title: Second messengers in platelet aggregation evoked by serotonin and A23187, a calcium ionophore.
abstract: We investigated the combined effect of 5-hydroxytryptamine (5-HT, serotonin) and calcium ionophore (A23187) on human platelet aggregation. Aggregation, monitored at 37 degrees C using a Dual-channel Lumi-aggregometer, was recorded for 5 min after challenge by a change in light transmission as a function of time. 5-HT (2-200 microM) alone did not cause platelet aggregation, but markedly potentiated A23187 (low dose) induced aggregation. Inhibitory concentration (IC50) values for a number of compounds were calculated as means +/- SEM from dose-response determinations. Synergism between 5-HT (2-5 microM) and A23187 (0.5-2 microM) was inhibited by 5-HT receptor blockers, methysergide (IC50 = 18 microM) and cyproheptadine (IC50 = 20 microM), and calcium channel blockers (verapamil and diltiazem, IC50 = 20 microM and 40 microM respectively). Interpretation of the effects of these blockers is complicated by their lack of specificity. Similarly, U73122, an inhibitor of phospholipase C (PLC), blocked the synergistic effect at an IC50 value of 9.2 microM. Wortmannin, a phosphatidylinositide 3-kinase (PI 3-K) inhibitor, also blocked the response (IC50 = 2.6 microM). However, neither genistein, a tyrosine-specific protein kinase inhibitor, nor chelerythrine, a protein kinase C inhibitor, affected aggregation at concentrations up to 10 microM. We conclude that the synergistic interaction between 5-HT and ionophore may be mediated by activation of PLC/Ca2+ and PI 3-kinase signalling pathways, but definitive proof will require other enzyme inhibitors with greater specificity.
authors.affiliation: Department of Biological and Biomedical Sciences, The Aga Khan University, Karachi, Pakistan.
authors.forename: J D
authors.initials: JD
authors.lastname: Connor
authors.forename: H
authors.initials: H
authors.lastname: Rasheed
authors.forename: A H
authors.initials: AH
authors.lastname: Gilani
authors.forename: M
authors.initials: M
authors.lastname: Cheema
authors.forename: Z
authors.initials: Z
authors.lastname: Rizvi
authors.forename: S A
authors.initials: SA
authors.lastname: Saeed
mesh_terms: D000328:Adult; D000470:Alkaloids; D000730:Androstadienes / Q000494:pharmacology; D053119:Benzophenanthridines; D001792:Blood Platelets / Q000187:drug effects*; D000001:Calcimycin / Q000494:pharmacology*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D003533:Cyproheptadine / Q000494:pharmacology; D004110:Diltiazem / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004338:Drug Combinations; D004357:Drug Synergism; D004791:Enzyme Inhibitors; D004963:Estrenes / Q000494:pharmacology; D005260:Female; D019833:Genistein / Q000494:pharmacology; D006801:Humans; D066298:In Vitro Techniques; D007476:Ionophores / Q000494:pharmacology*; D008297:Male; D008784:Methysergide / Q000494:pharmacology; D010617:Phenanthridines / Q000494:pharmacology; D019869:Phosphatidylinositol 3-Kinases / Q000502:physiology*; D000081082:Phosphoinositide-3 Kinase Inhibitors; D010974:Platelet Aggregation / Q000187:drug effects*; D011760:Pyrrolidinones / Q000494:pharmacology; D015290:Second Messenger Systems / Q000187:drug effects*; D012701:Serotonin / Q000494:pharmacology*; D010738:Type C Phospholipases / Q000037:antagonists & inhibitors / Q000502:physiology*; D014700:Verapamil / Q000494:pharmacology; D000077191:Wortmannin
chemical_list: D000470:Alkaloids; D000730:Androstadienes; D053119:Benzophenanthridines; D000001:Calcimycin; D002121:Calcium Channel Blockers; D003533:Cyproheptadine; D004110:Diltiazem; D004338:Drug Combinations; D004791:Enzyme Inhibitors; D004963:Estrenes; D019833:Genistein; D007476:Ionophores; D008784:Methysergide; D010617:Phenanthridines; D019869:Phosphatidylinositol 3-Kinases; D000081082:Phosphoinositide-3 Kinase Inhibitors; D011760:Pyrrolidinones; D012701:Serotonin; D010738:Type C Phospholipases; D014700:Verapamil; D000077191:Wortmannin; C060229:1-(6-((3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl)-1H-pyrrole-2,5-dione; C016299:chelerythrine