Omega-conotoxin blockade distinguishes Ca from Na permeable states in neuronal calcium channels.
The blocking properties of the neurotoxic peptide omega-conotoxin GVIA (omega-CgTX), on neuronal Ca channels were investigated. In line with previous reports (Feldman et al. 1987; McCleskey et al. 1987), we found that micromolar concentrations of the toxin block selectively and persistently the high-threshold Ca channels of chick sensory neurons. The block by omega-CgTX could be partially relieved in low [Ca2+]o (less than 1 microM) toxin-free solutions, allowing Na ions to flow through open high-threshold Ca channels. Ca currents through these channels, however, remained permanently blocked on returning to normal Ca2+ toxin-free solutions. Also neurons which were preincubated with omega-CgTX in low Ca2+ (5 mM EGTA) failed to show high-threshold Ca currents during washing with normal Ca2+. Thus, appearance of Na currents through Ca channels in CgTX-pretreated cells was neither a consequence of unbinding of the toxin from its receptor site nor due to an interaction of EGTA with bound omega-CgTX. Na currents in CgTX-pretreated cells could be reversibly suppressed by bath applications of verapamil or by further addition of the toxin. At variance with Ca currents, block of Na currents by omega-CgTX was faster and reversible (KD 0.7 microM). Our data are consistent with the idea that neuronal Ca channels are in different conformational states when permeable to Ca2+ or Na+ ions and that omega-CgTX depresses persistently ion permeation primarily in the Ca-permeable state.
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Dipartimento di Anatomia e Fisiologia Umana, Torino, Italy.
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D000818:Animals; D015220:Calcium Channels / Q000187:drug effects*; D002642:Chick Embryo; D005727:Ganglia, Spinal / Q000187:drug effects*; D008978:Mollusk Venoms / Q000494:pharmacology*; D009474:Neurons / Q000187:drug effects*; D015222:Sodium Channels / Q000187:drug effects*; D020866:omega-Conotoxin GVIA
D015220:Calcium Channels; D008978:Mollusk Venoms; D015222:Sodium Channels; D020866:omega-Conotoxin GVIA