Activation characteristics of the calcium-dependent outward potassium current in Helix

The activation of calcium-dependent outward potassium current [IK(Ca)] by shortlasting Ca2+ inward currents was studied. These Ca2+ currents were produced either by small depolarizing pulses preceding the larger depolarizations or by interposed repolarizations (I.R.s) starting from depolarized membrane potentials. IK(Ca) then develops with a potential-invariant time course (half time 6-12 ms) and the normally bell-shaped isochronal IK(Ca)/V curve, measured at between 30 and 300 ms, is straightened. However, there Ca2+ - injecting pulses, of any amplitude and duration, do not increase the steady-state conductance to values beyond those measured with single step depolarizations to lower potentials. varied in length, activation of IK(Ca) increased linearly with the during depolarization to near the supposed calcium equilibrium potential with no further Ca2+ influx. When I.R.s are varied in length, activation of I(Ca) increases linearly with the amount of Ca2+ current. Fading of activation during I.R. follows a time course nearly ten times slower than activation and is not expressed in tail currents. The time course of IK(Ca) is described by a function defined only by voltage parameters of activation combined with a minimum activation time constant which is similar to that found in tail currents. Peak location and general form of the IK(Ca)/V relationship for times up to several hundred milliseconds are well predicted without the necessity of explicitly account for actual calcium entry.