Abstract

Stimulation of platelets by various agents such as thrombin, collagen and A23187 leads to a rapid Ca²⁺ - dependent, phospholipase A₂-mediated mobilization of arachidonic acid from certain phospholipids and the biotransformation of this fatty acid to prostaglandin endoperoxides, thromboxanes A₂ and B₂, prostaglandins, hydroxyacids and malonyldialdehyde (MDA). Local anesthetics (dibucaine, tetracaine, benzocaine, and QX572), chlorpromazine and propranolol are very effective inhibitors of MDA formation induced by thrombin, collagen and A23187, but they do not block conversion of exogenous arachidonate to MDA. This result indicates that these drugs do not affect the enzyme prostaglandin synthetase responsible for the initial steps in arachidonate conversion to PG endoperoxides and thromboxanes. Utilizing platelets whose phospholipids were prelabeled with ¹⁴C-arachidonate and then stimulated with thrombin or A23187 we have found that local anesthetics, chlorpromazine and propranolol block: (a) formation of ¹⁴C-thromboxane B₂, ¹⁴C-prostaglandins and ¹⁴C-hydroxyacids (12-L-hydroxy-5,8,10,14-eicosatetraenoic acid and 12-L-hydroxy-5,8,10-heptadecatrienoic acid) from endogenous (phospholipid) ¹⁴C-archidonate; and (b) block the fall in the ¹⁴C-arachidonate content of platelet phospholipids, especially phosphatidyl choline. Local anesthetics, chlorpromazine and propranolol did not inhibit the conversion of ¹⁴C-arachidonate to ^l4C-l2-L-hydroxy-5,8,10,14-eicosatetraenoic acid by a soluble fraction from platelets containing lipoxygenase activity. These results conclusively demonstrate that local anesthetics, chlorpromazine and propranolol inhibit the activation of PLA₂ by stimuli, which is the initial and rate-limiting step in arachidonate metabolism. To some extent the ability of local anesthetics, chlorpromazine and propranolol to inhibit platelet aggregation and the release reaction (secretion of ADP, serotonin, etc.) can undoubtedly be attributed to their effect on PLA₂ activation which prevents formation of the aggregating agents PG endoperoxides and thromboxane A₂. However, other actions of these drugs, notably their ability to block aggregation induced by thromboxane A₂ and by other aggregating agents, under conditions which are independent of PG endoperoxide and thromboxane A₂ production, indicates additional mechanisms of action which can prevent cellular adhesion and exocytosis. These other actions may be related to non-arachidonate mediated effects on Ca²⁺-dependent phospholipases, antagonism of Ca²⁺-dependent phenomena necessary for membrane adhesion and fusion, or interference with the process of intracellular Ca²⁺ release by stimuli.