SummaryThis work reports the functional studies of CHO cells coexpressing α-adrenergic (αAR) and human fibrinogen (Fg) receptors (integrin αIIbβ3). Stimulation of these cells with α-agonists produced a transient rise in the free cytosolic calcium (Ca++) accompanied by enhanced binding to soluble Fg, and these effects were prevented by specific αAR antagonists. The αadrenergic-induced activation of αIIbβ3 in CHO-αIIbβ3-αAR increased the rate of adhesion and extension of cells onto Fg coated plates, and also induced a soluble Fgand αIIbβ3-dependent formation of cell aggregates, whereas no effects were observed by the stimulation of CHO-αIIbβ3 cells. α-Adrenergic antagonists, the ligand mimetic peptide RGDS, pertussis toxin (PTX), or EDTA, they all prevented the α-adrenergic stimulation of adhesion and aggregation. However, inhibition of PKC prevented the α-adrenergic stimulation of cell adherence, whereas blocking the intracellular Ca++ mobilization impeded the stimulation of cell aggregation. The α-adrenergic activation was associated with phosphorylation of a protein of ∼100 kDa and proteins of the MAPK family. The former was selectively phosphorylated by α-adrenergic stimulation whereas the latter were phosphorylated by the binding of cells to Fg and markedly intensified by α-adrenergic stimulation.