Influence of some beta blockers (pindolol, atenolol, timolol and metoprolol) on aggregation and arachidonic acid metabolism in human platelets

The effects of four beta-adrenoceptor blocking agents on platelet aggregation, formation of thromboxane from exogenous arachidonic acid (AA) in platelets, on AA incorporation in platelet phospholipids, and on platelet phospholipase activity were studied. Of the four drugs pindolol inhibited thromboxane formation in a dose-related (0.25–1.0 mM) manner apparently by exerting its influence at the cyclooxygenase (CO) level. This drug also inhibited aggregation induced by AA, collagen, epinephrine and ADP. Atenolol and metoprolol though not showing inhibition of AA-induced aggregation did inhibit collagen- and ADP-induced aggregation; metoprolol reversed ADP-induced aggregation, and abolished second phase of epinephrine-induced aggregation. Timolol did not inhibit aggregation induced by all the aggregating agents. Atenolol inhibited (by ca. 1096) TxB₂ formation in platelets from exagenous as well as endogenous AA at rather high concentrations (1.0 mM). Metoprolol and Tmolol did not do so. The observations reported here can be best explained by taking into account the membrane stabilizing effects and lipophilic properties of the drugs.