The activation by Ca2+ of platelet phospholipase A2. Effects of dibutyryl cyclic adenosine monophosphate and 8-(N,N-diethylamino)-octyl-3,4,5-trimethoxybenzoate.
Thrombin-induced release of arachidonic acid from human platelet phosphatidylcholine is found to be more than 90% impaired by incubation of platelets with 1 mM dibutyryl cyclic adenosine monophosphate (Bt2 cyclic AMP) or with 0.6 mM 8-(N,N-diethylamino)-octyl-3,4,5-trimethoxybenzoate (TMB-8), an intracellular calcium antagonist. Incorporation of arachidonic acid into platelet phospholipids is not enhanced by Bt2 cyclic AMP. The addition of external Ca2+ to thrombin-treated platelets incubated with Bt2 cyclic AMP or TMB-8 does not counteract the observed inhibition. However, when divalent cation ionophore A23187 is employed as an activating agent, much less inhibition is produced by Bt2 cyclic AMP or TMB-8. The inhibition which does result can be overcome by added Ca2+. Inhibition of arachidonic acid liberation by Bt2 cyclic AMP, but not by TMB-8, can be overcome by high concentrations of A23187. When Mg2+ is substituted for Ca2+, ionophore-induced release of arachidonic acid from phosphatidylcholine of inhibitor-free controls is depressed and inhibition by Bt2 cyclic AMP is slightly enhanced. The phospholipase A2 activity of platelet lysates is increased by the presence of added Ca2+, however, the addition of either A23187 or Bt2 cyclic AMP is without effect on this activity. We suggest that Bt2 cyclic AMP may promote a compartmentalization of Ca2+, thereby inhibiting phospholipase A activity. The compartmentalization may be overcome by ionophore. By contrast, TMB-8 may immobilize platelet Ca2+ stores in situ or restrict access of Ca2+ to phospholipase A in a manner not susceptible to reversal by high concentrations of ionophore.
D001095:Arachidonic Acids / Q000097:blood; D001792:Blood Platelets / Q000201:enzymology*; D003994:Bucladesine / Q000494:pharmacology; D002118:Calcium / Q000037:antagonists & inhibitors / Q000097:blood*; D002413:Cations, Divalent; D000242:Cyclic AMP / Q000494:pharmacology; D004789:Enzyme Activation / Q000187:drug effects; D005707:Gallic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D006801:Humans; D008274:Magnesium / Q000494:pharmacology; D010740:Phospholipases / Q000097:blood*; D013917:Thrombin / Q000378:metabolism
D001095:Arachidonic Acids; D002413:Cations, Divalent; D005707:Gallic Acid; D003994:Bucladesine; D000242:Cyclic AMP; D010740:Phospholipases; D013917:Thrombin; D008274:Magnesium; D002118:Calcium