mu-Conotoxin GIIIB. Myotoxin II, Geographutoxin II, GTx-II. A Selective Blocker of Skeletal Muscle Nav1.4 Channels.mu-Conotoxin GIIIB was originally isolated from the marine snail Conus geographus and this is a synthetic version of the peptide. ??mu-Conotoxin GIIIB selectively blocks Nav1.4 channels which are expressed predominantly in muscle. It is a potent blocker and 600 nM completely blocked Nav1.4 related currents in rat skeletal muscle. Ion Channel Modulators; Voltage - Gated Na+ Channel Blockers.