Comparison of the effects of drugs on the aggregation of hamster platelets in vivo and in vitro.

1. A method is described for measuring the inhibitory effectiveness of drugs on the aggregation by ADP of hamster platelets in vivo.2. The method was used to compare the effects of several drugs, viz. adenosine, imipramine, desmethylimipramine and aspirin, on platelet aggregation in vivo with their in vitro effects measured photometrically.3. The concentrations of adenosine and imipramine present in the cheek pouch after 10 min infusions were measured using radioactively labelled drugs.4. The results show that adenosine (0.4 muM) inhibited platelet aggregation in vivo by 43%, whereas several times this concentration was required to produce the same inhibition in vitro.5. Imipramine and desmethylimipramine (0.4 muM) did not inhibit platelet aggregation in vivo; in vitro, however, desmethylimipramine caused up to 34% inhibition at concentrations as low at 0.25 muM.6. Aspirin (estimated 0.2 mM) inhibited platelet aggregation in vivo by 37% whereas similar inhibition in vitro required about 1 mM aspirin. Sodium salicylate was several times less potent than aspirin in vivo.