Purine and pyrimidine nucleotide receptors in the apical membranes of equine cultured epithelia

Purinergic signalling in epithelial ion transport: regulation of secretion and absorption

Intracellular ATP, the energy source for many reactions, is crucial for the activity of plasma membrane pumps and, thus, for the maintenance of transmembrane ion gradients. Nevertheless, ATP and other nucleotides/nucleosides are also extracellular molecules that regulate diverse cellular functions, including ion transport. In this review, I will first introduce the main components of the extracellular ATP signalling, which have become known as the purinergic signalling system. With more than 50 components or processes, just at cell membranes, it ranks as one of the most versatile signalling systems. This multitude of system components may enable differentiated regulation of diverse epithelial functions. As epithelia probably face the widest variety of potential ATP-releasing stimuli, a special attention will be given to stimuli and mechanisms of ATP release with a focus on exocytosis. Subsequently, I will consider membrane transport of major ions (Cl(-) , HCO(3)(-) , K(+) and Na(+) ) and integrate possible regulatory functions of P2Y2, P2Y4, P2Y6, P2Y11, P2X4, P2X7 and adenosine receptors in some selected epithelia at the cellular level. Some purinergic receptors have noteworthy roles. For example, many studies to date indicate that the P2Y2 receptor is one common denominator in regulating ion channels on both the luminal and basolateral membranes of both secretory and absorptive epithelia. In exocrine glands though, P2X4 and P2X7 receptors act as cation channels and, possibly, as co-regulators of secretion. On an organ level, both receptor types can exert physiological functions and together with other partners in the purinergic signalling, integrated models for epithelial secretion and absorption are emerging.

Zinc-cyclen coordination to UTP, TTP or pyrophosphate induces pyrene excimer emission

Pyrene labelled Zn(2+)-cyclen 1 and bis-Zn(2+)-bis-cyclen 2 complexes were synthesized. The reversible coordination at physiological pH of Zn(2+)-cyclens to phosphate anions and to imide moieties, as present in thymine and uracil nucleotides, is well known. In the presence of analytes bearing a phosphate and an imide or two phosphate groups the formation of a ternary complex consisting of two pyrene-labelled metal complexes and the analyte molecule, is observed. The close proximity of the pyrene labels in the complex induces pyrene excimer emission, which is observable by the unarmed eye. By this, the presence of UMP, UDP, UTP and TTP in buffered aqueous solution is signalled, while other nucleotides are not able to induce excimer emission. In the same way, Zn(2+)-cyclen-pyrene acts as luminescent chemosensor for PP(i) and fructose-1,6-bisphosphate in aqueous buffer.

A preliminary study of the short circuit current (Isc) responses of sweat gland cells from normal and anhidrotic horses to purinergic and adrenergic agonists

The causal factors of equine anhidrosis have not yet been elucidated but defective electrolyte transport mechanisms in the gland are likely to be involved. To investigate this possibility, experiments were performed on cultured equine sweat gland epithelia from five free-sweating UK horses (3 intact males, 2 mares, aged 2-4 years) and from three free-sweating Singapore horses (1 intact male, 2 mares, aged 3-5 years) and three anhidrotic (Singapore) horses (1 intact male, 1 gelding, 1 mare, aged 3-6 years). Cultured cells from each animal were grown on permeable supports and loaded into Ussing chambers to quantify transepithelial resistance and agonist-induced electrolyte transport by the short circuit current (Isc) technique. Transepithelial resistances across the layers of cultured cells were not significantly different between cells from UK and Singapore free-sweating horses, but were significantly reduced in anhidrotic animals. Purinergic agonists added to the apical and basolateral aspects of the cultured cells caused similar increases in Isc between the two populations of unaffected cells, but Isc increases were significantly reduced in anhidrotic animals. Beta-adrenergic agonist stimulation of the anhidrotic cell layers failed to elicit any change in Isc. These pilot results not only confirm earlier conclusions from anatomical findings that failure in the secretory process occurs in anhidrosis but also indicate that both of the known ion transport mechanisms are involved. The trigger for these failures warrants further investigation.

Equine sweating and anhidrosis Part 1 ? equine sweating

Sweating has a variety of functions in mammals including pheromone action, excretion of waste products and maintenance of the skin surface ecosystem. In a small number of mammalian species, which includes humans and the Equidae, it also has an important role in thermoregulation. This review is focused specifically on the thermoregulatory role of sweat in Equidae and the causes of sweating failure (anhidrosis). The first part describes the glandular appearance, sweat composition, and output rates; and considers the latest theories on the glandular control and secretory mechanisms. It is concluded that the glands are not directly innervated but are controlled by the interplay of neural, humoral and paracrine factors. The secretory mechanism is not as simple as previously thought and is mediated by the dynamic interaction of activating pathways, including autocrine control not only of the secretory process but probably also of secretory cell reproduction, growth, and death.

Adenosine-evoked Na+ transport in human airway epithelial cells

BACKGROUND AND PURPOSE

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

EXPERIMENTAL APPROACH

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

KEY RESULTS

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

CONCLUSIONS AND IMPLICATIONS

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

Immunolocalization of aquaporin-5 expression in sweat gland cells from normal and anhidrotic horses

Western blot analysis showed that sweat gland cells from freely sweating horses expressed the water channel aquaporin-5 (AQP-5). Immunohistochemistry revealed a strong AQP-5-like activity reaction at the apical membrane of the glandular secretory cells, which was absent from the surrounding myoepithelium and all other skin structures. In anhidrotic horses, AQP-5 was also found at the apical membrane of the luminal sweat gland cells, but the level of expression reduced with the length of time that the animal had displayed anhidrosis. The level of AQP-5 expression was substantially reduced in animals with long-term anhidrosis, hence implicating water channel impairment as a possible factor in the development of this disorder.

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.

Nucleotide-induced restoration of conjunctival chloride and fluid secretion in adenovirus type 5-infected pigmented rabbit eyes

We evaluated the role of extracellular UTP and other nucleotides in the regulation of chloride (JCl) and fluid secretion (JCl) across the pigmented rabbit conjunctiva. Jv was determined in freshly excised conjunctival tissues mounted between two buffer reservoirs maintained in an enclosed environment at 37 degrees C. Short circuit current (Isc) and 36Cl flux were measured using modified Ussing-type chambers. Fluid flux measurements were made with a pair of capacitance probes. After observing the baseline for 15 to 30 min, fluid flux was measured in the presence of mucosally applied nucleotides (10 microM) for a period of 30 min. Mucosal application of 10 microM each of UTP, UDP, ATP, ADP, AMP, adenosine, and ATP-gamma-S transiently stimulated fluid secretion across the conjunctiva to a significant extent for 10 to 15 min. Other nucleotides did not show any significant effect. The stimulation of fluid secretion correlated well with the stimulation in Isc (r2 = 0.85). UTP (0.1-1000 microM) led to a maximal increase in fluid secretion by 11.72 +/- 0.48 microl/(h x cm2) with an EC50 value of 10.39 +/- 1.08 microM. ATP (0.1-1000 microM) caused a maximal increase in fluid secretion by 11.89 +/- 0.88 microl/(h x cm2) with an EC50 value of 17.23 +/- 2.63 microM. Adenovirus type 5 (Ad5) infection significantly decreased both net 36Cl secretion across the conjunctiva by approximately 56% and the rate of fluid secretion by approximately 56%. UTP (10 microM), but not 1 mM 8-bromo-cAMP, was able to elicit a normal stimulatory response in the Ad5-infected tissues. In conclusion, mucosal application of purinergic nucleotides may be therapeutically important in restoring ion and fluid secretion in the diseased conjunctiva.

Multiple purinergic receptors lead to intracellular calcium increases in cultured rat Sertoli cells

Our previous study has demonstrated the activation of calcium-dependent Cl(-) secretion through P2Y(2) receptors by extracellular nucleotides. To evaluate the contribution and involvement of other receptor subtypes to this physiological response, the purpose of this study was to investigate the regulation of intracellular calcium concentration ([Ca(2+)](i)) by different purinergic agonists in cultured rat Sertoli cells. Cultured epithelia of Sertoli cells from immature rats were grown on glass coverslips and the purity was confirmed by immunohistochemistry. Intracellular calcium concentration was monitored by microspectrofluorimetric technique using calcium sensitive fluorescent dye Fura-2. Results showed that UTP and ATP consistently increased [Ca(2+)](i). Adenosine, AMP and 2-methylthio-ADP (2-MeSADP) failed to elicit a discernible response. Cross desensitization experiments suggested that the responses to UTP and ATP were mediated by a common receptor population. This is consistent with the presence of receptors belonging to the P2Y(2) subclass. Sertoli cells also responded to 2-methylthio-ATP (2-MeSATP) and ADP but not 2-methylthio-ADP (2-MeSADP) in a concentration-dependent manner. Moreover, the ATP-or UTP-pretreated epithelia continued to respond to ADP, indicating that these cells express at least one additional receptor population that allowed nucleotides to increase [Ca(2+)](i). Apart from the P2Y receptor agonists, the P2X(4) and P2X(7) agonist, 2' and 3'-O-(4-benzoylbenzoyl)-ATP (Bz-ATP), also evoked [Ca(2+)](i) increases in rat Sertoli cells. Messenger RNA transcript corresponding to P2Y(1), P2Y(2), P2X(4) and P2X(7) receptors genes were detected using RT-PCR. Taken together, these data indicate that both metabotropic receptors and ionotropic receptors are present in rat Sertoli cells through which extracellular nucleotides can act.

Equine anhidrosis

The molecular basis of the pathophysiology of anhidrosis is still not well understood. Therefore, treatments are more often based on clinical impressions than on scientific fact. Treatment options for this condition will improve only when more is known about the molecular events that cause anhidrosis, especially as they relate to beta2-receptor dysfunction and stimulus-secretion coupling in the sweat glands of affected horses. Although this additional information is being attained, sound environmental management will continue to be a very important aspect of the treatment of horses affected with anhidrosis.

Stimulation of Cl- secretion via membrane-restricted Ca2+ signaling mediated by P2Y receptors in polarized epithelia

Extracellular nucleotides such as ATP have been shown to regulate ion transport processes in a variety of epithelia. This effect is mediated by the activation of plasma membrane P2Y receptors, which leads to Ca(2+) signaling cascade. Ion transport processes (e.g. activation of apical calcium-dependent Cl(-) channels) are then stimulated via an increase in [Ca(2+)](i). Many polarized epithelia express apical and/or basolateral P2Y receptors. To test whether apical and basolateral stimulation of P2Y receptors elicit polarized Ca(2+) signaling and anion secretion, we simultaneously measured the two parameters in polarized epithelia. Although activation of P2Y receptors located at both apical and basolateral membranes evoked an increase in [Ca(2+)](i), only apical P2Y receptors-coupled Ca(2+) release stimulated an increase in anion secretion. Moreover, the calcium influx evoked by apical and basolateral P2Y receptor stimulation is predominately via the basolateral membrane domain. It appears that the apical P2Y receptor-regulated Ca(2+) release and activation of apical Cl(-) channels is compartmentalized in polarized epithelia with basolateral P2Y-stimulated Ca(2+) release failing to activate anion secretion. These data suggest that there may be two distinct ATP-releasable Ca(2+) pools, each coupled to apical and basolateral membrane receptor but linked to the same calcium influx pathway located at the basolateral membrane.

Stimulation of chloride secretion by baicalein in isolated rat distal colon

The effect of baicalein on mucosal ion transport in the rat distal colon was investigated in Ussing chambers. Mucosal addition of baicalein (1-100 microM) elicited a concentration-dependent short-circuit current (I(sc)) response. The increase in I(sc) was mainly due to Cl(-) secretion. The presence of mucosal indomethacin (10 microM) significantly reduced both the basal and subsequent baicalein-evoked I(sc) responses. The baicalein-induced I(sc) were inhibited by mucosal application of diphenylamine-2-carboxylic acid (100 microM) and glibenclamide (500 microM) and basolateral application of chromanol 293B (30 microM), a blocker of K(v)LQT1 channels and Ba(2+) ions (5 mM). Treatment of the colonic mucosa with baicalein elicited a threefold increase in cAMP production. Pretreating the colonic mucosa with carbachol (100 microM, serosal) but not thapsigargin (1 microM, both sides) abolished the baicalein-induced I(sc). Addition of baicalein subsequent to forskolin induced a further increase in I(sc). These results indicate that the baicalein evoked Cl(-) secretion across rat colonic mucosa, possibly via a cAMP-dependent pathway. However, the action of baicalein cannot be solely explained by its cAMP-elevating effect. Baicalein may stimulate Cl(-) secretion via a cAMP-independent pathway or have a direct effect on cystic fibrosis transmembrane conductance regulator.

Luminal P2Y2 receptor-mediated inhibition of Na+ absorption in isolated perfused mouse CCD

Extracellular nucleotides regulate renal transport. A luminal P2Y2 receptor in mouse cortical collecting duct (CCD) principal cells has been demonstrated elsewhere. Herein the effects of adenosine triphosphate (ATP) and uridine triphosphate (UTP) on electrogenic Na+ absorption in perfused CCD of mice kept on a low-NaCl diet were investigated. Simultaneously, transepithelial voltage (V(te)), transepithelial resistance (R(te)), and fura-2 [Ca2+]i fluorescence were measured. Baseline parameters were V(te), -16.5 +/- 1.2 mV; R(te), 80.8 +/- 7.1 Omega cm2; and equivalent short-circuit current (I(sc)), -261.0 +/- 25.1 microA/cm2 (n = 45). Amiloride (10 microM) almost completely inhibited I(sc) to -3.9 +/- 3.8 microA/cm2 (n = 10). Luminal ATP (100 microM) reduced V(te) from -16.5 +/- 2.1 to -12.5 +/- 1.93 and increased R(te) from 113.1 +/- 16.2 to 123.8 +/- 16.7 Omega cm2, which resulted in a 31.7% inhibition of amiloride-sensitive I(sc) (n = 12). Similarly, luminal UTP reversibly reduced V(te) from -22.0 +/- 2.1 to -13.6 +/- 2.1 mV and increased R(te) from 48.4 +/- 5.3 to 59.2 +/- 7.1 Omega cm2, which resulted in 49.1% inhibition of Na+ absorption (n = 6). In parallel, luminal ATP and UTP elevated [Ca2+]i in CCD, increasing the fura-2 ratio by 2.7 +/- 0.7 and 4.0 +/- 1.2, respectively. Basolateral ATP and UTP (100 microM) also inhibited amiloride-sensitive I(sc) by 21.8 (n = 14) and 20.1% (n = 8), respectively. Inhibition of luminal nucleotide-induced [Ca2+]i increase by Ca2+ store depletion with cyclopiazonic acid (3 microM) did not affect nucleotide-mediated inhibition of Na+ transport (n = 7). No evidence indicated the activation of a luminal Ca2+-activated Cl- conductance, a phenomenon previously shown in M-1 CCD cells (J Physiol 524: 77-99, 2000). In essence, these data indicate that luminal ATP and UTP, most likely via P2Y2 receptors, mediate inhibition of amiloride-sensitive I(sc) in perfused mouse CCD. This inhibition appears to occurs independently of an increase of cytosolic Ca2+.

Evidence for basolateral P2Y(6) receptors along the rat proximal tubule: functional and molecular characterization

In this study, the distribution of P2Y(6) receptor mRNA in rat nephron segments was investigated and a functional approach was used to analyze basolateral protein expression. Reverse transcription-PCR studies revealed more intense expression of P2Y(6) receptor mRNA in the proximal tubule and the thick ascending limb of Henle's loop, less intense expression in the thin descending limb and the cortical and outer medullary collecting ducts, and no detectable expression in either the thin ascending limb or the inner medullary collecting duct. Dose-dependent calcium responses to basolateral administration of UDP (a selective agonist for the P2Y(6) receptor) were observed in the proximal tubule but not in any of the other segments studied. In the proximal tubule, intracellular calcium concentration changes induced by UDP were associated with increased production of inositol phosphates, as were those induced by ATP and norepinephrine. However, UDP-induced intracellular calcium concentration changes were different, exhibiting no plateau after the initial peak; moreover, a single stimulation with a high concentration of UDP induced full desensitization of the UDP-sensitive calcium pathway but did not alter the responsiveness of the proximal tubule to ADP (a specific P2Y(1) receptor agonist), ATP or norepinephrine. In summary, this report demonstrates that P2Y(6) receptor mRNA is expressed in most segments of the rat nephron but that basolateral expression of the protein is restricted to the proximal tubule, where the receptor is coexpressed with the P2Y(1) receptor. The differences in the distributions of P2Y(6) receptor mRNA and UDP responses may indicate the presence of luminal receptors in other nephron segments.

Pharmacological characterisation of pyrimidinoceptor responses in NG108-15 cells

In the present study, the P2Y receptor(s) mediating the effects of the pyrimidines UTP and UDP on phospholipase C activation in the mouse neuroblastoma x rat glioma hybrid cell line NG108-15 was investigated. Reverse Transcriptase-Polymerase Chain Reaction (RT-PCR) analysis detected transcripts for the P2Y(6) and P2Y(2) receptors, but not for P2Y(1) and P2Y(4.) UTP and UDP were equipotent agonists and their effects were partially additive. Suramin, reactive blue 2 and pyridoxal phosphate-6-azophenyl-2',4'disulfonic acid (PPADS) antagonised the phospholipase C response to both UTP and UDP. High micromolar concentrations of adenosine, 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS-21680), 2',3'-O-isopropylideneadenosine (iPAdo) and adenosine 3':5'-cyclic monophosphate (3',5'-cAMP) were able to antagonise the effect of UTP on phospholipase C but not that of UDP. The additivity of the UTP and UDP responses, novel P2 receptor antagonist profile and the distinguishing action of adenosine may indicate the expression of a pyrimidine selective P2Y receptor in addition to the P2Y(6) type in these cells.

Multiple P2Y receptor subtypes in the apical membranes of polarized epithelial cells

Apical ATP, ATP, UTP and UDP evoked transient increases in short circuit current (I(SC), a direct measure of transepithelial ion transport) in confluent Caco-2 cells grown on permeable supports. These responses were mediated by a population of at least three pharmacologically distinct receptors. Experiments using cells grown on glass coverslips showed that ATP and UTP consistently increased intracellular free calcium ([Ca(2+)](i)) whilst sensitivity to UDP was variable. Cross desensitization experiments suggested that the responses to UTP and ATP were mediated by a common receptor population. Messenger RNA transcripts corresponding to the P2Y(2), P2Y(4) and P2Y(6) receptors genes were detected in cells grown on Transwell membranes by the reverse transcriptase - polymerase chain reaction. Identical results were obtained for cells grown on glass. Experiments in which I(SC) and [Ca(2+)](i) were monitored simultaneously in cells on Transwell membranes, confirmed that apical ATP and UTP increased both parameters and showed that the UDP-evoked increase in I(SC) was accompanied by a [Ca(2+)](i)-signal. Ionomycin consistently increased [Ca(2+)](i) in such polarized cells but caused no discernible change in I(SC). However, subsequent application of apical ATP or UTP evoked a small rise in I(SC) but no rise in [Ca(2+)](i). UDP evoked no such response. As well as evoking increases in [Ca(2+)](i), the ATP/UTP-sensitive receptors present in Caco-2 cells thus allow direct control over ion channels in the apical membrane. The UDP-sensitive receptors, however, appear to simply evoke a rise in [Ca(2+)](i).

The luminal P2Y receptor in the isolated perfused mouse cortical collecting duct

Extracellular nucleotides regulate renal ion transport. With the use of in vitro perfusion and [Ca(2+)](i) imaging, this study investigated whether mouse and rabbit cortical collecting ducts (CCD) respond to luminal nucleotides. In mouse CCD, luminal ATP (EC(50): 10 microM) and UTP (EC(50): 9.7 microM) increased [Ca(2+)](i) with an initial peak and a plateau. To make certain that basolateral P2 receptors were not activated by luminal nucleotides via leak diffusion, luminal trypsin (1 microM), a known agonist for basolateral proteinase-activated receptors, was perfused. Mouse CCD that were responsive to luminal ATP were nonresponsive to luminal trypsin but always showed [Ca(2+)](i) elevations by basolateral trypsin (10 or 100 nM). Luminal alpha,beta- and beta,gamma-methylene ATP, 2-methyl-S-ATP, ADP, UDP, and 2',3'-O-4-benzoylbenzoyl ATP had no effect (100 microM, n = 9). Without external Ca(2+), luminal ATP still stimulated a [Ca(2+)](i) increase. Mouse CCD also responded to basolateral ATP (EC(50): 23 microM) and UTP (EC(50): 23 microM) with smaller [Ca(2+)](i) elevations. Confocal microscopy of perfused CCD showed that luminal ATP (100 microM) rapidly increased [Ca(2+)](i) in nearly all cells (n = 6) and the same cells that responded to luminal ATP responded to basolateral ATP (100 microM). In contrast, rabbit CCD did not respond to luminal ATP/UTP (n = 8) despite ATP's known effect from the basolateral side (EC(50): 34 microM). These data indicate the expression of luminal P2Y receptors (probably P2Y(2)) in principal cells of mouse CCD but not in rabbit CCD.

Nucleotide-evoked ion transport and [Ca(2+)](i) changes in normal and hyperhidrotic human sweat gland cells

Apical and basolateral application of ATP and UTP evoked [Ca(2+)](i) and short circuit current (Isc) increases in normal and hyperhidrotic human eccrine sweat gland cells grown into functionally polarised epithelia on permeable supports. Basolateral application to hyperhidrotic cells exhibited a markedly greater increase in Isc than in normal cells. Hyperhidrotic cells also demonstrated differences from the normal in [Ca(2+)](i) and Isc responses to ATP when pre-treated with thapsigargin. The data demonstrate the presence of apical and basolateral receptors that allow nucleotides to increase [Ca(2+)](i) and Isc. The results suggest that changes from the normal in transepithelial ion transport contribute to the characteristic excessive fluid production of hyperhidrotic sweat glands.

Pyrimidinoceptors-mediated activation of Ca(2+)-dependent Cl(-) conductance in mouse endometrial epithelial cells

Previous studies have demonstrated the activation of endometrial Cl(-) secretion through P(2Y2) (P(2U)) purinoceptors by extracellular ATP. The present study further explored the presence of pyrimidine-sensitive receptors in the primary cultured mouse endometrial epithelial cells using the short-circuit current (I(SC)) and whole-cell patch-clamp techniques. UDP induced a transient increase in I(SC) in a concentration-dependent manner (EC(50) approximately 8.84 microM). The UDP-induced I(SC) was abolished after pretreating the epithelia with a calcium chelator, 1, 2-bis-(2-aminophenoxy)-ethane-N,N,N'N'tetraacetic acid-acetomethyl ester (BAPTA-AM), suggesting the dependence of the I(SC) on cytosolic free Ca(2+). The type of receptor involved was studied by cross-desensitization between ATP and UDP. ATP or UDP desensitized its subsequent I(SC) response. However, when ATP was added after UDP, or vice versa, a second I(SC) response was observed, indicating the activation of distinct receptors, possibly pyrimidine-sensitive receptors in addition to P(2Y2) (P(2U)) receptors. Similar results were observed in the patch-clamp experiments where UDP and ATP were shown to sequentially activate whole-cell current in the same cell. The UDP-activated whole-cell current exhibited outward rectification with delay activation and inactivation at depolarizing and hyperpolarizing voltages, respectively. In addition, the UDP-evoked whole-cell current reversed near the equilibrium potential of Cl(-) in the presence of a Cl(-) gradient across the membrane, and was sensitive to 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), indicating the activation of Ca(2+)-activated Cl(-) conductance. These characteristics were very similar to that of the ATP-activated whole-cell current. Taken together, our findings indicate the presence of distinct receptors, pyrimidinoceptors and P(2Y2) (P(2U)) receptors in mouse endometrial epithelial cells. These distinct receptors appear to converge on the same Ca(2+)-dependent Cl(-) channels.

Regulation of intracellular calcium by extracellular nucleotides in pig tracheal submucosal gland cells

The mechanisms by which extracellular nucleotides (ATP and UTP) regulate intracellular Ca2+ in cultured pig tracheal gland cells were studied. The calcium response induced by ATP or UTP was composed of a peak response and a steady plateau. In the absence of extracellular Ca2+, the peak response of the cells to both ATP and UTP was smaller, and no subsequent plateau was observed. After treatment of the cells with pertussis toxin, the peak response to UTP was significantly smaller and no plateau was seen even in the presence of extracellular Ca2+, but pertussis toxin did not change the effect of ATP. Pretreatment with U107, a phospholipase C inhibitor, almost abolished the calcium response to both ATP and UTP. Immunocytochemistry showed that in these cells, the IP3 receptor was localized in the cytoplasm (including the endoplasmic reticulum) of the cells. Our results indicate that both release of calcium from the intracellular store and Ca2+ influx across the cell membrane contribute to the mobilization of [Ca2+]i upon stimulation with nucleotides, that ATP and UTP regulate intracellular Ca2+ predominantly via the G protein-phospholipase C-IP3 pathway, and that ATP and UTP may act via distinct subtypes of P2Y receptors.

Cellular activation by Ca2+ release from stores in the endoplasmic reticulum but not by increased free Ca2+ in the cytosol

Ca(2+) release from intracellular stores and/or transmembrane influx can increase the cytosolic free Ca(2+) concentration ([Ca(2+)](i)). Such changes in [Ca(2+)](i) might transduce signals regulating transcription, motility, secretion, and so on. Surfactant secretagogues such as ATP and ionomycin stimulate the release and transmembrane influx of Ca(2+), both of which increase [Ca(2+)](i). The addition of surfactant protein A (SP-A) or depleting cellular Ca(2+) inhibited both surfactant secretion and Ca(2+) transients. Current results suggest that Ca(2+) signalling stimulates surfactant secretion by type II pneumocytes, but not via increased [Ca(2+)](i). Treatment of cells with a Ca(2+) chelator, bis-(o-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid acetoxymethyl ester (BAPTA-AM), stimulated secretion but decreased [Ca(2+)](i). Adding SP-A or depleting Ca(2+) inhibited BAPTA-AM-induced secretion. When studied directly, Ca(2+) in the endoplasmic reticulum store ([Ca(2+)](l)) decreased in response to BAPTA, ionomycin and thapsigargin, and increased in response to SP-A. Phorbol ester (PMA) induced surfactant secretion without altering [Ca(2+)](i) or [Ca(2+)](l) and was unaffected by Ca(2+) depletion. The addition of PMA to Ca(2+)-releasing secretagogues increased secretion, but combining two Ca(2+)-releasing secretagogues did not. These results suggest that (1) Ca(2+) signalling of type II cell surfactant secretion reflects changes in [Ca(2+)](l), not [Ca(2+)](i), (2) PMA elicits secretion differently from Ca(2+)-releasing secretagogues, and (3) SP-A inhibits secretion by enhancing Ca(2+) sequestration within endoplasmic reticulum stores. Whether other cell types signal via changes in [Ca(2+)](l) is unknown.

Regulation of epidermal homeostasis through P2Y2 receptors

1. Previous studies have indicated a role for extracellular ATP in the regulation of epidermal homeostasis. Here we have investigated the expression of P2Y2 receptors by human keratinocytes, the cells which comprise the epidermis. 2. Reverse transcriptase-polymerase chain reaction (RT - PCR) revealed expression of mRNA for the G-protein-coupled, P2Y2 receptor in primary cultured human keratinocytes. 3. In situ hybridization studies of skin sections revealed that P2Y2 receptor transcripts were expressed in the native tissue. These studies demonstrated a striking pattern of localization of P2Y2 receptor transcripts to the basal layer of the epidermis, the site of cell proliferation. 4. Increases in intracellular free Ca2+ concentration ([Ca2+]i) in keratinocytes stimulated with ATP or UTP demonstrated the presence of functional P2Y receptors. 5. In proliferation studies based on the incorporation of bromodeoxyuridine (BrdU), ATP, UTP and ATPgammaS were found to stimulate the proliferation of keratinocytes. 6. Using a real-time firefly luciferase and luciferin assay we have shown that under static conditions cultured human keratinocytes release ATP. 7. These findings indicate that P2Y2 receptors play a major role in epidermal homeostasis, and may provide novel targets for therapy of proliferative disorders of the epidermis, including psoriasis.

Desensitization of P2Y2 receptor-activated transepithelial anion secretion

Desensitization of P2Y2 receptor-activated anion secretion may limit the usefulness of extracellular nucleotides in secretagogue therapy of epithelial diseases, e.g., cystic fibrosis (CF). To investigate the desensitization process for endogenous P2Y2 receptors, freshly excised or cultured murine gallbladder epithelia (MGEP) were mounted in Ussing chambers to measure short-circuit current (Isc), an index of electrogenic anion secretion. Luminal treatment with nucleotide receptor agonists increased the Isc with a potency profile of ATP = UTP > 2-methylthioATP >> alpha,beta-methylene-ATP. RT-PCR revealed the expression of P2Y2 receptor mRNA in the MGEP cells. The desensitization of anion secretion required a 10-min preincubation with the P2Y2 receptor agonist UTP and increased in a concentration-dependent manner (IC50 approximately 10(-6) M). Approximately 40% of the anion secretory response was unaffected by maximal desensitizing concentrations of UTP. Recovery from UTP-induced desensitization was rapid (<10 min) at preincubation concentrations less than the EC50 (1.9 x 10(-6) M) but required progressively longer time periods at greater concentrations. UTP-induced total inositol phosphate production and intracellular Ca2+ mobilization desensitized with a concentration dependence similar to that of anion secretion. In contrast, maximal anion secretion induced by Ca2+ ionophore ionomycin was unaffected by preincubation with a desensitizing concentration of UTP. It was concluded that 1) desensitization of transepithelial anion secretion stimulated by the P2Y2 receptor agonist UTP is time and concentration dependent; 2) recovery from desensitization is prolonged (>90 min) at UTP concentrations >10(-5) M; and 3) UTP-induced desensitization occurs before the operation of the anion secretory mechanism.

Nucleotide-evoked calcium signals and anion secretion in equine cultured epithelia that express apical P2Y2 receptors and pyrimidine nucleotide receptors

1. Experiments with a spontaneously transformed equine epithelial cell line showed that certain nucleotides increased intracellular free calcium ([Ca2+]i) in cells plated on glass coverslips. The rank order of potency was ATP UTP > 5-Br-UTP, whilst UDP and ADP were ineffective. The response thus appears to be mediated by P2Y2 receptors. 2. Nucleotides also increased short circuit current (Isc) in cells grown into epithelial monolayers and the rank order of potency was UDP> UTP > 5-Br-UTP > ATP > ADP. The increase in [Ca2+]i and the rise in ISC thus have different pharmacological properties. Cross-desensitization experiments indicated that, as well as P2Y2 receptors, the monolayer cultures express at least one additional receptor population that allowed nucleotides to increase ISC. 3. The UDP-evoked increase in ISC was essentially abolished in BAPTA-loaded epithelia suggesting that this response is dependent upon increased [Ca2+]i. Moreover, experiments in which ISC and [Ca2+]i were measured simultaneously showed that the UDP- and ADP-evoked increases in ISC were accompanied by increases in [Ca2+]i. 4. When grown under conditions which favour the development of a polarized phenotype, these epithelial cells thus appear to express [Ca2+]i-mobilizing receptors sensitive to UDP and ADP that are not present in non-polarized cells on coverslips.