The effect of Tityus serrulatus scorpion toxin gamma on Na channels in neuroblastoma cells

Abstract

The effects of highly purified toxin gamma from the venom of the scorpion Tityus serrulatus (TiTx gamma) on nerve membrane ionic channels have been investigated using the suction electrodes voltage clamp technique on neuroblastoma cells. The amplitude of the normally voltage-dependent Na current is reversible reduced by approximately 50% after 15-105 nM TiTx gamma, whereas even the highest toxin concentrations have no significant effect on the outward K current in the presence of tetrodotoxin. TiTx gamma causes a transient inward current to appear at membrane potentials between -70 and -40 mV, a potential region in which no significant inward current is observed in control experiments. Tetrodotoxin (300 nM) rapidly blocks both the TiTx gamma-induced inward current and the remaining normally voltage-dependent Na current. The binding of radiolabelled TiTx gamma to the Na channels in the neuroblastoma cell membrane is prevented by native TiTx gamma with a K0.5 = 0.75 nM. Both activation and inactivation of the TiTx gamma-induced Na current are shifted 30-40 mV towards more negative potential values as compared to normally voltage-dependent Na current. The TiTx gamma-induced Na current exhibits sigmoidal activation kinetics and relatively slow, exponential inactivation kinetics. The local anesthetic procaine at an external concentration of 1 mM blocks more effectively the remaining normally voltage-dependent Na current than the TiTx gamma-induced Na current. Both Na current components are equally blocked by 1 mM of the local anesthetic propoxycaine. The relation between the effects of TiTx gamma on Nat channels and those of other known neurotoxins those of other known neurotoxins specific of this channel is discussed.(ABSTRACT TRUNCATED AT 250 WORDS)