Increased aminophospholipid translocase activity in human platelets during secretion

Abstract

Flourescent labeled analogs of phosphatidylcholine (NBD-PC) and phosphatidylserine (NBD-PS) were used to study transport of phospholipids from the outer to the inner leaflet of the plasma membrane of human platelets. Platelets were stimulated with thrombin or Ca 2+-ionophore at various extracellular [Ca 2+]. No significant transport of NBD-PC could be observed either in resting or stimulated platelets. NBD-PS transport in platelets stimulated with thrombin (with or without extracellular Ca 2+), or ionophore in the presence of EGTA, was enhanced 4-fold ( t sol1 2 ≈ 2 min ) compared to unstimulated controls ( t sol1 2 ≈ 8 min ). Stimulation with ionophore at extracellular [Ca 2+] exceeding 8 μM caused a gradual decrease in inward transport of NBD-PS. At 100 μM Ca 2+, NBD-PS transport becomes as slow as that of NBD-PC. We conclude that platelet activation by agonists that induce secretion without appreciable shedding is accompanied by an increase in translocase activity that maintains asymmetry during fusion which occurs during exocytosis.