Rapid and reversible block of N-type calcium channels (CaV 2.2) by omega-conotoxin GVIA in the absence of divalent cations.
Omega-conotoxin GVIA (omegaCGVIA) has been reported to be an irreversible blocker of N-type calcium channels (Ca(V) 2.2). However, recent studies have demonstrated that the omegaCGVIA off-rate is correlated with divalent cation concentration, because increasing [Ba2+]o accelerated the recovery from omegaCGVIA block. This predicts that the dissociation of omegaCGVIA from N-channels will be negligible in the absence of divalent cations. Surprisingly, we find that omegaCGVIA block is rapidly reversible in divalent cation-free (0 Ba2+) external solutions in which current was carried by MA+. The recovery followed a single-exponential time course with tau = 31 sec. Isochronic measurements showed that, at 2 min after the removal of toxin, current returned to 86% of control in 0 Ba2+ compared with 19% in 3 mm Ba2+. The off-rate of omegaCGVIA from N-channels was dependent on [Ba2+]o, because, at an intermediate concentration (3 microm Ba2+), N-current recovered with tau = 64 sec, significantly slower than that in 0 Ba2+ but faster than in 3 mm Ba2+. Recovery from omegaCGVIA block was also observed when Cs+ or Na+ carried the current in divalent cation-free conditions. The off-rate was sensitive to [Ba2+]o only during washout, because current recovered slowly in the presence of 3 mm Ba2+, even after it was blocked in 0 Ba2+. Assuming that the toxin is a pore blocker, our findings are consistent with a model in which Ba2+ interacts at a site on the extracellular surface of the channel to regulate omegaCGVIA dissociation from N-channels.
Liang
Haoya
H
Department of Physiology, Tulane University Health Science Center, New Orleans, Louisiana 70112, USA.
Elmslie
Keith S
KS
D000818:Animals; D001464:Barium / Q000494:pharmacology; D001665:Binding Sites / Q000187:drug effects / Q000502:physiology; D002118:Calcium / Q000378:metabolism; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D020864:Calcium Channels, N-Type / Q000378:metabolism*; D002413:Cations, Divalent / Q000378:metabolism*; D002414:Cations, Monovalent / Q000494:pharmacology; D002478:Cells, Cultured; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology; D007700:Kinetics; D008744:Methylamines / Q000378:metabolism; D009474:Neurons / Q000166:cytology / Q000187:drug effects / Q000502:physiology; D018408:Patch-Clamp Techniques; D011485:Protein Binding / Q000187:drug effects / Q000502:physiology; D011487:Protein Conformation / Q000187:drug effects; D011892:Rana catesbeiana; D020866:omega-Conotoxin GVIA / Q000493:pharmacokinetics / Q000494:pharmacology*
D002121:Calcium Channel Blockers; D020864:Calcium Channels, N-Type; D002413:Cations, Divalent; D002414:Cations, Monovalent; D008744:Methylamines; D001464:Barium; D020866:omega-Conotoxin GVIA; C027451:methylamine; D002118:Calcium