Secretion of β-thromboglobulin and Serotonin from Human Platelets Induced by Microbubbles Differs from Secretion...

The effect of nitrogen (N(2))-microbubbles on human platelets resembles that of common agonists in terms of aggregation, but displays one unusual feature in that cyclooxygenase-inhibitors (e.g. aspirin) poorly inhibit the reaction. In the present study, we have investigated the cyclooxygenase-dependence of microbubble-induced platelet secretion of β-thromboglobulin (β-TG) and (14)C-serotonin. The effect of microbubbles in stirred platelet-rich plasma (PRP) was compared with that of collagen and of polystyrene microspheres. The reduction in the number of single platelets induced by microbubbles was continuous with time and reached 60% after 30 min and was accompanied by secretion of (14)C-serotonin and β-TG. While 50% of the secretion of β-TG persisted in spite of aspirin treatment, secretion of (14)C-serotonin was completely abolished. The enhancing effect of epinephrine on microbubble-induced platelet secretion appeared to be prostanoid dependent. Platelet secretion of β-TG and (14)C-serotonin induced by a high dose of collagen was significantly reduced by aspirin, while a low dose of collagen induced a cyclooxygenase-independent secretion of β-TG and (14)C-serotonin. The reduction in single platelets when PRP was stirred with microspheres reached a mean value of 27% after 10 min but did not increase further with time. Microspheres caused a small but significant secretion of β-TG but no secretion of (14)C-serotonin. Aspirin did not reduce the loss in single platelets nor the secretion of β-TG. The present study suggests that cyclooxygenase independent α-granule secretion may facilitate and, at least in part, sustain the reaction between microbubbles and platelets. The difference in agonistic effect of polystyrene microspheres on platelets shows that polystyrene spheres are not suitable substitutes for microbubbles in in vitro or in vivo experiments.