CHARACTERIZATION OF THE EFFECT OF THE ADENOSINE AGONIST CYCLOHEXYLADENOSINE ON PLATELET ACTIVATING FACTOR-INDUCED INCREASES IN [CA-2+]I IN HUMAN PLATELETS INVITRO

Previous studies have shown that adenosine agonists acting at A-2 receptors inhibit platelet aggregation. Since an increase in cytosolic Ca2+ concentration (delta[Ca2+]i) is closely associated with the time frame of platelet aggregation, we have examined the effect of adenosine receptor function on induced increases of [Ca2+]i by a potent platelet activator, platelet activating factor (PAF). We loaded washed platelets with Fura-2, then induced increases in [Ca2+]i with various concentrations of PAF, and then determined EC50 values (PAF concentration at half-maximal response) and values for maximal response of delta[Ca2+]i (max-delta[Ca2+]i). The EC50 for PAF-delta[Ca2+]i was 112 +/- 37 (SD) pM and the max-delta[Ca2+]i was 284 +/- 138 (SD) nM. Our results show that PAF-delta[Ca2+]i was inhibited in a non-competitive manner by the adenosine receptor agonist cyclohexyladenosine (CHA) with an IC50 of 14.9-mu-M. This inhibition was partially reversed by theophylline, an adenosine receptor antagonist, with an IC50 of 19-mu-M. Based on the results of these studies together with evidence from other research groups that platelets do not possess A-1 receptors, our results suggest that CHA inhibited PAF-delta[Ca2+]i in platelets through an activation of A-2 receptors.