Adrenoceptor activity of muscarinic toxins identified from mamba venoms

BACKGROUND AND PURPOSE Muscarinic toxins (MTs) are snake venom peptides named for their ability to interfere with ligand binding to muscarinic acetylcholine receptors (mAChRs). Recent data infer that these toxins may have other G-protein-coupled receptor targets than the mAChRs. The purpose of this study was to systematically investigate the interactions of MTs with the adrenoceptor family members. EXPERIMENTAL APPROACH We studied the interaction of four common MTs, MT1, MT3, MT7 and MT alpha, with cloned receptors expressed in insect cells by radioligand binding. Toxins showing modest to high-affinity interactions with adrenoceptors were additionally tested for effects on functional receptor responses by way of inhibition of agonist-induced Ca2+ increases. KEY RESULTS All MTs behaved non-competitively in radioligand displacement binding. MT1 displayed higher binding affinity for the human alpha(2B)-adrenoceptor (IC50 = 2.3 nM) as compared with muscarinic receptors (IC50 >= 100 nM). MT3 appeared to have a broad spectrum of targets showing high-affinity binding (IC50 = 1-10 nM) to M-4 mAChR, alpha(1A)-, alpha(1D)- and alpha(2A)-adrenoceptors and lower affinity binding (IC50 >= 25 nM) to alpha(1B)-and alpha(2C)-adrenoceptors and M-1 mAChR. MT7 did not detectably bind to other receptors than M-1, and MT alpha was specific for the alpha(2B)-adrenoceptor. None of the toxins showed effects on beta(1)- or beta(2)-adrenoceptors. CONCLUSIONS AND IMPLICATIONS Some of the MTs previously found to interact predominantly with mAChRs were shown to bind with high affinity to selected adrenoceptor subtypes. This renders these peptide toxins useful for engineering selective ligands to target various adrenoceptors.