CALCIUM-DEPENDENT TRANSIENT POTASSIUM OUTWARD CURRENT IN THE MARINE CILIATE EUPLOTES-VANNUS

Abstract

In the marine hypotrichous ciliate Euplotes vannus, the transient K+ outward current, I(K fast), was studied by use of a single-microelectrode voltage-clamp equipment. Activation and inactivation kinetics, and steady-state inactivation are comparable to the properties of A-currents. Not typical for this type of current is its insensitivity to either 4-AP or 3,4-AP and its Ca2+ dependence which was derived from its inhibition by either extracellular Cd2+, La3+, D-600, or by intracellular BAPTA. Actual amplitudes of I(K fast) were obtained from a composite current, by subtraction of early parts of a slowly activating K+ current, I(K slow), and of the early, transient Ca2+ inward current, I(Ca fast), that is typical for ciliates. I(K fast) counteracts I(Ca fast) during the first milliseconds after onset of depolarization such that the composite current is purely outward directed.