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.