Calcium-dependent synergistic interaction of platelet activating factor and epinephrine in human platelet aggregation

To investigate the mechanism (s) involved in the synergistic interaction of platelet activating factor (PAF) and epinephrine.

Blood was obtained from healthy human subjects reported to be free of medications for at least two weeks before sampling. Aggregation was monitored at 37 oC using Dual-channel Lumi-aggregometer. The resulting aggregation was recorded for 5 min by the measurement of light transmission as a function of time.

Platelet aggregation mediated by subthreshold concentrations of PAF (5-8 nmol/L) plus epinephrine (0.5-2 micromol/L) was inhibited by ?2-receptor blocker, yohimbine, and PAF receptor antagonist WEB 2086. This synergism was inhibited by calcium channel blockers, verapamil and diltiazem. In addition, platelet aggregation by co-addition of PAF and epinephrine was also inhibited by very low concentrations of phospholipase C (PLC) inhibitor (U73122; IC50=0.2 micromol/L), the MAP kinase inhibitor, PD 98059 (IC50=3 micromol/L), and cyclooxygenase (COX-1) inhibitors including indomethacin (IC50=0.25 micromol/L), flurbiprofen (IC50=0.7 micromol/L), and piroxicam (IC50=7 micromol/L). However, COX-2 inhibitors, nimesulide (IC50=26 micromol/L), NS-398 (IC50=7 micromol/L), and etodolac (IC50=15 micromol/L) were also effective in inhibiting the aggregation. The inhibitors of protein kinase C (chelerythrine) and tyrosine kinase (genistien), and phosphatidylinositol 3-kinase inhibitor (wortmannin) had no significant effect on platelet aggregation induced by PAF and epinephrine.

The synergistic effect of PAF and epinephrine on human platelet aggregation is receptor-mediated and involves the activation of PLC/Ca2+, COX and MAP kinase signalling pathways.