title: Evidence that human platelet alpha-adrenergic receptors coupled to inhibition of adenylate cyclase are not associated with the subunit of adenylate cyclase ADP-ribosylated by cholera toxin.
abstract: Exposure of the alpha-adrenergic receptor of the human platelet to agonist prior to solubilization stabilizes a receptor complex of the alpha-adrenergic receptor with the GTP-binding protein(s) which modulates receptor affinity for agonists (Smith, S. K., and Limbird, L. E. (1981) Proc. Natl. Acad. Sci. U. S. A. 78, 4026-4030). The soluble alpha-adrenergic receptor is characterized by retention of sensitivity to GTP and a faster rate of sedimentation in sucrose gradients than antagonist-occupied or unoccupied receptors. The present studies were undertaken to determine whether the alpha-adrenergic receptor, which is coupled to inhibition of adenylate cyclase, contains the same GTP-binding protein that is involved in activation of adenylate cyclase. The GTP-binding protein that is coupled to activation of adenylate cyclase was labeled with [32P]ADP-ribose using cholera toxin. Incorporation of [32]ADP-ribose into a Mr = 42,000 peptide in human platelet membranes was paralleled by an enhancement of GTP-sensitive catalytic activity in the membranes. However, cholera toxin treatment did not modify alpha-receptor-mediated inhibition of adenylate cyclase or interaction of the alpha-receptor with agonist agents. Moreover, sucrose gradient centrifugation revealed that the [32P]ADP-ribosylated Mr = 42,000 subunit of the stimulatory GTP-binding protein did not appear to associate with the agonist-alpha-receptor complex. These data suggest that the GTP-binding protein that mediates GTP activation of adenylate cyclase in the human platelet membrane is distinct from the GTP-binding protein that modulates alpha-adrenergic receptor affinity for agonist agents and which associates with the receptor in the presence of agonists.
mesh_terms: D000246:Adenosine Diphosphate Ribose / Q000097:blood*; D000262:Adenylyl Cyclases / Q000097:blood*; D000527:Alprostadil; D001792:Blood Platelets / Q000378:metabolism*; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D002772:Cholera Toxin / Q000494:pharmacology*; D004837:Epinephrine / Q000494:pharmacology; D006160:Guanosine Triphosphate / Q000494:pharmacology; D006801:Humans; D007700:Kinetics; D008970:Molecular Weight; D009702:Nucleoside Diphosphate Sugars / Q000097:blood*; D011458:Prostaglandins E / Q000494:pharmacology; D011941:Receptors, Adrenergic / Q000378:metabolism*; D011942:Receptors, Adrenergic, alpha / Q000378:metabolism*
chemical_list: D009702:Nucleoside Diphosphate Sugars; D011458:Prostaglandins E; D011941:Receptors, Adrenergic; D011942:Receptors, Adrenergic, alpha; D000246:Adenosine Diphosphate Ribose; D006160:Guanosine Triphosphate; D002772:Cholera Toxin; D000262:Adenylyl Cyclases; D000527:Alprostadil; D004837:Epinephrine