Nucleotide coronary vasodilation in guinea pig hearts

How selective antagonists and genetic modification have helped characterise the expression and functions of vascular P2Y receptors

Vascular P2Y receptors mediate many effects, but the role of individual subtypes is often unclear. Here we discuss how subtype-selective antagonists and receptor knockout/knockdown have helped identify these roles in numerous species and vessels. P2Y1 receptor-mediated vasoconstriction and endothelium-dependent vasodilation have been characterised using the selective antagonists, MRS2179 and MRS2216, whilst AR-C118925XX, a P2Y2 receptor antagonist, reduced endothelium-dependent relaxation, and signalling evoked by UTP or fluid shear stress. P2Y2 receptor knockdown reduced endothelial signalling and endothelial P2Y2 receptor knockout produced hypertensive mice and abolished vasodilation elicited by an increase in flow. UTP-evoked vasoconstriction was also blocked by AR-C118925XX, but the effects of P2Y2 receptor knockout were complex. No P2Y4 receptor antagonists are available and P2Y4 knockout did not affect the vascular actions of UTP and UDP. The P2Y6 receptor antagonist, MRS2578, identified endothelial P2Y6 receptors mediating vasodilation, but receptor knockout had complex effects. MRS2578 also inhibited, and P2Y6 knockout abolished, contractions evoked by UDP. P2Y6 receptors contribute to the myogenic tone induced by a stepped increase in vascular perfusion pressure and possibly to the development of atherosclerosis. The P2Y11 receptor antagonists, NF157 and NF340, inhibited ATP-evoked signalling in human endothelial cells. Vasoconstriction mediated by P2Y12/P2Y13 and P2Y14 receptors was characterised using the antagonists, cangrelor, ticagrelor, AR-C67085 and MRS2211 or PPTN respectively. This has yet to be backed up by receptor knockout experiments. Thus, subtype-selective antagonists and receptor knockout/knockdown have helped identify which P2Y subtypes are functionally expressed in vascular smooth muscle and endothelial cells and the effects that they mediate.

Disentangling the Gordian knot of local metabolic control of coronary blood flow

Recognition that coronary blood flow is tightly coupled with myocardial metabolism has been appreciated for well over half a century. However, exactly how coronary microvascular resistance is tightly coupled with myocardial oxygen consumption (MV̇o₂) remains one of the most highly contested mysteries of the coronary circulation to this day. Understanding the mechanisms responsible for local metabolic control of coronary blood flow has been confounded by continued debate regarding both anticipated experimental outcomes and data interpretation. For a number of years, coronary venous Po₂ has been generally accepted as a measure of myocardial tissue oxygenation and thus the classically proposed error signal for the generation of vasodilator metabolites in the heart. However, interpretation of changes in coronary venous Po₂ relative to MV̇o₂ are quite nuanced, inherently circular in nature, and subject to confounding influences that remain largely unaccounted for. The purpose of this review is to highlight difficulties in interpreting the complex interrelationship between key coronary outcome variables and the arguments that emerge from prior studies performed during exercise, hemodilution, hypoxemia, and alterations in perfusion pressure. Furthermore, potential paths forward are proposed to help to facilitate further dialogue and study to ultimately unravel what has become the Gordian knot of the coronary circulation.

Integrative model of coronary flow in anatomically based vasculature under myogenic, shear, and metabolic regulation

Coronary blood flow is regulated to match the oxygen demand of myocytes in the heart wall. Flow regulation is essential to meet the wide range of cardiac workload. The blood flows through a complex coronary vasculature of elastic vessels having nonlinear wall properties, under transmural heterogeneous myocardial extravascular loading. To date, there is no fully integrative flow analysis that incorporates global and local passive and flow control determinants. Here, we provide an integrative model of coronary flow regulation that considers the realistic asymmetric morphology of the coronary network, the dynamic myocardial loading on the vessels embedded in it, and the combined effects of local myogenic effect, local shear regulation, and conducted metabolic control driven by venous O₂ saturation level. The model predicts autoregulation (approximately constant flow over a wide range of coronary perfusion pressures), reduced heterogeneity of regulated flow, and presence of flow reserve, in agreement with experimental observations. Furthermore, the model shows that the metabolic and myogenic regulations play a primary role, whereas shear has a secondary one. Regulation was found to have a significant effect on the flow except under extreme (high and low) inlet pressures and metabolic demand. Novel outcomes of the model are that cyclic myocardial loading on coronary vessels enhances the coronary flow reserve except under low inlet perfusion pressure, increases the pressure range of effective autoregulation, and reduces the network flow in the absence of metabolic regulation. Collectively, these findings demonstrate the utility of the present biophysical model, which can be used to unravel the underlying mechanisms of coronary physiopathology.

Open-loop (feed-forward) and feedback control of coronary blood flow during exercise, cardiac pacing, and pressure changes

A control system model was developed to analyze data on in vivo coronary blood flow regulation and to probe how different mechanisms work together to control coronary flow from rest to exercise, and under a variety of experimental conditions, including cardiac pacing and with changes in coronary arterial pressure (autoregulation). In the model coronary flow is determined by the combined action of a feedback pathway signal that is determined by the level of plasma ATP in coronary venous blood, an adrenergic open-loop (feed-forward) signal that increases with exercise, and a contribution of pressure-mediated myogenic control. The model was identified based on data from exercise experiments where myocardial oxygen extraction, coronary flow, cardiac interstitial norepinephrine concentration, and arterial and coronary venous plasma ATP concentrations were measured during control and during adrenergic and purinergic receptor blockade conditions. The identified model was used to quantify the relative contributions of open-loop and feedback pathways and to illustrate the degree of redundancy in the control of coronary flow. The results indicate that the adrenergic open-loop control component is responsible for most of the increase in coronary blood flow that occurs during high levels of exercise. However, the adenine nucleotide-mediated metabolic feedback control component is essential. The model was evaluated by predicting coronary flow in cardiac pacing and autoregulation experiments with reasonable fits to the data. The analysis shows that a model in which coronary venous plasma adenine nucleotides are a signal in local metabolic feedback control of coronary flow is consistent with the available data.

Cardiac purinergic signalling in health and disease

This review is a historical account about purinergic signalling in the heart, for readers to see how ideas and understanding have changed as new experimental results were published. Initially, the focus is on the nervous control of the heart by ATP as a cotransmitter in sympathetic, parasympathetic, and sensory nerves, as well as in intracardiac neurons. Control of the heart by centers in the brain and vagal cardiovascular reflexes involving purines are also discussed. The actions of adenine nucleotides and nucleosides on cardiomyocytes, atrioventricular and sinoatrial nodes, cardiac fibroblasts, and coronary blood vessels are described. Cardiac release and degradation of ATP are also described. Finally, the involvement of purinergic signalling and its therapeutic potential in cardiac pathophysiology is reviewed, including acute and chronic heart failure, ischemia, infarction, arrhythmias, cardiomyopathy, syncope, hypertrophy, coronary artery disease, angina, diabetic cardiomyopathy, as well as heart transplantation and coronary bypass grafts.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

Extracellular NM23 Signaling in Breast Cancer: Incommodus Verum

The notion that breast cancers can survive in an individual patient in a dormant state only to grow as metastatic disease in the future, is in our view incontrovertibly established. Convincing too is the evidence that surgery to remove the primary tumor often terminates dormancy resulting in accelerated relapses. Accepting that many deaths due to breast cancer might be averted were we to understand the cellular mechanisms underlying escape from dormancy, we have examined the extracellular signals produced by breast cancers derived from women with metastatic breast disease. In this perspective, we explore the role of extracellular nucleotide signaling that we have proposed constitutes a pathological axis from the transformed tumor cell to the endothelium in the service of intravasation, dissemination, extravasation and angiogenesis. A role for the dinucleotide kinase NM23/NDPK (nucleoside diphosphate kinase) secreted by breast tumor cells in the generation of signals that stimulate vascular leakiness, anti-thrombosis, endothelial migration and growth, constitutes a mechanistic basis for escape from latency and offers putative therapeutic targets for breast cancer management not previously appreciated.

Control of coronary blood flow during exercise

During exercise, coronary blood flow increases to match the augmented myocardial oxygen demand because of tachycardia. Coronary vasodilation during exercise is via a combination of feedforward and feedback control mechanisms. Feedforward control is mediated by sympathetic β-adrenoceptor vasodilation. Feedback vasodilator control is via a novel hypothesis where adenine nucleotides released from red blood cells act on endothelial purinergic receptors.

Functional contribution of P2Y1 receptors to the control of coronary blood flow

Activation of ADP-sensitive P2Y(1) receptors has been proposed as an integral step in the putative "nucleotide axis" regulating coronary blood flow. However, the specific mechanism(s) and overall contribution of P2Y(1) receptors to the control of coronary blood flow have not been clearly defined. Using vertically integrative studies in isolated coronary arterioles and open-chest anesthetized dogs, we examined the hypothesis that P2Y(1) receptors induce coronary vasodilation via an endothelium-dependent mechanism and contribute to coronary pressure-flow autoregulation and/or ischemic coronary vasodilation. Immunohistochemistry revealed P2Y(1) receptor expression in coronary arteriolar endothelial and vascular smooth muscle cells. The ADP analog 2-methylthio-ADP induced arteriolar dilation in vitro and in vivo that was abolished by the selective P2Y(1) antagonist MRS-2179 and the nitric oxide synthase inhibitor N(G)-nitro-l-arginine methyl ester. MRS-2179 did not alter baseline coronary flow in vivo but significantly attenuated coronary vasodilation to ATP in vitro and in vivo and the nonhydrolyzable ATP analog ATPγS in vitro. Coronary blood flow responses to alterations in coronary perfusion pressure (40-100 mmHg) or to a brief 15-s coronary artery occlusion were unaffected by MRS-2179. Our data reveal that P2Y(1) receptors are functionally expressed in the coronary circulation and that activation produces coronary vasodilation via an endothelium/nitric oxide-dependent mechanism. Although these receptors represent a critical component of purinergic coronary vasodilation, our findings indicate that P2Y(1) receptor activation is not required for coronary pressure-flow autoregulation or reactive hyperemia.

Extracellular purine metabolism and signaling of CD73-derived adenosine in murine Treg and Teff cells

CD73-derived adenosine acts as potent inhibitor of inflammation, and regulatory T cells (Treg) have been shown to express CD73 as a novel marker. This study explored the role of endogenously formed adenosine in modulating NF-κB activity and cytokine/chemokine release from murine Treg and effector T cells (Teff) including key enzymes/purinergic receptors of extracellular ATP catabolism. Stimulating murine splenocytes and CD4(+) T cells with anti-CD3/anti-CD28 significantly upregulated activated NF-κB in CD73(-/-) T cells (wild type: 4.36 ± 0.21; CD73(-/-): 6.58 ± 0.75; n = 4; P = 0.029). This was associated with an augmented release of proinflammatory cytokines IL-2, TNF-α, and IFN-γ. Similar changes were observed with the CD73 inhibitor APCP (50 μM) on NF-κB and IFN-γ in wild-type CD4(+) T-cells. Treatment of stimulated CD4(+) T-cells with adenosine (25 μM) potently reduced IFN-γ release which is mediated by adenosine A2a receptors (A2aR). AMP (50 μM) also reduced cytokine release which was not inhibited by APCP. In Teff, A2aR activation (CGS21680) potently inhibited the release of IL-1, IL-2, IL-3, IL-4, IL-12, IL-13, IFN-γ, TNF-α, granulocyte-macrophage colony-stimulating factor (GM-CSF), CCL3, and CCL4. However, in Treg, CGS21680 did not alter cytokine/chemokine release. In summary, CD73-derived adenosine tonically inhibits active NF-κB in CD4(+) T-cells, thereby modulating the release of a broad spectrum of proinflammatory cytokines and chemokines. Downregulation of P2X7 and upregulation of CD73 in Treg after antigenic stimulation may be an important mechanism to maintain the ability of Treg to generate immunosuppressive adenosine.

Adenine nucleotide control of coronary blood flow during exercise

The adenine nucleotide hypothesis postulates that the ATP released from red blood cells is broken down to ADP and AMP in coronary capillaries and that ATP, ADP, and AMP act on purinergic receptors on the surface of capillary endothelial cells. Purinergic receptor activation initiates a retrograde conducted vasodilator signal to the upstream arteriole that controls coronary blood flow in a negative feedback manner. A previous study (M. Farias 3rd, M. W. Gorman, M. V. Savage, and E. O. Feigl, Am J Physiol Heart Circ Physiol 288: H1586-H1590, 2005) demonstrated that coronary venous plasma ATP concentration increased during exercise and correlated with coronary blood flow. The present experiments test the adenine nucleotide hypothesis by examining the balance between oxygen delivery (via coronary blood flow) and myocardial oxygen consumption during exercise before and after purinergic receptor blockade. Dogs (n = 7) were chronically instrumented with catheters in the aorta and coronary sinus and a flow transducer around the circumflex coronary artery. During control treadmill exercise, myocardial oxygen consumption increased and the balance between oxygen delivery and myocardial oxygen consumption fell as indicated by a declining coronary venous oxygen tension. Blockade of P1 and P2Y(1) purinergic receptors combined with inhibition of nitric oxide synthesis significantly decreased the balance between oxygen delivery and myocardial oxygen consumption compared with control. The results support the hypothesis that ATP and its breakdown products ADP and AMP are part of a negative feedback control mechanism that matches coronary blood flow to myocardial oxygen consumption at rest and during exercise.

Purinergic mechanisms in breast cancer support intravasation, extravasation and angiogenesis

Several advances have recently expanded models of tumor growth and promoted the concept of tumor homeostasis, the hypothesis that primary tumors exert an anti-proliferative effect on both themselves and subclinical secondary metastases. Recent trials indicate that the characterization of tumor growth as uncontrolled is inconsistent with animal models, clinical models, and epidemiological models. There is a growing body of evidence which lends support to an updated concept of tumor growth: tumor homeostasis. In the case of breast cancer, if not all metastasizing tumors, these advances suggest an inconvenient truth. That is, if breast tumor cells metastasize to distant sites early in the tumorigenesis process, then removal of a breast tumor may hasten the development of its metastases. We explore the heretofore unappreciated notion that nucleotides generated by tumor cells following the secretion of an ADP-kinase can promote metastasis and support angiogenesis. Evidence is presented that blockade of the actions of nucleotides in the setting of newly diagnosed breast cancer may provide a useful adjunct to current anti-angiogenesis treatment.

Theoretical model of metabolic blood flow regulation: roles of ATP release by red blood cells and conducted responses

A proposed mechanism for metabolic flow regulation involves the saturation-dependent release of ATP by red blood cells, which triggers an upstream conducted response signal and arteriolar vasodilation. To analyze this mechanism, a theoretical model is used to simulate the variation of oxygen and ATP levels along a flow pathway of seven representative segments, including two vasoactive arteriolar segments. The conducted response signal is defined by integrating the ATP concentration along the vascular pathway, assuming exponential decay of the signal in the upstream direction with a length constant of approximately 1 cm. Arteriolar tone depends on the conducted metabolic signal and on local wall shear stress and wall tension. Arteriolar diameters are calculated based on vascular smooth muscle mechanics. The model predicts that conducted responses stimulated by ATP release in venules and propagated to arterioles can account for increases in perfusion in response to increased oxygen demand that are consistent with experimental findings at low to moderate oxygen consumption rates. Myogenic and shear-dependent responses are found to act in opposition to this mechanism of metabolic flow regulation.

Luminal ATP-induced contraction of rabbit pulmonary arteries and role of purinoceptors in the regulation of pulmonary arterial pressure

The effects of luminal ATP between rabbit pulmonary (PAs) and coronary arteries (CAs) were compared to understand the role of purinoceptors in the regulation of pulmonary arterial pressure (PAP) under hypoxia. Diameters of vessels were video analyzed under luminal perfusion. ATP-induced membrane currents and intracellular Ca(2+) signals ([Ca(2+)](i)) were compared in pulmonary (PASMCs) and coronary myocytes (CASMCs) using patch clamp and spectrofluorimetry. PAP was measured in perfused lungs under ventilation. Luminal ATP induced constriction of rabbit PAs in the presence of endothelium. In contrast, CAs showed dilating responses to luminal ATP even in the absence of endothelium. In PASMCs, both P2X-mediated inward current and P2Y-mediated store Ca(2+) release were consistently observed. In contrast, CASMCs showed neither P2X nor P2Y responses. In the perfused lungs, hypoxia-induced PAP increase was decreased by suramin, a purinergic antagonist. A luminal application of alpha,beta-meATP largely increased PAP, whereas UTP decreased PAP. The combined application of P2X- and P2Y-selective agonists (alpha,beta-meATP and UTP) increased PAP. However, the perfusion of ATP alone decreased PAP, and the ATP-induced PAP decrease was affected neither by adenosine receptor antagonist nor by nitric oxide synthase inhibitor. In summary, although the luminal ATP constricts isolated PAs and suramin attenuated the HPV of perfused lungs, the bimodal responses of PAP to purinergic agonists indicate that the luminal ATP regulates pulmonary circulation via complex signaling interactions in situ.

Human plasma ATP concentration

BACKGROUND

Human plasma ATP concentration is reported in many studies as roughly 1000 nmol/L. The present study tested the hypothesis that the measured plasma ATP concentration is lower if ATP release from formed blood elements is inhibited during blood sample processing. A second hypothesis was that pretreatment with aspirin to inhibit platelets would reduce the measured plasma concentration of ATP.

METHODS

Blood was sampled from the antecubital vein in 20 healthy individuals 30 and 60 min after ingestion of aspirin (325 mg) or placebo. Aliquots of each blood sample were added to the usual EDTA/saline solution to inhibit ATP catabolism, or to a new stabilizing solution designed to both stop ATP catabolism and inhibit ATP release from blood elements. The stabilizing solution contained NaCl, EDTA, tricine buffer, KCl, nitrobenzylthioinosine, forskolin, and isobutylmethylxanthine. Plasma ATP was measured with the luciferin-luciferase assay with standard additions in each sample to determine ATP content. Hemoglobin concentration was used as an index of sample hemolysis, and the plasma ATP concentration was corrected for the hemolysis component.

RESULTS

Aspirin pretreatment had no effect on plasma ATP concentrations. However, use of the stabilizing solution resulted in mean (SD) ATP concentrations 8-fold lower than the use of EDTA alone [28 (16) vs 236 (201) nmol/L; P <0.001].

CONCLUSION

When precautions are taken to inhibit ATP release from blood elements during sample preparation, human venous plasma ATP concentration is much lower than previously reported.

Pump-induced platelet aggregation with subsequent hypotension: Its mechanism and prevention with clopidogrel

OBJECTIVES

Use of extracorporeal circuits in cardiopulmonary bypass and hemodialysis often causes bleeding problems and hypotension. As shown previously, this might be caused by activation of blood platelets due to pumping. The present study investigates the mechanism of pump-induced platelet aggregation and its possible prevention.

METHODS AND RESULTS

Continuous measurement of platelet aggregation in an extracorporeal shunt from a carotid to a femoral artery in rats showed that aggregation during the first 10 minutes of pumping was not reduced by coating the tube with albumin or heparin nor by using dalteparin instead of unfractionated heparin as anticoagulant. Also, pump characteristics seemed unimportant because aggregation could already be elicited by single tube compression with one pump roller. It was calculated that during compression wall shear stress in the tube rises far beyond the values known to induce platelet aggregation, occurring also in clinically used roller pumps. A crucial role for adenosine diphosphate was demonstrated by blockade of platelet adenosine diphosphate-P2Y12 receptors with the clinically used drug clopidogrel (50 mg/kg intravenously, n = 8). This prevented platelet aggregation and the fall of systemic blood pressure (to 71% +/- 12% in controls, n = 6) during 2 hours of continuous pumping.

CONCLUSION

We conclude that pump-induced platelet aggregation is not caused by factors released from the tube or its coating but is initiated by short bouts of high shear stress, and its continuation is critically dependent on adenosine diphosphate. The latter might have clinical relevance for patients connected to extracorporeal systems.

ATP-loaded liposomes effectively protect mechanical functions of the myocardium from global ischemia in an isolated rat heart model

ATP-loaded liposomes (ATP-L) infused into Langendorff-instrumented isolated rat hearts protect the mechanical functions of the myocardium during ischemia/reperfusion. The left ventricular developed pressure (LVDP) at the end of the reperfusion in the ATP-L group recovered to 72% of the baseline (preservation of the systolic function) compared to 26%, 40%, and 51% in the groups treated with Krebs-Henseleit (KH) buffer, empty liposomes (EL), and free ATP (F-ATP), respectively. The ATP-L-treated group also showed a significantly lower left ventricular end diastolic pressure (LVEDP; better preservation of the diastolic function) after ischemia/reperfusion than controls. After incubating the F-ATP and ATP-L with ATPase, the protective effect of the F-ATP was completely eliminated because of ATP degradation, while the protective effect of the ATP-L remained unchanged. Fluorescence microscopy confirmed the accumulation of liposomes in ischemic areas, and the net ATP in the ischemic heart increased with ATP-L. Our results suggest that ATP-L can effectively protect myocardium from ischemic/reperfusion damage.

Dipyridamole, an Underestimated Vascular Protective Drug

Dipyridamole, developed almost half a century ago, acts by inhibiting nucleoside transport, which increases adenosine levels leading to inhibition of platelet aggregation and vasodilatation mainly in the coronary tree. It is a vaso-protective drug with proven efficacy in the prevention of strokes. Adenosine receptor 2 inhibitory purines, ubiquitously available in food and drink, inhibit the vasomotor effects of dipyridamole but not its action on platelet aggregation. This and the slow build-up of blood levels of dipyridamole after oral application may explain why incidents of drug-induced angina ("coronary steal") were never reported in the prevention trials. The prevention of arterial thrombosis and the positive remodeling of the arterial system (arteriogenesis) by elevated blood flows suggest that dipyridamole may be able to halt the progression of organ manifestations of atherosclerosis. Clinical trials for the secondary prevention of vascular occlusions in other vascular beds should be encouraged.

Plasma ATP during exercise: possible role in regulation of coronary blood flow

It was previously shown that red blood cells release ATP when blood oxygen tension decreases. ATP acts on microvascular endothelial cells to produce a retrograde conducted vasodilation (presumably via gap junctions) to the upstream arteriole. These observations form the basis for an ATP hypothesis of local metabolic control of coronary blood flow due to vasodilation in microvascular units where myocardial oxygen extraction is high. Dogs (n = 10) were instrumented with catheters in the aorta and coronary sinus, and a flow transducer was placed around the circumflex coronary artery. Arterial and coronary venous plasma ATP concentrations were measured at rest and during three levels of treadmill exercise by using a luciferin-luciferase assay. During exercise, myocardial oxygen consumption increased approximately 3.2-fold, coronary blood flow increased approximately 2.7-fold, and coronary venous oxygen tension decreased from 19 to 12.9 mmHg. Coronary venous plasma ATP concentration increased significantly from 31.1 to 51.2 nM (P < 0.01) during exercise. Coronary blood flow increased linearly with coronary venous ATP concentration (P < 0.01). Coronary venous-arterial plasma ATP concentration difference increased significantly during exercise (P < 0.05). The data support the hypothesis that ATP is one of the factors controlling coronary blood flow during exercise.

Antiaggregatory activity in human platelets of potent antagonists of the P2Y 1 receptor

Activation of the P2Y(1) nucleotide receptor in platelets by ADP causes changes in shape and aggregation, mediated by activation of phospholipase C (PLC). Recently, MRS2500(2-iodo-N(6)-methyl-(N)-methanocarba-2'-deoxyadenosine-3',5'-bisphosphate) was introduced as a highly potent and selective antagonist for this receptor. We have studied the actions of MRS2500 in human platelets and compared these effects with the effects of two acyclic nucleotide analogues, a bisphosphate MRS2298 and a bisphosphonate derivative MRS2496, which act as P2Y(1) receptor antagonists, although less potently than MRS2500. Improved synthetic methods for MRS2500 and MRS2496 were devised. The bisphosphonate is predicted to be more stable in general in biological systems than phosphate antagonists due to the non-hydrolyzable CP bond. MRS2500 inhibited the ADP-induced aggregation of human platelets with an IC(50) value of 0.95 nM. MRS2298 and MRS2496 also both inhibited the ADP-induced aggregation of human platelets with IC(50) values of 62.8 nM and 1.5 microM, respectively. A similar order of potency was observed for the three antagonists in binding to the recombinant human P2Y(1) receptor and in inhibition of ADP-induced shape change and ADP-induced rise in intracellular Ca(2+). No substantial antagonism of the pathway linked to the inhibition of cyclic AMP was observed for the nucleotide derivatives, indicating no interaction of these three P2Y(1) receptor antagonists with the proaggregatory P2Y(12) receptor, which is also activated by ADP. Thus, all three of the bisphosphate derivatives are highly selective antagonists of the platelet P2Y(1) receptor, and MRS2500 is the most potent such antagonist yet reported.

P2Y purine receptor responses and expression in the pulmonary circulation of juvenile rabbits

The purine nucleotide ATP mediates pulmonary vasodilation at birth by stimulation of P2Y purine receptors in the pulmonary circulation. The specific P2Y receptors in the pulmonary circulation and the segmental distribution of their responses remain unknown. We investigated the effects of purine nucleotides, ATP, ADP, and AMP, and pyrimidine nucleotides, UTP, UDP, and UMP, in juvenile rabbit pulmonary arteries for functional characterization of P2Y receptors. We also studied the expression of P2Y receptor subtypes in pulmonary arteries and the role of nitric oxide (NO), prostaglandins, and cytochrome P-450 metabolites in the response to ATP. In conduit size arteries, ATP, ADP, and AMP caused greater relaxation responses than UTP, UDP, and UMP. In resistance vessels, ATP and UTP caused comparable vasodilation. The response to ATP was attenuated by the P2Y antagonist cibacron blue, the NO synthase antagonist N(omega)-nitro-l-arginine methyl ester (l-NAME), and the cytochrome P-450 inhibitor 17-octadecynoic acid but not by the P2X antagonist alpha,beta-methylene ATP or the cyclooxygenase inhibitor indomethacin in conduit arteries. In the resistance vessels, l-NAME caused a more complete inhibition of the responses to ATP and UTP. Responses to AMP and UMP were NO and endothelium dependent, whereas responses to ADP and UDP were NO and endothelium independent in the conduit arteries. RT-PCR showed expression of P2Y(1), P2Y(2), and P2Y(4) receptors, but not P2Y(6) receptors, in lung parenchyma, pulmonary arteries, and pulmonary artery endothelial cells. These data suggest that distinct P2Y receptors mediate the vasodilator responses to purine and pyrimidine nucleotides in the juvenile rabbit pulmonary circulation. ATP appears to cause NO-mediated vasodilation predominantly through P2Y2 receptors on endothelium.