Comparison of Phorbol Ester-induced Responses in Rabbit and Human Platelets and Difference in Shape Change

Phorbol 12,13-dibutyrate (PDBu) induced aggregation and ATP release of washed human and rabbit platelets in a concentration-dependent manner. The EC(50) for PDBu-induced aggregation of washed human and rabbit platelets were 67.6 ± 3.3 and 32.7 ± 5.9 nM, respectively. PDBu and l-oleoyl-2-acetylglycerol (OAG) also caused a shape change in rabbit platelets detected by microscopic and turbidimetric techniques. Both 20K and 47K proteins of rabbit platelets were phosphorylated by PDBu treatment. All the PDBu-induced responses of human and rabbit platelets were blocked by a protein kinase C inhibitor, staurosporine, in a concentration-dependent manner. In both human and rabbit platelets, PDBu induced no phosphoinositide breakdown, caused no increase of [Ca(2+)](f) and induced no formation of TxB(2). Indomethacin, BN52021, nitroprusside and cytochalasin B did not inhibit, whereas verapamil, prostaglandin E, and EGTA inhibited PDBu-induced aggregation of washed human and rabbit platelets. Apyrase and creatine phosphate/creatine phosphokinase inhibited PDBu-induced aggregation, but did not affect the shape change or 20K myosin light chain phosphorylation of rabbit platelets. In Ca(2+)-free Tyrode's solution containing 1 mM EGTA, PDBu induced a shape change in rabbit platelets and this effect was inhibited by staurosporine and cytochalasin B. It is concluded that PDBu and OAG induce a shape change in rabbit platelets probably through phosphorylation of 20K myosin light chain.