Albumin dependence of Sph-1-P release from activated platelets.

(A) Platelet suspensions, containing 1% BSA (lower lanes) or not (upper lanes), were incubated with [³H]Sph for 6 minutes and further stimulated without (a and d) or with 0.5 U/mL of thrombin (b and c) or 1 μmol/L TPA (e and f) for 5 minutes. In c and f, 1 μmol/L staurosporine was added 2 minutes before stimulation; this protein kinase inhibitor, by itself, failed to affect Sph-1-P release. After incubation, samples were centrifuged, and lipids were extracted from the resultant medium supernatant (M) and cell pellet (C) to detect [³H]Sph-1-P release from platelets. (B) Thrombin- or TPA-induced [³H]Sph-1-P release from platelets pretreated without or with staurosporine in the absence (■) or presence (▪) of extracellular 1% BSA was calculated.