COMPARATIVE INVESTIGATION OF THE EFFECTS OF THE IMMUNOSUPPRESSANTS CYCLOSPORINE-A, CYCLOSPORINE-G, AND FK-506 ON PLATELET ACTIVATION

We have determined that ADP-induced platelet aggregation and secretion are enhanced by preincubation of human platelets with the immunosuppr essant cyclosporine A (CSA) (see accompanying article by Naik et al., 1993). In the present report we compare the effects of CSA with two ot her potent immunosuppressants, cyclosporine G (CSG), an analogue of CS A, and FK-506, on platelet activation. Preincubation of platelets with either CSA, CSG, or FK-506 resulted in platelets which exhibited hype raggregability when stimulated by ADP. CSG produced the lowest degree of hyperaggregation, whereas FK-506 produced the highest at an equival ent concentration. All three compounds enhanced the secretion of serot onin, with FK-506 producing the highest degree of serotonin release an d CSG producing the lowest amount. Platelet hyperaggregation and enhan ced 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 plate let aggregation, whereas CSG enhancement was only 20%. At a concentrat ion of 600 ng/ml, CSA produced 120% enhancement of ADP-induced platele t aggregation, whereas CSG produced only a 30% enhancement. Several po tential mechanisms responsible for the enhanced ADP-induced aggregatio n and secretion were investigated. Determination of the binding of two radiolabeled probes directed against the fibrinogen receptor, fibrino gen itself, and a monoclonal antibody (M.Ab.G10) revealed that the bin ding of fibrinogen to ADP-stimulated platelets was enhanced by 50% fol lowing the preincubation of platelets with 600 ng/ml CSA. Smaller, yet significant enhancement occurred in the presence of CSG. Similar resu lts were obtained when M.Ab.G10 was used. Preincubation of platelets w ith 600 ng/ml CSA or CSG resulted in 60% and 30% increase in total pro tein kinase C activity, respectively, following the addition of ADP. I n conclusion, this study has determined that preincubation of platelet s with CSA or FK-506 (CSG, to a smaller extent) results in significant enhancement of ADP-induced platelet aggregation and secretion. The hy peraggregation and hypersecretion observed may be due to an enhanced e xpression of fibrinogen receptors on the platelet surface resulting fr om an increase in protein kinase C activity.