INHIBITION OF PLATELET-AGGREGATION BY ADENOSINE RECEPTOR AGONISTS

2-(Ar)alkoxyadenosines, which are agonists selective for the A(2A)AR in PC12 cell and rat striatum membranes, are also agonists at the A(2)AR coupled to adenylate cyclase (AC) that mediates the inhibition of platelet aggregation. A panel of twelve well-characterized adenosine analogues stimulated human platelet AC and inhibited ADP-induced platelet aggregation at sub- to low-micromolar concentrations with a potency ranking CGS 21680 > adenosine > R-PIA. There were significant correlations between the EC(50) Of anti-aggregatory activity and either the EC(50) Of stimulation of platelet and PC12 cell AC (r(2) = 0.66 and 0.67, respectively) or the K-i of inhibition of [H-3]NECA binding to the rat striatum membranes (r(2) = 0.75). Likewise, platelet AC stimulation correlated well with stimulation of PC12 cell AC and with [H-3]NECA binding (r(2) = 0.94 and 0.91, respectively). Ten 2-(ar)alkoxyadenosines stimulated platelet AC at EC(50)s ranging between 0.16 and 2.3 mu M and inhibited platelet aggregation at EC(50)s ranging between 2 and 30 mu M. There were no correlations between the EC(50)s of anti-aggregatory activity and either the EC(50)s of the stimulation of platelet or PC12 AC (r(2) = 0.08 and 0.06, respectively) or with the K-i of the inhibition of [H-3]NECA binding to the A(2a)AR in rat striatum (r(2) = 0.02). The EC(50)s Of the stimulation of platelet AC correlated with those of the stimulation of PC 12 C (r(2) = 0.48), and also with the K-i of [H-3]NECA binding (r(2) = 0.71). Each of the 23 adenosines completely inhibited platelet aggregation and thus, functionally, all behaved as full agonists. As stimulants of PC12 cell AC, Group A and B analogues were equally efficacious. As stimulants of platelet AC, however, the efficacy relative to NECA (=1.0) of Group B analogues was significantly less than that of Group A analogues, 0.49+/-0.2 vs. 0.72+/-0.05, P<0.01. The partial agonist activity of Group B analogues at the platelet A(2)AR but full agonist activity at the PC12 cell A(2),AR, as well as the relatively low correlations between platelet AC stimulation and other indices of A(2a)AR agonist activity, suggest the platelet receptor is not a typical A(2a)AR. Further, the lack of a correlation between the platelet anti-aggregatory and AC stimulatory activity suggests that (a) the 2-(ar)alkoxyadenosines might affect platelet aggregation by mechanisms other than AC stimulation or (b) that the stimulation of the platelet membrane AC by 2-(ar)alkoxy-adenosines does not correspond to the accumulation of cyclic AMP in intact platelets.