereas trials in hypertension and renal failure have been equivocal.

Molecular aspects of acute inhibition of Na(+)-H(+) exchanger NHE3 by A(2)-adenosine receptor agonists

Adenosine regulates Na(+) homeostasis by its acute effects on renal Na(+) transport. We have shown in heterologously transfected A6/C1 cells (renal cell line from Xenopus laevis) that adenosine-induced natriuresis may be effected partly via A(2) adenosine receptor-mediated inactivation of the renal brush border membrane Na(+)-H(+) exchanger NHE3. In this study we utilized A6/C1 cells stably expressing wild-type as well as mutated forms of NHE3 to assess the molecular mechanism underlying A(2)-dependent control of NHE3 function. Cell surface biotinylation combined with immunoprecipitation revealed that NHE3 is targeted exclusively to the apical domain and that the endogenous Xenopus NHE is located entirely on the basolateral side of A6/C1 transfectants. Stimulation of A(2)-adenosine receptors located on the basolateral side for 15 min with CPA (N6-cyclopentyladenosine) acutely decreased NHE3 activity (microspectrofluorimety). This effect was mimicked by 8-bromo-cAMP and entirely blocked by pharmacological inhibition of PKA (with H89) or singular substitution of two PKA target sites (serine 552 and serine 605) on NHE3. Downregulation of NHE3 activity by CPA was attributable to a reduction of NHE3 intrinsic transport activity without change in surface NHE3 protein at 15 min. At 30 min, the decrease in transport activity was associated with a decrease in apical membrane NHE3 antigen. In conclusion, two highly conserved target serine sites on NHE3 determine NHE3 modulation upon A(2)-receptor activation and NHE3 inactivation by adenosine proceeds via two phases with distinct mechanisms.

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.

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.

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.

Receptors coupled to pertussis toxin-sensitive G-proteins traffic to opposite surfaces in Madin-Darby canine kidney cells. A1 adenosine receptors achieve apical and alpha 2A adrenergic receptors achieve basolateral localization

The alpha 2A adrenergic receptor (alpha 2AAR) previously was shown to be directly delivered to and retained on the lateral subdomain of renal epithelial cells. The present studies demonstrate that, in contrast, wild-type and epitope-tagged canine A1 adenosine receptors (A1AdoR) are apically enriched (65-83%) in Madin-Darby canine kidney (MDCKII) and porcine renal epithelial (LLC-PKI) cells, based on surface biotinylation strategies detecting photoaffinity-labeled A1AdoR. Confocal microscopy corroborated the apical enrichment of the epitopetagged A1AdoR. Metabolic labeling studies revealed that this steady-state polarization is achieved by direct delivery to both the apical (60-75%) and basolateral surface. Growth of A1AdoR-expressing cells as monolayers presence of A1AdoR antagonists, which decreased cell growth, suggesting that A1AdoR elicit MDCKII cell proliferation. The preferential apical but detectable basolateral localization of A1AdoR provides a molecular understanding of published reports that functional responses can be elicited following apical as well as basolateral delivery of adenosine agonists in varying renal preparations. These findings also suggest that receptor chimeras derived from the Gi/Go-protein-coupled alpha 2AAR and A1AdoR will be informative in revealing structural features critical for basolateral versus apical targeting.