Preferential Interaction of v-Conotoxins with Inactivated N-type Ca Channels

The selective block of N-type Ca channels by v-conotoxins has been a hallmark of these channels, critical in delineating their biological roles and molecular characteristics. Here we report that the v-conotoxin-channel interaction depends strongly on channel gating. N-type channels (a1B, a2, and b1) expressed in Xenopus oocytes were blocked with a variety of v-conotoxins, including v-CTx-GVIA, v-CTx-MVIIA, and SNX331, a derivative of v-CTx-MVIIC. Changes in holding potential (HP) markedly altered the severity of toxin block and the kinetics of its onset and removal. Notably, strong hyperpolarization renders v-conotoxin block completely reversible. These effects could be accounted for by a modulated receptor model, in which toxin dissociation from the inactivated state is ;60-fold slower than from the resting state. Because v-conotoxins act exclusively outside cells, our results suggest that voltagedependent inactivation of Ca channels must be associated with an externally detectable conformational change.