Journal
BIOCHIMICA ET BIOPHYSICA ACTA-MOLECULAR AND CELL BIOLOGY OF LIPIDS
Volume 1761, Issue 2, Pages 195-204Publisher
ELSEVIER
DOI: 10.1016/j.bbalip.2005.12.008
Keywords
platelet; thrombin; phosphatidylserine; Na+/H+ exchanger (NHE) phosphatidylinositol 4,5-bisphosphate (PIP2)
Funding
- NIAMS NIH HHS [AR38910, R01 AR038910, R01 AR038910-12] Funding Source: Medline
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Platelet membrane phosphaticlylserine (PS) exposure that regulates the production of thrombin represents an important link between platelet activation and the coagulation cascade. Here, we have evaluated the involvement of the Na+/H+ exchanger (NEE) in this process in human platelets. PS exposure induced in human platelets by thrombin, TRAP, collagen or TRAP(+) collagen was abolished in a Na+-free medium. Inhibition of the Na+/H+ exchanger (NHE) by 5-(N-Ethyl-N-Isopropyl) Amiloride (EIPA) reduced significantly PS exposure, whereas monensin or nigericin, which mimic or cause activation of NHE, respectively, reproduced the agonist effect. These data suggest a role for Na+ influx through NHE activation in the mechanism of PS exposure. This newly identified pathway does not discount a role for Ca whose cytosolic concentration varies together with that of Na+ after agonist stimulation. Ca2+ deprivation from the incubation medium only attenuated PS exposure induced by thrombin, measured from the uptake of FM1 -43 (a marker of phospholipid scrambling independent of external Ca2+). Surprisingly, removal of external Ca2+ partially reduced FMI-43 uptake induced by A23187, known as a Ca2+ ionophore. The residual effect can be attributed to an increase in [Na+], mediated by the ionophore due to a lack of its specificity. Finally, phosphatidylinositol 4,5-bisphosphate (PIP,), previously reported as a target for Ca2+ in the induction of phospholipid scrambling, was involved in PS exposure through a regulation of NHE activity. All these results would indicate that the mechanism that results in PS exposure uses redundant pathways inextricably linked to the physio-pathological requirements of this process. (c) 2005 Elsevier B.V. All rights reserved.
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