Evidence that rat hepatocytes co-express functional P2Y1 and P2Y2 receptors

Receptor-specific Ca2+ oscillation patterns mediated by differential regulation of P2Y purinergic receptors in rat hepatocytes

Extracellular agonists linked to inositol-1,4,5-trisphosphate (IP₃) formation elicit cytosolic Ca²⁺ oscillations in many cell types, but despite a common signaling pathway, distinct agonist-specific Ca²⁺ spike patterns are observed. Using qPCR, we show that rat hepatocytes express multiple purinergic P2Y and P2X receptors (R). ADP acting through P2Y1R elicits narrow Ca²⁺ oscillations, whereas UTP acting through P2Y2R elicits broad Ca²⁺ oscillations, with composite patterns observed for ATP. P2XRs do not play a role at physiological agonist levels. The discrete Ca²⁺ signatures reflect differential effects of protein kinase C (PKC), which selectively modifies the falling phase of the Ca²⁺ spikes. Negative feedback by PKC limits the duration of P2Y1R-induced Ca²⁺ spikes in a manner that requires extracellular Ca²⁺. By contrast, P2Y2R is resistant to PKC negative feedback. Thus, the PKC leg of the bifurcated IP₃ signaling pathway shapes unique Ca²⁺ oscillation patterns that allows for distinct cellular responses to different agonists.

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Distinct stereotypic Ca²⁺ oscillations are elicited by P2Y1 and P2Y2 receptors

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P2X receptors do not contribute to the generation of Ca²⁺ oscillations

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Agonist-specific Ca²⁺ spike shapes reflect discrete modes of PKC negative feedback

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Bifurcation of IP₃/PKC signaling yields unique Ca²⁺ oscillation signatures

Important roles of P2Y receptors in the inflammation and cancer of digestive system

Purinergic signaling is important for many biological processes in humans. Purinoceptors P2Y are widely distributed in human digestive system and different subtypes of P2Y receptors mediate different physiological functions from metabolism, proliferation, differentiation to apoptosis etc. The P2Y receptors are essential in many gastrointestinal functions and also involve in the occurrence of some digestive diseases. Since different subtypes of P2Y receptors are present on the same cell of digestive organs, varying subtypes of P2Y receptors may have opposite or synergetic functions on the same cell. Recently, growing lines of evidence strongly suggest the involvement of P2Y receptors in the pathogenesis of several digestive diseases. In this review, we will focus on their important roles in the development of digestive inflammation and cancer. We anticipate that as the special subtypes of P2Y receptors are studied in depth, specific modulators for them will have good potentials to become promising new drugs to treat human digestive diseases in the near future.

Purinergic signalling in the liver in health and disease

Purinergic signalling is involved in both the physiology and pathophysiology of the liver. Hepatocytes, Kupffer cells, vascular endothelial cells and smooth muscle cells, stellate cells and cholangiocytes all express purinoceptor subtypes activated by adenosine, adenosine 5'-triphosphate, adenosine diphosphate, uridine 5'-triphosphate or UDP. Purinoceptors mediate bile secretion, glycogen and lipid metabolism and indirectly release of insulin. Mechanical stress results in release of ATP from hepatocytes and Kupffer cells and ATP is also released as a cotransmitter with noradrenaline from sympathetic nerves supplying the liver. Ecto-nucleotidases play important roles in the signalling process. Changes in purinergic signalling occur in vascular injury, inflammation, insulin resistance, hepatic fibrosis, cirrhosis, diabetes, hepatitis, liver regeneration following injury or transplantation and cancer. Purinergic therapeutic strategies for the treatment of these pathologies are being explored.

Calcium-dependent regulation of glucose homeostasis in the liver

A major role of the liver is to integrate multiple signals to maintain normal blood glucose levels. The balance between glucose storage and mobilization is primarily regulated by the counteracting effects of insulin and glucagon. However, numerous signals converge in the liver to ensure energy demand matches the physiological status of the organism. Many circulating hormones regulate glycogenolysis, gluconeogenesis and mitochondrial metabolism by calcium-dependent signaling mechanisms that manifest as cytosolic Ca(2+) oscillations. Stimulus-strength is encoded in the Ca(2+) oscillation frequency, and also by the range of intercellular Ca(2+) wave propagation in the intact liver. In this article, we describe how Ca(2+) oscillations and waves can regulate glucose output and oxidative metabolism in the intact liver; how multiple stimuli are decoded though Ca(2+) signaling at the organ level, and the implications of Ca(2+) signal dysregulation in diseases such as metabolic syndrome and non-alcoholic fatty liver disease.

Calcium signaling in the liver

Intracellular free Ca(2+) ([Ca(2+)]i) is a highly versatile second messenger that regulates a wide range of functions in every type of cell and tissue. To achieve this versatility, the Ca(2+) signaling system operates in a variety of ways to regulate cellular processes that function over a wide dynamic range. This is particularly well exemplified for Ca(2+) signals in the liver, which modulate diverse and specialized functions such as bile secretion, glucose metabolism, cell proliferation, and apoptosis. These Ca(2+) signals are organized to control distinct cellular processes through tight spatial and temporal coordination of [Ca(2+)]i signals, both within and between cells. This article will review the machinery responsible for the formation of Ca(2+) signals in the liver, the types of subcellular, cellular, and intercellular signals that occur, the physiological role of Ca(2+) signaling in the liver, and the role of Ca(2+) signaling in liver disease.

Activated hepatic stellate cells upregulate transcription of ecto-5'-nucleotidase/CD73 via specific SP1 and SMAD promoter elements

Adenosine is a potent modulator of liver fibrosis and inflammation. Adenosine has been shown to regulate such diverse activities as chemotaxis, contraction, and matrix production in hepatic stellate cells (HSC). Ecto-5'-nucleotidase/CD73 [EC 3.1.3.5] is the rate-limiting enzyme in adenosine production. Cd73-deficient mice are resistant to experimental liver fibrosis and have impaired adenosine generation. However, cell-specific expression and regulation of CD73 within the fibrotic liver have not been defined. In particular, prior evidence demonstrating that liver myofibroblasts, the cells believed to be responsible for matrix formation in the liver, express CD73 is lacking. Thus we tested the hypothesis that HSC and portal fibroblasts (PF), cells that undergo differentiation into liver myofibroblasts, express CD73 in a regulated fashion. We found that CD73 is weakly expressed in quiescent HSC and PF but is markedly upregulated at the transcriptional level in myofibroblastic HSC and PF. We furthermore found that CD73 protein and its functional activity are strongly increased in fibrous septa in rats subjected to experimental fibrosis. To determine the mechanism for the upregulation of Cd73 gene, we cloned the rat Cd73 promoter and then used serial truncation and site-directed mutagenesis to identify key regulatory elements. We identified two consensus SP1 motifs and one SMAD binding site, each of which was necessary for Cd73 gene upregulation. In conclusion, activated HSC upregulate Cd73 gene expression, via specific SP1 and SMAD promoter elements, after myofibroblastic differentiation. The ecto-5'-nucleotidase/CD73 enzyme is a novel cellular marker of activated liver myofibroblasts in vivo and in vitro and thus represents a promising molecular target for antifibrotic therapies in liver diseases.

Calcium-dependent physiologic and pathologic stimulus-metabolic response coupling in hepatocytes

A recurrent paradigm in calcium signaling is the coordination of the target response of the calcium signal with activation of metabolic energy production to support that response. This occurs in many tissues, including cardiac and skeletal muscle where contractile activity and ATP production are coordinately regulated by the frequency and amplitude of calcium transients, endocrine and exocrine cells that use calcium to drive the secretory process, and hepatocytes where the downstream targets of calcium include both catabolic and anabolic processes. The primary mechanism by which calcium enhances the capacity for energy production is through calcium-dependent stimulation of mitochondrial oxidative metabolism, achieved by increasing NADH production and respiratory chain flux. Although this enhances energy supply, it also has the potential for deleterious consequences resulting from increased generation of reactive oxygen species (ROS). The negative consequences of calcium-dependent mitochondrial activation can be ameliorated when the underlying cytosolic calcium signals occur as brief calcium spikes or oscillations, with signal strength encoded through the spike frequency (frequency modulation). Frequency modulation increases signal fidelity, and reduces pathological effects of calcium, including excess mitochondrial ROS production and apoptotic or necrotic outcomes. The present article reviews these issues using data obtained in hepatocytes under physiologic and pathologic conditions.

The birth and postnatal development of purinergic signalling

The purinergic signalling system is one of the most ancient and arguably the most widespread intercellular signalling system in living tissues. In this review we present a detailed account of the early developments and current status of purinergic signalling. We summarize the current knowledge on purinoceptors, their distribution and role in signal transduction in various tissues in physiological and pathophysiological conditions.

ATP release after partial hepatectomy regulates liver regeneration in the rat

BACKGROUND & AIMS

Paracrine interactions are critical to liver physiology, particularly during regeneration, although physiological involvement of extracellular ATP, a crucial intercellular messenger, remains unclear. The physiological release of ATP into extracellular milieu and its impact on regeneration after partial hepatectomy were investigated in this study.

METHODS

Hepatic ATP release after hepatectomy was examined in the rat and in human living donors for liver transplantation. Quinacrine was used for in vivo staining of ATP-enriched compartments in rat liver sections and isolated hepatocytes. Rats were treated with an antagonist for purinergic receptors (Phosphate-6-azo(benzene-2,4-disulfonic acid), PPADS), and liver regeneration after hepatectomy was analyzed.

RESULTS

A robust and transient ATP release due to acute portal hyperpressure was observed immediately after hepatectomy in rats and humans. Clodronate liposomal pre-treatment partly inhibited ATP release in rats. Quinacrine-stained vesicles, co-labeled with a lysosomal marker in liver sections and isolated hepatocytes, were predominantly detected in periportal areas. These vesicles significantly disappeared after hepatectomy, in parallel with a decrease in liver ATP content. PPADS treatment inhibited hepatocyte cell cycle progression after hepatectomy, as revealed by a reduction in bromodeoxyuridine incorporation, phosphorylated histone 3 immunostaining, cyclin D1 and A expression and immediate early gene induction.

CONCLUSION

Extracellular ATP is released immediately after hepatectomy from hepatocytes and Kupffer cells under mechanical stress and promotes liver regeneration in the rat. We suggest that in hepatocytes, ATP is released from a lysosomal compartment. Finally, observations made in living donors suggest that purinergic signalling could be critical for human liver regeneration.

The role of purinergic signaling in the liver and in transplantation: effects of extracellular nucleotides on hepatic graft vascular injury, rejection and metabolism

Extracellular nucleotides (e.g. ATP, UTP, ADP) are released by activated endothelium, leukocytes and platelets within the injured vasculature and bind specific cell-surface type-2 purinergic (P2) receptors. This process drives vascular inflammation and thrombosis within grafted organs. Importantly, there are also vascular ectonucleotidases i.e. ectoenzymes that hydrolyze extracellular nucleotides in the blood to generate nucleosides (viz. adenosine). Endothelial cell NTPDase1/CD39 has been shown to critically modulate levels of circulating nucleotides. This process tends to limit the activation of platelet and leukocyte expressed P2 receptors and also generates adenosine to reverse inflammatory events. This vascular protective CD39 activity is rapidly inhibited by oxidative reactions, such as is observed with liver ischemia reperfusion injury. In this review, we chiefly address the impact of these signaling cascades following liver transplantation. Interestingly, the hepatic vasculature, hepatocytes and all non-parenchymal cell types express several components co-ordinating the purinergic signaling response. With hepatic and vascular dysfunction, we note heightened P2- expression and alterations in ectonucleotidase expression and function that may predispose to progression of disease. In addition to documented impacts upon the vasculature during engraftment, extracellular nucleotides also have direct influences upon liver function and bile flow (both under physiological and pathological states). We have recently shown that alterations in purinergic signaling mediated by altered CD39 expression have major impacts upon hepatic metabolism, repair mechanisms, regeneration and associated immune responses. Future clinical applications in transplantation might involve new therapeutic modalities using soluble recombinant forms of CD39, altering expression of this ectonucleotidase by drugs and/or using small molecules to inhibit deleterious P2-mediated signaling while augmenting beneficial adenosine-mediated effects within the transplanted liver.

Cloning, purification, and identification of the liver canalicular ecto-ATPase as NTPDase8

Extracellular nucleotides regulate critical liver functions via the activation of specific transmembrane receptors. The hepatic levels of extracellular nucleotides, and therefore the related downstream signaling cascades, are modulated by cell-surface enzymes called ectonucleotidases, including nucleoside triphosphate diphosphohydrolase-1 (NTPDase1/CD39), NTPDase2/CD39L1, and ecto-5'-nucleotidase/CD73. The goal of this study was to determine the molecular identity of the canalicular ecto-ATPase/ATPDase that we hypothesized to correspond to the recently cloned NTPDase8. Human and rat NTPDase8 cDNAs were cloned, and the genes were located on chromosome loci 9q34 and 3p13, respectively. The recombinant proteins, expressed in COS-7 and HEK293T cells, were biochemically characterized. NTPDase8 was also purified from rat liver by Triton X-100 solubilization, followed by DEAE, Affigel Blue, and concanavalin A chromatographies. Importantly, NTPDase8 was responsible for the major ectonucleotidase activity in liver. The ion requirement, apparent K(m) values, nucleotide hydrolysis profile, and preference as well as the resistance to azide were similar for recombinant NTPDase8s and both purified rat NTPDase8 and porcine canalicular ecto-ATPase/ATPDase. The partial NH(2)-terminal amino acid sequences of all NTPDase8s share high identity with the purified liver canalicular ecto-ATPase/ATPDase. Histochemical analysis showed high ectonucleotidase activities in bile canaliculi and large blood vessels of rat liver, in agreement with the immunolocalization of NTPDase1, 2, and 8 with antibodies developed for this study. No NTPDase3 expression could be detected in liver. In conclusion, NTPDase8 is the canalicular ecto-ATPase/ATPDase and is responsible for the main hepatic NTPDase activity. The canalicular localization of this enzyme suggests its involvement in the regulation of bile secretion and/or nucleoside salvage.

Calcium Dynamics: Spatio‐Temporal Organization from the Subcellular to the Organ Level

Many essential physiological processes are controlled by calcium. To ensure reliability and specificity, calcium signals are highly organized in time and space in the form of oscillations and waves. Interesting findings have been obtained at various scales, ranging from the stochastic opening of a single calcium channel to the intercellular calcium wave spreading through an entire organ. A detailed understanding of calcium dynamics thus requires a link between observations at different scales. It appears that some regulations such as calcium-induced calcium release or PLC activation by calcium, as well as the weak diffusibility of calcium ions play a role at all levels of organization in most cell types. To comprehend how calcium waves spread from one cell to another, specific gap-junctional coupling and paracrine signaling must also be taken into account. On the basis of a pluridisciplinar approach ranging from physics to physiology, a unified description of calcium dynamics is emerging, which could help understanding how such a small ion can mediate so many vital functions in living systems.

ATP and vasopressin activate a single type of store-operated Ca2+ channel, identified by patch-clamp recording, in rat hepatocytes

Hepatocytes are highly polarised epithelial cells that mediate a large number of metabolic pathways, the transcellular movement of numerous ions and metabolites, and the secretion of proteins from both basal and canalicular membrane regions. Hormone-induced changes in the concentration of intracellular Ca2+ play a central role in regulating these functions. Store-operated Ca2+ channels (SOCs) and other Ca2+-permeable channels in the plasma membrane which are activated by hormones are essential for regulating the amount of Ca2+ in the hepatocyte in order to allow these Ca2+ signalling processes to occur. However, the properties of hormone-activated Ca2+ channels in hepatocytes and in other epithelial cells are not well defined. In this study, we have investigated SOCs in cultured rat hepatocytes by patch-clamp recording using IP3 and hormones as activators. We show that IP3 activates a single type of SOC, which, on the basis of its high selectivity for Ca2+ over Na+, inhibition by La3+ and 2-aminoethyl diphenylborate (2-APB), and the time course of fast inactivation, is very similar to CRAC channel in mast cells and lymphocytes. Moreover, a current (ISOC) with properties identical to those of the IP3-activated current can be activated by physiological concentrations of ATP and vasopressin. It is concluded that SOCs with properties similar to those of CRAC channel are present in hepatocytes, highly differentiated primary cells, and these channels can be activated by hormones under conditions close to physiological.

Regulation of human hepatocytes by P2Y receptors: control of glycogen phosphorylase, Ca2+, and mitogen-activated protein kinases

In the rat both short-term liver function, such as glycogen metabolism, and long-term events such as proliferation after partial hepatectomy, are in part controlled by release of nucleotides such as ATP acting on hepatocyte P2Y(1) and P2Y(2) receptors (members of a family of P2Y receptors for extracellular nucleotides such as ATP and UTP). Here, we have studied P2Y receptor regulation of signaling pathways involved in glycogen phosphorylase activation and proliferation of primary human hepatocytes. Stimulation of cultured hepatocytes with either ATP and UTP, but not UDP or 2-methylthio ADP, led to concentration-dependent increases in cytosolic free Ca(2+) concentration ([Ca(2+)](c); EC(50) for ATP = 3.3 microM, for UTP = 2.3 microM) and [(3)H]inositol (poly)phosphates (EC(50) for ATP = 9.4 microM, for UTP = 15.4 microM). ATP and UTP also stimulated glycogen phosphorylase in human hepatocytes, each with a threshold for activation of less than 1 microM. Application of 2-methylthio ADP up to 100 microM was ineffective. Phosphorylation of both extracellular signal-related kinase and c-Jun N-terminal kinase was stimulated by ATP and UTP, but not by 2-methylthio ADP or UDP, either alone or when costimulated with epidermal growth factor. In conclusion, in human hepatocytes P2Y receptors control both glycogen metabolism and proliferation-associated responses such as increased [Ca(2+)](c) and mitogen-activated protein kinase cascades. Regulation seems to be primarily through P2Y(2) receptors. In contrast with previous studies on rat hepatocytes, there is an absence of responses mediated by P2Y(1) receptors.

Control of hepatocyte metabolism by sympathetic and parasympathetic hepatic nerves

More than any other organ, the liver contributes to maintaining metabolic equilibrium of the body, most importantly of glucose homeostasis. It can store or release large quantities of glucose according to changing demands. This homeostasis is controlled by circulating hormones and direct innervation of the liver by autonomous hepatic nerves. Sympathetic hepatic nerves can increase hepatic glucose output; they appear, however, to contribute little to the stimulation of hepatic glucose output under physiological conditions. Parasympathetic hepatic nerves potentiate the insulin-dependent hepatic glucose extraction when a portal glucose sensor detects prandial glucose delivery from the gut. In addition, they might coordinate the hepatic and extrahepatic glucose utilization to prevent hypoglycemia and, at the same time, warrant efficient disposal of excess glucose.

On the encoding and decoding of calcium signals in hepatocytes

Many different agonists use calcium as a second messenger. Despite intensive research in intracellular calcium signalling it is an unsolved riddle how the different types of information represented by the different agonists, is encoded using the universal carrier calcium. It is also still not clear how the information encoded is decoded again into the intracellular specific information at the site of enzymes and genes. After the discovery of calcium oscillations, one likely mechanism is that information is encoded in the frequency, amplitude and waveform of the oscillations. This hypothesis has received some experimental support. However, the mechanism of decoding of oscillatory signals is still not known. Here, we study a mechanistic model of calcium oscillations, which is able to reproduce both spiking and bursting calcium oscillations. We use the model to study the decoding of calcium signals on the basis of co-operativity of calcium binding to various proteins. We show that this co-operativity offers a simple way to decode different calcium dynamics into different enzyme activities.

Abundant and dynamic expression of G protein-coupled P2Y receptors in mammalian development

Extracellular ATP mediates diverse biological effects by activating two families of receptors, the P2X and P2Y receptors. There is growing evidence to show that activation of G protein-coupled P2Y receptors can produce trophic effects in many cell types. Yet the expression and function of the P2Y receptors in development has rarely been studied and has never been investigated in mammalian development. This study used the reverse transcription-polymerase chain reaction and immunohistochemistry to demonstrate the abundant and dynamic expression of P2Y receptors in rat development. These receptors were expressed in a wide range of embryonic structures, notably somites, skeletal muscle, the central and peripheral nervous system, the heart, lung, and liver. All the P2Y receptors studied were expressed as early as embryonic day 11, when most embryonic organs were far from being functional and still in the process of being formed. P2Y receptor proteins were strongly expressed in temporary, developmental structures that do not have a correlate in the adult animal, including the somites (P2Y1, P2Y2, and P2Y4) and the floor plate of the neural tube (P2Y1). P2Y receptors were also dynamically expressed, with receptor mRNA and protein being both up- and down-regulated at different developmental stages. The down-regulation of the P2Y1, 2, and 4 receptor proteins in skeletal muscle and heart, and the disappearance of the P2Y4 receptor from the brainstem and ventral white matter of the spinal cord postnatally, demonstrated that many P2Y receptors were likely to be involved in functions specific to embryonic life. Thus, these findings strongly suggest that P2Y receptors play an important role in the development of many tissues, and pioneer further studies into the role of purinergic signalling in development.

Regulation of rat hepatocyte function by P2Y receptors: focus on control of glycogen phosphorylase and cyclic AMP by 2-methylthioadenosine 5'-diphosphate

Hepatocyte function is regulated by several P2Y receptor subtypes. Here we report that 2-methylthioadenosine 5'-diphosphate (2-MeSADP), an agonist at P2Y(1), P2Y(12), and P2Y(13) receptors, potently (threshold 30 nM) stimulates glycogen phosphorylase in freshly isolated rat hepatocytes. Antagonism by N(6)-methyl 2'-deoxyadenosine 3',5'-bisphosphate (MRS 2179) confirms that this response is mediated by P2Y(1) receptors. In addition, in these cells, both 2-MeSADP and UTP inhibited glucagon-stimulated cyclic AMP accumulation. This inhibitory effect of 2-MeSADP was not reversed by the P2Y(1) antagonists, adenosine-3'-phosphate-5'-phosphate (A3P5P) or MRS 2179, both in the range 1 to 300 microM, indicating that it was not mediated by P2Y(1) receptors. This contrasts with the increase in cytosolic free Ca(2+) concentration ([Ca(2+)](c)) induced by 2-MeSADP, which has shown to be inhibited by A3P5P. Pertussis toxin abolished the inhibitory effect of both UTP and 2-MeSADP. After culture of cells for 48 h, the ability of 2-MeSADP to inhibit cyclic AMP accumulation was greatly diminished. Reverse transcriptase-polymerase chain reaction analysis revealed that during this culture period, there was a decline in the ability to detect transcripts for P2Y(12) and P2Y(13) receptors, both of which are activated by 2-MeSADP and negatively coupled to adenylyl cyclase. However, in freshly isolated cells, the P2Y(12) and P2Y(13) receptor antagonist, 2-propylthio-beta,gamma-dichloromethylene-d-ATP (AR-C67085) (10 nM to 300 microM) did not alter the ability of 2-MeSADP to inhibit glucagon-stimulated cyclic AMP accumulation. We conclude that 2-MeSADP regulates rat hepatocyte glycogen phosphorylase by acting on P2Y(1) receptors coupled to raised [Ca(2+)](c), and by inhibiting cyclic AMP levels by an unknown G(i)-coupled receptor subtype, distinct from P2Y(1), P2Y(12), or P2Y(13) receptors.

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.

P2X4, P2Y1 and P2Y2 receptors on rat alveolar macrophages

ATP receptors present on rat alveolar macrophages (NR8383 cells) were identified by recordings of membrane current, measurements of intracellular calcium, RT-PCR and immunocytochemistry. In whole-cell recordings with a sodium-based internal solution, ATP evoked an inward current at -60 mV. This reversed at 0 mV. The EC50 for ATP was 18 microM in normal external solution (calcium 2 mm, magnesium 1 mm). The currents evoked by 2',3-O-(4-benzoyl)benzoyl-ATP were about five-fold smaller than those observed with ATP. ADP, UTP and alphabeta-methylene-ATP (alphabetameATP) (up to 100 microM) had no effect. ATP-evoked currents were potentiated up to ten-fold by ivermectin and were unaffected by suramin (30-100 microM), pyridoxal-phosphate-6-azophenyl-(2,4-sulphonic acid) (30-100 microM), and brilliant blue G (1 microM). In whole-cell recordings with a potassium-based internal solution and low EGTA (0.01 mm), ATP evoked an inward current at -60 mV that was followed by larger outward current. ADP and UTP (1-100 microM) evoked only outward currents; these reversed polarity at the potassium equilibrium potential and were blocked by apamin (10 nm). Outward currents were also blocked by the phospholipase C inhibitor U73122 (1 microM), and they were not seen with higher intracellular EGTA (10 mm). Suramin (30 microM) blocked the outward currents evoked by ATP and UTP, but not that evoked by ADP. PPADS (10 microM) blocked the ADP-evoked outward current without altering the ATP or UTP currents. RT-PCR showed transcripts for P2X subunits 1, 4 and 7 (not 2, 3, 5, 6) and P2Y receptors 1, 2, 4 and 12 (not 6). Immunocytochemistry showed strong P2X4 receptor expression partly associated with the membrane, weak P2X7 staining that was not associated with the cell membrane, and no P2X1 receptor immunoreactivity. We conclude that rat alveolar macrophages express (probably homomeric) P2X4 receptors, but find no evidence for other functional P2X subtypes. The P2Y receptors are most likely P2Y1 and P2Y2 and these couple through phospholipase C to an increase in intracellular calcium and the opening of SK type potassium channels.

ADP stimulation of inositol phosphates in hepatocytes: role of conversion to ATP and stimulation of P2Y2 receptors

1 Accumulation of inositol (poly)phosphates (InsP(x)) has been studied in rat hepatocytes labelled with [(3)H]inositol. Stimulation with ADP resulted in a significant increase in total [(3)H]InsP(x), whereas 2-MeSADP had only a small effect and ADPbetaS was ineffective. UTP and ITP also stimulated substantial increases in [(3)H]InsP(x). 2 The dose-response curve to ADP was largely unaltered by the presence of the P2Y(1) antagonist, adenosine-3'-phosphate-5'-phosphate (A3P5P). Similarly, inclusion of MRS 2179, a more selective P2Y(1) antagonist, had no effect on the dose-response curve to ADP. 3 The inclusion of hexokinase in the assay reduced, but did not abolish, the response to ADP. 4 HPLC analysis revealed that ADP in the medium was rapidly converted to AMP and ATP. The inclusion of hexokinase removed ATP, but exacerbated the decline in ADP concentration, leading to increased levels of AMP. 2-MeSADP was stable in the medium and ATP was largely unaffected. 5 The addition of the adenylate kinase inhibitor, diadenosine pentaphosphate (Ap(5)A) significantly reduced the ADP response. HPLC analysis conducted in parallel demonstrated that this treatment inhibited conversion of ADP to ATP and AMP. 6 Inclusion of the P1 antagonist CGS 15943 had no effect on the dose-response curve to ADP. 7 These observations indicate that hepatocytes respond to ADP with an increase in inositol (poly)phosphates following conversion to ATP. P2Y(1) activation in hepatocytes does not appear to be coupled to inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)) production.

The role of P2Y1 purinergic receptors and cytosolic Ca2+ in hypotonically activated osmolyte efflux from a rat hepatoma cell line

Exposure of HTC rat hepatoma cells to a 33% decrease in extracellular osmolality caused the cytosolic Ca(2+) concentration ([Ca(2+)](i)) to increase transiently by approximately 90 nm. This rise in [Ca(2+)](i) was inhibited strongly by apyrase, grade VII (which has a low ATP/ADPase ratio) but not by apyrase grade VI (which has a high ATP/ADPase ratio) or hexokinase, indicating that extracellular ADP and/or ATP play a role in the [Ca(2+)](i) increase. The hypotonically induced rise in [Ca(2+)](i) was prevented by the prior discharge of the intracellular Ca(2+) store of the cells by thapsigargin. Removal of extracellular Ca(2+) or inhibition of Ca(2+) influx by 1-10 microm Gd(3+) depleted the thapsigargin-sensitive Ca(2+) stores and thereby diminished the rise in [Ca(2+)](i). The hypotonically induced rise in [Ca(2+)](i) was prevented by adenosine 2'-phosphate-5'-phosphate (A2P5P) and pyridoxyl-5'-phosphate-6-azophenyl-2',4'-disulfonate, inhibitors of purinergic P2Y(1) receptors for which ADP is a major agonist. Both inhibitors also blocked the rise in [Ca(2+)](i) elicited by addition of ADP to cells in isotonic medium, whereas A2P5P had no effect on the rise in [Ca(2+)](i) elicited by the addition of the P2Y(2) and P2Y(4) receptor agonist, UTP. HTC cells were shown to express mRNA encoding for rat P2Y(1), P2Y(2), and P2Y(6) receptors. Inhibition of the hypotonically induced rise in [Ca(2+)](i) blocked hypotonically induced K(+) ((86)Rb(+)) efflux, modulated the hypotonically induced efflux of taurine, but had no significant effect on Cl(-) ((125)I-) efflux. The interaction of extracellular ATP and/or ADP with P2Y(1) purinergic receptors therefore plays a role in the response of HTC cells to osmotic swelling but does not account for activation of all the efflux pathways involved in the volume-regulatory response.

Purinergic regulation of glucose and glutamine synthesis in isolated rabbit kidney-cortex tubules

The effects of extracellular purinergic agonists and their breakdown products on glucose and glutamine synthesis in rabbit kidney-cortex tubules incubated with aspartate + glycerol or alanine + glycerol + octanoate were investigated. A rapid extracellular degradation of ATP was accompanied by an accumulation of AMP, inosine, and hypoxanthine. Extracellular ATP and its breakdown products accelerated glucose synthesis in renal tubules, while ammonium released from adenine-containing compounds enhanced glutamine synthesis and diminished the degree of gluconeogenesis stimulation. In contrast to AMP and inosine, ATP evoked calcium signals, while both ATP and inosine decreased intracellular cAMP content and accelerated the flux through fructose-1,6-bisphosphatase as concluded from changes in gluconeogenic intermediates. Since (i) the activity of partially purified renal fructose-1,6-bisphosphatase was increased upon protein phosphatase-1 treatment and decreased following treatment of previously dephosphorylated enzyme with protein kinase A catalytic subunit and (ii) both 8-bromoadenosine 3',5'-cyclic monophosphate and 8-(4-chlorophenyltio)-cAMP inhibited renal glucose synthesis, it seems likely that in rabbit renal tubules ATP and inosine stimulate gluconeogenesis via cAMP decrease, which favors the appearance of a more active, dephosphorylated form of fructose-1,6-bisphosphatase, a key gluconeogenic enzyme.

Expression of the P2Y1 nucleotide receptor in chick muscle: its functional role in the regulation of acetylcholinesterase and acetylcholine receptor

In vertebrate neuromuscular junctions, ATP is stored at the motor nerve terminals and is co-released with acetylcholine during neural stimulation. Here, we provide several lines of evidence that the synaptic ATP can act as a synapse-organizing factor to induce the expression of acetylcholinesterase (AChE) and acetylcholine receptor (AChR) in muscles, mediated by a metabotropic ATP receptor subtype, the P2Y(1) receptor. The activation of the P2Y(1) receptor by adenine nucleotides stimulated the accumulation of inositol phosphates and intracellular Ca(2+) mobilization in cultured chick myotubes. P2Y(1) receptor mRNA in chicken muscle is very abundant before hatching and again increases in the adult. The P2Y(1) receptor protein is shown to be restricted to the neuromuscular junctions and colocalized with AChRs in adult muscle (chicken, Xenopus, and rat) but not in the chick embryo. In chicks after hatching, this P2Y(1) localization develops over approximately 3 weeks. Denervation or crush of the motor nerve (in chicken or rat) caused up to 90% decrease in the muscle P2Y(1) transcript, which was restored on regeneration, whereas the AChR mRNA greatly increased. Last, mRNAs encoding the AChE catalytic subunit and the AChR alpha-subunit were induced when the P2Y(1) receptors were activated by specific agonists or by overexpression of P2Y(1) receptors in cultured myotubes; those agonists likewise induced the activity in the myotubes of promoter-reporter gene constructs for those subunits, actions that were blocked by a P2Y(1)-specific antagonist. These results provide evidence for a novel function of ATP in regulating the gene expression of those two postsynaptic effectors.

Expression of the P2Y1 nucleotide receptor in chick muscle: its functional role in the regulation of acetylcholinesterase and acetylcholine receptor

In vertebrate neuromuscular junctions, ATP is stored at the motor nerve terminals and is co-released with acetylcholine during neural stimulation. Here, we provide several lines of evidence that the synaptic ATP can act as a synapse-organizing factor to induce the expression of acetylcholinesterase (AChE) and acetylcholine receptor (AChR) in muscles, mediated by a metabotropic ATP receptor subtype, the P2Y(1) receptor. The activation of the P2Y(1) receptor by adenine nucleotides stimulated the accumulation of inositol phosphates and intracellular Ca(2+) mobilization in cultured chick myotubes. P2Y(1) receptor mRNA in chicken muscle is very abundant before hatching and again increases in the adult. The P2Y(1) receptor protein is shown to be restricted to the neuromuscular junctions and colocalized with AChRs in adult muscle (chicken, Xenopus, and rat) but not in the chick embryo. In chicks after hatching, this P2Y(1) localization develops over approximately 3 weeks. Denervation or crush of the motor nerve (in chicken or rat) caused up to 90% decrease in the muscle P2Y(1) transcript, which was restored on regeneration, whereas the AChR mRNA greatly increased. Last, mRNAs encoding the AChE catalytic subunit and the AChR alpha-subunit were induced when the P2Y(1) receptors were activated by specific agonists or by overexpression of P2Y(1) receptors in cultured myotubes; those agonists likewise induced the activity in the myotubes of promoter-reporter gene constructs for those subunits, actions that were blocked by a P2Y(1)-specific antagonist. These results provide evidence for a novel function of ATP in regulating the gene expression of those two postsynaptic effectors.

Purinergic-independent calcium signaling mediates recovery from hepatocellular swelling: implications for volume regulation

Swelling of hepatocytes and other epithelia activates volume-sensitive ion channels that facilitate fluid and electrolyte efflux to restore cell volume, but the responsible signaling pathways are incompletely defined. Previous work in model HTC rat hepatoma cells has indicated that swelling elicits ATP release, which stimulates P2 receptors and activates Cl(-) channels, and that this mechanism is essential for hepatocellular volume recovery. Since P2 receptors are generally coupled to Ca(2+) signaling pathways, we determined whether hepatocellular swelling affected cytosolic [Ca(2+)], and if this involved a purinergic mechanism. Exposure of HTC cells to hypotonic media evoked an increase in cytosolic [Ca(2+)], which was followed by activation of K(+) and Cl(-) currents. Maneuvers that interfered with swelling-induced increases in cytosolic [Ca(2+)], including extracellular Ca(2+) removal and intracellular Ca(2+) store depletion with thapsigargin, inhibited activation of membrane currents and volume recovery. However, the swelling-induced increases in cytosolic [Ca(2+)] were unaffected by either extracellular ATP depletion with apyrase or blockade of P2 receptors with suramin. These findings indicate that swelling elicits an increase in hepatocellular Ca(2+), which is essential for ion channel activation and volume recovery, but that this increase does not stem from activation of volume-sensitive P2 receptors. Collectively, these observations imply that regulatory responses to hepatocellular swelling involve a dual requirement for a purinergic-independent Ca(2+) signaling cascade and a Ca(2+)-independent purinergic signaling pathway.

P2 purinoceptor-mediated control of rat cerebral (pial) microvasculature; contribution of P2X and P2Y receptors

Purine and pyrimidine nucleotides evoke changes in the vascular tone of medium to large cerebral vessels through the activation of P2 purinoceptors. We have applied P2 receptor drugs to rat pial arterioles and measured changes in arteriole diameter (o.d. 40-84 micrometer at rest), and recorded currents from arteriolar smooth muscle cells using patch-clamp techniques. Transient vasoconstrictions and rapidly inactivating currents were evoked by alpha,beta-methylene ATP (0.1-30 micrometer) and were sensitive to the P2 receptor antagonists suramin and iso-PPADS. UTP and UDP (0.1-1000 micrometer) evoked sustained suramin-sensitive vasoconstrictions. ATP (0.1-1000 micrometer) and 2-methylthioATP (2MeSATP, 300 micrometer) evoked transient vasoconstrictions followed by sustained vasodilatations. ADP application resulted in only vasodilatation (EC50 approximately 4 micrometer). Vasodilator responses to ATP, 2MeSATP or ADP were unaffected by suramin (100 micrometer). RT-PCR analysis indicated that P2X1-7 and P2Y1,2,6 RNA can be amplified from the pial sheet. Our results provide direct evidence for the presence of functional P2X receptors with a phenotype resembling the P2X1 receptor subtype on cerebral resistance arterioles. The pharmacological properties of the pyrimidine-evoked responses suggest that a combination of P2Y2- and P2Y6-like receptors are responsible for the sustained vasoconstrictions. It is therefore likely that the nucleotides and their associated receptors are involved in a complicated regulatory system to control cerebral blood pressure.

Switching from simple to complex oscillations in calcium signaling

We present a new model for calcium oscillations based on experiments in hepatocytes. The model considers feedback inhibition on the initial agonist receptor complex by calcium and activated phospholipase C, as well as receptor type-dependent self-enhanced behavior of the activated G(alpha) subunit. It is able to show simple periodic oscillations and periodic bursting, and it is the first model to display chaotic bursting in response to agonist stimulations. Moreover, our model offers a possible explanation for the differences in dynamic behavior observed in response to different agonists in hepatocytes.

Evidence that 2-methylthioATP and 2-methylthioADP are both agonists at the rat hepatocyte P2Y(1) receptor

In the absence of selective antagonists, pharmacological characterization of P2Y receptor subtypes has relied heavily upon their distinct agonist profiles. 2-methylthioADP (2-MeSADP) is a selective agonist for the P2Y(1) receptor. The agonist action of 2-MeSATP at the P2Y(1) receptor has recently been questioned. The effects of both 2-MeSADP and 2-MeSATP have been studied on rat hepatocytes injected with the bioluminescent Ca(2+) indicator, aequorin. Single hepatocytes generate series of repetitive transients in cytosolic free calcium concentration ([Ca(2+)](i)) when stimulated with agonists acting through the phosphoinositide signalling pathway. The transients induced by 2-MeSADP and 2-MeSATP in the same cell were indistinguishable, indicating that they act at a common receptor. In contrast the transients evoked by ATP and UTP had very different profiles. Treatment of 2-MeSATP with an ATP-regenerating system to remove contaminating 2-MeSADP did not abolish its agonist activity. Application of the P2Y(1) antagonist, adenosine-3'-phosphate-5'-phosphate (A3P5P) inhibited the transients induced by both 2-MeSADP and 2-MeSATP. In contrast the transients induced by ATP and UTP were enhanced by the addition of A3P5P. These results indicate that both 2-MeSADP and 2-MeSATP are agonists at the rat hepatocyte P2Y(1) receptor.