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

Low concentrations (4 to 10 mu M) of the trivalent cation gadolinium raised the input resistance and altered the membrane potential of Paramecium tetr ataurelia suggesting an interference with membrane channels. Current-clamp conditions revealed a concentration-dependent reduction of membrane rectifi cation, in particular in the hyperpolarizing direction. The graded action p otential and the early inward current seen under voltage-clamp conditions w ere depressed. Standardized applications of probes for focal mechanostimula tion showed a reduction in mechanoreceptor potentials. Reversal responses o f downward swimming cells in gadolinium transiently rose to higher frequenc y and settled at a reduced level thereafter explaining observations of a tr ansient increase in gravitaxis. Vertical swimming speed was reduced, wherea s the horizontal speed was largely unchanged. Gravity-induced modulation of swimming speed (gravikinesis) was depressed. The data indicate that gadoli nium at micromolar concentrations effectively interferes with various types of membrane channels in Paramecium and is therefore no specific channel in hibitor to characterize graviresponses in this ciliate.