?-conotoxin blockade distinguishes Ca from Na permeable states in neuronal calcium channels

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Abstract
The blocking properties of the neurotoxic peptide ω-conotoxin GVIA (ω-CgTX), on neuronal Ca channels were investigated. In line with previous reports (Feldman et at. 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 ω-CgTX could be partially relieved in low [Ca2+]0 (2+ toxin-free solutions. Also neurons which were preincubated with ω-CgTX in low Ca2+ (6 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 ω-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 ω-CgTX was faster and reversible (K D 0.7 μM). 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 ω-CgTX depresses persistently ion permeation primarity in the Ca-permeable state.