Desensitization of platelet-activating factor-stimulated protein phosphorylation in platelets

Treatment of 32P-labeled rabbit platelets with platelet-activating factor (PAF) caused a time- and dose-dependent phosphorylation of several proteins including five major phosphorylated proteins with apparent molecular weights of 20,000, 35,000, 40,000, 65,000, and 150,000. Both PAF and thrombin caused a rapid increase followed by a decrease in phosphorylation of proteins, indicating the occurrence of a phosphorylation-dephosphorylation process. Four separate PAF receptor antagonists, CV-3988, CV-6209, SRI-63-441, and SRI-63-675 drastically reduced the PAF-stimulated protein phosphorylation. The order of potency was SRI-63675 greater than SRI-63441 greater than or equal to CV-6209 greater than CV-3988. These antagonists had no effect on thrombin-stimulated protein phosphorylation. Pretreatment of platelets with PAF (0.1 nM) completely abolished any further protein phosphorylation by the same concentration of PAF. PAF pretreatment shifted the dose response of protein phosphorylation by about 2 log units, to the right. When platelets were treated with PAF (10 nM) for 10 min, this abolished phosphorylation of proteins by any concentration of PAF. These studies indicated a homologous desensitization of protein phosphorylation. Interestingly, PAF-pretreated platelets still exhibited phosphorylation of proteins by thrombin. On the other hand, a lack of protein phosphorylation by PAF or thrombin was observed in platelets preexposed to thrombin and this demonstrated a heterologous desensitization. It is concluded that phosphorylation of proteins by PAF is a PAF receptor-coupled event and that this process is desensitized in platelets preexposed to PAF. The fact that both the activation of phosphoinositide-specific phospholipase C and the phosphorylation of proteins are desensitized in PAF-pretreated platelets suggests that a close "regulatory" intercommunication between these processes exists.