Membrane excitability and membrane currents in the marine ciliate Euplotes vannus

Electrical properties of E. vannus were investigated by use of constant current injection, voltage-clamp, and isoosmotic ion substitution. The resting potential of approximately -40 mV was K(+) and Ca(2+)-dependent. Spontaneous depolarizations occurred frequently with peaks around -20 mV and durations from several hundred ms to several s. External Ba(2+) or internal Cs(+) induced all-or-none action potentials. Current stimuli induced Ca(2+)-dependent graded action potentials. Sr(2+) or Ba(2+), but not Mg(2+), instead of Ca(2+) increased the regenerative response. Repolarization occurred in two steps: a first fast and a second slow one. It was strongly modified by the Ca(2+) substitutes. A voltage-dependent small Ca(2+) inward current was activated at depolarizations beyond -20 mV. It triggered a fast and a slowly activating K(+) outward current and was itself short-circuited by the fast K(+) current. Therefore, it could only be measured when K(+) currents were not activated or inhibited. A slowly activating Na(+) inward current was identified that turned to outward direction after replacement of external Na(+) by choline(+). The K(+) outward currents differed in their sensitivity to external TEA(+) and in their inactivation kinetics. All currents were correlated to the voltage-dependent influx of Ca(2+).