Allosteric α1-adrenoreceptor antagonism by the conopeptide ρ-TIA

A peptide contained in the venom of the predatory marine snail Conus tulipa, rho-TIA, has previously been shown to possess alpha(1)-adrenoreceptor antagonist activity. Here, we further characterize its pharmacological activity as well as its structure-activity relationships. In the isolated rat vas deferens, rho-TIA inhibited alpha(1)-adrenoreceptor-mediated increases in cytosolic Ca2+ concentration that were triggered by norepinephrine, but did not affect presynaptic alpha(2)-adrenoreceptor-mediated responses. In radioligand binding assays using [I-125] HEAT, rho-TIA displayed slightly greater potency at the alpha(1B) than at the alpha(1A) or alpha(1D) subtypes. Moreover, although it did not affect the rate of association for [H-3] prazosin binding to the alpha(1B)-adrenoreceptor, the dissociation rate was increased, indicating non-competitive antagonism by rho-TIA. N- terminally truncated analogs of rho-TIA were less active than the full-length peptide, with a large decline in activity observed upon removal of the fourth residue of rho-TIA (Arg(4)). An alanine walk of rho-TIA confirmed the importance of Arg(4) for activity and revealed a number of other residues clustered around Arg(4) that contribute to the potency of rho-TIA. The unique allosteric antagonism of rho-TIA resulting from its interaction with receptor residues that constitute a binding site that is distinct from that of the classical competitive alpha(1)-adrenoreceptor antagonists may allow the development of inhibitors that are highly subtype selective.