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 iloprost and the aggregating agent alpha-thrombin by measuring the internal calcium ion concentration ([Ca(2+)]i) of human fura-2-loaded platelets. Iloprost (0.003-100 micrograms/l) did not modify the resting calcium level; when added 2 minutes before exposure of the platelets to a submaximally active concentration of alpha-thrombin (10 U/l), iloprost dose-dependently antagonized the increase in [Ca(2+)]i. To evaluate if iloprost retained this antagonistic effect even after a prolonged contact, which is well known to cause a "desensitization" phenomenon, platelets were preincubated with iloprost (35 micrograms/l) for 3 hours. After washout, the effect of newly added iloprost (0.01-100 micrograms/l) on the alpha-thrombin-induced increase in [Ca(2+)]i was tested. Iloprost was still able to antagonize the increase in [Ca(2+)]i induced by alpha-thrombin in "desensitized" platelets; however, the dose-inhibitory response curve was significantly shifted to the right when compared with that obtained in control platelets (i.e., platelets preincubated for 3 hours with iloprost's solvent), and the resulting IC50 was significantly higher: 1.78 versus 0.2 micrograms/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 increase in [Ca(2+)]i induced by alpha-thrombin in desensitized platelets.
Cecchi
E
E
Department of Preclinical and Clinical Pharmacology, M. Aiazzi Mancini Firenze, Italy.
Capone
L
L
Ruocco
C
C
Fazzini
A
A
Mugelli
A
A
Giotti
A
A
Failli
P
P
D001792:Blood Platelets / Q000187:drug effects* / Q000378:metabolism; D002118:Calcium / Q000378:metabolism*; D006801:Humans; D016285:Iloprost / Q000494:pharmacology*; D010975:Platelet Aggregation Inhibitors / Q000494:pharmacology*; D013917:Thrombin / Q000037:antagonists & inhibitors*
D010975:Platelet Aggregation Inhibitors; D013917:Thrombin; D016285:Iloprost; D002118:Calcium