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

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 IK(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. IK(Ca) is characterized by cell 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. IK(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 IK(Ca) disappears much faster than the intracellularly accumulated Ca2+, with a time constant which is similar to the minimum activation time constant.