Stimulus-aggregation coupling in platelets activated with PAF-acether.
Contrary to most agonists, platelet-activating factor (PAF-acether) induces a more pronounced aggregation at 22 degrees C than at 37 degrees C. A possible explanation was sought in the mechanism that couples the PAF-acether-receptor complex with exposure and occupation of fibrinogen binding sites. Comparison of studies performed at 37 degrees C with those at 22 degrees C revealed: a faster binding of [3H]PAF-acether to its receptors; more accumulation of 32P-labelled phosphatidylinositol 4-monophosphate and a slower but more abundant formation of phosphatidic acid that lasted for 5 min; a 1.4-fold increase in phosphorylation of the Mr 47,000 protein and a 2-fold increase in phosphorylation of the myosin light chain. In contrast, less secretion occurred and less [32P]phosphatidylinositol accumulated at 22 degrees C than at 37 degrees C, and also the increase in cytosolic Ca2+ content and the formation of thromboxane B2 were considerably lower. No differences were found in [32P]phosphatidylinositol 4,5-bisphosphate formation and arachidonate metabolism. Fibrinogen binding studies revealed two types of binding at both temperatures, a high-affinity and a low-affinity binding. There were 6-fold more low-affinity binding sites at 22 degrees C than at 37 degrees C, whereas high-affinity binding did not change. These data suggest that the better aggregation found at 22 degrees C is the result of exposure of an increased number of fibrinogen binding sites. The increased protein phosphorylation and phosphatidic acid accumulation and the faster binding of PAF-acether to its receptors which accompany the better aggregation responses at 22 degrees C suggest that these processes are involved in the regulation of exposure of fibrinogen binding sites.
D016718:Arachidonic Acid; D001095:Arachidonic Acids / Q000097:blood; D001792:Blood Platelets / Q000378:metabolism; D002118:Calcium / Q000097:blood; D003600:Cytosol / Q000378:metabolism; D005340:Fibrinogen / Q000097:blood; D006801:Humans; D007700:Kinetics; D008970:Molecular Weight; D010716:Phosphatidylinositols / Q000097:blood; D010750:Phosphoproteins / Q000097:blood; D010766:Phosphorylation; D010972:Platelet Activating Factor / Q000502:physiology*; D010974:Platelet Aggregation; D013696:Temperature; D013929:Thromboxane B2 / Q000097:blood
D001095:Arachidonic Acids; D010716:Phosphatidylinositols; D010750:Phosphoproteins; D010972:Platelet Activating Factor; D016718:Arachidonic Acid; D013929:Thromboxane B2; D005340:Fibrinogen; D002118:Calcium