The membrane ofParamecium generates a Ca-dependent Na current upon depolarization. There is, however, also a Na current upon hyperpolarization in this membrane. The second Na current was analyzed under voltage clamp and found to have properties identical to those of the first. Both currents could be carried by Na and Li ions and not by K, Cs or choline ion. They were eliminated by either EGTA injection into the cell or Ca removal from the bath. Both currents were eliminated by a single-gene mutation,fast-2, that had no effect on Ca currents. These findings strongly suggest that these two currents are through the same Ca-dependent Na conductance. A hyperpolarization-induced Ca current was also identified, which served to activate the second Na current. These observations support a model that theParamecium membrane has two Ca channels with different voltage dependencies and only one Na channel, which is elicited by a rise of the itternal free Ca2+ concentration. The function of the Ca-dependent Na conductance is discussed.
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Saimi, Y. Calcium-dependent sodium currents inParamecium: Mutational manipulations and effects of hyper- and depolarization. J. Membrain Biol. 92, 227–236 (1986). https://doi.org/10.1007/BF01869391
- Ca-dependent current
- Na current