1. Human platelet nucleotides were labelled by incubating platelet-rich plasma with [U-14C]adenine. With such platelets, the effects of prostaglandin E1, theophylline and aspirin were determined on collagen-induced platelet aggregation and release of platelet ATP and ADP. Intracellular changes of platelet radioactive nucleotides, particularly 3′:5′-cyclic AMP, were also determined both with and without collagen treatment. 2. Prostaglandin E1, theophylline and aspirin inhibited collagen-induced aggregation of platelets in a dose-dependent manner. Collagen-induced release of ATP and ADP and breakdown of radioactive ATP were also inhibited in a dose-dependent manner. 3. Prostaglandin E1 stimulated the formation of platelet radioactive 3′:5′-cyclic AMP in a dose-dependent manner. With a given dose of prostaglandin E1, maximum formation of radioactive 3′:5′-cyclic AMP occurred by 10–30s and thereafter the concentrations declined. The degree of inhibition of aggregation produced by prostaglandin E1, however, increased with its time of incubation in platelet-rich plasma before addition of collagen, so that there was an inverse relationship between the radioactive 3′:5′-cyclic AMP concentration measured at the time of collagen addition and the subsequent degree of inhibition of aggregation obtained. 4. Neither theophylline nor aspirin at a concentration in platelet-rich plasma of 1.7mm altered platelet radioactive 3′:5′-cyclic AMP contents. In the presence of prostaglandin E1, theophylline increased the concentration of radioactive 3′:5′-cyclic AMP over that noted with prostaglandin E1 alone, but aspirin did not. 5. Mixtures of prostaglandin E1 and theophylline had a synergistic effect on inhibition of platelet aggregation. The same was true to a lesser extent with mixtures of prostaglandin E1 and aspirin. Such mixtures also inhibited collagen-induced release of platelet ATP and ADP and breakdown of platelet radioactive ATP. 6. Certain concentrations of either theophylline or aspirin and mixtures of small concentrations of prostaglandin E1 with either theophylline or aspirin caused little or no increase of radioactive 3′:5′-cyclic AMP at the time of collagen addition, but inhibited aggregation to a marked degree, whereas higher concentrations of prostaglandin E1 alone caused a much greater increase of radioactive 3′:5′-cyclic AMP at the time of collagen addition but inhibited aggregation to a lesser extent. With these compounds there does not appear to be a correlation between these parameters.

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