Voltage Clamp Studies on the Slow Inward Current during the Excitation ofNitellopsis obtusa

Abstract

The ionic currents across the plasmalemma of Nitellopsis obtusa were measured in voltage clamp experiments. Depolarization of the cell by 30-100 mV from the level of the resting potential resulted in (1) a rapid inward current, (2) a subsequent slow inward current, and (3) a stationary outward current. The first current component changed sign at —20 to —30 mV. The second component decreased to a minimum at this clamped level. With increasing depolarizing steps some slow transient current component reappeared without changing sign. This transient inward current occurred also when the potential was clamped either at large depolarizing (+80 mV) or at large hyperpolarizing (—300 mV) potentials. In cases when the slow inward current component was evident cessation of protoplasmic streaming was observed. The ATPase inhibitor dicyclo hexylcarbodiimide (DCCD) at a concentration of 2 x 10~5 M in the external medium inhibited the slow transient inward current without affecting the first rapid current component. It is suggested that the irreversible slow transient current component reflects the onset of some active ion-transport system in the plasmalemma during cell excitation.