Pharmacological intervention with platelet phospholipase A2.

Metabolites of arachidonic acid are important regulatory substances in blood platelets. They participate in platelet adhesion and aggregation, and pharmacological intervention with arachidonate cascade is widely used in therapy of hyperactive platelets and in the prevention of thromboembolic complications. To verify and compare the effect of cationic amphiphilic drugs (CAD) from different pharmacological groups on activation of platelet phospholipase A2--the essential enzyme of arachidonic pathway in blood platelets. Blood platelets were isolated from human and rat blood by differential centrifugation and aggregation was measured in pretreated and subsequently stimulated platelets in a dual channel aggregometer and recorded on a linear recorder. Activity of cytosolic phospholipase A2 (cPLA2) was determined by means of arachidonic acid liberation from platelet membrane phospholipids incorporated as triciated radionuclide. The radioactivity was determined by liquid scintillation method in Packard TriCarb 2500T. Cationic amphiphilic drugs of the beta-adrenoceptor blocking group inhibited platelet aggregation in the rank order of potency: propranolol > alprenolol > metipranolol > atenolol. Similarly did the H1-histamine antagonists bromadryl and dithiaden as well as the antimalarial chloroquine show antiplatelet effect in vitro in the rank order of potency: dithiaden > or = bromadryl > or = chloroquine. Dose-dependent inhibition of aggregation was followed by inhibition of arachidonic acid liberation from membrane phospholipids of platelets stimulated at receptor site (thrombin) or by a stimulus bypassing membrane receptors (Ca2+ ionophore A23187). The rank order potency for inhibition of stimulated 3H-AA liberation from membrane phospholipids was: a) for BAB drugs: propranolol > or = alprenolol > metipranolol, b) for other drugs: dithiaden > bromadryl > chloroquine. The drugs investigated interference with liberation of stimulated arachidonic acid showed nonspecific inhibition of platelet cytosolic phospholipase A2 by these drugs at intracellular level. The results revealed that besides inhibition of cyclooxygenase pathway, and of receptors for ADP and glycoproteins Gp IIb/IIIa, interaction of drugs with cPLA2 may represent a further site for antiplatelet action. (Fig. 5, Ref. 39.)