Evidence that Na+/H+ exchange regulates receptor-mediated phospholipase A2 activation in human platelets

• Published in: The Journal of biological chemistry 1986-07, Vol.261 (19), p.8667-8673
• Main Authors: Sweatt, J D, Connolly, T M, Cragoe, E J, Limbird, L E
• Format: Article
• Language: English
• Subjects: Adenosine Diphosphate - pharmacology; Arachidonic Acid; Arachidonic Acids - blood; Biological and medical sciences; Blood coagulation. Blood cells; Blood Platelets - drug effects; Blood Platelets - metabolism; Calcimycin - pharmacology; Carrier Proteins - blood; Chlortetracycline - pharmacology; Enzyme Activation; Epinephrine - pharmacology; Fundamental and applied biological sciences. Psychology; Humans; Kinetics; Lysophospholipids; Methylamines - pharmacology; Molecular and cellular biology; Phosphates - blood; Phosphatidylinositols - biosynthesis; Phosphatidylinositols - blood; Phospholipases - blood; Phospholipases A - blood; Phospholipases A2; Platelet; Sodium-Hydrogen Exchangers
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1016/S0021-9258(19)84432-0

Data in the previous paper suggest that epinephrine can mobilize a small pool of arachidonic acid via an enzymatic pathway distinct from phospholipase C and that this pathway is blocked by perturbations that block Na+/H+ exchange. The present studies demonstrate that epinephrine and ADP stimulate a phosphatidylinositol-hydrolyzing phospholipase A2 activity in human platelets. This occurs even when measurable phospholipase C activation, platelet secretion, and secondary aggregation are blocked with the thromboxane A2 receptor antagonist SQ29548. Furthermore, perturbants of Na+/H+ exchange diminish lysophosphatidylinositol production in response to epinephrine, ADP, and thrombin, but not to the Ca2+ ionophore A23187. Artificial alkalinization of the platelet interior with methylamine reverses the effect of the Na+/H+ antiporter inhibitor, ethylisopropylamiloride, on thrombin-stimulated lysolipid production, suggesting that the alkalinization of the platelet interior which would occur secondary to activation of Na+/H+ exchange might play an important role in phospholipase A2 activation. In addition, treatment of platelets with methylamine increases the sensitivity of phospholipase A2 to activation by the Ca2+ ionophore A23187, suggesting that changes in pH and Ca2+ may regulate phospholipase A2 activity synergistically. Finally, epinephrine causes a prompt decrease in platelet-chlortetracyclin fluorescence even in the presence of cyclooxygenase inhibitors, suggesting that epinephrine is able to mobilize membrane-bound Ca2+ independent of phospholipase C activation. Taken together, the data suggest that epinephrine-provoked stimulation of phospholipase A2 activity may occur as a result of Ca2+ mobilization and a concomitant intraplatelet alkalinization resulting from accelerated Na+/H+ exchange.