Trypargine blocks the sodium channels only from the inside of squid giant axon

A new tetrahydro-beta-carboline, trypargine (TRG), specifically suppresses the Na current (INa) when applied to the internal surface of the squid axon membrane without affecting the K current (IK). The binding of TRG to its receptor is potential-dependent and occurs at a site about halfway through the membrane electric field from the outside; the dissociation constant is 11 microM at 0 mV.