Conus venom interaction with α2-adrenergic receptors in calf retina membranes

α₂-Adrenergic receptors were identified in calf retina membranes by the specific binding of the radiolabelled antagonist [³H]RX 781094. Crude venoms from various Conus species did not interact with the radioligand but were able to inhibit radioligand binding to the α₂-receptors with the following order of potency: C. planorbis (IC₅₀ = 2.1 μg protein/ml) ≈ C. tessulatus (IC₅₀ = 2.7) >C. eburneus (IC₅₀ = 19) >C. textile (IC₅₀ = 54) >C. geographus (IC₅₀ = 130). Venom from 17 other species was less or not active at all. Venom competition binding curves were steep and not affected by GTP. In contrast, the ( − )-epinephrine competition binding curve was shallow and underwent a rightward shift and steepening in the presence of GTP. The venom-α₂-receptor interaction was completely inhibited by the calcium chelating reagent EGTA. These data suggest that the venom of certain Conus species contains peptide toxins which are capable of shielding the binding site of α₂-receptors in an antagonistic manner.