title: Thromboxane A2/prostaglandin H2 mobilizes calcium in human blood platelets.
abstract: The present study investigated the mechanism by which thromboxane A2/prostaglandin H2 (TXA2/PGH2) stimulates platelet activation. Previous studies in isolated platelet vesicles have suggested that TXA2/PGH2 functions to release calcium from intraplatelet stores. On this basis, we investigated whether TXA2/PGH2 causes mobilization of calcium in intact platelets. Calcium redistribution was measured using the fluorescent probe, chlortetracycline (CTC), and a photon-counting microspectrofluorometer. Human platelet-rich plasma was incubated with CTC (50 microM) for 40 min at 25 degrees C. Shape change was induced with arachidonic acid (AA, 100 microM) or ADP (0.75-1.0 microM). It was found that AA addition resulted in a significant release of intraplatelet calcium. This release was blocked by inhibition of the cyclooxygenase with indomethacin (20 microM) or the specific TXA2/PGH2 antagonist, 13-azaprostanoic acid (13-APA, 100 microM). On the other hand, neither indomethacin nor 13-APA had any effect on calcium release stimulated by ADP. However, prostacyclin (13 nM) inhibited both AA- and ADP-induced calcium release. These findings provide evidence that cyclooxygenase products of AA, i.e., TXA2 and/or PGH2 directly caused the mobilization of intraplatelet calcium. Furthermore, this calcium mobilization appears to be mediated through a specific TXA2/PGH2 receptor interaction.
mesh_terms: D000244:Adenosine Diphosphate / Q000494:pharmacology; D016718:Arachidonic Acid; D001095:Arachidonic Acids / Q000494:pharmacology; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism*; D002118:Calcium / Q000097:blood*; D002751:Chlortetracycline / Q000494:pharmacology; D011464:Epoprostenol / Q000494:pharmacology; D006801:Humans; D007213:Indomethacin / Q000494:pharmacology; D010974:Platelet Aggregation / Q000187:drug effects; D011449:Prostaglandin Endoperoxides / Q000494:pharmacology*; D011450:Prostaglandin Endoperoxides, Synthetic / Q000494:pharmacology*; D044262:Prostaglandin H2; D011463:Prostaglandins H / Q000494:pharmacology*; D011466:Prostanoic Acids / Q000494:pharmacology; D013268:Stimulation, Chemical; D013928:Thromboxane A2 / Q000494:pharmacology*; D013931:Thromboxanes / Q000494:pharmacology*
chemical_list: D001095:Arachidonic Acids; D011449:Prostaglandin Endoperoxides; D011450:Prostaglandin Endoperoxides, Synthetic; D011463:Prostaglandins H; D011466:Prostanoic Acids; D013931:Thromboxanes; D016718:Arachidonic Acid; D044262:Prostaglandin H2; D013928:Thromboxane A2; D000244:Adenosine Diphosphate; C022574:13-azaprostanoic acid; D011464:Epoprostenol; D002118:Calcium; D002751:Chlortetracycline; D007213:Indomethacin