The ADP receptor P2Y(1) mediates t-PA release in pigs during cardiac ischemia

The Role of Extracellular Adenosine Triphosphate in Ischemic Organ Injury

OBJECTIVES

Ischemic tissue injury contributes to significant morbidity and mortality and is implicated in a range of pathologic conditions, including but not limited to myocardial infarction, ischemic stroke, and acute kidney injury. The associated reperfusion phase is responsible for the activation of the innate and adaptive immune system, further accentuating inflammation. Adenosine triphosphate molecule has been implicated in various ischemic conditions, including stroke and myocardial infarction.

STUDY SELECTION

Adenosine triphosphate is a well-defined intracellular energy transfer and is commonly referred to as the body's "energy currency." However, Laboratory studies have demonstrated that extracellular adenosine triphosphate has the ability to initiate inflammation and is therefore referred to as a damage-associated molecular pattern. Purinergic receptors-dependent signaling, proinflammatory cytokine release, increased Ca influx into cells, and subsequent apoptosis have been shown to form a common underlying extracellular adenosine triphosphate molecular mechanism in ischemic organ injury.

CONCLUSIONS

In this review, we aim to discuss the molecular mechanisms behind adenosine triphosphate-mediated ischemic tissue injury and evaluate the role of extracellular adenosine triphosphate in ischemic injury in specific organs, in order to provide a greater understanding of the pathophysiology of this complex process. We also appraise potential future therapeutic strategies to limit damage in various organs, including the heart, brain, kidneys, and lungs.

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.

Adenosine diphosphate reduces infarct size and improves porcine heart function after myocardial infarct

Acute myocardial infarction continues to be a major cause of morbidity and mortality. Timely reperfusion can substantially improve outcomes and the administration of cardioprotective substances during reperfusion is therefore highly attractive. Adenosine diphosphate (ADP) and uridine-5-triphoshate (UTP) are both released during myocardial ischemia, influencing hemodynamics. Both mediate the release of tissue plasminogen activator (t-PA), which can reduce infarct size (IS). The objective of this study was to investigate whether exogenous ADP and UTP administration during reperfusion could reduce myocardial IS and whether this correlated to t-PA release or improvements in hemodynamic responses. Hemodynamic variables and t-PA were measured in 22 pigs before, during, and after 45 min of left anterior coronary artery occlusion. During reperfusion, the pigs were randomized to 240 min of intracoronary infusion of ADP, UTP, or control (no intervention). Ischemic area compared to the area at risk [IS/AAR] was measured. [IS/AAR] was 52 ± 11% in the control animals. ADP decreased [IS/AAR] by 19% (P < 0.05), while UTP increased [IS/AAR] by 15% (P < 0.05). Cardiac output (CO) increased from 3.4 to 3.5 L/min (P < 0.05) and mean arterial pressure (MAP) decreased from 87 to 73 mmHg in the ADP group (P < 0.05). t-PA concentration increased in the ADP and UTP group from 2.0 ng/mL to 2.5 and 2.4 ng/mL, respectively (P < 0.05) but remained unchanged in the control group. In conclusion, intracoronary ADP infusion during reperfusion reduces IS by ∼20% independently from systemic release of t-PA. ADP-induced reduction in both preload and afterload could account for the beneficial myocardial effect.

Astrocytic P2Y(1) receptor is involved in the regulation of cytokine/chemokine transcription and cerebral damage in a rat model of cerebral ischemia

After brain ischemia, significant amounts of adenosine 5'-triphosphate are released or leaked from damaged cells, thus activating purinergic receptors in the central nervous system. A number of P2X/P2Y receptors have been implicated in ischemic conditions, but to date the P2Y(1) receptor (P2Y(1)R) has not been implicated in cerebral ischemia. In this study, we found that the astrocytic P2Y(1)R, via phosphorylated-RelA (p-RelA), has a negative effect during cerebral ischemia/reperfusion. Intracerebroventricular administration of the P2Y(1)R agonist, MRS 2365, led to an increase in cerebral infarct volume 72 hours after transient middle cerebral artery occlusion (tMCAO). Administration of the P2Y(1)R antagonist, MRS 2179, significantly decreased infarct volume and led to recovered motor coordination. The effects of MRS 2179 occurred within 24 hours of tMCAO, and also markedly reduced the expression of p-RelA and interleukin-6, tumor necrosis factor-α, monocyte chemotactic protein-1/chemokine (C-C motif) ligand 2 (CCL2), and interferon-inducible protein-10/chemokine (C-X-C motif) ligand 10 (CXCL10) mRNA. P2Y(1)R and p-RelA were colocalized in glial fibrillary acidic protein-positive astrocytes, and an increase in infarct volume after MRS 2365 treatment was inhibited by the nuclear factor (NF)-κB inhibitor ammonium pyrrolidine dithiocarbamate. These results provide evidence that the P2Y(1)R expressed in cortical astrocytes may help regulate the cytokine/chemokine response after tMCAO/reperfusion through a p-RelA-mediated NF-κB pathway.

Effects of nucleotides and nucleosides on coagulation

Nucleotides, including ADP, ATP and uridine triphosphate (UTP), are discharged profusely in the circulation during many pathological conditions including sepsis. Sepsis can cause hypotension and systemic activation of the coagulation and fibrinolytic systems in humans, which may cause disseminated intravascular coagulation. We investigated whether nucleotide-induced cardiovascular collapse as provoked by systemic infusion of adenosine, ADP, ATP, UTP and nitric oxide affected the haemostatic system as assessed by whole blood thromboelastography (TEG) analysis. Ten pigs received a randomized infusion of adenosine, ADP, ATP, UTP or nitric oxide until mean arterial pressure was reduced to approximately 40% of baseline simulating sepsis-induced hypotension. The effect of the infusions on the haemostatic system was evaluated by TEG, and endothelial release of tissue plasminogen activator and plasminogen activator inhibitor-1 was measured. In contrast to the other infused substrates, ADP caused a reduction in maximum amplitude (71.4 to 64.2; P < 0.05), and reduced the angle, representing the thrombus formation (75.6 to 66.4; P < 0.05), indicating hypocoagulation. Despite increases in t-PA release (2.1 to 2.7 ng/ml; P < 0.05) and reductions in plasminogen activator inhibitor (33.9 +/- 10.9-17.8 +/- 4.4 ng/ml; P < 0.05) no increased fibrinolysis was found when whole blood was evaluated by TEG. Circulating ADP induces hypocoagulation without signs of increased fibrinolysis as evaluated by TEG. The potential clinical significance of these findings should be investigated further because ADP discharged systemically may possibly contribute to the coagulopathy observed in severe sepsis.

P2 purinoceptor-mediated cardioprotection in ischemic-reperfused mouse heart

P2 purinoceptor modulation of injury during ischemia-reperfusion was studied in murine hearts. Effects of P2 agonism or antagonism, and interstitial accumulation of P2 agonists (UTP, ATP, and ADP), were assessed in Langendorff perfused hearts during 20 min of ischemia and 45 min of reperfusion. In control hearts, ventricular pressure development recovered to 68 +/- 4 mm Hg (63 +/- 3% baseline), diastolic pressure remained elevated (23 +/- 2 mm Hg), and 26 +/- 4 U/g lactate dehydrogenase (LDH) was released during reperfusion, evidencing necrosis. Treatment with 250 nM UTP improved pressure development (85 +/- 5 mm Hg, or 77 +/- 2%) and reduced diastolic contracture (by approximately 70%, to 7 +/- 1 mm Hg) and LDH loss (by approximately 60%, to 11 +/- 2 U/g). In contrast, P2Y1 agonism with 50 nM 2-methyl-thio-ATP (2-MeSATP) was ineffective. In the presence of the P2Y antagonist suramin (10 or 200 microM), UTP no longer improved postischemic outcomes. Ischemia also substantially elevated interstitial [UTP], [ATP], and [ADP], potentially activating P2 receptors. This was supported in part by effects of antagonists: 200 microM suramin worsened LDH efflux (53 +/- 9 IU/g) and contractile dysfunction (41 +/- 2 mm Hg diastolic pressure; 28 +/- 3 mm Hg developed pressure), as did P2Y antagonism with either 10 or 100 microM reactive blue 2. However, a 10 microM concentration of suramin failed to alter outcome. P2X antagonism with 10 microM pyridoxal phosphate-6-azo-(benzene-2,4-disulfonic acid and P2X1-selective pyridoxal-alpha5-phosphate-6-phenylazo-4'-carboxylic acid (MRS2159) (30 microM) was ineffective. Data collectively support cardioprotection with low concentrations of UTP, and they are consistent with P2Y2 involvement. Endogenous nucleotides may also play a protective role, as evidenced by effects of P2 antagonists, although this warrants further investigation.