TP Receptor Function

Studies of TP receptor binding have been carried out with several radioligands that have structural similarity to TXAj and PGHj. 

Studies of TP receptor agonist binding performed in other cells and tissues have revealed similar variability in regard to the presence of one or two binding affinities. Nonlinear analysis of [ I]-BOP binding to placental tissue indicated a superior fit to two affinity 

Secreted constituents of platelet storage granules, particularly ADP, amplify primary aggregation stimulated by weak agonists. An important event necessary for TP receptor- 

Earlier studies indicated that the G protein responsible for PLC-P activation was G, G or both G, and G (150). It was not possible to determine whether one or both ofthese G proteins was the precise mediator, since nearly all cells contain both Ga and Ga (126), and becattse it has been difficult to distinguish Ga from Ga by antisera, due to 

Transfected TP-a and TP-P receptors in CHO cells demonstrated different patterns of homologous desensitization. TP-a receptors, overexpressed in fibroblasts, decreased in number, as did native platelet receptors, following 24 hour exposure to I-BOP, but TP-P receptors increased following similar long-term exposure to I-BOP (160). Activation of PKC uncoupled calcitun mobilization from TP-P receptors to a greater extent than from TP-a receptors (160). These differences cannot be attributed to differences in phosphorylation, since both isoforms transfected into HEK cells were phosphorylated similarly on agonist stimulation (60). 

---

Despite the description of several examples of impaired functional and biochemical responses to TP receptor agonists in human and animal platelets (70-76), only one TP receptor mutation has been described (77). A single amino acid substitution (R -L) was found in hxrman TXR-a cDNA obtained by RT-PCR from platelets of patients with a dominantly inherited bleeding disorder (See V. ALTERED TP RECEPTOR FUNCTION, A. TP RECEPTOR MUTATION below). 

Hiosphoiylation of the third extracellular loop also appeared to play a role in TP receptor function (66,86) (Sec IV TP RECEPTOR FUNCTION, E. TP RECEPTOR PHOSPHORYLATION, 2.ROLE OF PHOSPHORYLATION IN TP RECEPTOR ACTIVATION below). 

Endothelium-dependent pulmonary artery contractions in response to arachidonic acid or methacholine appeared to be mediated by TXAj in rabbits (179). However, approximately 25% of normal New Zealand White rabbits ftuled to demonstrate pulmonary artery contraction in response to arachidonic acid, U46619 or I-BOP. These "unresponsive" rabbits were found to have only 23% of the TP receptors present on pulmonary and aortic crade membranes of "responsive" rabbits (179). Of interest was the feet that platelets fix>m "unresponsive" rabbits aggregated as well or better than platelets firom "responsive" animals vdien stirred with U46619, and no deficiency ofTP receptors nor binding affinity was found. These data sug sted that TP receptor expression may be tissue tecific. This conclusion was supported by the observation that ir bition of testosterone 5a-reductase in tats decreased TP receptor density on aortic membranes but not on platelets (180) (See V. ALTERED TP RECEPTOR FUNCTION, 3. DRUG EFFECTS, a. HORMONES below). 

TP receptor signaling has been extensively docnmented in vascnlar smooth mns-cle and platelets, but its characterization in hnman ASM cells has been more limited until recently. ASM cells express messenger RNA (mRNA) for both TP receptor isoforms, and functional receptors respond to agonist with an increase in intracellular Ca " concentration (200). As a consequence, besides potentiating the epidermal growth factor (EGF) mitogenic response independently from transactivation of the EGF receptor (EGER) (200), TP receptor stimulation induces a concentration-dependent increase in DNA synthesis. 

Burris TP, Pelton PD, Zhou L, et al. A novel method for analysis of nuclear receptor function at natural promoters peroxisome proliferator-activated receptor y agonist actions on a P2 gene expression detected using branched DNA messenger RNA quantitation. Mol Endocrinol 1999 13 410—417. 

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 pho horylation, as well as that of the parallel phosphorylation of phosphatidylinositol 34dnase (PIj4dnase), that occurs on agonist stimulation, is unknown, but p27 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. 

I-BOP-induced aortic contractions were decreased 50% in dexamethasone-treated rabbits, and this functional effect of corticosteroid administration was accompanied by an 25% reduction in aortic cell membrane TP receptor number (186). Although the functional and receptor changes were corrrelated, the magnitude of the reduction in receptor density was small. It is possible that the effects of corticosteroids on aortic contraction were due to an influence on post-receptor events as well as on receptors (186). 

---