ADRENALINE ACTIVATES HUMAN PLATELETS BUT IS NOT PER SE AN AGGREGATING AGENT. EFFECTS ON PLATELET MORPHOLOGY, MEMBRANEFLUIDITY, FIBRINOGEN BINDING, CYTOPLASMIC FREE CALCIUM AND PROTEIN PHOSPHORYLATION

Adrenaline (Adr) is generally considered as a full agonist able to induce in vitro the aggregation of human platelets and could play an important role in vivo in the appearance of thrombotic disorders when catecholamine levels are increased. Adr 2.5 M) induces the aggregation and secretion of 41 % of preloaded ³H-serotonin in human platelets in citrated plasma. This effect is not seen in plasma collected on 50 ATU/ml hirudin, and is due to the generation of traces of thrombin during blood collection and not to a direct effect of citrate itself, such asthe lowering of plasma free calcium. With washed human platelets suspended in Tyrode's buffer containing 2 mM Ca²⁺, 0.35 %albumin and apyrase, Adr (0.1 -100 M) doesnot cause shape change, aggregation or secretion of serotonin and does not modify platelet ultrastructure as judged by electron microscopy. Adr (1-100 M) does not change platelet membrane fluidity, as studied with the lipophilic fluorescent probe TMA-DPH. Adr has no direct effect on fibrinogen binding to intact platelets, intracellular Ca²⁺levels measured with quin2, or phosphorylation of 20 KD_a or 47 KD_apolypeptides, whereas all these parameters are modified after stimulation with ADP orthrombin. Adr potentiates the action of all types of aggregating agents on aggregation, secretion, intracellular Ca²⁺ levels,membrane fluidity, fibrinogen binding or protein phosphorylation. This effect is also seen with alpha2-adrenergic agonists (noradrenaline, alpha-methyl noradrenaline, clonidine) and is inhibited by alpha2-adrenergicantagonists such as yohimbine. The potentiation of platelet aggregation by Adr is not modified by prior incubation of the platelets with1mM aspirin for 15 min. This study shows that Adr alone does not induce modifications ofmorphology, metabolism or function of intactand functional washed human plateletsand that Adr cannot be considered per se as an aggregating agent. However, Adr interactswith alpha2-adrenergic receptors on human plateletsand potentiates biochemical and aggregatory responses induced by other platelet agonists.