Platelets agonist stimulation

Agonist stimulation of platelet receptors induces various internal signaling pathways transduced from the membrane into the cytoplasm, which result in platelet receptors GPIIb/llla activation. 

Adenosine, in addition to serving as a substrate for the generation of cAMP plays a physiologic role as a platelet inhibitor and a vasodilator and may attenuate neutrophil-mediated damage to endothelial cells, Adenosine diphosphate (ADP)— a potent platelet agonist—is converted to adenosine, which is taken up rapidly by cells, especially erythrocytes and endothelial cells, A small proportion is metabolized to the aforementioned cyclic nucleotides. The remainder is broken down to inosine and subsequently to xanthine. Dipyridamole inhibits the active transport of adenosine into cells, but does not interfere with the passive diffusion. Since the platelet inhibitory effects of adenosine proceed via stimulation of adenylate cyclase, these effects can also be amplified by dipyridamole, In circulating blood, the largest amount of adenosine is found in red blood cells, This may, in part, help explain why dipyridamole is much more effective in whole blood than in plasma. 

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). 

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. 

Other human subjects with defective TP receptor agonist-stimulated platelet aggre-gation have been described (70-73), but a specific signal transduction abnormality has been defined in only one individual (73). 

All agonist-induced platelet responses, including shape change, aggregation, adhesion and secretion are inhibited by increased intracellular cAMP levels (1). Synthesis of cAMP in the platelet is stimulated by the binding of mediators, such as prostacyclin and adenosine, to cell surfece receptors ( ) coupled to GTP binding proteins (Fig. 1). 

Borsch-Haubold AG, Bartoli F, Asselin J, Dudler T, Kramer RM. Apitz-Castro R, Watson SP, Gelb MH. (1998). Identification of the phosphmylation sites of cytosolic phospholipase A2 in agonist-stimulated human platelets and HeLa cells. J. Biol. Chem. 273,4449-4458. 

In vivo, basal or agonist-stimulated release of NO result in inhibition of platelet... 

Burroughs SF, Johnson GJ. Beta-lactam antibiotics inhibit agonist-stimulated platelet calcium influx. Thromb Haemost 1993 69(5) 503-8. 

Epinephrine-stimulated Ca2 influx. Our results with D2O also provided evidence that a different platelet agonist i.e. epinephrine, does not act through the same mechanism of Ca2 mobilization as does ADP. In this regard, D2O was not found to be effective in blocking epinephrine-induced platelet activation (29). Therefore, we investigated the possibility that epinephrine, in contrast to ADP, increases platelet membrane permeability to Ca2. Using trace amounts of 45Ca it was demonstrated that epinephrine does in fact induce a transmembrane Ca2 influx which is concomitant with the... 

Complex intracellular signalling pathways (known as inside-out signalling) ensuing from stimulation by platelet agonists are thought ultimately... 

P2Y receptors are activated by adenine and uridine nucleotides. Most of the known P2Y receptors have been detected in the nervous system [21]. The majority of P2Y receptors inhibit neuronal N-type Ca2+ channels and M-type K+ channels. P2Y1 receptors are found exclusively on platelets, on their precursor megakaryocyte cells and on certain other cultured hematopoietic cells, such as K562 leukemia cells. They can be distinguished from other P2 receptors in that ADP is the most potent natural agonist and ATP is a competitive antagonist. ADP acts via a G protein to inhibit cyclic AMP accumulation, mobilize intracellular Ca2+ and stimulate granule secretion. ADP... 

GEA-3175 is more stable than GEA-3162 in vitro but still retains its biological activity [95]. The release of NO and NO2 by GEA 3175 was increased 140-fold in the presence of human plasma, as analyzed by ozone chemiluminescence [94]. GEA 3175 inhibited agonist-induced platelet aggregation and induced a more than 4-fold increase in platelet cGMP without affecting cAMP levels [94]. Thrombin-stimulated rises in the cytosolic free Ca2+ concentration and secretion were dose-dependently inhibited by GEA 3175. GEA 3175 showed a reduced capacity to inhibit platelet aggregation of uremic platelets compared to controls [96]. 

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. 

---