Chemoreception in Paramecium tetraurelia: acetate and folate-induced membrane hyperpolarization

Acetic and folic acids hyperpolarize the membrane potential of Paramecium tetraurelia in a concentration-dependent manner. The membrane responses are accompanied by small changes in cell resistance, and are significantly reduced by increasing extracellular cation concentrations, suggesting that the attractants bring about the membrane potential change by increasing cell permeability to cations. The inability to show a reversal potential for the hyperpolarization to attractants suggests that the effects of cations on the response are non-specific, however. The possible roles of Ca++, K+, and Na+ in the attractant-induced responses were further investigated by applying acetate and folate to cells with genetic defects in specific ion conductances, by collapsing the driving forces for these ions, and by testing the effects of ion channel blockers on the responses. These studies suggest that the membrane responses to attractants are not due to the direct effects of increased or decreased membrane permeability to cations. Attempts to block the acetate and folate-induced hyperpolarization by collapsing surface potential or using a mutant with reduced surface charge were inconclusive, as were studies on the possible role of attractant transport in the membrane responses. We hypothesize that the membrane hyperpolarization may be due to either the indirect effects of increased calcium permeability, to extrusion of calcium through activation of a calcium pump, or to a proton efflux.