apyrase and Coronary-Disease

Blood platelets maintain vascular integrity and promote primary and secondary hemostasis following interruption of vessel continuity. Biochemical or physical damage to coronary, carotid, or peripheral arteries promotes excessive platelet activation and recruitment culminating in vascular occlusion and tissue ischemia. Currently, inadequate therapeutic approaches to stroke and coronary artery disease (CAD) are a public health issue. Following our demonstration of neutrophil leukotriene production from arachidonate released from activated aspirin-treated platelets, we studied interactions among platelets and other blood cells. This led to concepts of transcellular metabolism and thromboregulation. Thrombosis has a proinflammatory component whereby biologically active substances are synthesized by different cell types that could not individually synthesize the metabolite(s). Endothelium controls platelet reactivity via at least three biochemical systems: autacoids leading to production of prostacyclin and nitric oxide (NO) and endothelial ecto-adenosine phosphatase (ADPase)/CD39/nucleoside triphosphate diphosphohydrolase (NTPDase-1). The autacoids are fluid phase reactants, not produced by tissues in the basal state, but are only synthesized intracellularly and released upon interactions of cells with an agonist. When released, they exert fleeting actions in the immediate milieu and are rapidly inactivated. CD39 is an integral component of the endothelial cell (EC) surface and is substrate activated. It maintains vascular fluidity in the complete absence of prostacyclin and NO, indicating that the latter are ancillary components of hemostasis. Therapeutic implications for the autacoids have not been compelling because of their transient and local action and limited potency. Conversely, CD39, acting solely on the platelet releasate, is efficacious in animal models. It metabolically neutralizes a prothrombotic releasate via deletion of ADP-the major recruiting agent responsible for formation of an occlusive thrombus. In addition, solCD39 reduced adenosine triphosphate (ATP)- and ischemia-induced norepinephrine release in the heart. This action can prevent fatal arrhythmia. Moreover, solCD39 ameliorated the sequelae of stroke in cd39 null mice. Thus, CD39 represents the next generation of cardioprotective and cerebroprotective molecules. This article focuses on our interpretations of recent data and their implications for therapeutics.

Topics: Adenosine Diphosphate; Animals; Antigens, CD; Apyrase; Aspirin; Blood Vessels; Cardiotonic Agents; Cells, Cultured; Cerebral Infarction; Coronary Disease; Drug Evaluation, Preclinical; Endothelial Cells; Endothelium, Vascular; Fibrinolytic Agents; Humans; Mice; Mice, Knockout; Mutagenesis, Site-Directed; Neuroprotective Agents; Platelet Aggregation; Platelet Aggregation Inhibitors; Recombinant Proteins; Structure-Activity Relationship; Synaptosomes; Thrombosis

The goal of this study was to test the hypotheses that endogenous ADP plays an important role in vivo in mediating platelet aggregation and cyclic coronary artery blood flow variations (CFVs) in stenosed and endothelium-injured coronary arteries in an experimental canine model. Anesthetized animals were studied and coronary blood flow velocities monitored by a pulsed Doppler flow probe positioned around the left anterior descending coronary artery. CFVs were established by an external constrictor positioned at sites with injured endothelium. Apyrase, an ADP-removing enzyme, was infused into the left anterior descending coronary artery (0.3-1.8 units/min) 30 minutes or 2 hours after the establishment of CFVs. Complete abolition of CFVs was achieved in 81% (13/16) of dogs with 30-minute CFVs and in 83% (five of six) of dogs with 2-hour CFVs. In other dogs, a potent inhibitor of ADP-induced platelet aggregation, clopidogrel, was administered as a 10 mg/kg i.v. bolus and a 2.5 mg/kg/hr infusion 30 minutes and 3 hours after the establishment of CFVs. This treatment resulted in complete abolition of CFVs in 14 dogs (100%) with either 30-minute or 3-hour CFVs. Epinephrine was infused into some dogs after CFVs had ceased as a result of either apyrase or clopidogrel administration and into some dogs in whom SQ29548, a thromboxane A2 receptor antagonist, had been given when apyrase failed to abolish CFVs. Epinephrine restored CFVs in all dogs treated with apyrase alone, 67% (four of six) of dogs treated with the combination of apyrase and SQ29548, and 29% (two of seven) of dogs treated with clopidogrel. The plasma epinephrine levels required for CFV restoration were 20 times higher than baseline values in dogs receiving apyrase alone, 100 times higher when a combination of apyrase and SQ29548 had been given, and more than 5,000 times higher in dogs receiving clopidogrel. In vitro studies showed that apyrase only inhibited ADP-induced platelet aggregation, whereas clopidogrel not only inhibited ADP-induced platelet aggregation, but also reduced platelet aggregation induced by the thromboxane mimetic U46619 and serotonin. These data suggest that 1) ADP is an important mediator of platelet aggregation and CFVs in vivo and 2) combined inhibition of thromboxane A2 and ADP's effects provides marked protection against CFVs in experimentally stenosed and endothelium-injured canine coronary arteries. These data and our previous observations are consistent with the possibili

Topics: Adenosine Diphosphate; Animals; Apyrase; Bridged Bicyclo Compounds, Heterocyclic; Clopidogrel; Coronary Circulation; Coronary Disease; Coronary Vessels; Dogs; Endothelium, Vascular; Fatty Acids, Unsaturated; Female; Hydrazines; Male; Platelet Aggregation; Platelet Aggregation Inhibitors; Serotonin; Thromboxane A2; Ticlopidine

To investigate the clinical implications and mechanisms of spontaneous platelet aggregation (SPA) in man, 150 normal subjects, 22 patient controls and 130 patients with vascular insufficiency were studied. SPA was negative in normal subjects and patient controls whereas it was positive in 36 of 66 (54%) patients with transient ischemic attacks, 6 of 32 (19%) patients with stable angina, 7 of 10 (70%) patients with acute myocardial infarction and 11 of 14 (80%) patients with acute peripheral arterial insufficiency. The SPA was inhibited with aspirin in vivo, and inhibited competitively in vitro by low concentrations of aspirin, 2-chloroadenosine, prostaglandin E1 or apyrase but only by high concentrations of heparin or hirudin. Addition of platelet-poor plasma from patients with positive SPA did not cause normal platelets to aggregate. Treatment of patients who had acute peripheral arterial insufficiency with aspirin and dipyridamole prevented SPA with notable clinical improvement of the ischemic changes.

Topics: Adenosine; Adult; Aged; Angina Pectoris; Apyrase; Arterial Occlusive Diseases; Aspirin; Coronary Disease; Dipyridamole; Heparin; Hirudins; Humans; Ischemic Attack, Transient; Middle Aged; Myocardial Infarction; Platelet Aggregation; Prostaglandins E