Accumulation of the inositol phosphates in thrombin-stimulated, washed rabbit platelets in the presence of lithium

Abstract

Experiments with washed rabbit platelets demonstrate that stimulation with a low concentration of thrombin (0.1 unit/ml), that causes maximal aggregation and partial release of amine granule contents, also causes increased accumulation of [3H]inositollabelled inositol trisphosphate (InsP3) in the presence of 20mM-Li+. This concentration of Li+ was found to inhibit the degradation of inositol phosphates by phosphomonoesterases. This result indicates that phosphatidylinositol 4,5-bisphosphate [Ptdlns(4,5)P2] is degraded early after platelet stimulation with thrombin, although in a previous study we had found no decrease in amount. In the absence of Li+, the labelling of inositol bisphosphate (InsP2) increased more rapidly than that of InsP3, consistent with rapid degradation of InsP3 by phosphomonoesterase. After 30s the increase in InsP2 was augmented by Li+. This increase in InsP2 could have been due to increased degradation of phosphatidylinositol 4-phosphate or inhibition of breakdown of InsP2 to InsP with a lesser inhibition of breakdown of InsP3 to InsP2. The effect on InsP3 and InsP2 of stimulation of the platelets with 1.0 unit of thrombin/ml was comparable with the effect of the lower concentration of thrombin. Inositol phosphate (InsP) labelling did not increase in response to 0.1 unit of thrombin/ml, but increased when the platelets were stimulated with 1.0 unit of thrombin/ml. Whether the increase in InsP was due to increased degradation of phosphatidylinositol or a greater rate of breakdown of InsP2 to InsP than InsP to inositol cannot be determined in these experiments. These results indicate that degradation of Ptdlns(4,5)P2 is an early event in platelet activation by thrombin and that formation of inositol phosphates and 1,2-diacylglycerol rather than a decrease in Ptdlns(4,5)P2 may be the important change.

Ni‐mediated inhibition of human platelet adenylate cyclase by thrombin

“…With regard to the functional responses observed in intact platelets after stimulation with thrombin, e.g. platelet shape change, secretion, aggregation or increase in calcium concentration and phosphoinositide metabolism [28], further studies are required to answer the question whether and how the observed adenylate cyclase inhibition is related to these functional changes.…”

Involvement of phosphoinositide metabolism in potentiation by adrenaline of ADP-induced aggregation of rabbit platelets

“…ADP, which stimulates platelet aggregation without causing the secretion of platelet dense-granule contents, causes early changes in phosphoinositides; some of these changes may be due to elevation of cytosolic [Ca2+] (Lloyd et al, 1972(Lloyd et al, , 1973aLeung et al, 1983;Vickers et al, 1982Vickers et al, , 1986a. Turnover of PI has been found to be associated with stimulation of platelets by adrenaline (Deykin & Snyder, 1973;Wallace et al, 1982), although the times of incubation were long, making it difficult to link the PI changes with aggregation.…”

Association of phosphatidylinositol kinase and phosphatidylinositol 4‐phosphate kinase activities with the cytoskeleton in human platelets

“…Firstly, there have been numerous studies describing its hydrolysis by phospholipase C as one of the earliest events following the binding of a variety of agonists to their membrane receptors [1][2][3][4][5][6][7]. This contributes to an increase of intracytoplasmic Ca 2÷ [8,9] and to activation of calmodulin-dependent protein kinase and protein kinase C [10].…”