Synergistic activation by collagen and 15-hydroxy-9 alpha,11 alpha-peroxidoprosta-5,13-dienoic acid (PGH2) of phosphatidylinositol metabolism and arachidonic acid release in human platelets.

Abstract

A B S T R A CT Collagen stimulates the activation of phosphatidylinositol (PI)-specific phospholipase C (EC 3.1.4.10) in human platelets, as manifested by the disappearance of PI, the transient formation of diacylglycerol (DG), and release of myoinositol. Platelets exposed to collagen also form lysophosphatidylinositol (LPI). Maximum formation of DG occurs within 60 s of the addition of collagen and is in proportion to the concentration of collagen provided, up to 100 Mg!2 X 109 platelets/ml. Hydrolysis of PI, formation of DG, and release of arachidonic acid are all inhibited -68% by aspirin or indomethacin, both of which inhibit platelet cyclooxygenase. This inhibition is reversed by the product of cyclooxygenase activity, 15-hydroxy -9a,lla -peroxidoprosta -5,13 -dienoic acid (PGH2), or by the PGH2 analogue and agonist, U-46619. The counteracting effects of either PGH2 or the PGH2 analogue can be blocked, in turn, by a PGH2 antagonist, U-51605. Neither PGH2 nor its stable analogue is, by itself, an efficient stimulus for PI breakdown to DG and LPI in platelets. However, in conjunction with collagen, these agents synergistically promote the net breakdown of PI and the release of arachidonic acid in aspirin-treated platelets. Our findings thereby imply that PGH2 has an important role in regulating both the release of its precursor, arachidonic acid, and the metabolism of PI induced by collagen.Dibutyryl cyclic AMP or prostaglandin D2 (PGD2), a prostaglandin that elevates concentrations of cAMP in platelets by stimulating adenylate cyclase, inhibits the hydrolysis of PI induced by collagen by 70%. The activation of PI metabolism by collagen appears to be

Enhancement of collagen‐induced phosphoinositide turnover by thromboxane A2 analogue through Ca2+ mobilization in human platelets

“…Presumably, TXA2 is generated during the action of collagen, increases cytoplasmic free Ca'+, and finally stimulates collagen-initiated phosphoinositide turnover. It is evident from our results as well as earlier observations [4,17] that Ca2+ mobilization alone is not sufficient and the occupation of the receptor with collagen is absolutely necessary for the induction of phosphoinositide turnover.…”

“…These arachidonic acid metabolites have been shown to induce secretion and aggregation [2,3]. Based on these observations, it has been suggested that PGH2 and TXA2 may play a role of crucial importance in the action of collagen [2- [4] have shown that PGH2 and its stable analogue U46619 induce phosphoinositide turnover to a small extent but counteract the inhibition by aspirin of collagen-induced phosphoinositide turnover.…”

Mechanism of Collagen Activation in Human Platelets

“…Collagen-induced Production of TXA Enhances the Increase in [Ca 2ϩ ] i and [Na ϩ ] i -TXA, produced subsequent to platelet activation with collagen (48), enhances collagen-induced activation (6,(13)(14)(15). Inhibiting TXA production with aspirin reduced the collagen-induced increase in [Ca 2ϩ ] i and aggregation and was almost completely abolished by the chelation of extracellular Ca 2ϩ .…”

“…At high concentrations, collagen activation of platelets has been shown to proceed through activation of phospholipase C␥2 and subsequent cleavage of phosphatidylinositol 4,5-bisphosphate into inositol 1,4,5-trisphosphate and 1,2-diacylglycerol (6,7). Inositol 1,4,5-trisphosphate induces the release of calcium from the dense tubular system (8,9), whereas 1,2-diacylglycerol activates protein kinase C (10).…”

“…At lower concentrations, many of the effects of collagen are enhanced by its production of thromboxane A 2 (TXA) 1 (6,(13)(14)(15). The collagen-induced increase in [Ca 2ϩ ] i can be decreased by inhibiting the production of TXA via the pretreatment of platelets with cyclooxygenase inhibitors such as aspirin (11,16,17).…”