Adenosine action on platelets : pharmacological and functional studies in human platelets in vitro

The endogenous purine adenosine (ADO) is known to inhibit platelet aggregation in vitro through increased platelet levels of cyclic AMP. This effect is ADO-A2-receptor-mediated through stimulation of membrane adenylyl cyclase. The present in vitro study on human platelets was undertaken to evaluate the pharmacokinetics of ADO in platelet-rich plasma (PRP) and in whole blood. The platelet ADO receptor was studied by cyclic AMP determination in PRP and by anti-aggregation (ADP-induced) in whole blood, using ADO and stable ADO analogs.

The half-life of micromolar concentrations of ADO, was less than 15 seconds in whole blood and in the range from 5 to 10 minutes in PRP. By adding adenosine deaminase to PRP, the half-life of ADO could be shortened to that in whole blood. This PRP model that mimics ADO kinetics in whole blood can be useful for studies of ADO-induced cyclic AMP accumulation in platelets. Furthermore, the duration and magnitude of the cyclic AMP accumulation response to ADO in PRP and the antiaggregatory effect of ADO in whole blood (filtragometry) were found to be related to the plasma ADO concentration. The results obtained using the latter technique indicate that aggregation is already affected by ADO at a physiological concentration.

The functional platelet ADO receptor (antiaggregatory) was found to be of the A2A type, based on the potency order of ADO analogs: 5'-ethylcarboxamido-adenosine (NECA) > 2-phenylamino-adenosine (CV-1808) > 2-1p-(2-carboxyethyl)-phenethylaminol-5'-N-ethylcarboxamido-adenosine(CGS-21680) > ADO in whole blood impedance aggregometry. The relative potency of ADO analogs on cyclic AMP accumulation in PRP was: NECA >> 2-chloro-adenosine (2-CADO) = ADO > N6-phenylisopropyl-adenosine (R-PIA) = N6-cyclopentyl-adenosine(CPA) > CV-1808, which indicates the presence of a platelet ADO A2B receptor binding site. Clinical concentration of the ADO uptake inhibitor dipyridamole (2 IIM) caused a small inhibitory effect on whole blood ADP aggregation, as assessed by impedance aggregometry. The results of pharmacological ADO uptake inhibition suggest that a minor elevation of ADO in plasma may modify aggregation.

In conclusion, an ADO-mediated cyclic AMP elevation in platelets may only be demonstrated during the period when ADO is elevated in plasma. Therefore, due to the short half-life of ADO in blood, the effect on platelet function by exogenously administered, or endogenously formed, ADO is transient. It is suggested that locally formed ADO may operate as a modulator of platelet aggregability during the passage of blood through tissues.