G-PROTEIN COUPLING OF HUMAN PLATELET V(1) VASCULAR VASOPRESSIN RECEPTORS

We used several approaches to identify the G protein coupled to V1 vas cular arginine vasopressin (AVP) receptors of human platelets. In puri fied platelet membranes, high-affinity specific binding of [H-3]AVP bu t not that of the V1 vascular antagonist [H-3]d(CH2)5Tyr(Me)AVP was mo dulated by guanosine 5'-O-(3-thiotriphosphate) or sodium fluoride both in the presence and absence of MgCl2. AVP failed to modify the [alpha -P-32]GTP labeling pattern or the cytosolic translocation of the 24- t o 27-kDa GTP-binding proteins. AVP-stimulated GTPase activity of plate let membranes was blocked by antibodies specific for the COOH-terminal of the G(qalpha) protein. AVP increased labeling of a 42-kDa platelet membrane protein by the photoreactive GTP analogue [alpha-P-32]azidoa nilido GTP. Immunoblotting of platelet proteins with various G protein -specific antibodies revealed that the 42-kDa protein labeled with [al pha-P-32]azidoanilido GTP was immunoblotted only by antibodies specifi c for the a-subunit of G(q-11). Thus V1 vascular AVP receptors of huma n platelets are coupled in a divalent cation-dependent manner to a G p rotein belonging to the G(q-11) family.