Discovery and recognition of purine receptor subtypes on platelets

Abstract

The effects of purines on platelets have been known since the 1960s, when Born demonstrated aggregation induced by ADP and its inhibition by adenosine and by ATP. The inhibition by adenosine is not specific for ADP, and adenosine acts at a separate receptor to stimulate adenylate cyclase, which has an inhibitory effect on platelet function. Studies using selective agonists and antagonists have shown that the platelet receptor is of the A 2A subtype and this has been confirmed using A 2A knockout mice. The situation with ADP is more complex, and there has been controversy about the number of ADP receptors on platelets. ADP causes shape change, aggregation, mobilisation of calcium from intracellular stores, rapid calcium influx, and inhibition of adenylate cyclase, and the relationship between these is becoming clearer. Two cloned P2 receptors have been detected on platelets, P2X 1 and P2Y 1 , and a third P2Y receptor is thought to exist. The P2X 1 receptor is responsible for the rapid calcium influx and can be activated by ATP as well as by ADP, but is likely to be desensitised under normal experimental conditions and its pathophysiological role is uncertain. The P2Y 1 receptor is responsible for calcium mobilisation, shape change, and the initiation of aggregation, and these responses are abolished in P2Y 1 knockout mice, while the other P2Y receptor is responsible for inhibition of adenylate cyclase and is required for full aggregation. ATP is a competitive antagonist at both these P2Y receptors, while some nucleotide analogues can discriminate between them. Drug Dev.

Mechanisms Involved in Adenosine Triphosphate–Induced Platelet Aggregation in Whole Blood

“…10,16 It stimulates adenylate cyclase and increases the intracellular level of cAMP, which in turn inhibits calcium mobilization and other signal transduction pathways. We thought it possible that differences in response to ATP in whole blood and in PRP containing leukocytes might relate to the relative availability of generated adenosine in the 2 systems.…”

“…Reduced aggregation at very high concentrations of ATP may be a consequence of antagonism by ATP of ADP interaction with P2Y 1 or P2Y 12 receptors. 10,11 It is unlikely that this is an artifact of the high concentrations used, because addition of high concentrations of ADP to whole blood or PRP did not result in reduced platelet aggregation (data not shown). There seems to be little inhibition of this aggregation by adenosine generated from the ATP, probably because the adenosine is rapidly removed by the erythrocytes in the blood.…”

“…It is known that ATP can interact with P2X 1 receptors on platelets, causing a transient Ca 2ϩ mobilization, 8,9 but this does not result in platelet aggregation, and the importance of P2X 1 receptors to overall platelet function is unknown. It is also known that ATP acts as an antagonist of the effects of ADP at P2Y 1 and P2Y 12 receptors 10,11 and that high concentrations can inhibit ADP-induced platelet aggregation. 12 When considering the possible effects of ATP on platelets in vitro and in vivo, the presence of enzymes present on blood cells and endothelial cells and in plasma that metabolize ATP must be taken into account.…”

“…These include enzymes that convert ATP to ADP and ADP to AMP (NTPDase-1, also known as ATP diphosphohydrolase, CD 39 and EC 3.6.1.5), 13,14 ATP to AMP and ADP to AMP (5Ј-monophosphate phosphoanhydrolase/phosphodiesterase, NMPP), 15 and AMP to adenosine (5Ј-nucleotidase), 13 the latter being an inhibitor of platelet aggregation. 10,16 Also, adenosine can be taken up and neutralized by erythrocytes and other blood cells, thus limiting its potential inhibitory action. 17,18 Thus, the overall effect of ATP could depend on several competing influences.…”

Inhibitory Effects of Methyl Brevifolincarboxylate Isolated from Phyllanthus niruri L. on Platelet Aggregation

“…20) Moreover, adenosine provides useful information on platelet aggregatory behavior using the data output of the laser-scattering method when compared with other inhibitors including 1. Typical output of the aggregometer with the laser-scattering method showing changes in the formation of platelet aggregates by size was reported elsewhere.…”