Studies on the Effect of Adrenergic Blocking Drugs on Catecholamine‐Induced Platelet Aggregation and Uptake of Noradrenaline and 5‐Hydroxytryptamine

Adrenaline (A) and noradrenaline (NA) were both found to induce platelet aggregation in citrated platelet rich human plasma, adrenaline being about 8 times more effective than noradrenaline. No effect was obtained with isoproterenol. The aggregating effect of catecholamines was stereospecific, the l‐form of NA being 30 times as effective as the d‐form. Platelet aggregation induced by A and NA could be blocked by phentolamine, phenoxybenzamine and dibenamine in the concentrations 10‐6M, 10‐4M and 10‐4M respectively, as well as with some β‐receptor blocking agents, pronethalol and propranolol, although only in a 10‐3M concentration. Almost no effect was observed with MJ 1999. High concentrations (10‐3M) of both α‐ and (β‐receptor blocking agents could block adenosine disphosphate induced aggregation indicating an unspecific effect of the blockers at this concentration. Uptake of3H‐NA and14C‐5‐HT in the platelets was inhibited by both α‐ and β‐receptor blocking agents in concentrations from about 10‐5M. There was no obvious correlation between inhibition of uptake by α‐receptor blocking agents and their effect on catecholamine‐induced aggregation indicating that uptake of A or NA was not a necessary step in catecholamine mediated platelet aggregation.