Membrane excitability and membrane currents in the marine ciliate Euplotes vannus

Abstract

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 Ca2+-dependent. Spontaneous depolarizations occurred frequently with peaks around −20 mV and durations from several hundred ms to several s. External Ba2+ or internal Cs+ induced all-or-none action potentials. Current stimuli induced Ca2+-dependent graded action potentials. Sr2+ or Ba2+, but not Mg2+, instead of Ca2+ increased the regenerative response. Repolarization occurred in two steps: a first fast and a second slow one. It was strongly modified by the Ca2+ substitutes. A voltage-dependent small Ca2+ 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 Ca2+. © 1988, Gustav Fischer Verlag · Stuttgart · New York. All rights reserved.