The alpha adrenergic response of Down's syndrome platelets

The alpha adrenergic responsiveness and pharmacological characteristics of alpha adrenergic receptors were examined using platelets from normal diploid and Down's syndrome (trisomy 21) individuals. Lower than normal doses of epinephrine were required to aggregate Down's syndrome platelets and promote ATP secretion. The concentrations of other platelet aggregating agents (ADP or thrombin) did not distinguish normal from Down's syndrome. Analysis of the alpha adrenergic receptor density and ligand affinity using the radioligand dihydroergocryptine indicated that the Down's syndrome platelets were not significantly different from normal. Likewise, the biochemical characteristics of the adenylate cyclase enzyme, e.g., its sensitivity to stimulation by prostaglandin E1 or inhibition by epinephrine, were not different when comparing Down's syndrome with normal platelet membranes. These data suggest that the enhanced epinephrine sensitivity of Down's syndrome platelets is independent of any change in the alpha adrenergic receptor or the adenylate cyclase enzyme complex.