The effects of some beta lactam antibiotics on (3H)-methyl-yohimbine binding to intact human platelets.

Several antibiotics have been reported to cause a bleeding diathesis in man, characterized by reduced platelet aggregation. We investigated the effects of several of the penicillins and of moxalactam on the binding of (3H)-methyl-yohimbine to intact human platelets. The (3H)-methyl-yohimbine binding met the criteria for interaction at an alpha2 adrenergic binding site and showed low interindividual variability. Penicillin G, ticarcillin, carbenicillin, piperacillin and moxalactam all inhibited (3H)-methyl-yohimbine binding, but at concentrations far in excess of clinically achievable plasma levels. We conclude that these compounds exert their antiplatelet effects by a mechanism other than competitive inhibition of catecholamine binding.