OBJECTIVETo compare the effects of protein kinase C (PKC) and calcium (Ca2+) on platelet aggregation and platelet membrane surface GP I b expression in thrombin receptors activation, and investigate the role of Gq signal transduction pathway in such activation.

METHODSPeptide SFLLRN (PARI-AP) and AYPGKF (PAR4-AP) were used to stimulate platelet, and the effects of Ro-31-2220 (inhibitor of PKC) and BAPTA/AM (calcium chelator) on the platelet aggregation and GP I b were analyzed.

RESULTSEither 25 micromol/L PAR1 or 250 micromol/L PAR4 peptide could induce absolute platelet aggregation with a reversible internalization of GP I b. Platelet aggregation was inhibited by Ro-31-2220 or BAPTA while the morphological change curve still occurred upon PARs activation. In addition, Ro-31-2220 decreased GP I b centralization upon PAR1 stimulation [(87.00 +/- 0.04)% and (73.00 +/- 0.08)%, respectively at 1, 2 min, P<0.05], albeit it blocked the internalization of GP I b in PAR4 activation [(44.00 +/- 0.01)% and (46.00 +/- 0.05)%, respectively at 10, 30 min, P <0.05]. Meanwhile, GP I b internalization was blocked by BAPTA in both peptides [(94.00 +/- 0.08)% and (95.00 +/- 0.00)% at 1 min, (92.00 +/- 0.02)% and (94.00 +/- 0.01)% at 2 min, (91.00 +/- 0.02)% and (91.00 +/- 0.02)% at 5 min, (90.00 +/- 0.04)% and (87.00 +/- 0.03)% at 10 min, respectively, P <0.05].

CONCLUSIONPKC and calcium play an important role in thrombin receptor activation. Calcium is closely correlated with such activation, being similar in the two PARs signal pathways. PK C promotes GP I b centralization in PAR1 pathway and accelerates GP I b return to membrane surface in PAR4 pathway.