Properties of a calcium- and voltage-activated potassium current in Helix pomatia neurons

Abstract

A calcium- and voltage-dependent current was found to be the principal outward current in identified Helix neurons. The current depends on the presence of [Ca²⁺]₀, with half maximal activation at 1 mM [Ca²⁺]₀, and it saturates beyond about 5 mM. The current is termed I_K(Ca) since the charge carried by it corresponds to the amount of potassium ions transferred from the cell interior, as determined from the increase in K⁺ concentration in the external volume with K⁺ liquid ion-exchanger microelectrodes. I_K(Ca) is characterized by bell shaped isochronal I/V curves. The peaks of these curves move from +30 mV to about +70 mV with an increase of the time of measurement from 30–200 ms. I_K(Ca) rise times have a minimum of 10–15 ms at low depolarization around 0 mV, but increase about exponentially with more positive potentials. A tenfold decrease in [Ca²⁺]₀ over the range of 30 to 0.3 mM also produces an increase in rise time, equivalent to a positive shift of potential by 20 mV. On repolarization of the membrane I_K(Ca) disappears much faster than the intracellularly accumulated Ca²⁺, with a time constant which is similar to the minimum activation time constant.