Ba2+ influx measures the duration of membrane excitation in Paramecium

We have developed an assay for the duration of membrane excitation in Paramecium tetraurelia by tracing the influx of 133Ba2+. This assay is performed at physiological temperatures and in physiological solutions. Ba2+ enters the paramecia through the Ca channels, since mutants defective in Ca channels show no significant Ba2+ influx. Ba2+ enters only when the Ca channels are opened during excitation which can be triggered by Na+ or Ba2+ itself. The ionic species and relative concentrations determine the duration of the action potentials and hence the duration of spinning or backward swimming. The longer the average period of excitation, the larger the Ba2+ influx. In a Ba-Ca solution the cells spend 30% of their time in the excited state. The rate of Ba2+ entry into the paramecia in that state is 1.3 mM/min. Ba2+ influx occurs over a 50-fold range of Ba2+ concentration. There is very little Ba2+ efflux. The fate of the entered Ba2+, the consequences of the large and rapid influx, the advantages and drawbacks of the Ba2+ influx assay, and the possible use of the assay for Ca channel function in cell-free preparations are discussed.