Platelet beta-adrenoceptors.

Inhibition by isoprenaline of the aggregatory response of human and rat platelets induced by various excitatory agonists is blocked by beta 2-adrenoceptor antagonists. beta 1-Adrenoceptor antagonists are ineffective. beta 2-Adrenoceptor agonists cause inhibition of the response of human platelets to various excitatory agonists. The maximal extent of inhibition is less than that observed for isoprenaline. beta 1-Adrenoceptor agonists fail to cause detectable inhibition of this response. Neither beta 1 nor beta 2-adrenoceptor agonists cause inhibition of the response of rat platelets to excitatory agonists. Only beta 2-adrenoceptor agonists block the inhibitory response to isoprenaline. The extent of inhibition by isoprenaline is a function of the excitatory agonist used and in human platelets is correlated with the ability of that agonist to suppress elevated platelet cyclic adenosine 3',5'-monophosphate (cyclic AMP) levels. Inhibition by isoprenaline is prevented in the presence of an inhibitor of adenylate cyclase. Isoprenaline increases platelet cyclic AMP levels with an EC50 similar to that required to observe inhibition of the aggregatory response. These data indicate that human platelets carry beta 2-adrenoceptors whose occupancy causes inhibition of the response to excitatory agonists as a consequence of elevation of platelet cyclic AMP. The beta-adrenoceptor present on rat platelets also appears to be of the beta 2-subtype.