Epinephrine potentiates human platelet activation but is not an aggregating agent

Abstract

Epinephrine can in certain in vitro conditions induce the aggregation of human platelets and could play an important role in vivo in the appearance of thrombotic disorders when catecholamine levels are increased. This study examines some functional and biochemical responses to epinephrine. Epinephrine induces the aggregation and serotonin secretion of human platelets in citrated plasma. This is not due to a direct effect of citrate itself, such as the lowering of plasma free Ca2+ but more likely to the generation of traces of thrombin during blood collection, as suggested by abrogation of these platelet responses when hirudin was added before citrate. When washed human platelets suspended in Tyrode buffer containing 2 mM Ca2+, 0.35% albumin and apyrase, and 0.1-100 microM epinephrine were used, no shape change, aggregation, or secretion of serotonin was observed, nor was the platelet ultrastructure modified. Epinephrine does not modify platelet membrane fluidity, as studied with the lipophilic fluorescent probe trimethylammonium-diphenylhexatriene. It has no direct effect on fibrinogen binding to intact platelets, intracellular Ca2+ levels measured by quin2, or protein phosphorylation. Epinephrine potentiates the action of all types of aggregating agents on aggregation, secretion, intracellular Ca2+ levels, membrane fluidity, fibrinogen binding, or protein phosphorylation. These effects are mediated by alpha 2-adrenergic agonists and inhibited by alpha 2-adrenergic antagonists. This study shows that epinephrine alone does not induce modifications of morphology, metabolism, or function of intact and functional washed human platelets and that it cannot be considered per se as an aggregating agent. However, epinephrine interacts with alpha 2-adrenergic receptors on human platelets and potentiates biochemical and aggregatory responses induced by other platelet agonists.

Epinephrine sensitizes human platelets in vivo and in vitro as studied by fibrinogen binding and P-selectin expression.

“…However, in agreement with previous in vitro studies, 12 -" we found that low concentrations of Epi per se did not activate platelets in vitro. We have previously shown that Epi infusion in vivo enhances platelet aggregability at a dose of 0.4 nmol • kg" 1 • min" 1 .…”

“…At supraphysiological concentrations (^1 /imol/L) Epi per se has been reported to induce aggregation in citrated but not hirudin-treated plasma. 12 -0 - 30 Enhancement of platelet responsiveness to these high concentrations of Epi in citrated plasma may 12 or may not B be dependent on enhanced thrombin formation. However, with wholeblood flow cytometry and single-cell counting, Shattil et al" have demonstrated that fibrinogen receptor exposure and fibrinogen binding are enhanced by high Epi concentrations under both low-and high-calcium conditions.…”

“…10 -11 However, direct platelet activation by Epi is usually seen at supraphysiological concentrations (micromolar range) and experimentally only under certain conditions. 1243 Lower, more physiological concentrations of Epi can sensitize platelets to other agonists, such as ADP, in vitro, 12 ' 14 and Epi infusions causing high physiological levels of Epi in plasma also seem to activate platelets in vivo. 8 Thus, platelet volume increases, 15 serum thromboxane B?…”

Defective platelet aggregation and increased resistance to thrombosis in purinergic P2Y1 receptor–null mice

“…However, it is well-known that inhibition of adenylyl cyclase per se is not sufficient to trigger platelet aggregation (3), and agonists such as epinephrine, acting by activation of Gi only, do not promote platelet aggregation (32). However, we show here that a high concentration of ADP (100 µM) induces aggregation of P2Y 1 -/-mice platelets through activation of the P2cyc receptor ( Figure 4).…”

“…Platelets were directly fixed in the aggregation cuvette with 2.5% glutaraldehyde in 0.1 M cacodylate buffer, pH 7.2, containing 2% sucrose. Platelets were then processed as described previously (32). Ultrathin sections were observed under a Philips CM120 Biotwin electron microscope (Philips Consumer Electronics B.V., Eindhoven, The Netherlands) at 120 kV.…”