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

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Abstract
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 μm) 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 μm) did not inhibit platelet aggregation in vivo; in vitro, however, desmethylimipramine caused up to 34% inhibition at concentrations as low at 0·25 μm. 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.