A significant concentration-dependent difference was found between beta-adrenoceptor blocking drugs in their ability to inhibit A23187-induced isolated platelet aggregation. In the absence of extracellular calcium ions the following rank order of potency to inhibit calcium ionophore stimulated platelet aggregation was shown: propranolol > bevantolol > alprenolol > metipranolol > oxprenolol > atenolol > pindolol > metoprolol approximately sotalol approximately practolol. The interruption of induced aggregation, as well as inhibition of aggregation, in the absence of extracellular calcium ions indicated interference of inhibitory beta-adrenoceptor blocking drugs with intramembrane or intraplatelet calcium pools activated with A23187. This suggestion was supported by the reversal of the inhibitory effect of beta-adrenoceptor blocking drugs in the presence of extracellular calcium ions. The effect was dose dependent and occurred within 30 sec after calcium administration. The results indicated that inhibitory beta-adrenoceptor blocking drugs, possessing a cationic amphiphilic structure, suppressed calcium mobilization in A23187-stimulated platelets, most probably after entering platelets. This explains why lipophilic drugs are more effective than hydrophilic ones in calcium ionophore A23187-stimulated platelets.
Adrenergic beta-Antagonists Animals