AIM: To study the enhanced effects of 5 hydroxytryptamine (5 HT) on ADP induced aggregation. METHODS:
Platelet aggregation
was quantified by the light transmission, the cytosolic free calcium (
i) was measured by digital fluorescent microscopy, and inositol 1,4, 5 triphosphate (IP 3) was determined by receptor binding assay. RESULTS: In rabbit platelet rich plasma (PRP), 5 HT 0 03-3 μmol·L -1 induced a decrease in light transmission (DLT) in a concentration dependent manner with centralization of granules, as revealed by electron microscopy. The DLT was accompanied with neither platelet aggregation nor a release reaction. In single washed platelets loaded with Fura 2, 5 HT caused a concentration dependent elevation of i, and IP 3 level was also transiently increased in washed platelets at 15 s after stimulation by 5 HT. Adenosine diphosphate (ADP) also caused DLT transiently in PRP before its own aggregation without a release reaction. Pretreatment of PRP or washed platelets with 5 HT, the DLT by ADP was reduced concentration dependently and ADP induced aggregation and i mobilization were enhanced. CONCLUSION: The enhancement of ADP induced aggregation was attributed to the superimposition of the calcium release from the storage sites and calcium influx induced by ADP over the calcium release from the storage sites by 5 HT.