Primary role of calcium ions in arachidonic acid release from rat platelet membranes. Comparison with human platelet membranes

Abstract

The liberation of arachidonic acid (AA) was investigated in platelet membranes prelabelled with [3H]AA. In rat platelet membranes, Ca2+ at concentrations over several hundred nanomolar induced [3H]AA release, with a concurrent decrease in 3H radioactivity of phosphatidylethanolamine and phosphatidylcholine. Some 4-6% of total radioactivity incorporated into platelet membrane lipids was released at 1-10 microM-Ca2+, which is nearly equivalent to that attained in agonist-stimulated platelets. Formation of lysophospholipids in [3H]glycerol-labelled membranes and decrease in [3H]AA liberated by the phospholipase A2 inhibitors mepacrine and ONO-RS-082 suggest that [3H]AA release is mainly catalysed by phospholipase A2. In intact platelets agonist-stimulated [3H]AA release was markedly decreased in the absence of extracellular Ca2+ or in the presence of the intracellular Ca2+ chelator quin 2. These results indicate that in rat platelets the rise of intracellular Ca2+ plays a primary role in the activation of phospholipase A2. In contrast, Ca2+ even at high millimolar concentrations did not effectively stimulate [3H]AA release in human platelet membranes. Thus factor(s) additional to or independent of Ca2+ is required for the liberation of AA in human platelets.

Stimulated cholesterol-enriched platelets display increased cytosolic Ca2+ and phospholipase A activity independent of changes in inositol trisphosphates and agonist/receptor binding

“…The data in Table 2 would thus suggest another mechanism of PLA activation, perhaps potentiated by a rise in [Ca2+]i, which becomes more evident when AIF4is the agonist than when thrombin is the agonist. Several reports have suggested the possibility of PLA activation by Ca2+independent routes in human platelets [27,45,46] as well as by Ca2+-dependent routes [28,46,47], and regulation of PLA2 by a specific G-protein has been reported, as noted above in this Discussion [23,24]. The increased membrane microviscosity induced by cholesterol enrichment may promote G-protein/PLA interaction, resulting in larger accumulations of [3H]lysoPtdCho.…”

“…These authors have suggested that the phospholipid precursors of arachidonic acid might be alkylacyl-PtdCho or diacyl-phosphatidylethanolamine, the hydrolysis of the latter inducing an arachidonate transfer from diacyl-PtdCho. We cannot exclude the contribution of other phospholipid classes to arachidonate release in our study; however, it should be noted that PLA activation in human platelets differs from that in rat platelets [45], and our results (Table 1) indicate that the major lysoPtdCho that accumulates in thrombin-stimulated human platelets is the acyl, rather than the alkyl, form.…”

N‐(p‐Amylcinnamoyl)anthranilic Acid (ACA): A Phospholipase A2 Inhibitor and TRP Channel Blocker

“…With respect to the focus of this review on calcium‐permeable cation channels, it is of interest that both inhibitors have repeatedly been described to interfere with the receptor‐induced changes of [Ca 2+ ] i . In contrast to the consistent blocking effect of ONO‐RS‐082 on intracellular calcium signals in platelets (Francesconi et al 1995; Nakashima et al 1989; Tohmatsu et al 1989), the effects of ONO‐RS‐082 on intracellular calcium signals were nonhomogeneous in pancreatic acini (Nishino et al 1998; Tsunoda and Owyang, 1995). In VSMCs, ONO‐RS‐082 diminished receptor‐induced changes of [Ca 2+ ] i and subsequent calcium‐dependent phosphorylation of myosin light chains (Byron, 1996; Li et al 2001; Miura et al 1997; Shimomura et al 2004).…”

Roles of prostaglandin production and mitogen-activated protein kinase activation in hepatocyte growth factor—mediated rat hepatocyte proliferation

“…21.1 x lo4 t 1.9 X lo4 dpm, n = 24). The values are means SD (n = 4) from two experiments each performed in duplicate.…”