Allosteric α1-Adrenoreceptor Antagonism by the Conopeptide ρ-TIA

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