Inhibition of calcium channels in rat central and peripheral neurons by ω-conotoxin MVIIC

Abstract

Inhibition of voltage-dependent calcium channels by ω-conotoxin MVIIC (ω-CTx-MVIIC) was studied in various types of rat neurons. When studied with 5 mM Ba2+ as charge carrier, ω-CTx-MVIIC block of N-type calcium channels in sympathetic neurons was potent, with half-block at 18 nM. Block of N-type channels had a rapid onset (τ ~ 1 sec at 1 μM ω-CTx-MVIIC) and quick reversibility (τ ~ 30 sec). The rate of block was proportional to toxin concentration, consistent with 1:1 binding of toxin to channels, with a rate constant (k(on)) of ~ 1 x 106 M-1·sec-1. Both potency and rate of block were reduced dramatically with increasing concentrations of extracellular Ba2+. ω-CTx-MVIIC also blocked P-type calcium channels in cerebellar Purkinje neurons, but both development and reversal of block were far slower than for N-type channels. The rate of block was proportional to toxin concentration, with k(on) ~ 1.5 x 103 M-1·sec-1 at 5 mM Ba2+. From this value and an unblocking time constant of ~200 min, a dissociation constant of ~50 nM was estimated. Thus, block of P-type channels is potent but very slow. In hippocampal CA3 pyramidal neurons, ω-CTx-MVIIC blocked ~50% of the high-threshold calcium channel current; one component (~20%) was blocked with the rapid kinetics expected for N-type channels, whereas the other component was blocked slowly. The component blocked slowly was reduced but not eliminated by preexposure to 200 nM or 1 μM ω-Aga-IVA.