ATP inhibits Mg(2+) uptake in MDCT cells via P2X purinoceptors

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.

Extracellular Nucleotides and P2 Receptors in Renal Function

The understanding of the nucleotide/P2 receptor system in the regulation of renal hemodynamics and transport function has grown exponentially over the last 20 yr. This review attempts to integrate the available data while also identifying areas of missing information. First, the determinants of nucleotide concentrations in the interstitial and tubular fluids of the kidney are described, including mechanisms of cellular release of nucleotides and their extracellular breakdown. Then the renal cell membrane expression of P2X and P2Y receptors is discussed in the context of their effects on renal vascular and tubular functions. Attention is paid to effects on the cortical vasculature and intraglomerular structures, autoregulation of renal blood flow, tubuloglomerular feedback, and the control of medullary blood flow. The role of the nucleotide/P2 receptor system in the autocrine/paracrine regulation of sodium and fluid transport in the tubular and collecting duct system is outlined together with its role in integrative sodium and fluid homeostasis and blood pressure control. The final section summarizes the rapidly growing evidence indicating a prominent role of the extracellular nucleotide/P2 receptor system in the pathophysiology of the kidney and aims to identify potential therapeutic opportunities, including hypertension, lithium-induced nephropathy, polycystic kidney disease, and kidney inflammation. We are only beginning to unravel the distinct physiological and pathophysiological influences of the extracellular nucleotide/P2 receptor system and the associated therapeutic perspectives.

P2X6 Knockout Mice Exhibit Normal Electrolyte Homeostasis

ATP-mediated signaling is an important regulator of electrolyte transport in the kidney. The purinergic cation channel P2X6 has been previously localized to the distal convoluted tubule (DCT), a nephron segment important for Mg²⁺ and Na⁺ reabsorption, but its role in ion transport remains unknown. In this study, P2x6 knockout (P2x6^-/-) mice were generated to investigate the role of P2X6 in renal electrolyte transport. The P2x6^-/- animals displayed a normal phenotype and did not differ physiologically from wild type mice. Differences in serum concentration and 24-hrs urine excretion of Na⁺, K⁺, Mg²⁺ and Ca²⁺ were not detected between P2x6^+/+, P2x6^+/- and P2x6^-/- mice. Quantitative PCR was applied to examine potential compensatory changes in renal expression levels of other P2x subunits and electrolyte transporters, including P2x1-5, P2x7, Trpm6, Ncc, Egf, Cldn16, Scnn1, Slc12a3, Slc41a1, Slc41a3, Cnnm2, Kcnj10 and Fxyd2. Additionally, protein levels of P2X2 and P2X4 were assessed in P2x6^+/+ and P2x6^-/- mouse kidneys. However, significant changes in expression were not detected. Furthermore, no compensatory changes in gene expression could be demonstrated in heart material isolated from P2x6^-/- mice. Except for a significant (P<0.05) upregulation of P2x2 in the heart of P2x6^-/- mice compared to the P2x6^+/+ mice. Thus, our data suggests that purinergic signaling via P2X6 is not significantly involved in the regulation of renal electrolyte handling under normal physiological conditions.

The dark side of extracellular ATP in kidney diseases

Intracellular ATP is the most vital source of cellular energy for biologic systems, whereas extracellular ATP is a multifaceted mediator of several cell functions via its interaction, in an autocrine or paracrine manner, with P2 purinergic receptors expressed on the cell surface. These ionotropic and metabotropic P2 purinergic receptors modulate a variety of physiologic events upon the maintenance of a highly sensitive "set point," the derangement of which may lead to the development of key pathogenic mechanisms during acute and chronic diseases. Growing evidence suggests that extracellular ATP signaling via P2 purinergic receptors may be involved in different renal pathologic conditions. For these reasons, investigators and pharmaceutical companies are actively exploring novel strategies to antagonize or block these receptors with the goal of reducing extracellular ATP production or accelerating extracellular ATP clearance. Targeting extracellular ATP signaling, particularly through the P2X7 receptor, has considerable translational potential, given that novel P2X7-receptor inhibitors are already available for clinical use (e.g., CE224,535, AZD9056, and GSK1482160). This review summarizes the current evidence regarding the involvement of extracellular ATP and its P2 purinergic receptor-mediated signaling in physiologic and pathologic processes in the kidney; potential therapeutic options targeting extracellular ATP purinergic receptors are analyzed as well.

Effects of extracellular phosphate on gene expression in murine osteoblasts

That phosphate homeostasis is tightly linked to skeletal mineralization is probably best underscored by the fact that the phosphaturic hormone FGF23 is primarily expressed by terminally differentiated osteoblasts/osteocytes and that increased circulating FGF23 levels are causative for different types of hypophosphatemic rickets. In contrast, FGF23 inactivation results in hyperphosphatemia, and unexpectedly this phenotype is associated with severe osteomalacia in Fgf23-deficient mice. In this context it is interesting that different cell types have been shown to respond to extracellular phosphate, thereby raising the concept that phosphate can act as a signaling molecule. To identify phosphate-responsive genes in primary murine osteoblasts we performed genome wide expression analysis with cells maintained in medium containing either 1 or 4 mM sodium phosphate for 6 h. As confirmed by qRT-PCR, this analysis revealed that several known osteoblast differentiation markers (Bglap, Ibsp, and Phex) were unaffected by raising extracellular phosphate levels. In contrast, we found that the expression of Enpp1 and Ank, two genes encoding inhibitors of matrix mineralization, was induced by extracellular phosphate, while the expression of Sost and Dkk1, two genes encoding inhibitors of bone formation, was negatively regulated. The ability of osteoblasts to respond to extracellular phosphate was dependent on their differentiation state, and shRNA-dependent repression of the phosphate transporter Slc20a1 in MC3T3-E1 cells partially abolished their molecular response to phosphate. Taken together, our results provide further evidence for a role of extracellular phosphate as a signaling molecule and raise the possibility that severe hyperphosphatemia can negatively affect skeletal mineralization.

P2Y2 receptor activation inhibits the expression of the sodium-chloride cotransporter NCC in distal convoluted tubule cells

Luminal nucleotide stimulation is known to reduce Na(+) transport in the distal nephron. Previous studies suggest that this mechanism may involve the thiazide-sensitive Na(+)-Cl(-) cotransporter (NCC), which plays an essential role in NaCl reabsorption in the cells lining the distal convoluted tubule (DCT). Here we show that stimulation of mouse DCT (mDCT) cells with ATP or UTP promoted Ca(2+) transients and decreased the expression of NCC at both mRNA and protein levels. Specific siRNA-mediated silencing of P2Y2 receptors almost completely abolished ATP/UTP-induced Ca(2+) transients and significantly reduced ATP/UTP-induced decrease of NCC expression. To test whether local variations in the intracellular Ca(2+) concentration ([Ca(2+)]i) may control NCC transcription, we overexpressed the Ca(2+)-binding protein parvalbumin selectively in the cytosol or in the nucleus of mDCT cells. The decrease in NCC mRNA upon nucleotide stimulation was abolished in cells overexpressing cytosolic PV but not in cells overexpressing either a nuclear-targeted PV or a mutated PV unable to bind Ca(2+). Using a firefly luciferase reporter gene strategy, we observed that the activity of NCC promoter region from -1 to -2,200 bp was not regulated by changes in [Ca(2+)]i. In contrast, high cytosolic calcium level induced instability of NCC mRNA. We conclude that in mDCT cells: (1) P2Y2 receptor is essential for the intracellular Ca(2+) signaling induced by ATP/UTP stimulation; (2) P2Y2-mediated increase of cytoplasmic Ca(2+) concentration down-regulates the expression of NCC; (3) the decrease of NCC expression occurs, at least in part, via destabilization of its mRNA.

Prolactin promotes cartilage survival and attenuates inflammation in inflammatory arthritis

Chondrocytes are the only cells in cartilage, and their death by apoptosis contributes to cartilage loss in inflammatory joint diseases, such as rheumatoid arthritis (RA). A putative therapeutic intervention for RA is the inhibition of apoptosis-mediated cartilage degradation. The hormone prolactin (PRL) frequently increases in the circulation of patients with RA, but the role of hyperprolactinemia in disease activity is unclear. Here, we demonstrate that PRL inhibits the apoptosis of cultured chondrocytes in response to a mixture of proinflammatory cytokines (TNF-α, IL-1β, and IFN-γ) by preventing the induction of p53 and decreasing the BAX/BCL-2 ratio through a NO-independent, JAK2/STAT3-dependent pathway. Local treatment with PRL or increasing PRL circulating levels also prevented chondrocyte apoptosis evoked by injecting cytokines into the knee joints of rats, whereas the proapoptotic effect of cytokines was enhanced in PRL receptor-null (Prlr(-/-)) mice. Moreover, eliciting hyperprolactinemia in rats before or after inducing the adjuvant model of inflammatory arthritis reduced chondrocyte apoptosis, proinflammatory cytokine expression, pannus formation, bone erosion, joint swelling, and pain. These results reveal the protective effect of PRL against inflammation-induced chondrocyte apoptosis and the therapeutic potential of hyperprolactinemia to reduce permanent joint damage and inflammation in RA.

Inorganic phosphate induces mammalian growth plate chondrocyte apoptosis in a mitochondrial pathway involving nitric oxide and JNK MAP kinase

Chondrocytes in the hypertrophic zone of the growth plate undergo apoptosis during endochondral bone development via mechanisms that involve inorganic phosphate (Pi) and nitric oxide (NO). Recent evidence suggests that Pi-dependent NO production plays a role in apoptosis of cells in the resting zone as well. This study examined the mechanism by which Pi induces NO production and the signaling pathways by which NO mediates its effects on apoptosis in these cells. Pi decreased the number of viable cells based on MTT activity; the number of TUNEL-positive cells and the level of DNA fragmentation were increased, indicating an increase in apoptosis. Blocking NO production using the NO synthase (NOS) inhibitor L: -NAME or cells from eNOS(-/-) mice blocked Pi-induced chondrocyte apoptosis, indicating that NO production is necessary. NO donors NOC-18 and SNOG both induced chondrocyte apoptosis. SNOG also upregulated p53 expression, the Bax/Bcl-2 expression ratio, and cytochrome c release from mitochondria, as well as caspase-3 activity, indicating that NO induces apoptosis via a mitochondrial pathway. Inhibition of JNK, but not of p38 or ERK1/2, MAP kinase was able to block NO-induced apoptosis, indicating that JNK is necessary in this pathway. Pi elevates NO production via eNOS in resting zone chondrocytes, which leads to a mitochondrial apoptosis pathway dependent on JNK.

The emergence of phosphate as a specific signaling molecule in bone and other cell types in mammals

Although considerable advances in our understanding of the mechanisms of phosphate homeostasis and skeleton mineralization have recently been made, little is known about the initial events involving the detection of changes in the phosphate serum concentrations and the subsequent downstream regulation cascade. Recent data has strengthened a long-established hypothesis that a phosphate-sensing mechanism may be present in various organs. Such a phosphate sensor would detect changes in serum or local phosphate concentration and would inform the body, the local environment, or the individual cell. This suggests that phosphate in itself could represent a signal regulating multiple factors necessary for diverse biological processes such as bone or vascular calcification. This review summarizes findings supporting the possibility that phosphate represents a signaling molecule, particularly in bone and cartilage, but also in other tissues. The involvement of various signaling pathways (ERK1/2), transcription factors (Fra-1, Runx2) and phosphate transporters (PiT1, PiT2) is discussed.

Joint aging and chondrocyte cell death

Articular cartilage extracellular matrix and cell function change with age and are considered to be the most important factors in the development and progression of osteoarthritis. The multifaceted nature of joint disease indicates that the contribution of cell death can be an important factor at early and late stages of osteoarthritis. Therefore, the pharmacologic inhibition of cell death is likely to be clinically valuable at any stage of the disease. In this article, we will discuss the close association between diverse changes in cartilage aging, how altered conditions influence chondrocyte death, and the implications of preventing cell loss to retard osteoarthritis progression and preserve tissue homeostasis.

Effects of extracellular nucleotides on renal tubular solute transport

A range of P2 receptor subtypes has been identified along the renal tubule, in both apical and basolateral membranes. Furthermore, it has been shown that nucleotides are released from renal tubular cells, and that ectonucleotidases are present in several nephron segments. These findings suggest an autocrine/paracrine role for nucleotides in regulating tubular function. The present review catalogues the known actions of extracellular nucleotides on tubular solute transport. In the proximal tubule, there is firm evidence that stimulation of apical P2Y(1) receptors inhibits bicarbonate reabsorption, whilst basolaterally applied ATP has the opposite effect. Clearance studies suggest that systemic diadenosine polyphosphates profoundly reduce proximal tubular fluid transport, through as yet unidentified P2 receptors. To date, only circumstantial evidence is available for an action of nucleotides on transport in the loop of Henle; and no studies have been made on native distal tubules, though observations in cell lines suggest an inhibitory effect on sodium, calcium and magnesium transport. The nephron segment most studied is the collecting duct. Apically applied nucleotides inhibit the activity of small-conductance K(+) channels in mouse collecting duct, apparently through stimulation of P2Y(2) receptors. There is also evidence, from cell lines and native tissue, that apically (and in some cases basolaterally) applied nucleotides inhibit sodium reabsorption. In mice pharmacological profiling implicates P2Y(2) receptors; but in rats, the receptor subtype(s) responsible is/are unclear. Recent patch-clamp studies in rat collecting ducts implicate apical P2Y and P2X subtypes, with evidence for both inhibitory and stimulatory effects. Despite considerable progress, clarification of the physiological role of the tubular P2 receptor system remains some way off.

P2X7 receptor as sensitive flow sensor for ERK activation in osteoblasts

The involvement of the P2 receptor in the activation of ERK induced by a short transient fluid flow stimulation in MC3T3-E1 osteoblasts was examined in the current study. The ERK activation induced by this transient fluid flow stimulation was followed by an increase in c-fos mRNA expression. Suramin, a non-selective P2 receptor antagonist, and two different P2X7 receptor (P2X7R) antagonists, ATP analogue (oxidized ATP) and dye (Brilliant blue G), inhibited fluid flow-induced ERK activation. However, the P2Y receptor pathway inhibitor U73122 did not abolish this ERK activation. The P2X7R agonist 2',3'-O-(4-benzoylbenzoyl)-ATP (BzATP) significantly increased ERK activation and this activation could be completely inhibited by oxidized ATP and Brilliant blue G. Our results suggest that P2X7R is a highly sensitive P2 receptor for fluid flow-induced ERK activation in osteoblasts.

Purinergic signaling in the lumen of a normal nephron and in remodeled PKD encapsulated cysts

The nephron is the functional unit of the kidney. Blood and plasma are continually filtered within the glomeruli that begin each nephron. Adenosine 5' triphosphate (ATP) and its metabolites are freely filtered by each glomerulus and enter the lumen of each nephron beginning at the proximal convoluted tubule (PCT). Flow rate, osmolality, and other mechanical or chemical stimuli for ATP secretion are present in each nephron segment. These ATP-release stimuli are also different in each nephron segment due to water or salt permeability or impermeability along different luminal membranes of the cells that line each nephron segment. Each of the above stimuli can trigger additional ATP release into the lumen of a nephron segment. Each nephron-lining epithelial cell is a potential source of secreted ATP. Together with filtered ATP and its metabolites derived from the glomerulus, secreted ATP and adenosine derived from cells along the nephron are likely the principal two of several nucleotide and nucleoside candidates for renal autocrine and paracrine ligands within the tubular fluid of the nephron. This minireview discusses the first principles of purinergic signaling as they relate to the nephron and the urinary bladder. The review discusses how the lumen of a renal tubule presents an ideal purinergic signaling microenvironment. The review also illustrates how remodeled and encapsulated cysts in autosomal dominant polycystic kidney disease (ADPKD) and remodeled pseudocysts in autosomal recessive PKD (ARPKD) of the renal collecting duct likely create an even more ideal microenvironment for purinergic signaling. Once trapped in these closed microenvironments, purinergic signaling becomes chronic and likely plays a significant epigenetic and detrimental role in the secondary progression of PKD, once the remodeling of the renal tissue has begun. In PKD cystic microenvironments, we argue that normal purinergic signaling within the lumen of the nephron provides detrimental acceleration of ADPKD once remodeling is complete.

Renal expression of parvalbumin is critical for NaCl handling and response to diuretics

The distal convoluted tubule (DCT) plays an essential role in the reabsorption of NaCl by the kidney, a process that can be inhibited by thiazide diuretics. Parvalbumin (PV), a Ca(2+)-binding protein that plays a role in muscle fibers and neurons, is selectively expressed in the DCT, where its role remains unknown. We therefore investigated the renal phenotype of PV knockout mice (Pvalb(-/-)) vs. wild-type (Pvalb(+/+)) littermates. PV colocalized with the thiazide-sensitive Na(+)-Cl(-) cotransporter (NCC) in the early DCT. The Pvalb(-/-) mice showed increased diuresis and kaliuresis at baseline with higher aldosterone levels and lower lithium clearance. Acute furosemide administration increased diuresis and natriuresis/kaliuresis, but, surprisingly, did not increase calciuria in Pvalb(-/-) mice. NaCl supplementation of Pvalb(-/-) mice increased calciuria at baseline and after furosemide. The Pvalb(-/-) mice showed no significant diuretic response to hydrochlorothiazide, but an accentuated hypocalciuria. A decreased expression of NCC was detected in the early DCT of Pvalb(-/-) kidneys in the absence of ultrastructural and apoptotic changes. The PV-deficient mice had a positive Ca(2+) balance and increased bone mineral density. Studies in mouse DCT cells showed that endogenous NCC expression is Ca(2+)-dependent and can be modulated by the levels of PV expression. These results suggest that PV regulates the expression of NCC by modulating intracellular Ca(2+) signaling in response to ATP in DCT cells. They also provide insights into the Ca(2+)-sparing action of thiazides and the pathophysiology of distal tubulopathies.

Functional expression of ionotropic purinergic receptors on mouse taste bud cells

Neurotransmitter receptors on taste bud cells (TBCs) and taste nerve fibres are likely to contribute to taste transduction by mediating the interaction among TBCs and that between TBCs and taste nerve fibres. We investigated the functional expression of P2 receptor subtypes on TBCs of mouse fungiform papillae. Electrophysiological studies showed that 100 microm ATP applied to their basolateral membranes either depolarized or hyperpolarized a few cells per taste bud. Ca(2+) imaging showed that similarly applied 1 mum ATP, 30 microm BzATP (a P2X(7) agonist), or 1 microm 2MeSATP (a P2Y(1) and P2Y(11) agonist) increased intracellular Ca(2+) concentration, but 100 microm UTP (a P2Y(2) and P2Y(4) agonist) and alpha,beta-meATP (a P2X agonist except for P2X(2), P2X(4) and P2X(7)) did not. RT-PCR suggested the expression of P2X(2), P2X(4), P2X(7), P2Y(1), P2Y(13) and P2Y(14) among the seven P2X subtypes and seven P2Y subtypes examined. Immunohistostaining confirmed the expression of P2X(2). The exposure of the basolateral membranes to 3 mm ATP for 30 min caused the uptake of Lucifer Yellow CH in a few TBCs per taste bud. This was antagonized by 100 microm PPADS (a non-selective P2 blocker) and 1 microm KN-62 (a P2X(7) blocker). These results showed for the first time the functional expression of P2X(2) and P2X(7) on TBCs. The roles of P2 receptor subtypes in the taste transduction, and the renewal of TBCs, are discussed.

Fate of the hypertrophic chondrocyte: microenvironmental perspectives on apoptosis and survival in the epiphyseal growth plate

The goal of this review is to examine the fate of the hypertrophic chondrocyte in the epiphyseal growth plate and consider the impact of the cartilage microenvironment on cell survival and apoptosis. Early investigations pointed to a direct role of the hypertrophic chondrocyte in osteogenesis. The terminally differentiated cells were considered to undergo a dramatic change in shape, size, and phenotype, and assume the characteristics of an osteoblast. While some studies have supported the notion of transdifferentiation, much of the evidence in favor of reprogramming epiphyseal chondrocytes is circumstantial and based on microscopic evaluation of cells that are present at the chondro-osseous junction. Although these investigations provided a novel perspective on endochondral bone formation, they were flawed by the failure to consider the importance of stem cells in osseous tissue formation. Subsequent studies indicated that many, if not all, of the cells of the cartilage plate die through the induction of apoptosis. With respect to agents that mediate apoptosis, at the chondro-osseous junction, solubilization of mineral and hydrolysis of organic matrix constituents by septoclasts generates high local concentrations of ions, peptides, and glycans, and secreted matrix metalloproteins. Individually, and in combination, a number of these agents serve as potent chondrocyte apoptogens. We present a new concept: hypertrophic cells die through the induction of autophagy. In the cartilage microenvironment, combinations of local factors cause chondrocytes to express an initial survival phenotype and oxidize their own structural macromolecules to generate ATP. While delaying death, autophagy leads to a state in which cells are further sensitized to changes in the local microenvironment. One such change is similar to ischemia reperfusion injury, a condition that leads to tissue damage and cell death. In the growth cartilage, an immediate effect of this type of injury is sensitization to local apoptogens. These two concepts (type II programmed cell death and ischemia reperfusion injury) emphasize the importance of the local microenvironment, in particular pO(2), in directing chondrocyte survival and apoptosis.

Purinergic Signaling Along the Renal Tubule: The Current State of Play

ATP-sensitive P2 receptors have been described in all mammalian nephron segments. A reductionist approach has demonstrated purinergic control of renal transporters in cell lines and heterologous expression systems. This brief update focuses on what is known of these receptors in native nephron segments (excluding the glomerulus) and outlines recent advances and possible future directions.

Genes for magnesium transport

We know very little about the regulation of magnesium uptake and the control of magnesium homeostasis. After years of relative neglect, however, rapid progress is now being made in understanding the molecular biology of magnesium transport in eukaryotes. Several new gene families have been implicated, and tools are in place for the dissection of the biochemical and biological roles played by the encoded proteins.

The fate of the terminally differentiated chondrocyte: evidence for microenvironmental regulation of chondrocyte apoptosis

Chondrocytes contained within the epiphyseal growth plate promote rapid bone growth. To achieve growth, cells activate a maturation program that results in an increase in chondrocyte number and volume and elaboration of a mineralized matrix; subsequently, the matrix is resorbed and the terminally differentiated cells are deleted from the bone. The major objective of this review is to examine the fate of the epiphyseal chondrocytes in the growing bone. Current studies strongly suggest that the terminally differentiated epiphyseal cells are deleted from the cartilage by apoptosis. Indeed, morphological, biochemical, and end-labeling techniques confirm that death is through the apoptotic pathway. Since the induction of apoptosis is spatially and temporally linked to the removal of the cartilage matrix, current studies have examined the apoptogenic activity of Ca(2+)-, Pi-, and RGD-containing peptides of extracellular matrix proteins. It is observed that all of these molecules are powerful apoptogens. With respect to the molecular mechanism of apoptosis, studies of cell death with Pi as an apoptogen indicate that the anion is transported into the cytosol via a Na(+/)Pi transporter. Subsequently, there is activation of caspases, generation of NO, and a decrease in the thiol reserve. Finally, we examine the notion that chondrocytes transdifferentiate into osteoblasts, and briefly review evidence for, and the rationale of, the transdifferentiation process. It is concluded that specific microenvironments exist in cartilage that can serve to direct chondrocyte apoptosis.

Molecular physiology of P2X receptors

P2X receptors are membrane ion channels that open in response to the binding of extracellular ATP. Seven genes in vertebrates encode P2X receptor subunits, which are 40-50% identical in amino acid sequence. Each subunit has two transmembrane domains, separated by an extracellular domain (approximately 280 amino acids). Channels form as multimers of several subunits. Homomeric P2X1, P2X2, P2X3, P2X4, P2X5, and P2X7 channels and heteromeric P2X2/3 and P2X1/5 channels have been most fully characterized following heterologous expression. Some agonists (e.g., alphabeta-methylene ATP) and antagonists [e.g., 2',3'-O-(2,4,6-trinitrophenyl)-ATP] are strongly selective for receptors containing P2X1 and P2X3 subunits. All P2X receptors are permeable to small monovalent cations; some have significant calcium or anion permeability. In many cells, activation of homomeric P2X7 receptors induces a permeability increase to larger organic cations including some fluorescent dyes and also signals to the cytoskeleton; these changes probably involve additional interacting proteins. P2X receptors are abundantly distributed, and functional responses are seen in neurons, glia, epithelia, endothelia, bone, muscle, and hemopoietic tissues. The molecular composition of native receptors is becoming understood, and some cells express more than one type of P2X receptor. On smooth muscles, P2X receptors respond to ATP released from sympathetic motor nerves (e.g., in ejaculation). On sensory nerves, they are involved in the initiation of afferent signals in several viscera (e.g., bladder, intestine) and play a key role in sensing tissue-damaging and inflammatory stimuli. Paracrine roles for ATP signaling through P2X receptors are likely in neurohypophysis, ducted glands, airway epithelia, kidney, bone, and hemopoietic tissues. In the last case, P2X7 receptor activation stimulates cytokine release by engaging intracellular signaling pathways.

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.