Differences in thromboxane A2 synthesis by megakaryocytes and platelets.

The capacity for thromboxane A2 synthesis in response to exogenous arachidonic acid, calcium ionophore A23187, thrombin, and collagen was studied during megakaryocyte maturation. Studies were performed in (1) isolated megakaryocytes not separated, (2) isolated megakaryocytes separated into subgroups at different stages of maturation, and (3) washed platelets. When comparisons were based on equal amounts of cell protein (10(5) megakaryocytes vs 10(8) platelets), isolated megakaryocytes, not separated into subgroups, responded to exogenous arachidonic acid with synthesis of thromboxane A2 equal to that of platelets from the same animals at their respective times of maximum synthesis (30 minutes vs 10 minutes). In similar fashion, megakaryocytes and platelets synthesized thromboxane A2 from endogenous arachidonic acid at the same minimum concentration of A23187, 0.1 mumol/L, and showed equal maximum synthesis at 1 mumol/L (167 +/- 9 pmol and 150 +/- 18 pmol, respectively). In contrast, maximum thromboxane A2 synthesis in response to thrombin (10 U/ml) was three times higher in platelets than in megakaryocytes (230 +/- 15 pmol and 74 +/- 5 pmol, respectively), and synthesis in response to collagen (20 micrograms/ml) was 20 times higher in platelets (130 +/- 20 pmol vs 7 +/- 1.2 pmol). When synthesis was studied in isolated megakaryocytes at different stages of maturation, the capacity for thromboxane A2 synthesis was established in immature megakaryocytes but was not fully developed in the most immature megakaryocytes. Synthesis in response to thrombin was not significantly enhanced by megakaryocyte maturation. Thus the ability to metabolize arachidonic acid occurs early during megakaryocyte maturation, but the ability to respond to thrombin and collagen is only fully established in platelets.