INACTIVATION OF DELAYED OUTWARD CURRENT STUDIED BY MEANS OF REPEATED VOLTAGE-CLAMP PULSES

Neurons under voltage-clamp generate prolonged outward currents in response to depolarizing command pulses. Delayed outward current raises to a peak and then declines to a non-zero steady-state. The present research studied the time decay of this current by means of repeated voltage-clamp steps and substituting calcium with barium in the external medium. The experiments were performed on neurons of the abdominal ganglia of land snail Helix pomatia. The neurons were impaled with the microelectrodes and voltage-clamped by means of a conventional type electronic circuit. At a fixed holding potential the outward current increases with increasing membrane potential or, at a given clamp potential, a reduction of the outward current is observed when increasing holding potential. Substitution of Ca with Ba results in a decrease of the total delayed outward current as the membrane potential is made more positive. The deficit in net outward charge transfer, under voltage clamp condition, is related to the inward flow of Ca++ ions. Ba++ does not activate the calcium dependent potassium current and with depolarizing clamps from -40 mV, to inactivate the transient outward current, it is possible to measure the voltage dependent component of delayed outward current. Barium seems to affect both the voltage - and Ca-dependent components of the delayed potassium current