Diversity of signaling underlying responses mediated by selective activation of the platelet thromboxane A2 (TXA2) receptor; pharmacological and kinetic studies in vitro.
Selective activation of the platelet TXA2 receptor is sufficient to mediate concurrent aggregation, deaggregation and shape change (SC) responses without activation of known Gi-coupled receptors (Platelets 2003; 14: 89). However, Gi-coupled receptor activation strongly influences the hemostasis response in vivo. This study investigated the modulatory effects of two signaling pathways related to Gi-coupled receptor activation, stimulation of phosphoinositide 3-kinases (PI3Ks) and inhibition of adenylyl cyclase (AC), on the aggregation, deaggregation and SC components of the platelet activation response. A novel turbidimetric approach was applied to separate these responses and to characterize their pharmacology and kinetics. The SC response was more sensitive to TXA2 receptor activation (lower EC50 value) but less sensitive to a TXA2 receptor antagonist (higher Kd value) than the net aggregation response. Epinephrine and sulprostone, agonists of the Gi-coupled alpha2A-adrenoceptor and EP3 receptor, respectively, amplified the SC, decelerated deaggregation and enhanced net aggregation responses. SQ22536 and 2',5'-dideoxyadenosine, inhibitors of AC activity, elicited smaller but qualitatively similar effects. The PI3K inhibitor wortmannin did not affect the SC response but accelerated deaggregation and inhibited net aggregation. These data are consistent with a differential modulation of the platelet SC response by each pathway associated with Gi-coupled receptor activation, while both pathways cooperatively enhance the net aggregation response by decelerating deaggregation. We propose that the TXA2 receptor mediated concurrent platelet aggregation and SC responses, that are differentially modulated by different signaling pathways, provide a model for studying the underlying cellular pharmacology of platelet physiology.
Maayani
Saul
S
Department of Anesthesiology, The Mount Sinai School of Medicine, New York, NY 10029, USA. saul.maayani@mssm.edu
Schwarz
Todd E
TE
Patel
Nayana D
ND
Craddock-Royal
Barbara D
BD
Tagliente
Thomas M
TM
D019796:15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5,13-dienoic Acid / Q000494:pharmacology; D000730:Androstadienes / Q000494:pharmacology; D001241:Aspirin / Q000494:pharmacology; D001792:Blood Platelets / Q000166:cytology / Q000502:physiology*; D048429:Cell Size; D004837:Epinephrine / Q000494:pharmacology; D006801:Humans; D007700:Kinetics; D015539:Platelet Activation / Q000502:physiology*; D010974:Platelet Aggregation / Q000187:drug effects / Q000502:physiology*; D044244:Receptors, Thromboxane A2, Prostaglandin H2 / Q000097:blood / Q000187:drug effects / Q000502:physiology*; D012016:Reference Values; D000077191:Wortmannin
D000730:Androstadienes; D044244:Receptors, Thromboxane A2, Prostaglandin H2; D019796:15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5,13-dienoic Acid; D001241:Aspirin; D000077191:Wortmannin; D004837:Epinephrine