The role of extracellular Ca2+ and Na+ in paf-acether-induced human platelet activation.
The dependence of paf-acether (paf)-induced human platelet activation on extracellular Ca2+ and Na+ was examined by quantitating aggregation, secretion and thromboxane (Tx) formation in the presence of physiological and/or low concentrations of Ca2+ and/or Na+. In the presence of 2 mM Ca2+ and 140 mM Na+, paf induced a dose-dependent reversible aggregation and less than 25% of [14C]serotonin release. These responses were insensitive to aspirin or Tx antagonist SQ 29548 treatment and negligible amounts of Tx were formed. In low Ca2+ buffer, paf induced irreversible aggregation and the 14C-serotonin release could exceed 60%. These increases in platelet response were associated with the formation of Tx and were suppressed by aspirin and SQ 29548 treatments, or by substituting NaCl with N-methylglucamine hydrochloride. Thus, in low Ca2+ medium, Tx synthesis is favored during platelet activation and is dependent on Na+ concentrations. A decrease in extracellular Na+ inhibited the paf-acether-induced Tx synthesis observed in low Ca2+ medium but not that induced by the Tx direct precursor, arachidonic acid (AA). Therefore, the increase observed in low Ca2+ medium, no longer seen when the Na+ level is decreased is not related to an impairment of the cyclooxygenase activity but rather implicates an effect on the activity of phospholipase A2. A decrease in extracellular Na+ (2 mM Ca2+ present), inhibited [14C]serotonin release induced by paf from platelets which had, or had not been, treated with aspirin. In this medium, the AA-induced release reaction was also affected whereas Tx formation was not altered, thus suggesting that other mechanisms involved in platelet response apart from Tx synthesis are dependent on extracellular Na+.
Keraly
C L
CL
INSERM U 200, Clamart, France.
Delautier
D
D
Benveniste
J
J
D002118:Calcium / Q000502:physiology*; D006801:Humans; D066298:In Vitro Techniques; D010972:Platelet Activating Factor / Q000494:pharmacology*; D015539:Platelet Activation / Q000187:drug effects*; D010974:Platelet Aggregation / Q000187:drug effects; D012701:Serotonin / Q000378:metabolism; D012964:Sodium / Q000502:physiology*; D013931:Thromboxanes / Q000096:biosynthesis
D010972:Platelet Activating Factor; D013931:Thromboxanes; D012701:Serotonin; D012964:Sodium; D002118:Calcium