Stimulation of insulin secretion and improvement of glucose tolerance in rat and dog by the P2y-purinoceptor agonist, adenosine-5'-O-(2-thiodiphosphate)

Involvement of P2Y signaling in the restoration of glucose-induced insulin exocytosis in pancreatic β cells exposed to glucotoxicity

Purinergic P2Y receptors, by binding adenosine triphosphate (ATP), are known for enhancing glucose-stimulated insulin secretion (GSIS) in pancreatic β cells. However, the impact of these receptors in the actin dynamics and insulin granule exocytosis in these cells is not established, neither in normal nor in glucotoxic environment. In this study, we investigate the involvement of P2Y receptors on the behavior of insulin granules and the subcortical actin network dynamics in INS-1 832/13 β cells exposed to normal or glucotoxic environment and their role in GSIS. Our results show that the activation of P2Y purinergic receptors by ATP or its agonist increase the insulin granules exocytosis and the reorganization of the subcortical actin network and participate in the potentiation of GSIS. In addition, their activation in INS-1832/13 β-cells, with impaired insulin secretion following exposure to elevated glucose levels, restores GSIS competence through the distal steps of insulin exocytosis. These results are confirmed ex vivo by perifusion experiments on islets from type 2 diabetic (T2D) Goto-Kakizaki (GK) rats. Indeed, the P2Y receptor agonist restores the altered GSIS, which is normally lost in this T2D animal model. Moreover, we observed an improvement of the glucose tolerance, following the acute intraperitoneal injection of the P2Y agonist concomitantly with glucose, in diabetic GK rats. All these data provide new insights into the unprecedented therapeutic role of P2Y purinergic receptors in the pathophysiology of T2D.

Role of purines in regulation of metabolic reprogramming

Purines, among most influential molecules, are reported to have essential biological function by regulating various cell types. A large number of studies have led to the discovery of many biological functions of the purine nucleotides such as ATP, ADP, and adenosine, as signaling molecules that engage G protein-coupled or ligand-gated ion channel receptors. The role of purines in the regulation of cellular functions at the gene or protein level has been well documented. With the advances in multiomics, including those from metabolomic and bioinformatic analyses, metabolic reprogramming was identified as a key mechanism involved in the regulation of cellular function under physiological or pathological conditions. Recent studies suggest that purines or purine-derived products contribute to important regulatory functions in many fundamental biological and pathological processes related to metabolic reprogramming. Therefore, this review summarizes the role and potential mechanism of purines in the regulation of metabolic reprogramming. In particular, the molecular mechanisms of extracellular purine- and intracellular purine-mediated metabolic regulation in various cells during disease development are discussed. In summary, our review provides an extensive resource for studying the regulatory role of purines in metabolic reprogramming and sheds light on the utilization of the corresponding peptides or proteins for disease diagnosis and therapy.

PI3K is involved in P2Y receptor-regulated cAMP /Epac/Kv channel signaling pathway in pancreatic β cells

P2Y receptors (P2YR) are a family of purinergic G protein-coupled receptors, which could be stimulated by extracellular nucleotides. In pancreatic β cells, activation of P2YR has long been shown to stimulate insulin secretion in a glucose-dependent manner. Previously, we reported that P2YR-modulated insulin secretion is mediated by a cAMP/Epac/Kv channel pathway. However, the interaction between Epac and the Kv channel in P2YR-modulated insulin secretion remains unclear. In this study, we used patch-clamp technique and insulin secretion assay to investigate the potential molecules that may link Epac to Kv channel inhibition induced by P2YR activation. We identified that phosphatidylinositide 3-kinase, which mediates P2YR-regulated insulin secretion, is a critical mediator between Epac and the Kv channel.

P2Y purinergic receptor-regulated insulin secretion is mediated by a cAMP/Epac/Kv channel pathway

Enhancement of insulin secretion is a major therapeutic approach for type 2 diabetes (T2D). Activation of P2Y purinergic receptor (P2YR) causes potentiation of insulin secretion in a glucose-dependent manner, making it a promising therapeutic target for T2D. Here we show that activation of P2YR to potentiate insulin secretion is mediated by adenylyl cyclase/cyclic AMP (cAMP) and the downstream effector, exchange protein directly activated by cAMP (Epac), leading to inhibition of voltage-dependent potassium (Kv) channels. P2YR-mediated Kv channel inhibition results in prolongation of action potential duration, and in turn elevates intracellular Ca(2+) level and insulin secretion. Taken together, the data indicate that cAMP/Epac/Kv channel pathway mediates P2YR-regulated insulin secretion, which may have important therapeutic implications for T2D.

Impairment of vesicular ATP release affects glucose metabolism and increases insulin sensitivity

Neuroendocrine cells store ATP in secretory granules and release it along with hormones that may trigger a variety of cellular responses in a process called purinergic chemical transmission. Although the vesicular nucleotide transporter (VNUT) has been shown to be involved in vesicular storage and release of ATP, its physiological relevance in vivo is far less well understood. In Vnut knockout (Vnut(-/-)) mice, we found that the loss of functional VNUT in adrenal chromaffin granules and insulin granules in the islets of Langerhans led to several significant effects. Vesicular ATP accumulation and depolarization-dependent ATP release were absent in the chromaffin granules of Vnut(-/-) mice. Glucose-responsive ATP release was also absent in pancreatic β-cells in Vnut(-/-) mice, while glucose-responsive insulin secretion was enhanced to a greater extent than that in wild-type tissue. Vnut(-/-) mice exhibited improved glucose tolerance and low blood glucose upon fasting due to increased insulin sensitivity. These results demonstrated an essential role of VNUT in vesicular storage and release of ATP in neuroendocrine cells in vivo and suggest that vesicular ATP and/or its degradation products act as feedback regulators in catecholamine and insulin secretion, thereby regulating blood glucose homeostasis.

Preliminary study characterizing the use of sitagliptin for glycemic control in healthy Beagle dogs with normal gluco-homeostasis

Sitagliptin is a dipeptidyl peptidase-4 inhibitor aimed at treating Type 2 diabetes mellitus (T2DM) and T1DM, by increasing blood levels of Glucagon-like peptide 1 (GLP-1) and insulin. The objective of this preliminary study is to characterize Sitagliptin’s ability for glycemic control, in healthy dogs under an oral glucose tolerance test (OGTT) environment. Overall, Sitagliptin did not result in any significant changes to temporal glucose and insulin concentrations. However, a ~55% increase in median total GLP-1 AUC_0–120min was observed, as compared to baseline control in healthy dogs (n=5), thus indicating a similar mode of action of Sitagliptin between healthy dogs and humans. Future studies to validate the use of Sitagliptin with dogs suffering from insulin independent diabetes are warranted.

Purinergic signalling in endocrine organs

There is widespread involvement of purinergic signalling in endocrine biology. Pituitary cells express P1, P2X and P2Y receptor subtypes to mediate hormone release. Adenosine 5'-triphosphate (ATP) regulates insulin release in the pancreas and is involved in the secretion of thyroid hormones. ATP plays a major role in the synthesis, storage and release of catecholamines from the adrenal gland. In the ovary purinoceptors mediate gonadotrophin-induced progesterone secretion, while in the testes, both Sertoli and Leydig cells express purinoceptors that mediate secretion of oestradiol and testosterone, respectively. ATP released as a cotransmitter with noradrenaline is involved in activities of the pineal gland and in the neuroendocrine control of the thymus. In the hypothalamus, ATP and adenosine stimulate or modulate the release of luteinising hormone-releasing hormone, as well as arginine-vasopressin and oxytocin. Functionally active P2X and P2Y receptors have been identified on human placental syncytiotrophoblast cells and on neuroendocrine cells in the lung, skin, prostate and intestine. Adipocytes have been recognised recently to have endocrine function involving purinoceptors.

Characterization of the use of liraglutide for glycemic control in healthy and Type 1 diabetes mellitus suffering dogs

Glucagon-like peptide 1 (GLP-1) is a glucose-lowering, intestinal-derived factor with multiple physiological effects, making it attractive for diabetes therapy. However, the therapeutic potential of endogenous GLP-1 is limited, because of rapid inactivation by dipeptidyl peptidase-4. Recently, enhanced incretin preparations, such as liraglutide, have emerged, which are more resistant to degradation and longer lasting. Liraglutide is a long-acting acylated human GLP-1 receptor agonist, with a 97% amino acid sequence identity to endogenous human GLP-1, and 100% amino acid sequence homology with canine GLP-1. Since liraglutide has yet to be examined for use in dogs, and the incretin effect has been reported to exist in dogs, we sought to initially characterize liraglutide's ability for glycemic control in healthy dogs, under an oral glucose tolerance test (OGTT) environment initially. This was followed up a more realistic scenario involving food with insulin injection +/- liraglutide injection resulting in a glucose curve based study involving dogs suffering from Type 1 diabetes mellitus (T1DM). Overall, liraglutide had a stabilizing effect on glucose levels, maintaining circulating levels between 77.0 and 137.0mg/ml throughout the OGTT test period, resulting in a significant reduction of 13.8% in glucose AUC0-120 min (total area under the curve for 0-120 min) as compared to baseline control in healthy dogs (n=5). Interestingly, the liraglutide associated reduction in circulating glucose was not accompanied by any significant increase in insulin. Moreover, T1DM dogs (n=4) responded favorably to liraglutide treatment, which lead to a significant reduction of 46.0% in glucose AUC0-12h (total area under the curve for 0-12h), and a significant reduction of 66.5% in serum glucose as compared to baseline controls (insulin treatment only). Therefore, liraglutide's prandial glucagon suppressive ability appears to play a key role in its glucose-lowering capability, and offers great potential for use with dogs suffering from T1DM.

Identification of a promising drug candidate for the treatment of type 2 diabetes based on a P2Y(1) receptor agonist

The activation by extracellular nucleotides of pancreatic P2Y receptors, particularly, the P2Y(1)R subtype, increases insulin secretion. Therefore, we developed analogues of the P2Y(1)R receptor agonist 2-MeS-ADP, as potential antidiabetic drugs. Analogue 3A was found to be a potent P2Y(1)R agonist (EC(50) = 0.038 μM vs 0.0025 μM for 2-MeS-ADP) showing no activity at P2Y(2/4/6)Rs. Analogue 3A was stable at pH 1.4 (t(1/2) = 7.3 h) and resistant to hydrolysis vs 2-MeS-ADP by alkaline phosphatase (t(1/2) = 6 vs 4.5 h), human e-NPP1 (4% vs 16% hydrolysis after 20 min), and human blood serum (30% vs 50% hydrolysis after 24 h). Intravenous administration of 3A in naive rats decreased blood glucose from 155 mg/dL to normal values, ca. 87 mg/dL, unlike glibenclamide, leading to subnormal values (i.e., 63 mg/dL). Similar observations were made for streptozotocin (STZ)-treated and db(+)/db(-) mouse models. Furthermore, 3A inhibits platelet aggregation in vitro and elongates bleeding time in mice (iv administration of 30 mg of 3A/kg), increasing bleeding time to 16 vs 9 min for Prasugrel. Oral administration of 30 mg/kg 3A to rats increased tail bleeding volume, similar to aspirin. These findings suggest that 3A may be an effective treatment for type 2 diabetes by reducing both blood glucose levels and platelet aggregation.

Purinergic signalling in the pancreas in health and disease

Pancreatic cells contain specialised stores for ATP. Purinergic receptors (P2 and P1) and ecto-nucleotidases are expressed in both endocrine and exocrine calls, as well as in stromal cells. The pancreas, especially the endocrine cells, were an early target for the actions of ATP. After the historical perspective of purinergic signalling in the pancreas, the focus of this review will be the physiological functions of purinergic signalling in the regulation of both endocrine and exocrine pancreas. Next, we will consider possible interaction between purinergic signalling and other regulatory systems and their relation to nutrient homeostasis and cell survival. The pancreas is an organ exhibiting several serious diseases - cystic fibrosis, pancreatitis, pancreatic cancer and diabetes - and some are associated with changes in life-style and are increasing in incidence. There is upcoming evidence for the role of purinergic signalling in the pathophysiology of the pancreas, and the new challenge is to understand how it is integrated with other pathological processes.

P2 purinergic signalling in the pancreatic beta-cell: control of insulin secretion and pharmacology

Extracellular adenosine triphosphate is able to modulate pancreatic beta-cell function, acting on P2 purinergic ionotropic (P2X) and metabotropic (P2Y) receptors. Physiologically, ATP entrains beta-cells into a common rhythm by coordinating Ca(2+) oscillations; it plays a central role in insulin secretion pulsatility. ATP also triggers a positive feedback signal amplifying glucose-induced insulin release, which argues for a potential pharmacological application. ATP has consistently been shown to increase cytoplasmic free calcium concentration, notably in human tissue. Acting on P2X receptors, of which different molecular subtypes are expressed in beta-cells, it leads to a transient insulin release that may involve a closure of K(ATP) channels or a rapidly decaying inward current. Activation of G-protein-coupled P2Y receptors triggers different signalling pathways and amplifies insulin release in a glucose-dependent way. It has recently been shown that pancreatic beta-cells express different molecular subtypes of receptors, which may explain the complex interaction of P2Y ligands on high- and low-affinity binding sites. Despite the complexity of this purinergic pharmacology, consistent pre-clinical data suggest the potential of P2Y receptor agonists as drug candidates for type 2 diabetes.

Purinergic receptors in the endocrine and exocrine pancreas

The pancreas is a complex gland performing both endocrine and exocrine functions. In recent years there has been increasing evidence that both endocrine and exocrine cells possess purinergic receptors, which influence processes such as insulin secretion and epithelial ion transport. Most commonly, these processes have been viewed separately. In beta cells, stimulation of P2Y(1) receptors amplifies secretion of insulin in the presence of glucose. Nucleotides released from secretory granules could also contribute to autocrine/paracrine regulation in pancreatic islets. In addition to P2Y(1) receptors, there is also evidence for other P2 and adenosine receptors in beta cells (P2Y(2), P2Y(4), P2Y(6), P2X subtypes and A(1) receptors) and in glucagon-secreting alpha cells (P2X(7), A(2) receptors). In the exocrine pancreas, acini release ATP and ATP-hydrolysing and ATP-generating enzymes. P2 receptors are prominent in pancreatic ducts, and several studies indicate that P2Y(2), P2Y(4), P2Y(11), P2X(4) and P2X(7) receptors could regulate secretion, primarily by affecting Cl(-) and K(+) channels and intracellular Ca(2+) signalling. In order to understand the physiology of the whole organ, it is necessary to consider the full complement of purinergic receptors on different cells as well as the structural and functional relation between various cells within the whole organ. In addition to the possible physiological function of purinergic receptors, this review analyses whether the receptors could be potential therapeutic targets for drug design aimed at treatment of pancreatic diseases.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

Pathophysiology and therapeutic potential of purinergic signaling

The concept of a purinergic signaling system, using purine nucleotides and nucleosides as extracellular messengers, was first proposed over 30 years ago. After a brief introduction and update of purinoceptor subtypes, this article focuses on the diverse pathophysiological roles of purines and pyrimidines as signaling molecules. These molecules mediate short-term (acute) signaling functions in neurotransmission, mechanosensory transduction, secretion and vasodilatation, and long-term (chronic) signaling functions in cell proliferation, differentiation, and death involved in development and regeneration. Plasticity of purinoceptor expression in pathological conditions is frequently observed, including an increase in the purinergic component of autonomic cotransmission. Recent advances in therapies using purinergic-related drugs in a wide range of pathological conditions will be addressed with speculation on future developments in the field.

P2Y purinergic potentiation of glucose-induced insulin secretion and pancreatic beta-cell metabolism

Purine nucleotides and their analogs increase insulin secretion through activation of pancreatic beta-cell P2Y receptors. The present study aimed at determining the role of glucose metabolism in the response to P2Y agonists and whether ATP-activated K+ channels (KATP channels) are involved in this response. The experiments were performed in the rat isolated pancreas, perfused with a Krebs-bicarbonate buffer supplemented with 2 g/l bovine serum albumin under dynamic glucose conditions from 5 mmol/l baseline to 11 mmol/l. ADPbetaS (0.5 micromol/l) was selected as a stable and selective P2Y agonist. This compound, ineffective on the 5 mmol/l glucose background, induced a significant threefold increase in insulin release triggered by the glucose challenge. The effect of ADPbetaS was markedly reduced (P <0.001) in the presence of an inhibitor of glucose metabolism. In addition to glucose, the ADP analog also amplified the beta-cell insulin response to 15 mmol/l methyl pyruvate (P <0.05), but it was ineffective on the insulin response to 2.5 mmol/l methyl succinate. A nonmetabolic stimulus was applied using tolbutamide (185 micromol/l). Insulin secretion induced by the KATP channel blocker was strongly reinforced by ADPbetaS (P <0.001), which prompted us to check a possible interplay of KATP channels in the effect of ADPbetaS. In the presence of diazoxide 250 micromol/l and 21 mmol/l KCl, ADPbetaS still amplified the second phase of glucose-induced insulin secretion (P <0.001). We conclude that P2Y receptor activation is able to promote insulin secretion through a mechanism, involving beta-cell metabolism and a rise in intracellular calcium; this effect does not result from a direct inhibitory effect on KATP channels.

P2X and P2Y purinoceptor expression in pancreas from streptozotocin-diabetic rats

The expression of the nucleotide receptors P2X1, P2X2, P2X7, P2Y1, P2Y2 and P2Y4, in the pancreas of the streptozotocin-induced diabetic rat was investigated using immunohistochemistry. In diabetic animals, P2X7 receptor expression, normally located in the outer periphery of the islet, was increased and located inside the islet. Double-labelling experiments, using antibodies raised against insulin, somatostatin and glucagon, showed, for the first time, an increase in immunostaining for P2X7 receptors on islet glucagon-containing alpha cells (which had migrated to the interior), while no P2X7 receptors were found in beta and delta cells. P2Y1 receptors were present in intra-islet capillaries, while P2Y4 receptors were found on both alpha and beta cells. P2Y1 and P2Y2 receptor expression was also found in pancreatic duct cells and P2X1, P2X2, P2Y1 and P2Y2 receptors were identified in small blood vessels.

Role of adenine nucleotides in insulin secretion from MIN6 pseudoislets

Insulin secretion from MIN6 cells configured as cell aggregates by culture on a gelatin substrate (pseudoislets) is enhanced compared to that of MIN6 cells grown as monolayers on tissue culture plastic, indicating the importance of beta-cell-to-beta-cell proximity for insulin release. In this study we have shown that glucose induced a biphasic release of insulin from pseudoislets, whereas the amplitude and duration of the responses of equivalent monolayer cells were much reduced. Purinergic aqonists have been implicated in intercellular communication between beta-cells, so we investigated whether adenine nucleotides co-released with insulin are responsible for the enhanced secretory responses of pseudoislets. We have demonstrated that MIN6 cells express purinergic A(1) and P2Y receptors, and that adenine nucleotides increased [Ca(2+)](i) with an efficacy of agonists being ATP > ADP > AMP. However, neither suramin nor the more selective A(1) antagonist 1,3-dipropyl-8-cyclopentylxanthine reduced glucose-induced insulin secretion from pseudoislets, and stimulation of monolayer cells with a range of adenine nucleotides did not enhance glucose-induced secretion. These results suggest that enhanced secretion from MIN6 pseudoislets is not due to increased paracrine/autocrine action of adenine nucleotides.

Effect of purinergic agonists and antagonists on insulin secretion from INS-1 cells (insulinoma cell line) and rat pancreatic islets

The effects of purinergic agonists on insulin release are controversial in the literature. In our studies (mainly using INS-1 cells, but also using rat pancreatic islets), ATP had a dual effect on insulin release depending on the ATP concentration: increasing insulin release (EC50 approximately/= 0.0032 microM) and inhibiting insulin release (EC50 approximately/= 0.32 microM) at both 5.6 and 8.3 mM glucose. This is compatible with the view that either two different receptors are involved, or the cells desensitize and (or) the effect of an inhibitory degradation product such as adenosine (ectonucleotidase effect) emerges. The same dual effects of ATP on insulin release were obtained using rat pancreatic islets instead of INS-1 cells. ADPbetaS, which is less degradable than ATP and rather specific for P2Y1 receptors, had a dual effect on insulin release at 8.3 mM glucose: stimulatory (EC50 approximately/= 0.02 microM) and inhibitory (EC50 approximately/= 0.32 microM). The effectiveness of this compound indicates the possible involvement of a P2Y1 receptor. 2-Methylthio-ATP exhibited an insulinotropic effect at very high concentrations (EC50 approximately/= 15 microM at 8.3 mM glucose). This indicated that distinct P2X or the P2Y1 receptor may be involved in these insulin-secreting cells. UTP increased insulin release (EC50 approximately/= 2 microM) very weakly, indicating that a P2U receptor (P2X3 or possibly a P2Y2 or P2Y4) are not likely to be involved. Suramin (50 microM) antagonized the insulinotropic effect of ATP (0.01 microM) and UTP (0.32 microM). Since suramin is not selective, the data indicated that various P2X and P2Y receptors may be involved. PPADS (100 microM), a P2X and P2Y1,4,6 receptor antagonist, was ineffective using either low or high concentrations of ATP and ADPbetaS, which combined with the suramin data hints at a P2Y receptor effect of the compounds. Adenosine inhibited insulin release in a concentration-dependent manner. DPCPX (100 microM), an adenosine (A1) receptor antagonist, inhibited the inhibitory effects of both adenosine and of high concentrations of ATP. Adenosine deaminase (1 U/mL) abolished the inhibitory effect of high ATP concentrations, indicating the involvement of the degradation product adenosine. Repetitive addition of ATP did not desensitize the stimulatory effect of ATP. U-73122 (2 microM), a PLC inhibitor, abolished the ATP effect at low concentrations. The data indicate that ATP at low concentrations is effective via P2Y receptors and the PLC-system and not via P2X receptors; it inhibits insulin release at high concentrations by being metabolized to adenosine.

Vasodilatation of intrapulmonary arteries to P2-receptor nucleotides in normal and pulmonary hypertensive newborn piglets

1. The vasodilator responses of isolated intrapulmonary arteries (IPA) to P2-receptor agonists were investigated during adaptation to extrauterine life in the piglet. The effect of pulmonary hypertension on the normal response was determined after exposing newborn animals to chronic hypobaric hypoxia (51 kPa) for 3 days. 2. Adenosine 5'-triphosphate (ATP), 2-methylthioATP (2-meSATP), adenosine 5-O-(2-thiodiphos-phate) (ADPbetaS) and uridine 5'-triphosphate (UTP) induced a relaxation in normal newborn piglet IPA pre-contracted with prostaglandin F2alpha (PGF2alpha). The relaxations were not affected by removal of the endothelium. The responses to ATP and ADPbetaS increased significantly with age. 3. The relaxation responses of IPA to ATP, 2-meSATP and ADPbetaS continued to increase normally after birth in an hypoxic environment. 4. The results of the study show that vasodilatation of porcine intrapulmonary vessels to nucleotides increased during development from foetus to adult; that the vasodilatation to purines was mediated by P2Y-receptors on the vascular smooth muscle rather than on the endothelium; and that the P2Y-receptor mediated relaxation of IPA remained normal in the pulmonary hypertensive neonate.

2-thioether 5'-O-(1-thiotriphosphate)adenosine derivatives as new insulin secretagogues acting through P2Y-Receptors

P2-Receptors (P2-Rs) represent significant targets for novel drug development. P2-Rs were identified also on pancreatic B cells and are involved in insulin secretion. Therefore, novel P2Y-R ligands, 2-thioether 5'-O-phosphorothioate adenosine derivatives (2-RS-ATP-alpha-S), were synthesized as potential insulin secretagogues. An efficient synthesis of these nucleotides and a facile method for separation of the chiral products are described. The enzymatic stability of the compounds toward pig pancreas type I ATPDase was evaluated. The rate of hydrolysis of 2-hexylthio-5'-O-(1-thiotriphosphate)adenosine (2-hexylthio-ATP-alpha-S) isomers by ATPDase was 28% of that of ATP. Some 2-thioether 5'-(monophosphorothioate)adenosine derivatives (2-RS-AMP-S) exerted an inhibitory effect on ATPDase. The apparent affinity of the compounds to P2Y(1)-R was determined by measurement of P2Y-R-promoted phospholipase C activity in turkey erythrocyte membranes. 2-RS-ATP-alpha-S derivatives were agonists, stimulating the production of inositol phosphates with K(0.5) values in the nanomolar range. 2-RS-AMP-S derivatives were full agonists, although 2 orders of magnitude less potent. All the compounds were more potent than ATP. The effect on insulin secretion and pancreatic flow rate was evaluated on isolated and perfused rat pancreas. A high increase, up to 500%, in glucose-induced insulin secretion was due to addition of 2-hexylthio-ATP-alpha-S in the nanomolar concentration range, which represents 100-fold enhancement of activity relative to ATP. 2-Hexylthio-AMP-S was 2.5 orders of magnitude less effective.

P2Y purinoceptors in gastric gland plasma membranes

This autoradiographic study of sections of the rabbit stomach fundus labelled with [35S]dATP alpha S, a radioligand for P2Y purinoceptors, has demonstrated a discrete pattern of distribution of the binding sites, i.e., the specific binding was only over the mucosa, but not over the muscular layer. Radioligand binding assays carried out on gastric gland plasma membranes showed that the binding process was saturable and a high density of a homogeneous population of binding sites was observed. These binding sites presented high affinity with a value of Kd = 4.1 +/- 0.8 nM and the maximum density of the binding sites was 16.8 +/- 1.6 pmol/mg protein. The displacement by purinoceptor ligands showed the following order of potency: ATP = 2-methylthio ATP > > alpha, beta-methylene ATP > > adenosine. Neither UTP nor pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) were able to displace the binding. The data support the presence of P2Y purinceptors in rabbit gastric glands.

Molecular cloning and characterization of a novel orphan receptor (P2P) expressed in human pancreas that shows high structural homology to the P2U purinoceptor

Here we report the cloning of a gene encoding a new member of the superfamily of G protein-coupled receptors. The gene encodes a protein of 365 amino acids closely resembling two recently cloned nucleotide binding receptors, called P2U and P2Y purinoceptors (71% and 49% sequence identity within the transmembrane domains, respectively). Our studies show that this new putative purinoceptor (designated P2P) is encoded by an intronless single copy gene that is exclusively expressed in pancreas, in contrast to the P2U and the P2Y purinoceptors which are widely distributed throughout the periphery. The identification of a pancreas-specific human putative P2 purinoceptor makes it attractive to speculate that the reported actions of ADP/ATP analogues in pancreas on insulin secretion are mediated through this receptor.

Preservation of insulin secretory responses to P2 purinoceptor agonists in Zucker diabetic fatty rats

The role of P2 purinoceptor agonists in regulatory insulin secretion in Zucker diabetic fatty (ZDF) rats was studied using the isolated perfused pancreas and intracellular Ca2+ concentration ([Ca2+]i) microfluorimetry. The relative potency of different purinoceptor agonists to stimulate the insulin secretory process was consistent with the conclusion that responses in [Ca2+]i and insulin secretion are mediated by the P2y subtype of purinoceptors. Additional studies using specific antagonists of the Ca2+ signaling pathway indicated that activation of P2y purinoceptor releases Ca2+ from intracellular stores and promotes Ca2+ entry through voltage-independent rather than voltage-dependent Ca2+ channels on the beta-cell membrane. Perfused pancreas and isolated islets from ZDF rats demonstrated markedly reduced or absent insulin secretion and [Ca2+]i responses to glucose and KCl. In contrast, responses to P2y purinoceptor agonists were normal, indicating that the secretion coupling pathway activated by these agonists is preserved in glucose-unresponsive islets from diabetic animals. These observations raise the possibility that the purinoceptor pathway may play an important role in regulating insulin secretion in hyperinsulinemic non-insulin-dependent diabetes mellitus.

Purinergic receptors on insulin-secreting cells

The insulin secreting B cell is fitted with the two types of purinergic receptors: P2 (for ATP and/or ADP) and P1 (for adenosine). The activation of P2 purinoceptors by ATP or ADP evokes a biphasic stimulation of insulin secretion from isolated perfused rat pancreas; this stimulation is dose-dependent between 10(-6) and 10(-4) M. Non hydrolysable structural analogues are also effective, and the relative potency of various agonists (2-methylthio ATP >> ATP = ADP = alpha, beta-methylene ATP >> AMP) gave evidence for a P2y purinoceptor subtype. Proposed mechanisms include both an increased Ca2+ uptake and an increased intracellular Ca2+ mobilization via the hydrolysis of polyphosphoinositides. ATP (or ADP) potentiates physiological insulin-secreting agents (glucose and acetylcholine) and P2 purinoceptors could play a physiological role in the stimulation of insulin secretion. The activation of P1 purinoceptors (adenosine receptors) decreases insulin secretion. Using structural analogues of adenosine, the receptor was characterized as an A1 subtype; it is coupled to a pertussis toxin sensitive G protein and it inhibits adenylate cyclase. It is of physiological relevance that the B cell has the two types of purinoceptors with opposite effects. Recently, a metabolically stable structural analogue of ADP, adenosine-5'-0-(2-thiodiphosphate) or ADP beta S, has been described as a potent secretory agent, effective at nanomolar concentrations on isolated perfused rat pancreas. In vivo, this substance is able to increase insulin secretion and to improve glucose tolerance after IV administration in rats and oral administration in dogs. Furthermore in streptozotocin-induced diabetes. ADP beta S retains its insulin secreting effects. These results suggest that P2y purinoceptors could be a new target for antidiabetic drugs.

Purinoceptors: are there families of P2X and P2Y purinoceptors?

There has been an exponential growth in interest in purinoceptors since the potent effects of purines were first reported in 1929 and purinoceptors defined in 1978. A distinction between P1 (adenosine) and P2 (ATP/ADP) purinoceptors was recognized at that time and later, A1 and A2, as well as P2x and P2y subclasses of P1 and P2 purinoceptors were also defined. However, in recent years, many new subclasses have been claimed, particularly for the receptors to nucleotides, including P2t, P2z, P2u(n) and P2D, and there is some confusion now about how to incorporate additional discoveries concerning the responses of different tissues to purines. The studies beginning to appear defining the molecular structure of P2-purinoceptor subtypes are clearly going to be important in resolving this problem, as well as the introduction of new compounds that can discriminate pharmacologically between subtypes. Thus, in this review, on the basis of this new data and after a detailed analysis of the literature, we propose that: (1) P2X(ligand-gated) and P2Y(G-protein-coupled) purinoceptor families are established; (2) four subclasses of P2X-purinoceptor can be identified (P2X1-P2X4) to date; (3) the variously named P2-purinoceptors that are G-protein-coupled should be incorporated into numbered subclasses of the P2Y family. Thus: P2Y1 represents the recently cloned P2Y receptor (clone 803) from chick brain; P2Y2 represents the recently cloned P2u (or P2n) receptor from neuroblastoma, human epithelial and rat heart cells; P2Y3 represents the recently cloned P2Y receptor (clone 103) from chick brain that resembles the former P2t receptor; P2Y4-P2Y6 represent subclasses based on agonist potencies of newly synthesised analogues; P2Y7 represents the former P2D receptor for dinucleotides. This new framework for P2 purinoceptors would be fully consistent with what is emerging for the receptors to other major transmitters, such as acetylcholine, gamma-aminobutyric acid, glutamate and serotonin, where two main receptor families have been recognised, one mediating fast receptor responses directly linked to an ion channel, the other mediating slower responses through G-proteins. We fully expect discussion on the numbering of the different receptor subtypes within the P2X and P2Y families, but believe that this new way of defining receptors for nucleotides, based on agonist potency order, transduction mechanisms and molecular structure, will give a more ordered and logical approach to accommodating new findings. Moreover, based on the extensive literature analysis that led to this proposal, we suggest that the development of selective antagonists for the different P2-purinoceptor subtypes is now highly desirable, particularly for therapeutic purposes.