ILOPROST ANTAGONIZES THE INCREASE IN INTERNAL CALCIUM-CONCENTRATION INDUCED BY ALPHA-THROMBIN IN HUMAN PLATELETS - A STUDY OF DESENSITIZATION

We studied the interaction between the synthetic prostacyclin analog i loprost and the aggregating agent alpha-thrombin by measuring the inte rnal calcium ion concentration ([Ca2+]i) of human fura-2-loaded platel ets. Iloprost (0.003-100 mu g/l) did not modify the resting calcium le vel; when added 2 minutes before exposure of the platelets to a submax imally active concentration of alpha-thrombin (10 U/l), iloprost dose- dependently antagonized the increase in [Ca2+]i. To evaluate if ilopro st retained this antagonistic effect even after a prolonged contact, w hich is well known to cause a ''desensitization'' phenomenon, platelet s were preincubated with iloprost (35 mu g/l) for 3 hours. After washo ut, the effect of newly added iloprost (0.01-100 mu g/l) on the alpha- thrombin-induced increase in [Ca2+]i was tested. Iloprost was still ab le to antagonize the increase in [Ca2+]i induced by ol-thrombin in ''d esensitized'' platelets; however, the dose-inhibitory response curve w as significantly shifted to the right when compared with that obtained in control platelets (i.e., platelets preincubated for 3 hours with i loprost's solvent), and the resulting IC50 was significantly higher: 1 .78 versus 0.2 mu g/l (p < 0.001). Since the maximal inhibitory effect of iloprost could also be reached under these experimental conditions , we conclude that iloprost retains its ability to antagonize the incr ease in [Ca2+]i induced by alpha-thrombin in desensitized platelets.