Inactivation and Activation of Inward Current During Adaptation to Potassium Ions in Paramecium caudatum

Abstract

Paramecium cells adapted to 2 mM K⁺ were voltage clamped and transient inward currents induced by step-depolarizations were studied. When the external K⁺ concentration was increased to 8 mM, the inward current was largely suppressed. Similar suppression of the inward current was induced by positive shifts of the holding potential above the resting level. The suppression of inward current is, therefore, primarily caused by depolarizing changes in the membrane potential. During adaptation to 8 mM K⁺, the inward current recovered to about 70% of that in the cells adapted to 2 mM K⁺, the resting membrane repolarized, and the steady state inactivation of the inward current was shifted to a more positive level than the cells adapted to 2 mM K⁺. When mutant cells which lack depolarization-sensitive Ca-current were adapted to 8 mM K⁺, a step-depolarization induced a fast activation of outward current, which would subtract from the inward current in the wild-type cells, suggesting that the transient inward current of cells adapted to 8 mM K⁺ is partially reduced by the fast activation of outward current.