Evidence that Na+/H+ exchange regulates receptor-mediated phospholipase A2 activation in human platelets.
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.
D000244:Adenosine Diphosphate / Q000494:pharmacology; D016718:Arachidonic Acid; D001095:Arachidonic Acids / Q000097:blood; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism*; D000001:Calcimycin / Q000494:pharmacology; D002352:Carrier Proteins / Q000097:blood*; D002751:Chlortetracycline / Q000494:pharmacology; D004789:Enzyme Activation; D004837:Epinephrine / Q000494:pharmacology; D006801:Humans; D007700:Kinetics; D008246:Lysophospholipids; D008744:Methylamines / Q000494:pharmacology; D010710:Phosphates / Q000097:blood; D010716:Phosphatidylinositols / Q000096:biosynthesis / Q000097:blood; D010740:Phospholipases / Q000097:blood*; D010741:Phospholipases A / Q000097:blood*; D054467:Phospholipases A2; D017923:Sodium-Hydrogen Exchangers
D001095:Arachidonic Acids; D002352:Carrier Proteins; D008246:Lysophospholipids; D008744:Methylamines; D010710:Phosphates; D010716:Phosphatidylinositols; D017923:Sodium-Hydrogen Exchangers; C025449:lysophosphatidylinositol; D016718:Arachidonic Acid; D000001:Calcimycin; D000244:Adenosine Diphosphate; C027451:methylamine; D010740:Phospholipases; D010741:Phospholipases A; D054467:Phospholipases A2; D002751:Chlortetracycline; D004837:Epinephrine