Platelet-activating factor (PAF)-induced platelet aggregation. Modulation by plasma adenosine and methylxanthines.

This study examined the role of plasma adenosine in the modulation of platelet-activating factor (PAF) activity on platelet aggregation and serotonin (5-HT) release in human platelet-rich plasma (PRP). In addition, the effects of methylxanthines (e.g. theophylline and caffeine) were studied on PAF-induced platelet aggregation in PRP isolated from blood samples from healthy subjects. Also, PAF-induced platelet aggregation was examined in PRP samples of patients receiving theophylline treatment. These studies demonstrate that plasma adenosine levels (0.1 to 0.3 microM) play a key role in negative modulation of PAF activity on platelet aggregation and 5-HT release. After depletion of plasma adenosine, the platelet-aggregating activity of PAF was increased greatly (> 10-fold). PAF at concentrations of 0.1 to 12 microM caused no 5-HT release in PRP containing normal amounts of adenosine (blood collected in the presence of 2'-deoxycoformycin and dilazep), whereas PAF at 0.1 microM caused 5-HT release (45%) in adenosine-depleted PRP, demonstrating that plasma adenosine is much more inhibitory of 5-HT release than platelet aggregation. The adenosine antagonists theophylline (50 microM), caffeine (50 microM) and a xanthine derivative, 3,7-dimethyl-l-propargylxanthine (DMPX, 10 microM) (a more specific adenosine A2 receptor antagonist), potentiated PAF activity on platelet aggregation in PRP samples containing adenosine. Also, patients receiving theophylline treatments showed significantly greater platelet aggregation induced by PAF in their PRP samples. PAF induced a rapid increase (80% in 15 sec) in intracellular Ca2+ mobilization, which was strongly inhibited by adenosine (IC50, 0.3 microM). Our studies suggest that agents that can increase plasma adenosine levels (e.g. inhibitors of adenosine uptake and adenosine metabolism) or methylxanthines may be useful in altering (inhibiting or enhancing, respectively) PAF actions on platelets and other tissues.