Alkaline phosphatase prevents platelet stimulation by thromboxane‐mimetics

Abstract

1 The effects of alkaline phosphatase on platelet aggregation, secretion and thromboxane B2 (TxB2) generation induced by the full dose-range of common platelet agonists were studied in human platelet-rich plasma and washed platelets. 2 Platelet aggregation and adenosine 5'-triphosphate (ATP) secretion induced by threshold and supramaximal concentrations of arachidonate and stable TxA2 and prostaglandin endoperoxide-mimetics (compounds U46619 and EP171) were abolished in the presence of alkaline phosphatase (0.5-1 u ml 1), even though the synthesis of TxB2 persisted. In contrast, platelet aggregation by PAF-acether and by supramaximal concentrations of thrombin as well as the primary wave of aggregation by adenosine diphosphate (ADP) and adrenaline were unaffected by alkaline phosphatase under conditions where the secondary wave of aggregation by ADP was blocked. 3 Alkaline phosphatase, unlike prostacyclin, failed to raise the adenosine 3': 5'-cyclic monophosphate (cyclic AMP) content of the platelets. Also, the pretreatment of platelets by inorganic phosphate or by ATP plus creatine phosphate/creatine phosphokinase reversed the inhibitory effect of alkaline phosphatase.4 Experiments performed in the guinea-pig in vivo showed that alkaline phosphatase was effective on thrombocytopenia induced by arachidonate. 5 Our results provide the first direct evidence for a specific inhibitory effect of alkaline phosphatase at a site sensitive to TxA2 and prostaglandin endoperoxides and suggest that its phosphorylation/ dephosphorylation state may play an important role in modulating platelet activation. These results also suggest the presence of ecto-protein kinases on membrane platelets.

Cellular localization of endothelial alkaline phosphatase reaction product and enzyme protein in the myocardium.

“…These findings point to a possible involvement of AP in antiinflammatory responses, which is supported by observations of an inhibitory effect of AP on stimulation of platelet aggregation by thomboxane mhetics (24). Although there is limited in vivo evidence to support this hypothesis, these results suggest a physiological role of AP on endothelial cell luminal surfaces as inhibitor of platelet aggregation.…”

Evidence for cAMP-dependent Platelet Ectoprotein Kinase Activity That Phosphorylates Platelet Glycoprotein IV (CD36)

“…The phosphorylation and dephosphorylation of proteins are critical to the regulation of cellular functions, particularly in blood platelets (4 -7). The dephosphorylating enzyme alkaline phosphatase, which removes phosphate groups from the external platelet membrane, prevents platelet aggregation and secretion by thromboxane mimetics (1). In addition, acid phosphatases that dephosphorylate the ectodomain of CD36, a collagen and thrombospondin receptor, decreased platelet aggregation to collagen and ADP (8).…”

“…It is well known that intracellular protein kinase A (PKA) 1 is also important in the regulation of various platelet functions (11)(12)(13), but ecto-PKA activity in the plasma membrane has not been demonstrated in blood platelets. The rate-limiting step for the ectophosphorylation activity depends on ATP and presumably on cAMP.…”

“…Tel: 33-1-40613498; Fax: 33-1-45688703. 1 The abbreviations used are: PKA, protein kinase A; PAGE, polyacrylamide gel electrophoresis; PGI2, prostaglandin I2 (prostacyclin); PKI, protein kinase A inhibitor; PRP, platelet-rich plasma. …”

Signal transduction involved in the platelet adenylate cyclase sensitization associated with PGH2/TxA2 receptor desensitization

“…Alkaline phosphatase, a non-specific dephosphorylating enzyme, can modulate platelet activation by arachidonic acid derivatives (Hatmi et al, 1991). This enzyme, which had no effect by itself, greatly enhanced platelet cAMP production in response to PGI 2 .…”