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

Abstract

The activation of calcium-dependent outward potassium current [I_K(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. I_K(Ca) then develops with a potential-invariant time course (half time 6–12 ms) and the normally bell-shaped isochronal I_K(Ca)/V curve, measured at between 30 and 300 ms, is straightened. However, these 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 I_K(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 I_K(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 I_K(Ca)/V relationship for times up to several hundred milliseconds are well predicted without the necessity to explicitly account for actual calcium entry.