Differences between platelet phosphoinositide metabolism stimulated by thrombin or SFLLRN are not accounted for by interaction of thrombin with glycoprotein Ib

Abstract

The formation of inositol phosphates was compared in aspirin-treated, washed human platelets suspended in Tyrode's-albumin solution containing 2 mM calcium and stimulated with SFLLRN (thrombin receptor-activating peptide) or thrombin. SFLLRN (20 µM) and thrombin (1 U/ml) resulted in maximal irreversible aggregation and 80-90% secretion of dense granule contents. SFLLRN (50-100 µM) caused larger increases at 10 sec than 20 µM SFLLRN in the formation of inositol trisphosphate (IP 3 , measured as [ 3 H]inositol label). These increases were not significantly less than those caused by thrombin (1 unit/ml). However, whereas the labeling of IP 3 increased from 10-60 sec with thrombin, with SFLLRN it was much less at 60 sec than that at 10 sec. The decrease was not due to degradation of SFLLRN by ectopeptidases, since it was not prevented by amastatin, an inhibitor of ectopeptidases. Degradation of glycoprotein Ib (GPIb) with an O-sialogly-coprotein endopeptidase did not affect the thrombin-stimulated labeling of inositol phosphates , indicating that binding to GPIb is not involved in the sustained thrombin-induced formation of inositol phosphates. The finding that the thrombin-stimulated formation of IP 3 was not dependent on Ca 2+ in the medium (EGTA added) indicates that the transient SFLLRN-induced formation of IP 3 is not due to failure to cause Ca 2+ influx. The finding that formation of IP 3 was transient in SFLLRN-stimulated platelets, whereas platelet ag-gregation and secretion were maximal, indicates that the sustained activation of phos-pholipase C caused by thrombin may have roles related to later processes in which platelets participate. Am.

Apolipoprotein(a) Enhances Platelet Responses to the Thrombin Receptor–Activating Peptide SFLLRN

“…For example, primary ADP-induced aggregation, which is not influenced by r-apo(a), does not involve the activation of intracellular phospholipase C, 46 whereas SFLLRN-induced responses, which are enhanced by both r-apo(a) and Lp(a), involve activation of phospholipase C, leading to hydrolysis of phosphatidylinositol 4,5-bisphosphate. 47,48 Stimulation of platelets with thrombin also activates phospholipase C, and Gries et al 16 reported that maximal platelet aggregation stimulated by 0.1 U/mL thrombin was not affected by Lp(a). However, it is not possible to observe an enhancement of a response that is already maximal.…”

Protease-activated receptor 1 is the primary mediator of thrombin-stimulated platelet procoagulant activity

“…Several discrepancies in platelet responses have been noted when platelets are stimulated by thrombin versus SFLLRN in vitro (8)(9)(10)(11)(12). These differences may be due in part to the presence of both PAR1 and PAR4 on human platelets (5,6).…”