ADP-INDUCED, PAF-INDUCED AND ADRENALINE-INDUCED PLATELET-AGGREGATION AND THROMBOXANE FORMATION ARE NOT AFFECTED BY A THROMBOXANE RECEPTOR ANTAGONIST AT PHYSIOLOGICAL EXTERNAL CA++ CONCENTRATIONS

Thromboxane (TX) receptor antagonists are of considerable clinical interest in prevention of acute thrombembolic vessel occlusion. This study demonstrates that the selective TX receptor antagonist, daltroban, at a concentration (10 mu M) that does not inhibit TX synthesis, markedly inhibits ADP-, PAF- and adrenaline-induced platelet secretion and TX formation. With the exception of ADP-induced platelet secretion, these actions are only detectable in citrated platelet-rich plasma but not in plasma anticoagulated by hirudin. Since TX antagonists are supposed to act at physiological external Ca++ concentrations in the clinics, it is questionable whether in vitro studies in Ca++-deprived media are the optimum model to evaluate the clinical potential of these compounds.