Electrical properties of Paramecium caudatum: all-or-none electrogenesis

Summary Specimens of Paramecium immersed in solutions of CaCl₂ show graded electrogenesis in response to imposed transmembrane current. However, when BaCl₂ in a final concentration of 0.25 mM is added to a 1 mM CaCl₂ solution, an outward current pulse of 10^-10 amp or greater elicits an all-or-none transient reversal in membrane potential having a duration of about 40 msec. An increase of [Ba⁺⁺] results in (a) lower resting potential, (b) positive shift in critical firing level, (c) increased overshoot of the action potential, (d) decreased hyperpolarizing afterpotential, and (e) increased duration of the action potential (a.p.). If [Ca⁺⁺] is increased along with [Ba⁺⁺] so as to keep the ratio [Ba⁺⁺]/[Ca⁺⁺] constant, the same results are obtained except that the duration of the a. p. remains unaltered. Thus, effects a-d appear to be related to [Ba⁺⁺] and not to [Ca⁺⁺] or [Cl^-]. The degree of overshoot in 1 mM Ca is linearly related to log [Ba⁺⁺] with a slope of approximately 22 mv. With the ratio [Ba⁺⁺]/[Ca⁺⁺] constant, the slope closely approaches the ideal value of 29 mv. The evidence indicates that prolongation of the action potential is due to a delayed onset of Ba inactivation, and that this in turn is a function of surface-bound Ba. Other features of the action potential are absolute refractoriness during its rising and plateau phases, relative refractoriness lasting several seconds, and repetitive firing in response to steady current depolarization. The response is unaffected by TTX and TEA. Mn prolongs the action potential. Sr has an action similar to Ba, whereas the addition of K, Na, Rb, or Mg to the basic calcium medium is unaccompanied by all-or-none electrogenesis.On leave of absence from the Zoological Institute, Faculty of Science, University of Tokyo.Support came from National Science Foundation grant GB-5752x, U.S.P.H.S. grant NB-03664, and in part from Office of Naval Research grant Nonr 4785(00) administered by the Marine Biological Laboratory, Woods Hole.