Signal transduction system in epinephrine stimulated platelets; comparison between epinephrine sensitive and insensitive platelets.

We recently reported the high prevalence of impaired platelet responsiveness only to epinephrine in healthy Japanese. This abnormality was associated with a 50% decrease in the number of alpha 2-adrenergic receptors. Platelets from non-responders (NR) do not undergo secondary platelet aggregation even after exposure to 100 microM epinephrine, but they can potentiate the effect of ADP to provoke platelet aggregation. To further define the nature of the defect and to delineate controversial steps of epinephrine stimulated signal transduction, a signaling pathway of epinephrine was investigated in platelets from NR and R(normal responder to epinephrine). In a unique particle counting apparatus, epinephrine initially triggered the formation of small platelet aggregates composing of 10-1000 cells from both R and NR, but the aggregates became larger (4600 > cells) only in platelets from R. Thus, platelets from NR lack the ability to form larger aggregates. A similar defect was reproduced by treating normal platelets with aspirin. In the presence of fibrinogen, platelets from NR lacked phospholipase A2 activation, determined by arachidonic acid liberation in the presence of inhibitors to cyclooxygenase and lipoxygenase. In the absence of fibrinogen, aggregation and phospholipase A2 activation were not evident in R and NR. The surface expression of GPIIb/IIIa was markedly decreased in platelets from NR after stimulation by epinephrine, in comparison with those from R. The resting level and epinephrine stimulated increase in cAMP were not significantly different between NR and R. Incubating R platelets with a half saturating dose of yohimbine rendered them insensitive to epinephrine. These results indicated that the impaired platelet aggregation induced by epinephrine was due to the impaired surface exposure of glycoproteins GPIIbIIIa integral to the activation of phospholipase A2, which requires the full and normal occupancy of the alpha 2-adrenergic receptor by epinephrine.