Previous Page that neither the affinity nor the number of [125I]-BOP platelet binding sites was altered by pretreatment with alkaline phosphatase (164). Alkaline phosphatase significantly decreased I-BOP-stimulated GTPase activity, and addition of inorganic phosphate to platelets exposed to alkaline phosphatase and I-BOP resulted in aggregation within seconds, plus restoration of increased I-BOP-induced GTPase activity. These studies indicated that the phosphor-ylation state of TP receptors, or closely associated proteins, influenced TP receptor-mediated platelet activation, but the mechanism responsible was not defined.

In addition to the serine-threonine phosphorylation sites modulated by PKC and PKA, a basal state of tyrosine phosphorylation exists in platelet TP receptors, as in bradykinin receptors, and this phosphorylation increases on agonist stimulation (166). The tyrosine kinase responsible for this phosphorylation, as well as that of the parallel phosphorylation of phosphatidylinositol 34rinase (PI3-kinase), that occurs on agonist stimulation, is unknown, but p27,1'lt• known to be activated by TP receptor agonists (167), is a candidate (166). It is also possible that PI} kinase itself might phosphorylate TP receptors. The functional role of tyrosine kinase-mediated phosphorylation of TP receptors is unknown.

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