CONCENTRATION-DEPENDENT EFFECTS OF BA-2+ ON ACTION-POTENTIAL AND MEMBRANE CURRENTS IN THE CILIATE STYLONYCHIA

1. 1. Action potentials and ionic membrane currents of the hypotrich ciliate Stylonychia mytilus were investigated in solutions of various Ba2+ and Ca2+ content, keeping the total concentrations of divalent cations constant. 2. 2. All ionic currents-early inward, delayed outward and the current upon membrane hyperpolarization-appeared to be affected by Ba2+. 3. 3. Both the inward and the outward steady-state rectification of the membrane were considerably reduced by Ba2+. 4. 4. Due to a reduced K+ outward current and possibly a decreased inward current inactivation an increasingly large net inward current became apparent in Ba2+ solutions. 5. 5. The maximum of the early inward current-voltage relationship was reduced and shifted along the voltage-axis in dependence of the Ba2+/Ca2+ concentration ratio. 6. 6. The changes in the membrane currents following gradual substitution of Ca2+ by Ba2+ appear to be responsible for the observed increase in both the amplitude and the duration of the action potentials. 7. 7. Our results suggest that the membrane currents are altered due to differences in the interaction of Ca2+ and Ba2+ with ionic channels. © 1984.