A barbiturate-induced potassium permeability increase in the myelinated nerve membrane

Effects of the highly lipid soluble barbiturates methohexital and thiopental on potential clamped myelinated nerve fibres were analyzed. Both were found to affect the potassium as well as sodium transport system. The methohexital effect on the potassium system was complex. At low potential values it reduced the permeability, while at high values it increased the permeability as well as slowed down the kinetics. It antagonized the rectification of IK at high potential values caused by increased axoplasmic [Na+]. The effect may be described as a combination of a decreased permeability constant (PK) and a methohexital induced potassium permeability with a potential and time dependence different to the ordinary potassium system. Thiopental reduced PK only. The effect on the sodium system was similar for the studied barbiturates and similar to that described for other barbiturates: a decrease of the permeability constant (PNa) and a shift of the h infinity-U relation in negative direction along the potential axis.