The Platelet-Stimulating Effect of Adrenaline Through α2-Adrenergic Receptors Requires Simultaneous Activation by a True Stimulatory Platelet Agonist

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Abstract
The stimulating effect of adrenaline on human platelet phospholipase C (PLC) activation and responses in vitro (shape change, aggregation and dense granule secretion) was investigated with respect to its dependence on exogenously added agonists. All experiments were performed with human gel-filtered platelets pretreated with acetylsalicylic acid to prevent endogenous stimulation by the arachidonate pathway. (1) Preliminary experiments demonstrated the presence of trace amounts of extracellular ADP (0.05-0.58 μM) in non-stimulated platelet suspensions; ADP was effectively converted to ATP by the enzyme system creatine phosphate (CP)/creatine phosphokinase (CPK). (2) The adrenaline-induced optical aggregation and single particle (platelet) disappearance in the presence of trace amounts of ADP were almost abolished by the ADP-scavanger system CP/ CPK. (3) The response of CP/CPK-treated thrombin- or platelet-activating factor (PAF)-stimulated platelets was markedly increased by a subsequent addition of adrenaline. When hirudin or BN 50726 was added just prior to adrenaline to terminate the activation by thrombin or PAF, respectively, the stimulating effect of adrenaline was also abolished. (4) CP/CPK-treated, PAF-stimulated platelets rapidly developed decreased responsiveness to a subsequent addition of PAF. When adrenaline was added instead of a second addition of PAF, the stimulating effect of adrenaline was gradually decreased and prevented in parallel with the homologous desensitization of PAF. (5) The weak platelet agonist serotonin by itself induced only shape change in CP/CPK-treated platelets. Adrenaline failed to enhance the extent of this serotonin-induced platelet activation. (6) These results strongly suggest that adrenaline per se is not a platelet agonist in vitro but acts to enhance the stimulation induced by true agonists.