Procoagulant surface exposure and apoptosis in rabbit platelets: association with shortened survival and steady-state senescence

Background: The signal(s) for removal of senescent platelets from the circulation are not fully understood; phosphatidylserine (PS) expression on platelets and another marker of apoptosis, loss of mitochondrial inner membrane potential (DeltaPsi(m)), have been implicated in platelet clearance. Objective: To investigate whether shortened platelet survival and steady-state platelet senescence are associated with increased surface exposure of PS and DeltaPsi(m) collapse. Methods: Survival of in-vitro biotinylated rabbit platelets treated with thrombin or Ca2+-ionophore A23187 was tracked by flow cytometry after injection. Steady-state platelet senescence was investigated by infusing biotin to label a platelet cohort. PS expression and DeltaPsi(m) of in-vitro biotinylated platelets and of the aging platelet cohort biotinylated in-vivo were measured by flow cytometry using annexin V-FLUOS and the DeltaPsi(m)-sensitive dye CMXRos, respectively. Results: Although PS expression, DeltaPsi(m) and survival of thrombin-degranulated platelets were similar to those of control platelets, increasing concentrations of A23187 caused increased surface exposure of PS and progressive shortening of platelet survival; only one-sixth of PS-expressing platelets also exhibited DeltaPsi(m) loss. The cohort of senescent, biotinylated platelets remaining in the circulation at 96 h had increased exposure of PS and collapsed DeltaPsi(m); of the 17% of PS-expressing platelets, one-third did not exhibit DeltaPsi(m) loss. There was also an increase in platelets with collapsed DeltaPsi(m) but not expressing PS. Conclusions: Platelets with shortened survival and senescent platelets have increased surface exposure of PS, that may be involved in their clearance. PS expression can occur independently of DeltaPsi(m) collapse and conversely, in aged platelets, DeltaPsi(m) loss can occur independently of PS expression.