Alpha-adrenergic stimulation of phosphatidylinositol synthesis in human platelets as an alpha-2 effect secondary to platelet aggregation.

Epinephrine and adenosine diphosphate (ADP) stimulated 3H-glycerol uptake into phosphatidylinositol of human platelets. Yohimbine, an alpha-2 adrenoceptor antagonist, markedly reduced epinephrine-stimulated 3H-glycerol uptake into phosphatidylinositol; while prazosin, an alpha-1 antagonist, was without effect. Likewise, yohimbine, but not prazosin, blocked epinephrine-induced platelet aggregation. Furthermore, clonidine, a specific agonist for alpha-2 adrenoceptors, stimulated incorporation of 3H-glycerol into phosphatidylinositol and promoted platelet aggregation in the presence of low concentrations of ADP. These studies indicate that the effects of epinephrine on platelet aggregation and phosphatidylinositol synthesis are mediated through alpha-2 adrenoceptors. Further, since the stimulation of phosphatidylinositol synthesis seen with epinephrine was also observed with ADP, this suggests that the increased 3H-glycerol labeling is an indirect result of platelet aggregation.