Introduction to Purinergic Signaling

Purinergic signaling was proposed in 1972, after it was demonstrated that adenosine 5'-triphosphate (ATP) was a transmitter in nonadrenergic, noncholinergic inhibitory nerves supplying the guinea-pig taenia coli. Later, ATP was identified as an excitatory cotransmitter in sympathetic and parasympathetic nerves, and it is now apparent that ATP acts as a cotransmitter in most, if not all, nerves in both the peripheral nervous system and central nervous system (CNS). ATP acts as a short-term signaling molecule in neurotransmission, neuromodulation, and neurosecretion. It also has potent, long-term (trophic) roles in cell proliferation, differentiation, and death in development and regeneration. Receptors to purines and pyrimidines have been cloned and characterized: P1 adenosine receptors (with four subtypes), P2X ionotropic nucleotide receptors (seven subtypes) and P2Y metabotropic nucleotide receptors (eight subtypes). ATP is released from different cell types by mechanical deformation, and after release, it is rapidly broken down by ectonucleotidases. Purinergic receptors were expressed early in evolution and are widely distributed on many different nonneuronal cell types as well as neurons. Purinergic signaling is involved in embryonic development and in the activities of stem cells. There is a growing understanding about the pathophysiology of purinergic signaling and there are therapeutic developments for a variety of diseases, including stroke and thrombosis, osteoporosis, pain, chronic cough, kidney failure, bladder incontinence, cystic fibrosis, dry eye, cancer, and disorders of the CNS, including Alzheimer's, Parkinson's. and Huntington's disease, multiple sclerosis, epilepsy, migraine, and neuropsychiatric and mood disorders.

Purinergic signalling: pathophysiology and therapeutic potential

The article begins with a review of the main conceptual steps involved in the development of our understanding of purinergic signalling, including non-adrenergic, non-cholinergic (NANC) neurotransmission; identification of ATP as a NANC transmitter; purinergic cotransmission; recognition of two families of purinoceptors [P1 (adenosine) and P2 (ATP/ADP)]; and, later, cloning and characterisation of P1 (G protein-coupled), P2X (ion channel) and P2Y (G protein-coupled) receptor subtypes. Further studies have established the involvement of ATP in synaptic neurotransmission in both ganglia and in the central nervous system; long-term (trophic) purinergic signalling in cell proliferation, differentiation and death occurring in development and regeneration; and short-term purinergic signalling in neurotransmission, neuromodulation and secretion. ATP is released from most cell types in response to gentle mechanical stimulation and is rapidly degraded to adenosine by ecto-nucleotidases. This review then focuses on the pathophysiology of purinergic signalling in a wide variety of systems, including urinogenital, cardiovascular, airway, musculoskeletal and gastrointestinal. Consideration is also given to the involvement of purinoceptors in pain, cancer and diseases of the central nervous system. Purinergic therapeutic approaches for the treatment of some of these diseases are discussed.

Purinergic signaling in the airways

Evidence for a significant role and impact of purinergic signaling in normal and diseased airways is now beyond dispute. The present review intends to provide the current state of knowledge of the involvement of purinergic pathways in the upper and lower airways and lungs, thereby differentiating the involvement of different tissues, such as the epithelial lining, immune cells, airway smooth muscle, vasculature, peripheral and central innervation, and neuroendocrine system. In addition to the vast number of well illustrated functions for purinergic signaling in the healthy respiratory tract, increasing data pointing to enhanced levels of ATP and/or adenosine in airway secretions of patients with airway damage and respiratory diseases corroborates the emerging view that purines act as clinically important mediators resulting in either proinflammatory or protective responses. Purinergic signaling has been implicated in lung injury and in the pathogenesis of a wide range of respiratory disorders and diseases, including asthma, chronic obstructive pulmonary disease, inflammation, cystic fibrosis, lung cancer, and pulmonary hypertension. These ostensibly enigmatic actions are based on widely different mechanisms, which are influenced by the cellular microenvironment, but especially the subtypes of purine receptors involved and the activity of distinct members of the ectonucleotidase family, the latter being potential protein targets for therapeutic implementation.

Molecular pharmacology, physiology, and structure of the P2Y receptors

The P2Y receptors are a widely expressed group of eight nucleotide-activated G protein-coupled receptors (GPCRs). The P2Y(1)(ADP), P2Y(2)(ATP/UTP), P2Y(4)(UTP), P2Y(6)(UDP), and P2Y(11)(ATP) receptors activate G(q) and therefore robustly promote inositol lipid signaling responses. The P2Y(12)(ADP), P2Y(13)(ADP), and P2Y(14)(UDP/UDP-glucose) receptors activate G(i) leading to inhibition of adenylyl cyclase and to Gβγ-mediated activation of a range of effector proteins including phosphoinositide 3-kinase-γ, inward rectifying K(+) (GIRK) channels, phospholipase C-β2 and -β3, and G protein-receptor kinases 2 and 3. A broad range of physiological responses occur downstream of activation of these receptors ranging from Cl(-) secretion by epithelia to aggregation of platelets to neurotransmission. Useful structural models of the P2Y receptors have evolved from extensive genetic analyses coupled with molecular modeling based on three-dimensional structures obtained for rhodopsin and several other GPCRs. Selective ligands have been synthesized for most of the P2Y receptors with the most prominent successes attained with highly selective agonist and antagonist molecules for the ADP-activated P2Y(1) and P2Y(12) receptors. The widely prescribed drug, clopidogrel, which results in irreversible blockade of the platelet P2Y(12) receptor, is the most important therapeutic agent that targets a P2Y receptor.

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.

Purinergic signalling

While there were early papers about the extracellular actions of purines, the role of ATP as a purinergic neurotransmitter in nonadrenergic, noncholinergic nerves in the gut and bladder in 1972 was a landmark discovery, although it met considerable resistance for the next 20 years. In the early 1990s, receptors for purines were cloned: four P1 receptor subtypes and seven P2X ionotropic and eight P2Y metabotropic receptor subtypes are currently recognized and characterized. The mechanisms underlying ATP release and breakdown are discussed. Purines and pyrimidines have major roles in the activities of non-neuronal cells as well as neurons. This includes fast signalling roles in exocrine and endocrine secretion, platelet aggregation, vascular endothelial cell-mediated vasodilation and nociceptive mechanosensory transduction, as well as acting as a cotransmitter and neuromodulator in most, if not all, nerve types in the peripheral and central nervous systems. More recently, slow (trophic) purinergic signalling has been implicated in cell proliferation, migration, differentiation and death in embryological development, wound healing, restenosis, atherosclerosis, ischaemia, cell turnover of epithelial cells in skin and visceral organs, inflammation, neuroprotection and cancer.

International Union of Pharmacology LVIII: update on the P2Y G protein-coupled nucleotide receptors: from molecular mechanisms and pathophysiology to therapy

There have been many advances in our knowledge about different aspects of P2Y receptor signaling since the last review published by our International Union of Pharmacology subcommittee. More receptor subtypes have been cloned and characterized and most orphan receptors de-orphanized, so that it is now possible to provide a basis for a future subdivision of P2Y receptor subtypes. More is known about the functional elements of the P2Y receptor molecules and the signaling pathways involved, including interactions with ion channels. There have been substantial developments in the design of selective agonists and antagonists to some of the P2Y receptor subtypes. There are new findings about the mechanisms underlying nucleotide release and ectoenzymatic nucleotide breakdown. Interactions between P2Y receptors and receptors to other signaling molecules have been explored as well as P2Y-mediated control of gene transcription. The distribution and roles of P2Y receptor subtypes in many different cell types are better understood and P2Y receptor-related compounds are being explored for therapeutic purposes. These and other advances are discussed in the present review.

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.

Historical review: ATP as a neurotransmitter

Purinergic signalling is now recognized to be involved in a wide range of activities of the nervous system, including neuroprotection, central control of autonomic functions, neural-glial interactions, control of vessel tone and angiogenesis, pain and mechanosensory transduction and the physiology of the special senses. In this article, I give a personal retrospective of the discovery of purinergic neurotransmission in the early 1970s, the struggle for its acceptance for approximately 20 years, the expansion into purinergic cotransmission and its eventual acceptance when receptor subtypes for ATP were cloned and characterized and when purinergic synaptic transmission between neurons in the brain and peripheral ganglia was described in the early 1990s. I also discuss the current status of the field, including recent interest in the pathophysiology of purinergic signalling and its therapeutic potential.

Term-dependency of P2 receptor-mediated contractile responses of isolated human pregnant uterus

OBJECTIVE

The aim of the study was to test the hypothesis that in the human uterus, the effectiveness of P2 receptor-mediated contractile responses is up-regulated during pregnancy.

STUDY DESIGN

Experiments were performed on myometrial samples obtained from women undergoing caesarean section at 28-30 weeks of pregnancy (3 women, Group 1), 32-34 weeks of pregnancy (6 women, Group 2) and 38-41 weeks of pregnancy (16 women, Group 3). Concentration-response relationships for a non-selective P2 receptor agonist, adenosine 5'-triphosphate (ATP), a selective P2X receptor agonist, alpha,beta-methylene-ATP (alpha,beta-meATP), and a frequency-response relationship for non-adrenergic non-cholinergic (NANC) electrical field stimulation (EFS) were obtained using routine pharmacological organ bath technique. Effects of pyridoxalphosphate azophenyl-2',4'-disulphonic acid (PPADS, 10(-5) M), a P2 receptor antagonist, were also evaluated. Parametric Student's t-test, non-parametric Wilcoxon T-test, Mann-Whitney U-test, two-way analysis of variance (ANOVA) and Krushkal-Wallis tests were used for statistical analysis.

RESULTS

ATP (10(-6) to 3 x 10(-4) M), alpha,beta-meATP (10(-7) to 3 x 10(-5) M) and EFS (2-32 Hz) evoked contractions of isolated pregnant uterus in all three groups. Uterus responses to ATP were not correlated with the term of pregnancy while the amplitude of uterine contractions to alpha,beta-meATP and EFS was higher in full term pregnancy than in earlier pregnancy. PPADS antagonized uterus responses to alpha,beta-meATP and EFS, but not to ATP, in all three groups.

CONCLUSION

P2X receptor-mediated contractions of human pregnant uterus to alpha,beta-meATP and EFS, but not to ATP, are increased with the progression of pregnancy.

Potentiation of uterine effects of prostaglandin F2{alpha} by adenosine 5'-triphosphate

OBJECTIVE

To investigate the interaction of exogenous adenosine 5'-triphosphate (ATP), a P2 receptor agonist, with prostaglandin F(2alpha) (PGF(2alpha)) on pregnant women in labor as well as on isolated human pregnant uterus preparations.

METHODS

For an in vitro study, myometrial samples were obtained from 27 women undergoing elective cesarean delivery at term. Concentration-response relationships for ATP (10(-8) -3 x 10(-4) mol/L), PGF(2alpha) (10(-9) -10(-5) mol/L), and their combination were obtained by using routine pharmacological organ bath technique. An in vivo study was performed with 34 pregnant women with dysfunctional abnormalities of the active stage of labor who were randomly allocated into 2 study groups. The women in the control group (18 patients) received intravenous prostaglandin F(2alpha) at an initial rate of 7.5 mug/min, whereas the women in the ATP group (16 patients) received prostaglandin F(2alpha) concomitantly with ATP (0.45 nmol/min, intravenously).

RESULTS

Adenosine 5'-triphosphate at concentrations of 10(-6) -3 x 10(-4) mol/L and PGF(2alpha) at concentrations of 10(-8) -10(-5) mol/L caused concentration-dependent contractions of isolated smooth muscle preparations of the human pregnant uterus. At concentrations of 10(-6) mol/L and below, ATP had no effects on mechanical activity of the isolated uterus, but at concentrations of 10(-7) mol/L and 10(-6) mol/L, it significantly potentiated the contractile responses of the uterus induced by PGF(2alpha) (P < .05, 2-way analysis of variance). Patients receiving intravenous infusion of ATP as a supplement to PGF(2alpha) treatment, compared with those without ATP, had a significantly shorter interval from the start of the treatment to full cervical dilatation (3.31 +/- 1.49 hours and 4.67 +/- 1.11 hours in ATP and control groups, respectively; P = .014, Wilcoxon Mann-Whitney test). The total dose of prostaglandin received was significantly lower in the ATP group than that of controls (1,489.8 +/- 699.9 mug and 3,394.2 +/- 1,951.9 mug, respectively; P = .003, Wilcoxon Mann-Whitney test). No side effects of ATP treatment were observed during or after infusion.

CONCLUSION

Adenosine 5'-triphosphate potentiates effects of PGF(2alpha) on pregnant human uterus in vitro and in vivo and thus could be a useful supplemental drug to increase uterine contractility at labor.

P2Y2 nucleotide receptor up-regulation in submandibular gland cells from the NOD.B10 mouse model of Sjögren's syndrome

Sjögren's syndrome (SS) is an autoimmune disease that specifically targets exocrine glands, including salivary glands, and results in an impairment of secretory function. P2Y(2) nucleotide receptors for extracellular ATP and UTP are up-regulated in response to stress or injury in a variety of tissues including submandibular glands (SMGs) [Ahn JS, Camden JM, Schrader AM, Redman RS, Turner JT. Reversible regulation of P2Y(2) nucleotide receptor expression in the duct-ligated rat submandibular gland. Am J Physiol Cell Physiol 2000;279:C286-94; Hou M, Malmsjö M, Möller S, Pantev E, Bergdahl A, Zhai X-H, et al. Increase in cardiac P2X(1)- and P2Y(2)-receptor mRNA levels in congestive heart failure. Life Sci 1999;65:1195-206; Kishore BK, Wang Z, Rab H, Haq M, Soleimani M. Upregulation of P2Y(2) purinoceptor during ischemic reperfusion injury (IRI): possible relevance to diuresis of IRI. J Am Soc Nephrol 1998;9:581 (abstract); Koshiba M, Apasov S, Sverdlov V, Chen P, Erb L, Turner JT, et al. Transient up-regulation of P2Y(2) nucleotide receptor mRNA expression is an immediate early gene response in activated thymocytes. Proc Natl Acad Sci U S A 1997;94:831-6; Turner JT, Landon LA, Gibbons SJ, Talamo BR. Salivary gland P2 nucleotide receptors. Crit Rev Oral Biol Med 1999;10:210-24; Seye CI, Gadeau AP, Daret D, Dupuch F, Alzieu P, Capron L, et al. Overexpression of the P2Y(2) purinoceptor in intimal lesions of the rat aorta. Arterioscler Thromb Vasc Biol 1997;17:3602-10; Seye C, Kong Q, Erb L, Garrad RC, Krugh B, Wang M, et al. Functional P2Y(2) nucleotide receptors mediate uridine 5'-triphosphate-induced intimal hyperplasia in collared rabbit carotid arteries. Circulation 2002;106:2720-6].

OBJECTIVE

To assess whether P2Y(2) receptor expression is up-regulated in SMGs of the NOD.B10 mouse model of primary SS as compared to SMGs of normal C57BL/6 mice.

DESIGN

SMG cells were isolated from normal C57BL/6 and diseased NOD.B10 mice. P2Y(2) receptor mRNA expression was determined by reverse transcription-polymerase chain reaction (RT-PCR) and in situ hybridization, whereas functional P2Y(2) receptor activity was analyzed by measuring UTP-induced increases in [Ca(2+)](i).

RESULTS

In contrast to SMG cells from C57BL/6 mice, SMG cells from 4- to 19-week-old NOD.B10 mice exhibited increased P2Y(2) receptor mRNA localized to both ductal and acinar cell types. The levels of mRNA for other uridine nucleotide receptors, i.e., P2Y(4) and P2Y(6) receptors, showed no significant differences between SMG cells of C57BL/6 and NOD.B10 mice, suggesting that only the P2Y(2) receptor was up-regulated in NOD.B10 mice. Moreover, P2Y(2) receptor activity in SMG cells from NOD.B10 mice increased with age (i.e., disease progression).

CONCLUSION

P2Y(2) receptor up-regulation in SMGs is associated with the SS phenotype in NOD.B10 mice, which encourages further attempts to determine the role of this pathway in the development of SS.

UTP and diadenosine tetraphosphate accelerate wound healing in the rabbit cornea

Nucleotides are naturally occurring substances present in tear film that can stimulate tear secretion in animals and humans. We investigated whether certain nucleotides can affect the rate of wound healing in the cornea of white rabbits. In the absence of any added compound, the rate of healing was 72.4 +/- 2.2 microm h(-1). Of all the tested nucleotides, UTP and Ap(4)A were the most active ones, maximally increasing the rate of healing to 121.6 +/- 3.7 and 93.7 +/- 3.2 microm h(-1), respectively. Responses to UTP were dose dependent. UTP had a pD(2) value of 8.9 +/- 0.1 (EC(50): 1.25 nM). P2 purinoceptor antagonists such as suramin and reactive blue-2, inhibited the effect of UTP indicating the involvement of P2Y receptors. Mitogen-activated protein kinase (MAPK) cascade inhibitors also abolished the effects of UTP, suggesting that P2Y receptors are coupled to the MAPK cascade, and that this is involved in controlling the rate of epithelial cell migration.

Adenosine tetraphosphate, Ap4, a physiological regulator of intraocular pressure in normotensive rabbit eyes

Adenosine 5' tetraphosphate, Ap(4), is a natural nucleotide present in many biological systems. This nucleotide has been found as a constituent of the nucleotide pool present in the aqueous humor of New Zealand rabbits. HPLC analysis confirmed its identity and calculated its concentration levels to be 197 +/- 21 nM. When applied topically to the rabbit eyes, this mononucleotide produced a reduction in the intraocular pressure, which was dose-dependent. The pD(2) value calculated from the dose-response curve was 7.28 +/- 0.47, which is equivalent to 52.48 nM. The time course of such intraocular pressure reduction presented a maximal decrease of IOP to 75.1 +/- 2.3% compared with the vehicle control value (100%), and the effect lasted for more than 2 h. Cross-desensitization studies demonstrated that Ap(4) effect was mediated via a P2X receptor in this system. P2 receptor antagonists suramin, pyridoxal phosphate 6-azophenyl-2',4'-disulfonic acid (PPADS), and reactive blue 2 (RB-2) showed that only the latter was able to revert the effect of Ap(4). Antagonists of adrenoceptors and cholinoceptors were able to partially reverse the effect of this nucleotide; this might indicate a connection with the neural mechanisms that control the intraocular pressure.

Purinergic Receptors Are Part of a Signaling System for Keratinocyte Proliferation, Differentiation, and Apoptosis in Human Fetal Epidermis

We have investigated the expression of P2X5, P2X7, P2Y1, and P2Y2 receptor subtypes in 8- to 11-wk-old human fetal epidermis in relation to markers of proliferation (proliferating cell nuclear antigen (PCNA) and Ki-67), keratinocyte differentiation (cytokeratin K10 and involucrin), and markers of apoptosis (TdT-mediated dUTP nick end labeling (TUNEL) and anti-caspase-3). Immunohistochemistry showed that each of the four receptors was expressed in spatially distinct zones of the developing epidermis: P2Y1 receptors were found in the basal layer, P2X5 receptors were predominantly in the basal and intermediate layers, and both P2Y2 and P2X7 receptors were in the periderm. Colocalization experiments suggested different functional roles for these receptors. P2Y1 receptors were found in fetal keratinocytes positive for PCNA and Ki-67, suggesting a role in proliferation. P2X5 receptors double labeled with differentiated fetal keratinocytes that were positive for cytokeratin K10, suggesting a role in differentiation. P2X7 receptors colocalized with anti-caspase-3 antibody and were also expressed in periderm cells positive for TUNEL, suggesting a role in periderm cell apoptosis. P2Y2 receptors were found only in periderm cells and may have a role in chloride and fluid secretion into the amniotic fluid.

Localization of ocular P2Y2 receptor gene expression by in situ hybridization

The objective of this study was to determine the cellular localization of P2Y(2) receptor gene expression in rabbit and primate ocular tissues using the technique of non-isotopic in situ hybridization. Fresh frozen whole eye from a New Zealand White rabbit and whole eye and eyelid from a rhesus macaque were cut into 5 microm thick sections and mounted onto glass slides. In situ hybridization was performed on ocular cryosections using digoxigenin-labeled P2Y(2) receptor riboprobes. Alkaline phosphatase-conjugated anti-digoxigenin antibody was used to localize riboprobe hybridization, which was subsequently visualized by staining with a precipitating alkaline phosphatase substrate. Cytoplasmic staining indicative of antisense riboprobe hybridization to P2Y(2) receptor mRNA was observed in the palpebral and bulbar conjunctival epithelium, including goblet cells, the corneal epithelium, and in meibomian gland sebaceous and ductal cells. Staining was also observed in both layers of the ciliary body epithelium, subcapsular epithelium of the lens, and corneal endothelium. In the posterior eye, staining was observed in various layers of the retina, including ganglion cell, inner nuclear, inner segment and retinal pigment epithelium layers, in the optic nerve head, and in a variety of structures within the choroid. No specific staining of sense riboprobe was seen on any of the ocular structures. These results demonstrate that the P2Y(2) receptor gene is expressed in a variety of ocular cells types and suggest that P2Y(2) receptors are associated with diverse physiological functions throughout the eye.

Selective P2Y2 receptor agonists stimulate vaginal moisture in ovariectomized rabbits

OBJECTIVE

To determine the expression of P2Y(2) receptors in vaginal and cervical tissues and the effects of P2Y(2) receptor agonists INS45973 and INS365 on vaginal moisture.

DESIGN

Pilot in vivo and histological study using animal subjects.

SETTING

Experimental laboratory research.

ANIMAL(S)

Female New Zealand White rabbits were used for in vivo studies and female cynomolgus monkey (Macaca fascicularis) was used for in situ hybridization.

INTERVENTION(S)

Rabbits were kept intact or ovariectomized. Two weeks after ovariectomy, animals received daily vaginal instillation of vehicle or drugs for 16 days.

MAIN OUTCOME MEASURE(S)

Vaginal moisture was assessed in rabbits on 4 separate days during the treatment period. The P2Y(2) receptor mRNA distribution was assessed by in situ hybridization of monkey vagina and cervix.

RESULT(S)

Compared to control, vaginal moisture was significantly diminished in ovariectomized animals treated with vehicle. INS365 (8.1%) and INS45973 (0.9%) increased vaginal moisture in ovariectomized animals to levels that were comparable to or significantly higher than control animals, respectively. In situ hybridization studies indicated that P2Y(2) receptor mRNA was localized to endocervical and cervical gland, epithelium, and stratified squamous epithelium of the vagina.

CONCLUSION(S)

INS45973 and INS365 may interact with P2Y(2) receptors in the cervix and vagina to stimulate vaginal moisture in the estrogen (E)-deprived state. The P2Y(2) receptor agonists provide a potential nonhormonal alternative for treating vaginal dryness in postmenopausal women.

INS365 suppresses loss of corneal epithelial integrity by secretion of mucin-like glycoprotein in a rabbit short-term dry eye model

P2Y2 receptor agonists, like UTP and ATP, stimulate mucin secretion from goblet cells in vitro. Therefore, mucin stimulants could be good candidates for the treatment of dry eye syndrome because mucin increases the tear film stability and protects against desiccation of ocular surface. INS365 is a more stable P2Y2 receptor agonist than UTP. In the present study, we evaluated, in normal rabbit eyes, its effectiveness to release mucin from goblet cells and to protect the corneal damage induced by desiccation. For mucin secretion, impression cytology was performed following the instillation of INS365 solution or saline into the conjunctival sac. The specimens were stained with periodic acid and Schiff (PAS) reagent, and then the staining area was calculated using computer software. INS365 dose-dependently decreased the PAS staining area of conjunctival goblet cells from 2 to 15 min post-application. Furthermore, we utilized the rabbit short-term dry eye model to evaluate if INS365 eyedrops could protect against any of the damage produced by blockage of blinking with ocular speculum. INS365 significantly suppressed corneal damage at concentrations of more than 0.1% w/v. These results suggest that this P2Y2 agonist is a good candidate for the treatment of dry eye disease.