Fluorimetric determination of the resting potential changes associated with the chemotactic response in Paramecium

(1) Evidence is presented which indicates that the carbocyanine dye (3,3'-dipropyl thiadicarbocyanine) can be used as a spectroscopic probe for monitoring the resting potential across tha plasma membrane of the ciliated protozoan Paramecium. (2) The dye at low concentrations (less than or equal to 1 muM) does not affect either the viability or the motility of the cells, nor does it induce a chemotactic response. (3) The fluorescence of the dye bound to the cells alters as the potential across the membrane is changed by increasing the external cation concentration. (4) The absorbance of the bound dye also changes in response to an alteration of the membrane potential. (5) The membrane potential changes as measured by the fluorescence method have been correlated with the measurements of the potential estimated by microelectrode methods. (6) Both cations which induce a negative chemotactic response in Paramecium (K+, Na+, Ba2+) and several non-toxic cations bring about a rapid depolarization of the plasma membrane. The significance of these rapid changes in relation to the swimming behaviour of the ciliate is discussed.