title: Comparative investigation of the effects of the immunosuppressants cyclosporine A, cyclosporine G, and FK-506 on platelet activation.
abstract: We have determined that ADP-induced platelet aggregation and secretion are enhanced by preincubation of human platelets with the immunosuppressant cyclosporine A (CSA) (see accompanying article by Naik et al., 1993). In the present report we compare the effects of CSA with two other potent immunosuppressants, cyclosporine G (CSG), an analogue of CSA, and FK-506, on platelet activation. Preincubation of platelets with either CSA, CSG, or FK-506 resulted in platelets which exhibited hyperaggregability when stimulated by ADP. CSG produced the lowest degree of hyperaggregation, whereas FK-506 produced the highest at an equivalent concentration. All three compounds enhanced the secretion of serotonin, with FK-506 producing the highest degree of serotonin release and CSG producing the lowest amount. Platelet hyperaggregation and enhanced secretion were both time- and dose-dependent. Comparative studies performed with CSA and CSG indicated that a short preincubation period with CSA (2.5 min) resulted in a 90% enhancement of ADP-induced platelet aggregation, whereas CSG enhancement was only 20%. At a concentration of 600 ng/ml, CSA produced 120% enhancement of ADP-induced platelet aggregation, whereas CSG produced only a 30% enhancement. Several potential mechanisms responsible for the enhanced ADP-induced aggregation and secretion were investigated. Determination of the binding of two radiolabeled probes directed against the fibrinogen receptor, fibrinogen itself, and a monoclonal antibody (M.Ab.G10) revealed that the binding of fibrinogen to ADP-stimulated platelets was enhanced by 50% following the preincubation of platelets with 600 ng/ml CSA. Smaller, yet significant enhancement occurred in the presence of CSG. Similar results were obtained when M.Ab.G10 was used. Preincubation of platelets with 600 ng/ml CSA or CSG resulted in 60% and 30% increase in total protein kinase C activity, respectively, following the addition of ADP. In conclusion, this study has determined that preincubation of platelets with CSA or FK-506 (CSG, to a smaller extent) results in significant enhancement of ADP-induced platelet aggregation and secretion. The hyperaggregation and hypersecretion observed may be due to an enhanced expression of fibrinogen receptors on the platelet surface resulting from an increase in protein kinase C activity.
authors.affiliation: Department of Anatomy and Cell Biology, SUNY Health Science Center, Brooklyn 11203.
authors.forename: J B
authors.initials: JB
authors.lastname: Fernandes
authors.forename: U P
authors.initials: UP
authors.lastname: Naik
authors.forename: M S
authors.initials: MS
authors.lastname: Markell
authors.forename: E
authors.initials: E
authors.lastname: Kornecki
mesh_terms: D000911:Antibodies, Monoclonal; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism; D016572:Cyclosporine / Q000494:pharmacology*; D006801:Humans; D066298:In Vitro Techniques; D015539:Platelet Activation / Q000187:drug effects*; D010974:Platelet Aggregation / Q000187:drug effects; D011493:Protein Kinase C / Q000378:metabolism; D012701:Serotonin / Q000378:metabolism; D016559:Tacrolimus / Q000494:pharmacology*
chemical_list: D000911:Antibodies, Monoclonal; D012701:Serotonin; C045571:cyclosporin G; D016572:Cyclosporine; D011493:Protein Kinase C; D016559:Tacrolimus