α_2A‐Adrenergic receptors activate protein kinase C in human platelets via a pertussis toxin‐sensitive G‐protein

4,4'-Diisothiocyanato-stilbene-2,2'-disulfonic acid (DIDS) stimulates human platelets via α_2A-adrenergic receptor-mediated activation of protein kinase C (PKC) independent of the phospholipase C pathway. Here we show, that in permeabilized platelets activation of PKC by DIDS (20 μM), measured as ³²P incorporation in pleckstrin, is completely inhibited by guanosine 5'-(2-O-thio)diphosphate (200 μM), an inhibitor of heterotrimeric G-proteins. Also pertussin toxin (4 ), which ADP-ribosylates the α-subunits of G_i's and G_o, prevents pleckstrin phosphorylation by DIDS. N-Ethylmaleimide (50 μM), which uncouples G_i from α_2A-adrenoceptors, inhibits pleckstrin phosphorylation by DIDS in intact platelets. Activation of PKC by 55 nM phorbol 12-myristate 13-acetate and 500 nM platelet-activating factor are not disturbed by NEM. DIDS inhibits by 40 ± 5% (n = 4) the pertussis toxin-catalyzed [³P]ADP-ribosylation of a 41 kDa protein fraction previously shown to contain the α-subunits ofG_iα-1, G_iα-2 and G_iα-3. Thus, the α_2A-adrenergic receptor activates PKC via a G-protein of the G_i-family.