Identification of a purine (P2) receptor linked to ion transport in a cultured renal (MDCK) epithelium

Purinergic signalling in the kidney in health and disease

The involvement of purinergic signalling in kidney physiology and pathophysiology is rapidly gaining recognition and this is a comprehensive review of early and recent publications in the field. Purinergic signalling involvement is described in several important intrarenal regulatory mechanisms, including tuboglomerular feedback, the autoregulatory response of the glomerular and extraglomerular microcirculation and the control of renin release. Furthermore, purinergic signalling influences water and electrolyte transport in all segments of the renal tubule. Reports about purine- and pyrimidine-mediated actions in diseases of the kidney, including polycystic kidney disease, nephritis, diabetes, hypertension and nephrotoxicant injury are covered and possible purinergic therapeutic strategies discussed.

Agonists that increase [Ca²⁺](i) halt the movement of acidic cytoplasmic vesicles in MDCK cells

Translocation of vesicles within the cytoplasm is essential to normal cell function. The vesicles are typically transported along the microtubules to their destination. The aim of this study was to characterize the vesicular movement in resting and stimulated renal epithelial cells. MDCK cells loaded with either quinacrine or acridine orange, dyes taken up by acidic vesicles, were observed at 37°C in semiopen perfusion chambers. Time-lapse series were analyzed by Imaris software. Our data revealed vigorous movement of stained vesicles in resting MDCK cells. These movements seem to require intact microtubules because nocodazole leads to a considerable reduction of the vesicular movements. Interestingly, we found that extracellular ATP caused the vesicular movement to cease. This observation was obvious in time lapse. Similarly, other stimuli known to increase the intracellular Ca²⁺ concentration ([Ca²⁺](i)) in MDCK cells (increment in the fluid flow rate or arginine vasopressin) also reduced the vesicular movement. These findings were quantified by analysis of single vesicular movement patterns. In this way, ATP was found to reduce the lateral displacement of the total population of vesicles by 40%. Because all these perturbations increase [Ca²⁺](i), we speculated that this increase in [Ca²⁺](i) was responsible for the vesicle arrest. Therefore, we tested the effect of the Ca²⁺ ionophore, ionomycin (1 μM), which in the presence of extracellular Ca²⁺ resulted in a considerable and sustained reduction of vesicular movement amounting to a 58% decrease in average lateral vesicular displacement. Our data suggest that vesicles transported on microtubules are paused when subjected to high intracellular Ca²⁺ concentrations. This may provide an additional explanation for the cytotoxic effect of high [Ca²⁺](i).

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.

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.

Mitochondria play an important role in adenosine-induced ATP release from Madin-Darby canine kidney cells

We previously found that adenosine stimulates ATP release from Madin-Darby canine kidney (MDCK) cells, by activating an Ins(1,4,5)P(3) sensitive-calcium (Ca(2+)) pathway through the stimulation of A(1) receptors. Thus, we investigated the intracellular pathway of ATP efflux after the rise in intracellular Ca(2+) in MDCK cells. Adenosine evoked an increase in mitochondrial Ca(2+) using Rhod-2/AM, a mitochondrial Ca(2+) indicator. Adenosine-induced ATP release was inhibited by mitochondrial modulators, such as oxidative phosphorylation modulators (carbonyl cyanide 3-chlorophenylhydrazone and oligomycin), mitochondrial ADP/ATP carrier inhibitors (N-ethylmaleimide, carboxyatractyloside and bongkrekic acid), a mitochondrial Na(+)-Ca(2+) exchange inhibitor (CGP-37157). In addition, mitochondrial modulators significantly reduced intracellular ATP content. On the other hand, 2-deoxy-glucose (2-DG) induced a greater decrease in intracellular ATP content than mitochondrial modulators. ATP release was still induced by adenosine in the presence of 5mM 2-DG. These results suggest that mitochondria play an important role in the signaling pathway of adenosine-triggered ATP release in MDCK cells.

Cl- secretion in ATP-treated renal epithelial C7-MDCK cells is mediated by activation of P 2Y1 receptors, phospholipase A2 and protein kinase A

This study examines the mechanism of P 2Y-induced Cl- secretion in monolayers of C7-Madin-Darby canine kidney (MDCK) cells triggered by basolateral application of ATP and measured as transcellular short current (I(SC)). Both ATP-induced arachidonic acid (AA) synthesis and I(SC) in ATP-treated cells were abolished by the phosholipase A2 (PLA2) inhibitor, AACOCF3. The cyclo-oxygenase inhibitor indomethacin decreased I(SC) and cAMP production in ATP-treated cells with an IC50 of approximately 0.3 microm. ATP led to rapid activation of cAMP-dependent protein kinase A (PKA), as estimated by phosphorylation of a vasodilator-stimulated phosphoprotein. PKA activity and I(SC) evoked by ATP, as well as by prostaglandin E1 (PGE1), were diminished in the presence of the PKA inhibitor H-89 or an adenovirus-mediated expression of PKA-inhibitor protein, PKI. In contrast, indomethacin completely blocked the increment of PKA and I(SC) triggered by ATP and AA, but did not affect PKA activation and I(SC) detected with PGE1. The kinetics of [Ca2+]i elevation in ATP- and thapsigargin-treated cells were similar and suppressed by the Ca(2+)i chelator BAPTA. Neither baseline nor maximal increment of ATP-induced I(SC) was affected by thapsigargin and BAPTA. Real-time PCR showed that C7 cells express more mRNA for P 2Y1 and P 2Y2 than for other P 2Y receptor subtypes. The rank order of potency (2MeSATP > ATP > ADP >> UTP) indicates that P 2Y1 rather than P 2Y2 receptors contribute to PKA and I(SC) activation. Viewed collectively, these data show that Cl- secretion in C7-MDCK monolayers treated with basolateral ATP is triggered by P 2Y1 receptors and is mediated by subsequent [Ca2+]i-independent activation of PLA2 and PKA.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Control of epithelial transport via luminal P2 receptors

P2 membrane receptors are specifically activated by extracellular nucleotides like ATP, ADP, UTP, and UDP. P2 receptors are subdivided into metabotropic P2Y and ionotropic P2X receptors. They are expressed in all tissues and induce a variety of biological effects. In epithelia, they are found in both the basolateral and the luminal membranes. Their widespread luminal expression in nearly all transporting epithelia and their effect on transport are summarized. The P2Y(2) receptor is a prominent luminal receptor in many epithelia. Other luminal P2 receptors include the P2X(7), P2Y(4), and P2Y(6) receptors. Functionally, luminal P2Y(2) receptor activation elicits differential effects on ion transport. In nearly all secretory epithelia, intracellular Ca(2+) concentration-activated ion conductances are stimulated by luminal nucleotides to induce Cl(-), K(+), or HCO(3)(-) secretion. This encompasses respiratory and various gastrointestinal epithelia or tissues like the conjunctiva of the eye and the epithelium of sweat glands. In the distal nephron, all active transport processes appear to be inhibited by luminal nucleotides. P2Y(2) receptors inhibit Ca(2+) and Na(+) absorption and K(+) secretion. Commonly, in all steroid-sensitive epithelia (lung, distal nephron, and distal colon), luminal ATP/UTP inhibits epithelial Na(+) channel-meditated Na(+) absorption. ATP is readily released from epithelial cells onto their luminal aspect, where ecto-nucleotidases promote their metabolism. Adenosine generated by the action of 5'-nucleotidase may elicit further effects on ion transport, often opposite those of ATP. ATP release from epithelia continues to be poorly understood. Integrated functional concepts for luminal P2 receptors are suggested: 1) luminal P2 receptors are part of an epithelial "secretory" defense mechanism; 2) they may be involved in the regulation of cell volume when transcellular solute transport is out of balance; 3) ATP and adenosine may be important autocrine/paracrine regulators mediating cellular protection and regeneration after ischemic cell damage; and 4) ATP and adenosine have been suggested to mediate renal cyst growth and enlargement in polycystic kidney disease.

Autocrine extracellular purinergic signaling in epithelial cells derived from polycystic kidneys

ATP and its metabolites are potent autocrine agonists that act extracellularly within tissues to affect epithelial function. In polycystic kidneys, renal tubules become dilated and/or encapsulated as cysts, creating abnormal microenvironments for autocrine signaling. Previously, our laboratory has shown that high-nanomolar to micromolar quantities of ATP are released from cell monolayers in vitro and detectable in cyst fluids from microdissected human autosomal dominant polycystic kidney (ADPKD) cysts. Here, we show enhanced ATP release from autosomal recessive polycystic kidney (ARPKD) and ADPKD epithelial cell models. RT-PCR and immunoblotting for P2Y G protein-coupled receptors and P2X purinergic receptor channels show expression of mRNA and/or protein for multiple subtypes from both families. Assays of cytosolic Ca(2+) concentration and secretory Cl(-) transport show P2Y and P2X purinergic receptor-mediated stimulation of Cl(-) secretion via cytosolic Ca(2+)-dependent signaling. Therefore, we hypothesize that autocrine purinergic signaling may augment detrimentally cyst volume expansion in ADPKD or tubule dilation in ARPKD, accelerating disease progression.

Extracellular nucleotide signaling along the renal epithelium

During the past two decades, several cell membrane receptors, which preferentially bind extracellular nucleotides, and their analogs have been identified. These receptors, collectively known as nucleotide receptors or "purinergic" receptors, have been characterized and classified on the basis of their biological actions, their pharmacology, their molecular biology, and their tissue and cell distribution. For these receptors to have biological and physiological relevance, nucleotides must be released from cells. The field of extracellular ATP release and signaling is exploding, as assays to detect this biological process increase in number and ingenuity. Studies of ATP release have revealed a myriad of roles in local regulatory (autocrine or paracrine) processes in almost every tissue in the body. The regulatory mechanisms that these receptors control or modulate have physiological and pathophysiological roles and potential therapeutic applications. Only recently, however, have ATP release and nucleotide receptors been identified along the renal epithelium of the nephron. This work has set the stage for the study of their physiological and pathophysiological roles in the kidney. This review provides a comprehensive presentation of these issues, with a focus on the renal epithelium.

P2Y receptors of MDCK cells: epithelial cell regulation by extracellular nucleotides

1. Madin-Darby canine kidney (MDCK) cells, a well- differentiated renal epithelial cell line derived from distal tubule/collecting duct, respond to extracellular nucleotides by altering ion flux and the production of arachidonic acid-derived products, in particular prostaglandin E2 (PGE2). Our work has defined the receptors and signalling events involved in such responses. 2. We have found evidence for expression of at least three P2Y receptor subtypes (P2Y1, P2Y2 and P2Y11) in MDCK-D1 cells, a subclone from parental MDCK. 3. These receptors appear to couple to increases in calcium and protein kinase C activity, probably via a Gq/G11-mediated activation of phospholipase C. 4. In addition, P2Y receptor activation can promote a prominent increase in cAMP. This includes both a P2Y2 receptor-mediated cyclo-oxygenase (COX)-dependent component and another COX-independent component mediated by other P2Y receptors. 5. We have documented that changing media in which cells are grown releases ATP and, in turn, activates P2Y receptors. Such release of ATP contributes in a major way to basal cAMP levels in these cells. 6. The data indicate that MDCK cells are a useful model to define the regulation of epithelial cells by extracellular nucleotides. Of particular note, spontaneous or stretch-induced release of ATP and subsequent activation of one or more P2Y receptors contributes to establishing the basal activity of signalling pathways.

P2Y(2) receptor of MDCK cells: cloning, expression, and cell-specific signaling

Madin-Darby canine kidney (MDCK)-D1 cells, a canine renal epithelial cell line, co-express at least three different P2Y receptor subtypes: P2Y(1), P2Y(2), and P2Y(11) (24). Stimulation of P2Y receptors in these cells results in the release of arachidonic acid (AA) and metabolites and the elevation of intracellular cAMP. To define in more precise terms the signaling contributed by the MDCK-D1 P2Y(2) (cP2Y(2)) receptor, we have cloned and heterologously expressed it in CF2Th (canine thymocyte) cells, a P2Y(2)-null cell. Analysis by RT-PCR indicated that canine P2Y(2) receptors are expressed in skeletal muscle, spleen, kidney, lung, and liver. When expressed in CF2Th cells, cP2Y(2) receptors promoted phospholipase C-mediated phosphatidylinositol (PI) hydrolysis [uridine 5'-triphosphate > or = ATP > adenosine 5'-diphosphate > 2MT-ATP] and mobilization of intracellular Ca(2+). In contrast to their actions in MDCK-D1 cells, cP2Y(2) receptors did not stimulate formation of cAMP or AA release when expressed in CF2Th cells. The data indicate that cell setting plays an essential role in the ability of P2Y receptors to regulate AA release and cAMP formation. In particular, renal epithelial cells preferentially express components critical for cP2Y(2)-induced cAMP formation, including the expression of enzymes involved in the generation and metabolism of AA and receptors that respond to PGE(2).

ATP activates cAMP production via multiple purinergic receptors in MDCK-D1 epithelial cells. Blockade of an autocrine/paracrine pathway to define receptor preference of an agonist

Extracellular nucleotides regulate function in many cell types via activation of multiple P2-purinergic receptor subtypes. However, it has been difficult to define which individual subtypes mediate responses to the physiological agonist ATP. We report a novel means to determine this by exploiting the differential activation of an autocrine/paracrine signaling pathway. We used Madin-Darby canine kidney epithelial cells (MDCK-D1) and assessed the regulation of cAMP formation by nucleotides. We found that ATP, 2-methylthio-ATP (MT-ATP) and UTP increase cAMP production. The cyclooxygenase inhibitor indomethacin completely inhibited UTP-stimulated, did not inhibit MT-ATP-stimulated, and only partially blocked ATP-stimulated cAMP formation. In parallel studies, ATP and UTP but not MT-ATP stimulated prostaglandin production. By pretreating cells with indomethacin to eliminate the P2Y2/prostaglandin component of cAMP formation, we could assess the indomethacin-insensitive P2 receptor component. Under these conditions, ATP displayed a ten-fold lower potency for stimulation of cAMP formation compared with untreated cells. These data indicate that ATP preferentially activates P2Y2 relative to other P2 receptors in MDCK-D1 cells (P2Y1 and P2Y11, as shown by reverse transcriptase polymerase chain reaction) and that P2Y2 receptor activation is the principal means by which ATP increases cAMP formation in these cells. Blockade of autocrine/paracrine signaling can aid in dissecting the contribution of multiple receptor subtypes activated by an agonist.

Capacitative Ca2+ entry (CCE) induced by luminal and basolateral ATP in polarised MDCK-C7 cells is restricted to the basolateral membrane

In previous studies we have characterised various properties of capacitative Ca2+ entry (CCE) in different epithelia. After Ca2+ store depletion with PLC/InsP3-coupled agonists or by inhibition of store Ca2+ uptake, with for example thapsigargin, Ca2+ influx is activated. This leads to a sustained cellular response (e.g. NaCl secretion). In the present study, we have investigated CCE in polarised MDCK-C7 cells grown on permeable supports in a chamber allowing for separate luminal and basolateral perfusion. The transepithelial resistance (Rte) and voltage (Vte) were measured simultaneously to verify the tightness of the epithelial monolayers. MDCK-C7 cells grew to very tight monolayers (Rto > 3000 omega.cm2). Apical ATP (100 mumol/l) led to a biphasic [Ca2+]i increase. Removal of apical Ca2+ in the continuous presence of ATP did not reduce the stimulated plateau. However, removal of Ca2+ from the basolateral side rapidly and completely interrupted the [Ca2+]i plateau to below basal values ([Ca2+]i decrease during plateau phase after removal of basolateral Ca2+ = 213 +/- 15 nmol/l, n = 9). Furthermore, MDCK-C7 responded to basolateral ATP (100 mumol/l) with a biphasic [Ca2+]i transient. Again the plateau phase of the ATP-induced [Ca2+]i effect was fully dependent on the presence of basolateral but not apical Ca2+ ([Ca2+]i decrease during plateau phase after removal of basolateral Ca2+ = 196 +/- 5 nmol/l, n = 10). Receptor-independent depletion of cytosolic Ca2+ stores with thapsigargin from both sides led to a rise in [Ca2+]i, which was also exclusively dependent on the presence of basolateral Ca2+ (n = 8). These data indicate that MDCK-C7 cells express luminal and basolateral P2-receptors coupled to PLC/InsP3/Ca2+. ATP applied from both sides induced a sustained [Ca2+]i plateau which was due to transmembrane Ca2+ influx. The ATP- and thapsigargin-induced Ca2+ influx pathway was exclusively located in the basolateral membrane.

Adenine nucleotide and protein kinase C regulation of renal tubular epithelial cell wound healing

The present studies were done to determine the effect of selected adenine nucleotides on healing of wounds made by mechanically denuding areas in confluent monolayers of renal tubular epithelial cells. We found that hydrolyzable and nonhydrolyzable forms of ATP but not UTP stimulated healing of LLC-PK1 cell wounds, while both ATP and UTP promoted healing of MDCK cell wounds, suggesting that different subtypes of purinoceptors regulated wound healing in these cells. Stimulation of wound healing by ATP was equivalent in control cells and in cells in which irradiation suppressed proliferation, suggesting a prominent role for cell migration in the healing process. Since ATP receptors are often linked to activation of protein kinase C, the effect of a protein kinase C activator (4 beta-phorbol 12-myristate 13-acetate, PMA) on wound healing was studied. Long-term (24 hr) exposure to PMA inhibited while short-term (30-120 min) exposure to PMA enhanced cell wound healing. Two chemically and mechanistically dissimilar protein kinase C inhibitors (calphostin C and chelerythrine) inhibited LLC-PK1 and MDCK cell wound healing, and calphostin C prevented ATP enhancement of LLC-PK1 healing. These observations suggest a role for protein kinase C in regulation of basal and adenine nucleotide-stimulated wound healing. Adenosine triphosphate did not affect cell-cell adhesion of either LLC-PK1 or MDCK cells. Adenine nucleotides and PMA enhanced and calphostin C inhibited short-term adhesion of LLC-PK1 and MDCK cells to plastic and to other substrates such as fibronectin, laminin and collagen type IV.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium-dependent regulation of membrane ion permeability in a cell line derived from the equine sweat gland epithelium

We measured the rates of 125I- and 86Rb+ efflux from preloaded, cultured equine sweat gland cells. The calcium ionophore ionomycin increased the efflux of both isotopes. Anion efflux was unaffected by Ba2+, but this cation inhibited 86Rb(+)-efflux, suggesting that [Ca2+]i-activated potassium channels were present. Activation of these channels was not, however, important for the efflux of anions. We measured 125I- efflux from valinomycin-depolarised cells in which anion cotransport was inhibited. Changes in 125I- efflux reflect changes in anion permeability under these conditions, and ionomycin caused a clear permeability increase that was abolished by the anion channel blocker diphenylamine-2-carboxylate. ATP and UTP increased the efflux of both isotopes, suggesting that type P2U purine receptors allow these nucleotides to regulate membrane permeability.

Regulation of transepithelial ion transport by two different purinoceptors in the apical membrane of canine kidney (MDCK) cells

1. The effect of extracellular nucleotides on the transepithelial ion transport of Madin Darby canine kidney cells (MDCK) was investigated. Cells were grown up to confluency on permeable supports and the short circuit current (ISC) was measured with an Ussing chamber-like mini-perfusion system. 2. Apical ATP stimulated a biphasic ISC increase consisting of a first rapid and transient peak followed by a broader one. 3. The first peak evoked by ATP was reversibly blocked by basilen blue (BB) in a concentration-dependent fashion, with an EC50 of 7.5 microM. 4. The P2 gamma receptor agonist, 2-methylthioATP (2-MeSATP) caused a single transient ISC increase that was completely blocked by pretreatment with BB. On the contrary, the P2x agonist, alpha, beta-methylene ATP (alpha, beta-meATP) was almost completely ineffective on ISC. UTP essentially induced a monophasic response the time-course of which resembled that of the second peak stimulated by ATP. The agonist potency order was 2-MeSATP > or = ATP >> UTP, alpha, beta-meATP for the first peak and UTP > or = ATP > 2-MeSATP > alpha, beta-meATP for the second peak. 5. Monolayer incubation with the membrane permeable calcium chelator [bis-o-aminophenoxy)-ethane-N,N,N',N',-tetraacetic acid, tetra(acetoximethyl)-ester] (BAPTA/AM) inhibited the ATP-evoked first peak. 6. The non-hydrolyzable ATP analogue, adenosine-5'-O-(3-thio)-trisphosphate (ATP-gamma-S) elicited a biphasic response similar to that of ATP. The P1 receptor agonist, 2-chloroadenosine and CGS-21680, were almost unable to induce an ISC increase.2+ increase. The second induces prostaglandin synthesis probably through a P2U receptor activation.

Nucleotides mobilize intracellular calcium stores of renal proximal cells in primary culture: existence of a suramin-sensitive mechanism

Changes of intracellular calcium concentrations [Ca2+]i were measured in primary cultured rabbit proximal convoluted tubules (PCT). A dual-excitation, digital-imaging inverted microscope was used to monitor the fura-2 fluorescence. The basal calcium level was 106 +/- 11 nM (n = 36). The stimulatory effects of adenosine triphosphate (ATP), adenosine diphosphate (ADP) and adenosine were studied. ATP and ADP induced transient increases of [Ca2+]i (1059 +/- 115% of the resting level (n = 29), and 659 +/- 134% (n = 10), respectively) by releasing calcium from cytoplasmic stores. Adenosine had less effect (279 +/- 48% of the resting level, n = 3). In the same conditions the ATP antagonist suramin (100 microM) inhibited the action of ATP and ADP to 231 +/- 52% (n = 3), and 308 +/- 29% (n = 4) of the resting level, respectively, but did not modify that of adenosine (281 +/- 72%, n = 3). A pretreatment (500 ng/ml for 2 h at 37 degrees C) of the culture with the toxin of Bordetella pertussis completely blocked the ATP response. Our results are evidence for the presence of a functional suramin-sensitive ATP and ADP puriceptor in cultured renal proximal cells. A pertussis-toxin-sensitive G protein is linked to the transduction mechanism. This receptor is distinct from an adenosine puriceptor also found in the proximal monolayer.

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

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

Ionic dependence of the extracellular ATP-induced permeabilization of transformed mouse fibroblasts: role of plasma membrane activities that regulate cell volume

Extracellular ATP rendered the plasma membrane of transformed mouse fibroblasts permeable to normally impermeant molecules. This permeability change was prevented by increasing the ionic strength of the isotonic medium with NaCl. Conversely, the cells exhibited increased sensitivity to ATP when the NaCl concentration was decreased below isotonicity, when the KCl concentration was increased above 5 mM while maintaining isotonicity, and when the pH of the medium was raised above 7.0. These conditions as well as the addition of ATP itself caused cell swelling. However, the effect of ATP was independent of cell volume and dependent upon the ionic strength and not the osmolarity of the medium since 1) addition of sucrose to isotonic medium did not prevent permeabilization although media made hypertonic with either sucrose or NaCl caused a decrease in cell volume; and 2) addition of sucrose or NaCl to hypotonic media caused a decrease in cell volume, but only NaCl addition decreased the response to ATP. Conditions that have been shown to inhibit plasma membrane proteins that play a reciprocal role in cell volume regulation had reciprocal effects on the permeabilization process, even though the effect of ATP was independent of cell volume. For example, inhibition of the Na+,K+-ATPase by ouabain increased sensitivity of cells to ATP while conditions which inhibit Na+,K+,Cl- -cotransporter activity, such as treatment of the cells with the diuretics furosemide or bumetanide or replacement of sodium chloride in the medium with sodium nitrate or thiocyanate, inhibited permeabilization. The furosemide concentration that inhibited permeabilization was greater than the concentration that inhibited Na+,K+,Cl- -cotransporter-mediated 86Rb+ (K+) uptake, suggesting that the effect of furosemide on the permeabilization process may not be specific for the Na+,K+,Cl- -cotransporter.

Effect of extracellular adenosine triphosphate on electrical properties of subconfluent Madin-Darby canine kidney cells

1. The present study has been performed to test for an influence of extracellular ATP on the potential differences across the cell membrane (PD) in subconfluent MDCK cells utilizing conventional microelectrodes. 2. In the absence of ATP, the mean measured PD was -47.5 +/- 0.3 mV (+/- S.E.M., n = 320). Application of 10 mumol/l ATP leads to rapid (less than 2 s) hyperpolarization of the cell membrane by -18.5 +/- 0.4 mV (n = 221), reduction of input resistance by 14 +/- 1 M omega (n = 106) and increase of the sensitivity of PD to alterations of extracellular potassium. 3. The concentration needed for half-maximal effect (K1/2) of ATP is approximately 0.5 mumol/l. ATP-gamma-S (K1/2 approximately 0.4 mumol/l) aand ADP (K1/2 approximately 0.9 similarly effective, whereas up to 1 mmol/l AMP or adenosine does not significantly alter PD. Application of 10 mumol/l theophylline, 1 mumol/l phentolamine and 10 mumol/l indomethacin does not blunt the hyperpolarizing effect of ATP. 4. The ATP-induced hyperpolarization is completely abolished in the presence of 1 mmol/l quinidine but only incompletely by 0.1 mmol/l quinidine or 1 mmol/l barium. In calcium-free extracellular fluid (1 mmol/l EDTA added) PD is 18.5 +/- 1.7 mV (n = 18). With reduced extracellular calcium, the hyperpolarizing effect of ATP is blunted (-12.3 +/- 1.6 mV, n = 18) and only transient. 5. In conclusion, ATP hyperpolarizes MDCK cells by increasing the potassium conductance. The activation of potassium channels requires calcium.

Control of anion and fluid secretion by apical P2-purinoceptors in the rat epididymis

1. Exogenous adenosine triphosphate (ATP) stimulated the short circuit current (SCC) in primary monolayer cultures of rat epididymal cells when added to the apical but not to the basolateral side of the monolayers. Half-maximal stimulation was achieved at 5 x 10(-8) M ATP. 2. The increase in SCC induced by ATP was dependent on the presence of extracellular Cl in the bathing solutions. 3. The effects of other adenosine derivatives, and purine and pyrimidine nucleotides were studied. Their orders of potency in stimulating SCC were: ATP greater than adenosine diphosphate much greater than adenosine monophosphate, adenosine, and ATP greater than inosine triphosphate greater than guanosine triphosphate greater than cytidine triphosphate. These results indicate that ATP interacts with a P2-purinoceptor at the apical membrane of the epididymal cells. 4. The SCC response to ATP was not blocked by 8-phenyltheophylline, a P1-purinoceptor antagonist or by propranolol. Although pretreatment of the cultures with piroxicam abolished the SCC response to bradykinin, it did not affect the response to ATP. This indicates that the SCC response to ATP was not mediated by an increase in the synthesis of prostaglandins. 5. Serosal to mucosal Cl flux (Js-m Cl) and net water flux were measured in the luminally perfused rat epididymis in vivo. ATP (1 microM) added to the luminal perfusion solution caused an increase in Js-m Cl and net water secretion by the epididymal duct. 6. Since spermatozoa contain a high concentration of ATP, it is proposed that ATP released from spermatozoa may affect anion and fluid secretion by the epididymis. The control of secretion via the apical purinoceptors offers a means by which spermatozoa regulate the fluidity of their own environment.

Role of adenine compounds in autonomic neurotransmission

Clearly adenine compounds exert numerous effects throughout the autonomic nervous system. The responses of various peripheral tissues to purines are summarized in Table 2. The evidence supporting a possible excitatory neurotransmitter function for ATP is very good in the vas deferens and good in both the bladder detrusor and certain blood vessels. ATP may also be an excitatory neurotransmitter in the colon, hepatocytes and frog atrium. These responses appear to be mediated by P2x-purinoceptors. There is good evidence supporting a role for ATP as an inhibitory neurotransmitter in the taenia coli and duodenum, and some support in the anal sphincter and possibly the rabbit portal vein; these responses appear to be mediated by P2y-purinoceptors. There is good evidence against ATP being an inhibitory neurotransmitter in the stomach fundic muscle and ileum. ATP (or more likely its metabolite adenosine) may act as an inhibitory neurotransmitter by interacting with postsynaptic P1-purinoceptors in cultured sympathetic neurones and also in the parasympathetic vesicle ganglion of the cat. It seems likely that ATP released from heart, platelets or vascular endothelium could be an endogenous relaxant of blood vessels through its actions on the endothelium. Although the addition of exogenous adenosine affects many tissues, evidence supporting modulatory functions for endogenous extracellular adenosine has only been clearly demonstrated in the ileum, gallbladder, vas deferens, fallopian tubes, kidney, blood vessels, carotid sinus, heart and adipose tissue. Both ATP and adenosine, released during periods of hypoxia or ischemia, could exert negative inotropic, chronotropic and dromotropic actions in the heart. In many cases, the potential sources of extracellular purines have not been established. This is particularly important when attempting to establish a neurotransmitter function for ATP in a tissue. For instance, the one outstanding piece of evidence required to confirm that ATP is an excitatory neurotransmitter released from sympathetic nerves in blood vessels is the unequivocal demonstration that it is, in fact, released from the sympathetic nerves when they are stimulated. To date, only the release of radiolabeled metabolites of ATP, possibly from post- rather than presynaptic sites, has been detected. Studies of the release of ATP are complicated by its rapid degradation extracellularly by ecto-ATPase. Unfortunately, there are no specific inhibitors of ecto-ATPase available at present, but one hopes that a suitable inhibitor will be developed shortly.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanism of action of ATP on intestinal epithelial cells. Cyclic AMP-mediated stimulation of active ion transport

ATP, ADP and AMP but not adenosine increased cyclic AMP in dispersed enterocytes prepared from guinea pig small intestine. This action of ATP was augmented by IBMX and was reproduced by App(NH)p or App(CH2)p. ATP also increased the formation of cyclic [14C]AMP in enterocytes that had been preincubated with [14C]adenine. Gpp(NH)p and NaF each caused persistent activation of adenylate cyclase in plasma membranes from enterocytes and ATP caused significant augmentation of this persistent activation. In addition to increasing cellular cyclic AMP and augmenting Gpp(NH)p and NaF-stimulated persistent activation of adenylate cyclase, ATP increased the Isc across mounted strips of small intestine and inhibited net absorption of fluid and electrolytes in segments of everted small intestine. These results indicate that intestinal epithelial cells possess a receptor that interacts with ATP and other adenine nucleotides and that receptor occupation by ATP causes activation of adenylate cyclase, increased cyclic AMP and changes in active ion transport across intestinal mucosa.

Cultured monolayers of MDCK cells: a novel model system for the study of epithelial development and function

The application of cell culture techniques to the study of epithelial transport function is illustrated by the renal-derived MDCK system. MDCK cells display a typical epithelial morphology with an asymmetric localisation of the (Na+-K+)-ATPase to the lateral interspace membranes. Transepithelial ion transport is observed (using cell monolayers grown on permeable millipore filters clamped into standard Ussing chambers) which is sensitive to hormonal stimulation by adrenaline, exogenous ATP, PGE1, vasopressin and aldosterone. The development of epithelial characteristics in culture is discussed.

Stimulation of Cl- secretion by exogenous ATP in cultured MDCK epithelial monolayers

Cultures epithelial monolayers of MDCK cells were grown upon Millipore filter supports and mounted in Ussing chambers for ion-transport studies. Addition of exogenous ATP to the basal bathing solutions resulted in a stimulation of the short-circuit current which was due to both an increased transmonolayer p.d. and an increased conductance. Measurements of tracer Na+ and Cl- fluxes demonstrate that the ATP-stimulated short-circuit current, results from basal to apical Cl- movement (secretion) across the cultured monolayer. ATP-stimulated net Cl- secretion was inhibited by furosemide (1 x 10(-4) M) added to the basal bathing solution and by elevating the basal medium K+ concentration from 5.4 to 54 mM. Both furosemide and elevated basal K+ exert their inhibitory action upon the ATP-dependent short circuit current primarily by abolishing the electrogenic component without affecting the increased transmonolayer conductance. Hyperpolarization of the transmonolayer potential difference by applied currents also reduces the ATP dependent increase in the short-circuit current. The increased short-circuit current was insensitive to replacement of medium Na+ by choline+, but was linearly related to Cl- concentration with isethionate (2-hydroxyethanesulphonate) replacements. NO3-, I-, and the thiocyanate anion were all ineffective substitutes for Cl- whereas Br- and acetate were only partially effective. Sodium thiocyanate (10 mM) in the presence of NaCl inhibited the ATP-stimulated short-circuit current.