title: Promiscuous and reversible blocker of presynaptic calcium channels in frog and crayfish neuromuscular junctions from Phoneutria nigriventer spider venom.
abstract: Peptide channel blockers found in venoms of many predators are useful pharmacological tools and potential therapeutic agents. The venom of the Brazilian spider Phoneutria nigriventer contains a fraction, omega-phonetoxin-IIA (omega-Ptx-IIA, 8360 MW), which blocks Ca2+ channels. At frog neuromuscular junctions (NMJ) bathed in normal Ca2+ (1.8 mM) saline, omega-Ptx IIA did not affect spontaneous transmitter release but reversibly reduced evoked transmitter release by 75 and 95% at 12 and 24 nM, respectively. In contrast, toxin effects were irreversible in low-Ca2+ (0.5 mM) saline. Ca2+ imaging in normal-Ca2+ saline showed that omega-Ptx-IIA partially blocked stimulus-dependent presynaptic Ca2+ signals, and the blockade was almost completely reversible. Increases in spontaneous release frequency induced by high extracellular K+ were blocked by omega-Ptx-IIA. Therefore omega-Ptx-IIA blocks N-type Ca2+ channels, which admit Ca2+ that triggers transmitter release at the frog NMJ. Additional evidence predicts that omega-Ptx-IIA binds to N-type Ca2+ channels at a different site from that of omega-Conotoxin-GVIA. omega-Ptx-IIA also gave a low-affinity partial blockade of transmitter release and presynaptic Ca2+ signals at crayfish NMJs where P-type channels are blocked by omega-agatoxin-IVA. The Ca2+-dependent reversibility and promiscuity of this toxin may make it highly useful experimentally and therapeutically.
authors.affiliation: Department of Physiology, University of Toronto, Toronto, Ontario M5S 1A8, Canada.
authors.forename: Lanfranco R P
authors.initials: LR
authors.lastname: Troncone
authors.forename: John
authors.initials: J
authors.lastname: Georgiou
authors.forename: Shao-Ying
authors.initials: SY
authors.lastname: Hua
authors.forename: Donald
authors.initials: D
authors.lastname: Elrick
authors.forename: Ivo
authors.initials: I
authors.lastname: Lebrun
authors.forename: Fabio
authors.initials: F
authors.lastname: Magnoli
authors.forename: Milton P
authors.initials: MP
authors.lastname: Charlton
mesh_terms: D000818:Animals; D003400:Astacoidea; D002121:Calcium Channel Blockers / Q000302:isolation & purification / Q000494:pharmacology*; D015220:Calcium Channels / Q000502:physiology*; D009469:Neuromuscular Junction / Q000187:drug effects* / Q000502:physiology; D017729:Presynaptic Terminals / Q000187:drug effects* / Q000502:physiology; D011894:Rana pipiens; D013111:Spider Venoms / Q000302:isolation & purification / Q000494:pharmacology*; D013112:Spiders
chemical_list: D002121:Calcium Channel Blockers; D015220:Calcium Channels; D013111:Spider Venoms