Effects of gadolinium on electrical membrane properties and behaviour in Paramecium tetraurelia

Low concentrations (4 to 10 μM) of the trivalent cation gadolinium raised the input resistance and altered the membrane potential of Paramecium tetraurelia suggesting an interference with membrane channels. Current-clamp conditions revealed a concentration-dependent reduction of membrane rectification, in particular in the hyperpolarizing direction. The graded action potential and the early inward current seen under voltage-clamp conditions were depressed. Standardized applications of probes for focal mechanostimulation showed a reduction in mechanoreceptor potentials. Reversal responses of downward swimming cells in gadolinium transiently rose to higher frequency and settled at a reduced level thereafter explaining observations of a transient increase in gravitaxis. Vertical swimming speed was reduced, whereas the horizontal speed was largely unchanged. Gravity-induced modulation of swimming speed (gravikinesis) was depressed. The data indicate that gadolinium at micromolar concentrations effectively interferes with various types of membrane channels in Paramecium and is therefore no specific channel inhibitor to characterize graviresponses in this ciliate.