Effects of rubidium, caesium, strontium, barium and lanthanum on ionic currents in myelinated nerve fibres from Xenopus laevis.

A comparative analysis of the effects of externally applied Rb+, Cs+, Ba2+ and La3+ on myelinated fibres under potential clamp conditions was made. The ions Cs+, Ba2+ and La3+ (belonging to groups 1a, 2a and 3a of period 6 in the periodic system) were found to affect the K system in specific ways. Ba2+ and La3+ reduced the outward K+ current associated with positive potential steps; the effect of La3+ being larger than that of Ba2+. The effect of Ba2+ depended on the K+ concentration. Cs+, Ba2+ and La3+ reduced the inward K+ tail current at repolarisation in solutions with high K+ concentration. Cs+ and Ba2+ caused a rectification of the K+ current, i.e. they reduced the inward tail current proportionally more than the outward steady state current. The effect of Ba2+ could not be described as a simple reduction of the permeability constant PK. Rb+ was found to affect the time course of the K system by increasing the rate constant beta n. Sr2+, Ba2+ and La3+ reduced the permeability constant PNa. These ions also shifted potential dependent parameters in general along the potential axis. The effect of La3+ was larger than that of Sr2+ and Ba2+. Cs+, Ba2+ and La3+ reduced the p-current, Ba2+ and La3+ the leak conductance.