title: Adrenoceptor activity of muscarinic toxins identified from mamba venoms.
abstract: OBJECTIVE
  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.
  
  
  METHODS
  We studied the interaction of four common MTs, MT1, MT3, MT7 and MTα, 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.
  
  
  RESULTS
  All MTs behaved non-competitively in radioligand displacement binding. MT1 displayed higher binding affinity for the human α(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 M₄ mAChR, α(1A)-, α(1D)- and α(2A)-adrenoceptors and lower affinity binding (IC₅₀ ≥ 25 nM) to α(1B)- and α(2C)-adrenoceptors and M₁ mAChR. MT7 did not detectably bind to other receptors than M₁, and MTα was specific for the α(2B)-adrenoceptor. None of the toxins showed effects on β₁- or β₂-adrenoceptors.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Biosciences, Biochemistry, Åbo Akademi University, Turku, Finland.
authors.forename: K
authors.initials: K
authors.lastname: Näreoja
authors.forename: J P
authors.initials: JP
authors.lastname: Kukkonen
authors.forename: S
authors.initials: S
authors.lastname: Rondinelli
authors.forename: D M
authors.initials: DM
authors.lastname: Toivola
authors.forename: J
authors.initials: J
authors.lastname: Meriluoto
authors.forename: J
authors.initials: J
authors.lastname: Näsman
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D002478:Cells, Cultured; D017815:Elapidae; D005260:Female; D006801:Humans; D020128:Inhibitory Concentration 50; D018721:Muscarinic Agonists / Q000378:metabolism / Q000494:pharmacology; D011485:Protein Binding; D011869:Radioligand Assay / Q000379:methods; D051381:Rats; D017207:Rats, Sprague-Dawley; D011941:Receptors, Adrenergic / Q000235:genetics / Q000378:metabolism*; D011976:Receptors, Muscarinic / Q000235:genetics / Q000378:metabolism*; D012910:Snake Venoms / Q000737:chemistry / Q000494:pharmacology*; D018411:Spodoptera
chemical_list: D018721:Muscarinic Agonists; D011941:Receptors, Adrenergic; D011976:Receptors, Muscarinic; D012910:Snake Venoms; D002118:Calcium