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

The ATP-Releasing Maxi-Cl Channel: Its Identity, Molecular Partners and Physiological/Pathophysiological Implications

The Maxi-Cl phenotype accounts for the majority (app. 60%) of reports on the large-conductance maxi-anion channels (MACs) and has been detected in almost every type of cell, including placenta, endothelium, lymphocyte, cardiac myocyte, neuron, and glial cells, and in cells originating from humans to frogs. A unitary conductance of 300-400 pS, linear current-to-voltage relationship, relatively high anion-to-cation selectivity, bell-shaped voltage dependency, and sensitivity to extracellular gadolinium are biophysical and pharmacological hallmarks of the Maxi-Cl channel. Its identification as a complex with SLCO2A1 as a core pore-forming component and two auxiliary regulatory proteins, annexin A2 and S100A10 (p11), explains the activation mechanism as Tyr23 dephosphorylation at ANXA2 in parallel with calcium binding at S100A10. In the resting state, SLCO2A1 functions as a prostaglandin transporter whereas upon activation it turns to an anion channel. As an efficient pathway for chloride, Maxi-Cl is implicated in a number of physiologically and pathophysiologically important processes, such as cell volume regulation, fluid secretion, apoptosis, and charge transfer. Maxi-Cl is permeable for ATP and other small signaling molecules serving as an electrogenic pathway in cell-to-cell signal transduction. Mutations at the SLCO2A1 gene cause inherited bone and gut pathologies and malignancies, signifying the Maxi-Cl channel as a perspective pharmacological target.

Cell Volume-Activated and Volume-Correlated Anion Channels in Mammalian Cells: Their Biophysical, Molecular, and Pharmacological Properties

There are a number of mammalian anion channel types associated with cell volume changes. These channel types are classified into two groups: volume-activated anion channels (VAACs) and volume-correlated anion channels (VCACs). VAACs can be directly activated by cell swelling and include the volume-sensitive outwardly rectifying anion channel (VSOR), which is also called the volume-regulated anion channel; the maxi-anion channel (MAC or Maxi-Cl); and the voltage-gated anion channel, chloride channel (ClC)-2. VCACs can be facultatively implicated in, although not directly activated by, cell volume changes and include the cAMP-activated cystic fibrosis transmembrane conductance regulator (CFTR) anion channel, the Ca²⁺-activated Cl^- channel (CaCC), and the acid-sensitive (or acid-stimulated) outwardly rectifying anion channel. This article describes the phenotypical properties and activation mechanisms of both groups of anion channels, including accumulating pieces of information on the basis of recent molecular understanding. To that end, this review also highlights the molecular identities of both anion channel groups; in addition to the molecular identities of ClC-2 and CFTR, those of CaCC, VSOR, and Maxi-Cl were recently identified by applying genome-wide approaches. In the last section of this review, the most up-to-date information on the pharmacological properties of both anion channel groups, especially their half-maximal inhibitory concentrations (IC₅₀ values) and voltage-dependent blocking, is summarized particularly from the standpoint of pharmacological distinctions among them. Future physiologic and pharmacological studies are definitely warranted for therapeutic targeting of dysfunction of VAACs and VCACs.

α-Ketoglutarate stimulates pendrin-dependent Cl- absorption in the mouse CCD through protein kinase C

α-Ketoglutarate (α-KG) is a citric acid cycle intermediate and a glutamine catabolism product. It is also the natural ligand of 2-oxoglutarate receptor 1 (OXGR1), a G_q protein-coupled receptor expressed on the apical membrane of intercalated cells. In the cortical collecting duct (CCD), Cl^-/[Formula: see text] exchange increases upon α-KG binding to the OXGR1. To determine the signaling pathway(s) by which α-KG stimulates Cl^- absorption, we examined α-KG-stimulated Cl^- absorption in isolated perfused mouse CCDs. α-KG increased electroneutral Cl^- absorption in CCDs from wild-type mice but had no effect on Cl^- absorption in pendrin knockout mice. Because G_q protein-coupled receptors activate PKC, we hypothesized that α-KG stimulates Cl^- absorption through PKC. If so, PKC agonists should mimic, whereas PKC inhibitors should abolish, α-KG-stimulated Cl^- absorption. Like α-KG, PKC agonist (phorbol-12,13-dibutyrate, 500 nM) application increased Cl^- absorption in wild-type but not in pendrin null CCDs. Moreover, PKC inhibitors (2.5 mM GF109203X and 20 nM calphostin C), Ca²⁺ chelators (BAPTA, 10-20 μM), or PKC-α or -δ gene ablation eliminated α-KG-stimulated Cl^- absorption. We have shown that STE20/SPS-1-related proline-alanine-rich protein kinase (SPAK) gene ablation increases urinary α-KG excretion, renal pendrin abundance, and CCD Cl^- absorption. However, in SPAK null CCDs, Cl^- absorption was not activated further by luminal α-KG application nor was Cl^- absorption reduced with the PKC inhibitor GF109203 . Thus SPAK gene ablation likely acts through a PKC-independent pathway to produce a chronic adaptive increase in pendrin function. In conclusion, α-KG stimulates pendrin-dependent Cl^-/[Formula: see text] exchange through a mechanism dependent on PKC and Ca²⁺ that involves PKC-α and PKC-δ.

Recent advances in distal tubular potassium handling

It is well known that sodium reabsorption and aldosterone play important roles in potassium secretion by the aldosterone-sensitive distal nephron. Sodium- and aldosterone-independent mechanisms also exist. This review focuses on some recent studies that provide novel insights into the sodium- and aldosterone-independent potassium secretion by the aldosterone-sensitive distal nephron. In addition, we discuss a study reporting on the regulation of the mammalian potassium kidney channel ROMK by intracellular and extracellular magnesium, which may be important in the pathogenesis of persistent hypokalemia in patients with concomitant potassium and magnesium deficiency. We also discuss outstanding questions and propose working models for future investigation.

Synthesis and evaluation of new N6-substituted adenosine-5'-N-methylcarboxamides as A3 adenosine receptor agonists

A number of N(6)-substituted adenosine-5'-N-methylcarboxamides were synthesised and their pharmacology, in terms of their receptor affinity, selectivity and cardioprotective effects, were explored. The first series of compounds, 4a-4f and 5a-5f, showed modest receptor affinity for the A(3)AR with K(i) values in the low to mid muM range. However, the incorporation of a 4-(2-aminoethyl)-2,6-di-tert-butylphenol group in the N(6)-position (in compounds 4g and 5g) significantly improved the affinity with K(i) values of 30 and 9 nM, respectively. Improvements in affinity, as well as selectivity were seen when a functionalized linker was introduced. The N(6)-phenyl series, compounds 7a-7d, demonstrated low to mid nanomolar receptor affinities (K(i)=2.3-45.0 nM), with 7b displaying 109-fold selectivity for the A(3)AR (vs A(1)). The N(6)-benzyl series 9a-9c also proved to be potent and selective A(3)AR agonists and the longer chain length linker 13 was tolerated at the A(3)AR without abrogation of affinity or selectivity. Cardioprotection was demonstrated by a simulated ischaemia cell culture assay, whereby 7b, 7c, 9a, 9b and 9c all showed cardioprotective effects at 100 nM comparable or better than the benchmark A(3)AR agonist IB-MECA, but which were indistinguishable by statistical analysis. For example, compound 9c reduced cell death by 68.0+/-3.6%.

Attenuation of renal ischemia-reperfusion injury by postconditioning involves adenosine receptor and protein kinase C activation

SUMMARY

Significant organ injury occurs after transplantation and reflow (i.e., reperfusion injury). Postconditioning (PoC), consisting of alternating periods of reperfusion and re-occlusion at onset of reperfusion, attenuates reperfusion injury in organs including heart and brain. We tested whether PoC attenuates renal ischemia-reperfusion (I/R) injury in the kidney by activating adenosine receptors (AR) and protein kinase C (PKC). The single kidney rat I/R model was used. Groups: (1) sham: time-matched surgical protocol only. In all others, the left renal artery (RA) was occluded for 45 min and reperfused for 24 h. (2) CONTROL: I/R with no intervention at R. All antagonists were administered 5 min before reperfusion. (3) PoC: I/R + four cycles of 45 s of R and 45 s of re-occlusion before full R. (4) PoC + ARi: PoC plus the AR antagonist 8-rho-(sulfophenyl) theophylline (8-SPT). (5) PoC + PKCi: PoC plus the PKC antagonist chelerythrine (Che). In shams, plasma blood urea nitrogen (BUN mg/dl) at 24 h averaged 23.2 +/- 5.3 and creatinine (Cr mg/dl) averaged 1.28 +/- 0.2. PoC reduced BUN (87.2 +/- 10 in CONTROL vs. 38.8 +/- 9, P = 0.001) and Cr (4.2 +/- 0.6 in CONTROL vs. 1.5 +/- 0.2, P < 0.001). 8-SPT and Che reversed renal protection indices after PoC. I/R increased apoptosis, which was reduced by PoC, which was reversed by 8-SPT and Che. Postconditioning attenuates renal I/R injury by adenosine receptor activation and PKC signaling.

Activation of maxi-anion channel by protein tyrosine dephosphorylation

The maxi-anion channel with a large single-channel conductance of >300 pS, and unknown molecular identity, is functionally expressed in a large variety of cell types. The channel is activated by a number of experimental maneuvers such as exposing cells to hypotonic or ischemic stress. The most effective and consistent method of activating it is patch membrane excision. However, the activation mechanism of the maxi-anion channel remains poorly understood at present. In the present study, involvement of phosphorylation/dephosphorylation in excision-induced activation was examined. In mouse mammary fibroblastic C127 cells, activity of the channel was suppressed by intracellular application of Mg-ATP, but not Mg-5'-adenylylimidodiphosphate (AMP-PNP), in a concentration-dependent manner. When a cocktail of broad-spectrum tyrosine phosphatase inhibitors was applied, channel activation was completely abolished, whereas inhibitors of serine/threonine protein phosphatases had no effect. On the other hand, protein tyrosine kinase inhibitors brought the channel out of an inactivated state. In mouse adult skin fibroblasts (MAFs) in primary culture, similar maxi-anion channels were found to be activated on membrane excision, in a manner sensitive to tyrosine phosphatase inhibitors. In MAFs isolated from animals deficient in receptor protein tyrosine phosphatase (RPTP)zeta, activation of the maxi-anion channel was significantly slower and less prominent compared with that observed in wild-type MAFs; however, channel activation was restored by transfection of the RPTPzeta gene. Thus it is concluded that activation of the maxi-anion channel involves protein dephosphorylation mediated by protein tyrosine phosphatases that include RPTPzeta in mouse fibroblasts, but not in C127 cells.

The maxi-anion channel: a classical channel playing novel roles through an unidentified molecular entity

The maxi-anion channel is widely expressed and found in almost every part of the body. The channel is activated in response to osmotic cell swelling, to excision of the membrane patch, and also to some other physiologically and pathophysiologically relevant stimuli, such as salt stress in kidney macula densa as well as ischemia/hypoxia in heart and brain. Biophysically, the maxi-anion channel is characterized by a large single-channel conductance of 300-400 pS, which saturates at 580-640 pS with increasing the Cl(-) concentration. The channel discriminates well between Na(+) and Cl(-), but is poorly selective to other halides exhibiting weak electric-field selectivity with an Eisenman's selectivity sequence I. The maxi-anion channel has a wide pore with an effective radius of approximately 1.3 nm and permits passage not only of Cl(-) but also of some intracellular large organic anions, thereby releasing major extracellular signals and gliotransmitters such as glutamate(-) and ATP(4-). The channel-mediated efflux of these signaling molecules is associated with kidney tubuloglomerular feedback, cardiac ischemia/hypoxia, as well as brain ischemia/hypoxia and excitotoxic neurodegeneration. Despite the ubiquitous expression, well-defined properties and physiological/pathophysiological significance of this classical channel, the molecular entity has not been identified. Molecular identification of the maxi-anion channel is an urgent task that would greatly promote investigation in the fields not only of anion channel but also of physiological/pathophysiological signaling in the brain, heart and kidney.

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.

Adenosine: trigger and mediator of cardioprotection

Adenosine, a purine nucleoside, is ubiquitous in the body, and is a critical component of ATP. Its concentration jumps 100-fold during periods of oxygen depletion and ischemia. There are four adenosine receptors: A(1) and A(3) coupled to G(i/o) and the high-affinity A(2A) and low-affinity A(2B) coupled to G(s). Adenosine is one of three autacoids released by ischemic tissue which are important triggers of ischemic preconditioning (IPC). It is the A(1) and to some extent A(3) receptors which participate in the intracellular signaling that triggers cardioprotection. Unlike bradykinin and opioids, the other two autacoids, adenosine is not dependent on opening of mitochondrial K(ATP) channels or release of reactive oxygen species (ROS), but rather activates phospholipase C and/or protein kinase C (PKC) directly. Another signaling cascade at reperfusion involves activated PKC which initiates binding to and activation of an A(2) adenosine receptor that we believe is the A(2B). Although the latter is the low-affinity receptor, its interaction with PKC increases its affinity and makes it responsive to the accumulated tissue adenosine. A(2B) agonists, but not adenosine or A(1) agonists, infused at reperfusion can initiate this second signaling cascade and mimic preconditioning's protection. The same A(2B) receptors are critical for postconditioning's protection. Thus adenosine is both an important trigger and a mediator of cardioprotection.

Voltage-dependent anion channel (VDAC) participates in amyloid beta-induced toxicity and interacts with plasma membrane estrogen receptor alpha in septal and hippocampal neurons

Voltage-dependent anion channel (VDAC) is a porin known by its role in metabolite transport across mitochondria and participation in apoptotic processes. Although traditionally accepted to be located within mitochondrial outer membrane, some data has also reported its presence at the plasma membrane level where it seems to participate in regulation of normal redox homeostasis and apoptosis. Here, exposure of septal SN56 and hippocampal HT22 cells to specific anti-VDAC antibodies prior to amyloid beta (Abeta) peptide was observed to prevent neurotoxicity. In these cell lines, we identified a VDAC form associated with the plasma membrane that seems to be particularly abundant in caveolae. The two membrane-related isoforms of estrogen receptor alpha (mERalpha) (80 and 67 kDa), known in SN56 cells to participate in estrogen-induced neuroprotection against Abeta injury, were also observed to be present in caveolae. Interestingly, we demonstrated for the first time that both VDAC and mERalpha interact at the plasma membrane of these neurons as well as in microsomal fractions of the corresponding murine septal and hippocampal tissues. These proteins were also shown to associate with caveolin-1, thereby corroborating their presence in caveolar microdomains. Taken together, these results suggest that VDAC-mERalpha association at the plasma membrane level may participate in the modulation of Abeta-induced cell death.

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.

ATP release via anion channels

ATP serves not only as an energy source for all cell types but as an 'extracellular messenger' for autocrine and paracrine signalling. It is released from the cell via several different purinergic signal efflux pathways. ATP and its Mg(2+) and/or H(+) salts exist in anionic forms at physiological pH and may exit cells via some anion channel if the pore physically permits this. In this review we survey experimental data providing evidence for and against the release of ATP through anion channels. CFTR has long been considered a probable pathway for ATP release in airway epithelium and other types of cells expressing this protein, although non-CFTR ATP currents have also been observed. Volume-sensitive outwardly rectifying (VSOR) chloride channels are found in virtually all cell types and can physically accommodate or even permeate ATP(4-) in certain experimental conditions. However, pharmacological studies are controversial and argue against the actual involvement of the VSOR channel in significant release of ATP. A large-conductance anion channel whose open probability exhibits a bell-shaped voltage dependence is also ubiquitously expressed and represents a putative pathway for ATP release. This channel, called a maxi-anion channel, has a wide nanoscopic pore suitable for nucleotide transport and possesses an ATP-binding site in the middle of the pore lumen to facilitate the passage of the nucleotide. The maxi-anion channel conducts ATP and displays a pharmacological profile similar to that of ATP release in response to osmotic, ischemic, hypoxic and salt stresses. The relation of some other channels and transporters to the regulated release of ATP is also discussed.

Osmotic Diuretics Induce Adenosine A1 Receptor Expression and Protect Renal Proximal Tubular Epithelial Cells against Cisplatin-mediated Apoptosis

Osmotic diuretics are used successfully to alleviate acute tubular necrosis (ATN) produced by chemotherapeutic agents and aminoglycoside antibiotics. The beneficial action of these agents likely involves rapid elimination of the nephrotoxic agents from the kidney by promoting diuresis. Adenosine A1 receptor (A1AR) subtype present on renal proximal tubular epithelial and cortical collecting duct cells mediates the antidiuretic and cytoprotective actions of adenosine. These receptors are induced by activation of nuclear factor (NF)-kappaB, a transcription factor reported to mediate hyperosmotic stress-induced cytoprotection in renal medullary cells. In this study, we tested the hypothesis that induction of the A1AR in renal proximal tubular cells by NF-kappaB contributes to the cytoprotection afforded by osmotic diuretics. Exposure of porcine renal proximal tubular epithelial (LLC-PK1) cells to mannitol or NaCl produced a significant increase in A1AR. This increase was preceded by adenosine release and NF-kappaB activation. Expression of an IkappaB-alpha mutant, which acts as a superrepressor of NF-kappaB, abrogated the increase in A1AR. Cells exposed to mannitol demonstrated increased reactive oxygen species (ROS) generation, which was attenuated by inhibiting xanthine oxidase with allopurinol. Allopurinol attenuated both the increase in A1AR expression and NF-kappaB activation produced by osmotic diuretics, indicating a role of adenosine metabolites in these processes. Treatment of LLC-PK1 cells with cisplatin (8 microm) resulted in apoptosis, which was attenuated by mannitol but exacerbated by selective A1AR blockade. Administration of mannitol to mice increases A1AR expression and activation of NF-kappaB in renal cortical sections. Taken together, these data provide novel mechanisms of nephroprotection by osmotic diuretics, involving both activation and induction of the A1AR, the latter mediated through activation of a xanthine oxidase pathway leading to ROS generation and promoting activation of NF-kappaB.

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.

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.

Adenosine receptors: G protein-mediated signalling and the role of accessory proteins

Ever since the discovery of the effects of adenosine in the circulation, adenosine receptors continue to represent a promising drug target. Firstly, this is due to the fact that the receptors are expressed in a large variety of cells; in particular, the actions of adenosine (or, respectively, of the antagonistic methylxanthines) in the central nervous system, in the circulation, on immune cells and on other tissues can be beneficial in certain disorders. Secondly, there exists a large number of ligands, which have been generated by introducing several modifications in the structure of the lead compounds (adenosine and methylxanthine), some of them highly specific. Four adenosine receptor subtypes have been identified by molecular cloning; they belong to the family of G protein-coupled receptors, which transfer signals by activating heterotrimeric G proteins. It has been appreciated recently that accessory proteins impinge on the receptor/G protein interaction and thus modulate the signalling reaction. These accessory components may be thought as adaptors that redirect the signalling pathway to elicit a cell-specific response. Here, we review the recent literature on adenosine receptors and place a focus on the role of accessory proteins in the organisation of adenosine receptor signalling. These components have been involved in receptor sorting, in the control of signal amplification and in the temporal regulation of receptor activity, while the existence of others is postulated on the basis of atypical cellular reactions elicited by receptor activation.

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.

Volume-dependent ATP-conductive large-conductance anion channel as a pathway for swelling-induced ATP release

In mouse mammary C127i cells, during whole-cell clamp, osmotic cell swelling activated an anion channel current, when the phloretin-sensitive, volume-activated outwardly rectifying Cl(-) channel was eliminated. This current exhibited time-dependent inactivation at positive and negative voltages greater than around +/-25 mV. The whole-cell current was selective for anions and sensitive to Gd(3)+. In on-cell patches, single-channel events appeared with a lag period of approximately 15 min after a hypotonic challenge. Under isotonic conditions, cell-attached patches were silent, but patch excision led to activation of currents that consisted of multiple large-conductance unitary steps. The current displayed voltage- and time-dependent inactivation similar to that of whole-cell current. Voltage-dependent activation profile was bell-shaped with the maximum open probability at -20 to 0 mV. The channel in inside-out patches had the unitary conductance of approximately 400 pS, a linear current-voltage relationship, and anion selectivity. The outward (but not inward) single-channel conductance was suppressed by extracellular ATP with an IC(50) of 12.3 mM and an electric distance (delta) of 0.47, whereas the inward (but not outward) conductance was inhibited by intracellular ATP with an IC(50) of 12.9 mM and delta of 0.40. Despite the open channel block by ATP, the channel was ATP-conductive with P(ATP)/P(Cl) of 0.09. The single-channel activity was sensitive to Gd(3)+, SITS, and NPPB, but insensitive to phloretin, niflumic acid, and glibenclamide. The same pharmacological pattern was found in swelling-induced ATP release. Thus, it is concluded that the volume- and voltage-dependent ATP-conductive large-conductance anion channel serves as a conductive pathway for the swelling-induced ATP release in C127i cells.

Protein kinase C and G(i/o) proteins are involved in adenosine- and ischemic preconditioning-mediated renal protection

Renal ischemic reperfusion (IR) injury is a significant clinical problem in anesthesia and surgery. Recently, it was demonstrated that both renal ischemic preconditioning (IPC) and systemic adenosine pretreatment protect against renal IR injury. In cardiac IPC, pertussis toxin-sensitive G-proteins (i.e., G(i/o)), protein kinase C (PKC), and ATP-sensitive potassium (K+(ATP)) channels are implicated in this protective signaling pathway. The aim of this study was to elucidate the signaling pathways that are responsible for renal protection mediated by both IPC and adenosine pretreatment. In addition, because A1 adenosine receptor antagonist failed to block renal IPC, whether activation of bradykinin, muscarinic, or opioid receptors can mimic renal IPC was tested because these receptors have been implicated in cardiac IPC. Rats were acutely pretreated with chelerythrine or glibenclamide, selective blockers of PKC and K+(ATP) channels, respectively, before IPC or adenosine pretreatment. Some rats were pretreated with pinacidil (K+(ATP)channel opener), bradykinin, methacholine, or morphine before renal ischemia. Twenty-four h later, plasma creatinine was measured. Separate groups of rats received pertussis toxin intraperitoneally 48 h before being subjected to the above protective protocols. IPC and adenosine pretreatment protected against renal IR injury. Pretreatment with pertussis toxin and chelerythrine abolished the protective effects of both renal IPC and adenosine. However, glibenclamide pretreatment had no effect on either renal IPC or adenosine-induced renal protection, indicating no apparent role for K+(ATP) channels. Moreover, pinacidil, bradykinin, methacholine, and morphine failed to protect renal function. Therefore, the conclusion is that cellular signal transduction pathways of renal IPC and adenosine pretreatment in vivo involve G(i/o) proteins and PKC but not K+(ATP) channels. Unlike cardiac IPC, bradykinin, muscarinic, and opioid receptors do not mediate renal IPC.

Complex host cell responses to antisense suppression of ACHE gene expression

3'-End-capped, 20-mer antisense oligodeoxynucleotides (AS-ODN) protected with 2'-O-methyl (Me) or phosphorothioate (PS) substitutions were targeted to acetylcholinesterase (AChE) mRNA and studied in PC12 cells. Me-modified AS-ODN suppressed AChE activity up to 50% at concentrations of 0.02-100 nM. PS-ODN was effective at 1-100 nM. Both AS-ODN displayed progressively decreased efficacy above 10 nM. In situ hybridization and confocal microscopy demonstrated dose-dependent decreases, then increases, in AChE mRNA. Moreover, labeling at nuclear foci suggested facilitated transcription or stabilization of AChE mRNA or both under AS-ODN. Intracellular concentrations of biotinylated oligonucleotide equaled those of target mRNA at extracellular concentrations of 0.02 nM yet increased only 6-fold at 1 microM ODN. Above 50 nM, sequence-independent swelling of cellular, but not nuclear, volume was observed. Our findings demonstrate suppressed AChE expression using extremely low concentrations of AS-ODN and attribute reduced efficacy at higher concentrations to complex host cell feedback responses.

Extracellular adenosine modulates a volume-sensitive-like chloride conductance in immortalized rabbit DC1 cells

Cl(-) currents induced by cell swelling were characterized in an immortalized cell line (DC1) derived from rabbit distal bright convoluted tubule by the whole cell patch-clamp techniques and by (125)I(-) efflux experiments. Exposure of cells to a hypotonic shock induced outwardly rectifying Cl(-) currents that could be blocked by 0.1 mM 5-nitro-2-(3-phenylpropyl-amino)benzoic acid, 1 mM DIDS, and by 1 mM diphenylamine-2-carboxylate. (125)I(-) efflux experiments showed that exposure of the monolayer to a hypotonic medium increased (125)I(-) loss. Preincubation of cells with LaCl(3) or GdCl(3) prevented the development of the response. The addition of 10 microM adenosine to the bath medium activated outwardly rectifying whole cell currents similar to those recorded after hypotonic shock. This conductance was inhibited by the A(1)-receptor antagonist 8-cyclopentyl-1,3-diproxylxanthine (DPCPX), LaCl(3), or GdCl(3) and was activated by GTPgammaS. The selective A(1)-receptor agonist N(6)-cyclopentyladenosine (CPA) mimicked the effect of hypotonicity on (125)I(-) efflux. The CPA-induced increase of (125)I(-) efflux was inhibited by DPCPX and external application of LaCl(3) or GdCl(3). Adenosine also enhanced Mn(2+) influx across the apical membrane. Overall, the data show that DC1 cells possess swelling- and adenosine-activated Cl(-) conductances that share identical characteristics. The activation of both conductances involved Ca(2+) entry into the cell, probably via mechanosensitive Ca(2+) channels. The effects of adenosine are mediated via A(1) receptors that could mediate the purinergic regulation of the volume-sensitive Cl(-) conductance.

Similarity of A(3)-adenosine and swelling-activated Cl(-) channels in nonpigmented ciliary epithelial cells

Chloride release from nonpigmented ciliary epithelial (NPE) cells is a final step in forming aqueous humor, and adenosine stimulates Cl(-) transport by these cells. Whole cell patch clamping of cultured human NPE cells indicated that the A(3)-selective agonist 1-deoxy-1-(6-[([3-iodophenyl]methyl)amino]-9H-purin-9-yl)-N-methyl-be ta-D-ribofuranuronamide (IB-MECA) stimulated currents (I(IB-MECA)) by approximately 90% at +80 mV. Partial replacement of external Cl(-) with aspartate reduced outward currents and shifted the reversal potential (V(rev)) from -23 +/- 2 mV to -0.0 +/- 0.7 mV. Nitrate substitution had little effect. Perfusion with the Cl(-) channel blockers 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) and niflumic acid inhibited the currents. Partial Cl(-) replacement with aspartate and NO(3)(-), and perfusion with NPPB, had similar effects on the swelling-activated whole cell currents (I(Swell)). Partial cyclamate substitution for external Cl(-) inhibited inward and outward currents of both I(IB-MECA) and I(Swell). Both sets of currents also showed outward rectification and inactivation at large depolarizing potentials. The results are consistent with the concept that A(3)-subtype adenosine agonists and swelling activate a common population of Cl(-) channels.

Cellular and molecular therapeutic targets for treatment of contractile dysfunction after cardioplegic arrest

Transient left ventricular (LV) dysfunction can occur after hypothermic hyperkalemic cardioplegic arrest. This laboratory has developed an isolated LV myocyte system of simulated cardioplegic arrest and rewarming in order to examine cellular and molecular events that may contribute to the LV dysfunction after cardioplegic arrest. Contractile function was examined using high-speed video microscopy after reperfusion and rewarming. After cardioplegic arrest and reperfusion, indices of myocyte contractility were reduced by over 40% from normothermic control values. The capacity of the myocyte to respond to an inotropic stimulus was examined through beta-adrenergic receptor stimulation with isoproterenol. After cardioplegic arrest, the contractile response to isoproterenol was reduced by over 50% from normothermic values. The next series of studies focused upon preventing these changes in myocyte contractile processes after cardioplegic arrest. First, the cardioplegic solutions were augmented with adenosine or an ATP-sensitive potassium channel opener, aprikalim. Both adenosine and aprikalim augmentation significantly improved myocyte function compared with cardioplegia alone values. A potential intracellular mechanism for the protective effects of either adenosine or the ATP-sensitive potassium channel is the activation of protein kinase C (PKC). A brief period of PKC activation before cardioplegic arrest provided protective effects on myocyte contractility with subsequent reperfusion and rewarming. In another set of studies, the potential protective effects of the active form of thyroid hormone (T3) were examined. In myocytes pretreated with T3, myocyte contractile function and beta-adrenergic responsiveness were significantly improved after hypothermic cardioplegic arrest and rewarming. Thus, endogenous means of providing improved myocardial protection during prolonged cardioplegic arrest can be achieved through a brief period of PKC activation or pretreatment with T3. Future studies, which more carefully deduce the basis for these pretreatment effects, will likely yield novel methods by which to protect myocyte contractile processes during cardioplegic arrest.

Immortalized kidney epithelial cells as tools for hormonally regulated ion transport studies

The development of transgenic mice carrying the simian virus-40 large T antigen gene or the temperature-sensitive simian virus-40 large T antigen gene, either alone or placed under the control of the 5'-regulatory regions of tissue-specific or ubiquitous genes, has permitted the production of differentiated, polarized kidney epithelial cells. This review covers the immortalized cell lines issued from the various parts of the renal tubule and, in particular, the recently established collecting duct cell lines that have been used as ex-vivo cell models to analyze the regulation of ion transport processes by hormones.

H-K-ATPase in the RCCT-28A rabbit cortical collecting duct cell line

In the present study, we demonstrate that the rabbit cortical collecting duct cell line RCCT-28A possesses three distinct H-K-ATPase catalytic subunits (HKalpha). Intracellular measurements of RCCT-28A cells using the pH-sensitive dye 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF) indicated that the mechanism accounting for recovery from an acid load exhibited both K+ dependence and sensitivity to Sch-28080 characteristic of H-K-ATPases. Recovery rates were 0.022 +/- 0.005 pH units/min in the presence of K+, 0.004 +/- 0.002 in the absence of K+, and 0.002 +/- 0.002 in the presence of Sch-28080. The mRNAs encoding the HKalpha1 subunit and the H-K-ATPase beta-subunit (HKbeta) were detected by RT-PCR. In addition, two HKalpha2 species were found by RT-PCR and 5' rapid amplification of cDNA ends (5'-RACE) in the rabbit renal cortex. One was homologous to HKalpha2 cDNAs generated from other species, and the second was novel. The latter, referred to as HKalpha2c, encoded an apparent 61-residue amino-terminal extension that bore no homology to reported sequences. Antipeptide antibodies were designed on the basis of this extension, and these antibodies recognized a protein of the appropriate mass in both rabbit renal tissue samples and RCCT-28A cells. Such findings constitute very strong evidence for expression of the HKalpha2c subunit in vivo. The results suggest that the rabbit kidney and RCCT-28A cells express at least three distinct H-K-ATPases.

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.

Cell-specific signaling and structure-activity relations of parathyroid hormone analogs in mouse kidney cells

PTH is an 84-amino acid protein. Occupancy of its cognate receptor generally results in activation of adenylyl cyclase and/or phosphoinositide-specific phospholipase Cbeta (PLCbeta). In the kidney, PTH receptors are present on proximal and distal tubule cells. In proximal tubules, PTH induces calcium signaling, typified by a transient rise in intracellular calcium ([Ca2+]i) and inositol trisphosphate formation, but does not affect calcium absorption. By contrast, in distal tubules, PTH increases calcium absorption that is associated with a slow and sustained rise in [Ca2+]i, but does not stimulate phospholipase C (PLC) or cause inositol trisphosphate accumulation. Nonetheless, stimulation of distal calcium transport requires activation of protein kinase C (PKC) and protein kinase A. We now characterize the origin of the differential effects of ligand occupancy by using synthetic human PTH analogs that preferentially activate adenylyl cyclase and/or PLCbeta. We further tested the hypothesis that phospholipase D is responsible for PKC activation in distal tubule cells. PTH-(1-31) increased [Ca2+]i in distal tubule but not in proximal tubule cells, whereas PTH-(3-34) caused a partial increase in [Ca2+]i in proximal cells, but had no effect in distal cells. PTH-(7-34) blocked increases in [Ca2+]i in distal tubule cells stimulated by PTH-(1-34) and PTH-(1-31). The PLC inhibitor U73122 abolished the PTH-induced rise in [Ca2+]i and inositol trisphosphate formation by proximal tubule cells, but had no effect on PTH-stimulated Ca2+ uptake by distal tubule cells. These results support the view that activation of PKC by PTH in distal tubule cells does not involve PLCbeta. PTH did, however, activate phospholipase D with attendant formation of diacylglycerol in distal cells. As activation of PKC is required for induction of calcium transport by PTH, we conclude that PTH receptors are capable of activating multiple phospholipases and that the structural requirements for such activation differ in proximal and distal tubule cells.

CFTR is a conductance regulator as well as a chloride channel

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

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.

Stretch-induced protection shares a common mechanism with ischemic preconditioning in rabbit heart

We sought to determine whether stretch-induced preconditioning may be related to activation of adenosine receptors, ATP-sensitive K+ (K+ATP) channels, and/or protein kinase C (PKC) in the rabbit heart. Anesthetized rabbits underwent 30 min of coronary artery occlusion followed by 3 h of reperfusion. Ischemic preconditioning was induced by one episode of 5 min of ischemia followed by 5 min of reperfusion, and stretch preconditioning was induced by a transient volume overload. The abilities of gadolinium (Gd3+), a blocker of stretch-activated channels, glibenclamide (Glib), a blocker of K+ATP channels, 8-(p-sulfophenyl)-theophylline (8-SPT), a blocker of adenosine receptors, and polymyxin B (PMXB), an antagonist of PKC, to prevent the infarct size-limiting effect of stretch-induced preconditioning were evaluated. Because the infarct size-reducing effect of stretch occurred in the absence of ischemia and was prevented by previous administration of Gd3+, Glib, 8-SPT, and PMXB, we propose that activation of mechanosensitive ion channels protects the rabbit heart from subsequent sustained ischemic insult, likely through a mechanism that involves downstream activation of PKC, adenosine receptors, and/or K+ATP channels.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

Cl- and K+ conductances activated by cell swelling in primary cultures of rabbit distal bright convoluted tubules

Ionic currents induced by cell swelling were characterized in primary cultures of rabbit distal bright convoluted tubule (DCTb) by the whole cell patch-clamp technique. Cl- currents were produced spontaneously by whole cell recording with an isotonic pipette solution or by exposure to a hypotonic stress. Initial Cl- currents exhibited outwardly rectifying current-voltage relationship, whereas steady-state currents showed strong decay with depolarizing pulses. The ion selectivity sequence was I- = Br- > Cl- >> glutamate. Currents were inhibited by 0.1 mM 5-nitro-2-(3-phenylpropylamino) benzoic acid and 1 mM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and strongly blocked by 1 mM diphenylamine-2-carboxylate. Currents were insensitive to intracellular Ca2+ but required the presence of extracellular Ca2+. They were not activated in cells pretreated with 200 nM staurosporine, 50 microM LaCl3, 10 microM nifedipine, 100 microM verapamil, 5 microM tamoxifen, and 50 microM dideoxyforskolin. Staurosporine, tamoxifen, verapamil, or the absence of external Ca2+ was without effect on the fully developed Cl- currents. Osmotic shock also activated K+ currents in Cl- free conditions. These currents were time independent, activated at depolarized potentials, and inhibited by 5 mM BaCl2. The activation of Cl- and K+ currents by an osmotic shock may be implicated in regulatory volume decrease in DCTb cells.

Expansion of the cellular content of ribonucleoside triphosphates induces cell shrinkage and KCl loss in Ehrlich ascites tumor cells and in Chinese hamster ovary cells

The conversion to corresponding triphosphate derivatives of various ribonucleosides has been studied in Ehrlich ascites tumor cells and in Chinese hamster ovary cells under conditions that are optimal for cellular uptake of orthophosphate. The initial cellular uptake of orthophosphate is followed by a cellular loss of Cl- which might be consistent with a H2PO4-/Cl- exchange mechanism. Subsequent addition of ribonucleosides to the medium leads to cellular accumulation of the corresponding triphosphate and to a concomitant loss of KCl and to sustained cell volume reduction. The latter two events are quite unspecific with regard to the nucleobase moiety of the ribonucleoside triphosphate accumulated (adenine, guanine and purine being almost equally effective) and they depend in a rather simple way on the increase of the cellular content of these compounds. The KCl loss seems to depend on opening of the separate K+ and Cl- channels. The pharmacological profile of the putative ion channels could not be identified in spite of experiments with conventional blockers. In the case of purine riboside the accumulation of the corresponding triphosphate and concomitant loss of KCl and cell water may be followed by a regain of cell volume due to a continued purine riboside triphosphate accumulation, which apparently depends on the uptake of orthophosphate by cotransport with Na+ and which for osmotic reasons is accompanied by the uptake of water and hence volume increase. The possibility that the nucleoside triphosphate induced opening of a putative Cl- channel may be due to a direct effect of triphosphate on a channel protein is discussed.

Adenosine stimulates Cl- channels of nonpigmented ciliary epithelial cells

Ciliary epithelial cells possess multiple purinergic receptors, and occupancy of A1 and A2 adenosine receptors is associated with opposing effects on intraocular pressure. Aqueous adenosine produced increases in short-circuit current across rabbit ciliary epithelium, blocked by removing Cl- and enhanced by aqueous Ba2+. Adenosine's actions were further studied with nonpigmented ciliary epithelial (NPE) cells from continuous human HCE and ODM lines and freshly dissected bovine cells. With gramicidin present, adenosine (> or = 3 microM) triggered isosmotic shrinkage of the human NPE cells, which was inhibited by the Cl- channel blockers 5-nitro-2-(3-phenylpropylamino)benzoate (NPPB) and niflumic acid. At 10 microM, the nonmetabolizable analog 2-chloroadenosine and AMP also produced shrinkage, but not inosine, UTP, or ATP. 2-Chloroadenosine (> or = 1 microM) triggered increases of whole cell currents in HCE cells, which were partially reversible, Cl- dependent, and reversibly inhibited by NPPB. Adenosine (> or = 10 microM) also stimulated whole cell currents in bovine NPE cells. We conclude that occupancy of adenosine receptors stimulates Cl- secretion in mammalian NPE 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.

Volume-sensitive chloride current in pigmented ciliary epithelial cells: role of phospholipases

The whole cell recording technique was used to examine an outwardly rectifying chloride current activated by hypotonic shock in bovine pigmented ciliary epithelial (PCE) cells. Removal of internal and external Ca2+ did not affect the activation of these currents, but they were abolished by the phospholipase C inhibitor neomycin. The current was blocked by 5-nitro-2-(3-phenylpropylamino)benzoic acid, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid, and 4,4'-disothiocyanostilbene-2,2'-disulfonic acid (DIDS) in a voltage-dependent manner, but tamoxifen, dideoxyforskolin, and quinidine did not affect it. This blocking profile differs from that of the volume-sensitive chloride channel in neighboring nonpigmented ciliary epithelial cells (Wu, J., J. J. Zhang, H. Koppel, and T. J. C. Jacob, J. Physiol, Lond. 491: 743-755, 1996), and this difference implies that the volume responses of the two cell types are mediated by different chloride channels (Jacob, T. J. C., and J. J. Zhang. J. Physiol. Lond. In press). Intracellular administration of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) to PCE cells induced a transient, time-independent, outwardly rectifying chloride current that closely resembled the current activated by hypotonic shock. DIDS produced a voltage-dependent block of the GTP gamma S-activated current similar to the block of the hypotonically activated current. Intracellular neomycin completely prevented activation of this current as did incubation of the cells in calphostin C. and inhibitor of protein kinase C (PKC). Removal of Ca2+ did not affect activation of the current by GTP gamma S but extended the duration of the response. Inhibition of phospholipase A2 (PLA2) with p-bromophenacyl bromide prevented the activation of the hypotonically induced current and also inhibited the current once activated by hypotonic solution. The findings imply that the hypotonic response in PCE cells is mediated by both phospholipase C (PLC) and PLA2. Both phospholipases generate arachidonic acid, and, in addition, the PLC pathway regulates the PLA2 pathway via a PKC-dependent phosphorylation of PLA2.

Heterogeneity of chloride channels in the apical membrane of isolated mitochondria-rich cells from toad skin

The isolated epithelium of toad skin was disintegrated into single cells by treatment with collagenase and trypsine. Chloride channels of cell-attached and excised inside-out apical membrane-patches of mitochondria-rich cells were studied by the patch-clamp technique. The major population of Cl- channels constituted small 7-pS linear channels in symmetrical solutions (125 mM Cl-). In cell-attached and inside-out patches the single channel i/V-relationship could be described by electrodiffusion of Cl- with a Goldmann-Hodgkin-Katz permeability of, PCl = 1.2 x 10(-14) - 2.6 x 10(-14) cm3. s-1. The channel exhibited voltage-independent activity and could be activated by cAMP. This channel is a likely candidate for mediating the well known cAMP-induced transepithelial Cl- conductance of the amphibian skin epithelium. Another population of Cl- channels exhibited large, highly variable conductances (upper limit conductances, 150-550 pS) and could be activated by membrane depolarization. A group of intermediate-sized Cl(-)-channels included: (a) channels (mean conductance, 30 pS) with linear or slightly outwardly rectifying i/V-relationships and activity occurring in distinct "bursts," (b) channels (conductance-range, 10-27 pS) with marked depolarization-induced activity, and (c) channels with unresolvable kinetics. The variance of current fluctuations of such "noisy" patches exhibited a minimum close to the equilibrium-potential for Cl-. With channels occurring in only 38% of sealed patches and an even lower frequency of voltage-activated channels, the chloride conductance of the apical membrane of mitochondria-rich cells did not match quantitatively that previously estimated from macroscopic Ussing-chamber experiments. From a qualitative point of view, however, we have succeeded in demonstrating the existence of Cl-channels in the apical membrane with features comparable to macroscopic predictions, i.e., activation of channel gating by cAMP and, in a few patches, also by membrane depolarization.

The role of swelling-induced anion channels during neuronal volume regulation

Regulation of cell volume is an essential function of most mammalian cells. In the cells of the central nervous system, maintenance of cell osmolarity and, hence, volume, is particularly crucial because of the restrictive nature of the skull. Cell volume regulation involves a variety of pathways, with considerable differences between cell types. One common pathway activated during hypo-osmotic stress involves chloride (Cl-) channels. However, hypo-osmotically stimulated anion permeability can be regulated by a diverse array of second messengers. Although neuronal swelling can occur in a number of pathological and nonpathological conditions, our understanding of neuronal volume regulation is limited. This article summarizes our current understanding of the role of anion channels during neuronal volume regulation.

Shrinkage activates a nonselective conductance: involvement of a Walker-motif protein and PKC

The ability of all cells to maintain their volume during an osmotic challenge is dependent on the regulated movement of salt and water across the plasma membrane. We demonstrate the phosphorylation-dependent gating of a nonselective conductance in Caco-2 cells during cellular shrinkage. Intracellular application of exogenous purified rat brain protein kinase C (PKC) resulted in the activation of a current similar to that activated during shrinkage with a Na(+)-to-Cl- permeability ratio of approximately 1.7:1. To prevent possible PKC- and/or shrinkage-dependent activation of cystic fibrosis transmembrane regulator (CFTR), which is expressed at high levels in Caco-2 cells, a functional anti-peptide antibody, anti-CFTR505-511, was introduced into the cells via the patch pipette. Anti-CFTR505-511, which is directed against the Walker motif in the first nucleotide binding fold of CFTR, prevented the PKC/shrink-age current activation. The peptide CFTR505-511 also induced current inhibition, suggesting the possible involvement of a regulatory element in close proximity to the channel that shares sequence homology with the first nucleotide binding fold of CFTR and whose binding to the channel is required for channel gating.

G protein G alpha i-2 inhibits outwardly rectifying chloride channels in human airway epithelial cells

Previously we demonstrated that the heterotrimeric G protein, G alpha i-2, inhibits cystic fibrosis transmembrane conductance regulator (CFTR) chloride (Cl-) channels in human airway epithelial cells (E. M. Schwiebert, F. Gesek, L. Ercolani, C. Wjasow, D. C. Gruenert, and B. A. Stanton. Am. J. Physiol. 267 (Cell Physiol. 36): C272-C281, 1994, and E. M. Schwiebert, N. L. Kizer, D. C. Gruenert, and B. A. Stanton. Proc. Natl. Acad. Sci. USA 89: 10623-10627, 1992). The goal of the present study was to determine if G proteins also regulate outwardly rectifying Cl- channels (ORCC), a distinct class of Cl- channels regulated defectively by protein kinase A (PKA) in cystic fibrosis (CF). To this end, we used the patch-clamp technique to study ORCC in a normal human airway epithelial cell line (9HTEo-) that expresses CFTR and ORCC. Stimulation of G proteins with GTP and GTP gamma S decreased the single-channel open probability (Po) of ORCC, whereas inhibition of G proteins by GDP beta S increased the Po. Moreover, pertussis toxin (PTX), an uncoupler of Gi and G(o) subclasses of heterotrimeric G proteins, also increased the Po. Purified G alpha i-2 decreased the Po. In contrast, other PTX-sensitive G proteins, G alpha i-1, G alpha i-3, and G alpha o, had no effect on Po. We propose that G alpha i-2 couples to a receptor whose agonist negatively regulates ORCC in human airway epithelial cells.

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.

Role of adenosine on glucagon-induced cAMP in a human cortical collecting duct cell line

The hormonal responsiveness profile of the cortical collecting duct varies from one species to another. To identify the hormones and agonists that modulate the functions of this tubule segment in the human species, we generated a cell line (HCD) immortalized by SV40 virus. The tubular origin of this cell line was assessed by the expression of collecting duct-specific antigens and the ability of vasopressin to increase by nine-fold cAMP synthesis. Glucagon and adenosine stimulated cAMP synthesis, and atrial natriuretic peptide stimulated cGMP synthesis in a concentration-dependent manner. Bradykinin, adenosine and angiotensin increased intracellular calcium concentration ([Ca2+]i). Because adenosine can regulate tubular functions, we examined its role on glucagon-induced cAMP synthesis. Using adenosine analogs, we demonstrated that HCT cells both expressed adenosine type-2 (A2) receptors which stimulated cAMP production, and adenosine type-1 (A1) receptors linked to [Ca2+]i increase which inhibited glucagon-stimulated cAMP synthesis. The inhibitory effect was abolished by pertussis toxin, and was neither due to [Ca2+]i increase nor to protein kinase C activation, which indicated that some A1 adenosine receptors were directly negatively coupled to adenylyl cyclase. These results suggest that adenosine can modify human cortical collecting duct functions in opposite ways according to the adenosine receptor activated.