Regulation of Platelet Function By Adenosine Receptors

Introduction: We have recently shown that severe trauma and hemorrhage lead to inhibition of platelet aggregation and an elevation in cyclic adenosine monophosphate (cAMP). Adenosine is one of the few humoral agents known to stimulate cAMP in platelets. Because adenosine is released from damaged tissue, it may contribute to the platelet dysfunction seen after severe trauma. Platelets have four adenosine receptors (A1, A2a, A2b and A3). These receptors are G-Protein Coupled Receptors and have been proposed to stimulate adenylyl cyclase and increase intracellular cAMP. Although studies have shown that stimulate A2a can inhibit platelet aggregation and elevate cAMP, there is little data elucidating the function of the other receptors. Objective: Define which adenosine receptors affects platelet aggregation and cAMP production. Methods: Platelet-rich plasma (PRP) was isolated from whole blood of human volunteers, and centrifuged at 200g for 10min. Light transmission aggregometry was performed using a plate reader (Synergy Neo2 Multimode Reader, BioTek) with constant agitation. PRP was stimulated with adenosine diphosphate (ADP) with or without various adenosine agonists or antagonists, including the non-metabolizable adenosine agonist 5-(N-ethyl-carboxamido) adenosine (NECA), antagonists to receptors A1 (DPCPX), A2a (Sch 58261), A2b (GS 6201) and A3 (MRS 1220), or agonists for A2a (CGS 21680) A2b (BAY 60-6583) or agonist A1 (CCPA), A2a (CGS 21680), A2b (Bay 60-6583), A3 (2-Cl-IB-Meca). Cyclic AMP was extracted from 100ul of PRP after adding 1ml of EtOH, 10mM ammonium formate, with 10ug/ml cGMP-Br as an internal control. Samples were centrifuged at 20K g for 10min, and supernatant dried. Samples were brought up in 200ul of 0.1% formic acid for analysis by Reverse Phase liquid chromatography/ Tandem Mass Spectroscopy (Quantiva, ThrermoFisher). N-8/group. Results: Adenosine diphosphate (100uM) leads to platelet aggregation (change in mAbsorbance units, Table 1). The adenosine agonist NECA inhibited aggregation to ADP and elevated cAMP in a dose dependent manner (pg/ml per 1000 plt, Table 1). Platelet aggregation was inhibited and cAMP was elevated after stimulation with agonists for adenosine receptor A2a agonist, but not A1, A2b, or A3 (Table 2). Antagonists for A2a, but not A1, A2b, A3, blocked NECA inhibition of ADP aggregation (Table 3). Agonist for adenosine receptor A2a inhibited the ADP-induced aggregation and elevated cAMP in a dose response manner (Table 4). Discussion: Adenosine inhibits platelet aggregation to ADP. The mechanism appears to be due to elevation in intracellular cAMP, and works through the A2a receptor. These data suggest that the A2a receptor could be potential target for a resuscitation strategy that could attenuate or prevent platelet dysfunction after trauma by preventing stimulation of adenylate cyclase and synthesis of cAMP. This study was funded by the US Army medical Research and Development Command. Disclosures No relevant conflicts of interest to declare.

Adenosine Regulation of cAMP through Phosphodiesterases

Introduction: Adenosine, an autacoid and metabolite of ATP, has been known to have anti-platelet properties. Of the 4 adenosine receptors (ARs), only A2A AR have been implicated in adenosines anti-platelet properties in human. A2A AR is a G-Protein Coupled Receptors associated with a stimulatory G-Protein (Gs) that can activate adenylyl cyclase (AC) and increase intracellular cAMP. An elevation of cAMP has been shown to inhibit platelet aggregation to natural stimuli. Regulation of intracellular cAMP is balanced between synthesis by adenylate cyclase and degradation by phosphdiesterases (PDE). There are 3 PDE subtypes found in platelets: PDE2, PDE3, and PDE5. However, it is not know which subtype(s) is (are) responsible for regulating cAMP level in human platelets after adenosine stimulation. Materials and Methods: Platelet-rich plasma (PRP) was isolated from whole blood of human volunteers, and centrifuged at 200g for 10min. Light transmission aggregometry was performed after stimulation of platelets with 100uM ADP, with or without NECA (non-specific AR agonist), DPCPX (A1 AR antagonist), and Sch 58261 (A2A AR antagonist). PRP treated with NECA, DPCPX, Sch 58261, and PDE inhibitors (EHNA, E in figures, for PDE2, Trequinsin, T in figures, for PDE3, and 4-{[3'4'-(methylenedioxy) benzyl]amino}-6-methoxyqunazolin, 4 in figures, for PDE 5). Cyclic AMP was measured in platelets after treatment by liquid chromatography/ Tandem Mass Spectroscopy (Quantiva, ThrermoFisher) after treated with these drugs. Results: ADP-induced platelet aggregation was inhibited in a dose dependent manner by the non-specific adenosine agonist, NECA (Figure 1) and the effect was blocked by A2A specific antagonist Sch 58261, not by the A1 AR antagonist, DPCPX (Figure 2). NECA inhibition of platelet aggregation was likely due to an elevation of intracellular cAMP (1 uM, 5min incubation, Figure 3). Inhibition of PDE3 alone, significantly increased intracellular cAMP, suggesting that basal PDE3 activity is present. PDE 3 inhibition combined with NECA elevated cAMP even higher than PDE inhibition or NECA alone (Figure 3), suggesting that NECA (A2A stimulation) effects PDE activity. Inhibition of PDE2 or 5 had no effect on basal or NECA stimulated cAMP (Figure 3). Inhibition of all 3 PDE (2,3,5) combined with NECA elevated cAMP to levels higher then NECA+ PDE3 inhibition, again suggesting that NECA maybe effecting the activity of the PDEs (Figure 3). The potentiation of cAMP by PDE3 inhibition + NECA was block by A2A, but not A1 antagonist (Figure 4) suggesting that the nonspecific adenosine agonist is elevating cAMP through A2A. Conclusion: 1. In human platelets, NECA stimulates cAMP through A2A receptors and this elevation is likely due to an elevation in adenylate cyclase via Gs coupled to A2A. PDE3 is basally active and likely regulated by adenosine receptors. Disclosures No relevant conflicts of interest to declare.

Abstract 5255: Differential Effect Of A Novel Adenosine-Agonist On Stimulation-Induced Norepinephrine Release From Rat Hearts

Presynaptic modulation of norepinephrine (NE) release plays a major role in the regulation of cardiac sympathetic activity. We tested the effect of the novel, highly selective adenosine A1 subtype agonist BAY 68–4986 on stimulation-induced NE release. Isolated rat hearts (Wistar or SHR) were perfused with a modified Krebs-Henseleit solution (Langendorff technique). Exocytotic NE release was induced by electrical field stimulation with 5 V, 6 Hz for 1 minute. NE was detected by HPLC. Two stimulations (S1, S2) were performed to compare intraindividually NE release under control conditions (S1) and with BAY 68–4986 30ug/ml, 300 ug/ml or CCPA 10–6M (S2). Neuronal uptake was inhibited by desipramine (100 nM) in order to prevent amine elimination and nonexocytotic release of NE. Stimulation of adenosine A1 receptors with BAY 68–4986 led to a concentration dependent decrease of the stimulation-induced NE release only in SHR rats, whereas no effect could be seen in Wistar rats (SHR: S2/S1 = 0.90 +/− 0.08 with 30 ug/l, S2/S1 = 0.54 +/− 0.02 with 300 ug/l, Wistar: S2/S1 = 1.05 +/− 0.12 with 30 ug/l, S2/S1 = 1.03 +/− 0.09 with 300 ug/l, p=0.003). The effect of BAY 68–4986 on stimulation-induced NE release in SHR was similar to the effect of 10–6 M CCPA (S2/S1 = 0.59 ± 0.05). This differential effect in genetically different rats was also observed in vivo, with a blunted increase of the heart rate during stress reaction only in SHR rats, when treated with BAY 68–4986. We could demonstrate that the novel, highly selective A1-adenosine agonist BAY 68–4986 has a differential effect on stimulation-induced NE release and stress responde in SHR rats compared to Wistar rats.