The dual nature of extracellular ATP as a concentration-dependent platelet P2X1 agonist and antagonist

Abstract

Patient groups subject to higher occurrence of stroke (e.g., people with diabetes, cystic fibrosis, pulmonary hypertension) have reduced release of ATP from their erythrocytes (ERYs) when subjected to flow-induced deformation or pharmacological stimuli. These same groups also have platelets that are more adhesive in comparison to controls. Here we show platelet aggregation, and inhibition of that aggregation, is affected by free Ca(2+) entering the platelet through the ATP-gated P2X1 receptor. The addition of ATP (10 microM) increased the platelet NO by 26.7 +/- 7.7%. This value was decreased significantly to below basal levels in the presence of NF 449 (p < 0.001), an inhibitor of the P2X1 receptor on the platelet. Aggregation profiles measured in the presence of ATP revealed that when the P2X1 receptor was blocked, or when the measurements were performed in Ca(2+) free buffer, platelet aggregation was nearly eliminated. Our findings employing standard aggregation measurements suggest that ATP behaves as a platelet inhibitor below 1.6 x 10(-19) moles ATP per platelet; however, above this value, ATP behaves as a platelet activator. These findings suggesting a dual nature of ATP with regard to platelet behavior were confirmed by passing platelets over endothelial cells that were coated in the channels of a microfluidic device. Importantly, it was determined that ERY-derived ATP release was a major determinant of platelet adhesion to the endothelium. These findings may have implications in anti-platelet drug design as most current therapies focus on the inhibition of P2Y-type receptors. Moreover, through the use of microfluidic technologies, we have provided in vitro evidence for a possible relationship between ERY properties and platelet behavior in vivo.

A Molecular Level Understanding of Zinc Activation of C-peptide and its Effects on Cellular Communication in the Bloodstream

“…Recently, in vitro studies involving immobilized endothelial cells revealed that C-peptide may also have an effect on platelet adhesion [47]. This finding is in contrast to a previous report that Cpeptide does not have any direct effects on platelets [48].…”

“…However, recently, our group has demonstrated that ATP does indeed participate in platelet activity. In fact, it was shown that below normal levels of ATP binding to the P2X receptor resulted in a decrease in NO production by platelets, resulting in an increase in platelet aggregation [47]. However, as ATP levels increased, both the NO production and aggregation rates of platelets decreased.…”

“…Recent data has also suggested that ATP may be a determinant in platelet activity in vivo [47]. ATP acts on platelets through the P2X receptor, which regulates a calcium ion channel in platelets.…”

“…However, when platelets are added to a sample of RBCs containing C-peptide, the platelet behavior is altered. Specifically, in a recent study, platelets were pumped across endothelial cells immobilized in flow channels having a diameter of about 200 micrometers [47]. The number of platelets adhering to this endothelium per unit area depended upon the amount of ATP present in the media with the platelets.…”

Use of the red blood cell as a simple drug target and diagnostic by manipulating and monitoring its ability to release adenosine triphosphate (ATP)

“…Current efforts, some described here, are also attempting to provide evidence that the RBC may be a determinant in platelet function via ATP binding to platelet receptors [46]. Other results demonstrate that the RBC may be serving as a mediator of drug efficacy [44]. lar levels of ATP in the RBC).…”