Inactivation of delayed potassium current in cultured bovine chromaffin cells

Abstract

Potassium currents of cultured bovine chromaffin cells in the presence of CdCl2 (0.5 mM) or CoCl2 (2–4 mM) may be described as a component that follows an n4 kinetics over the first 100 ms. However, during prolonged voltage clamp steps (5–20 s) this current reaches a peak and then declines to a non-zero steady-state level. In the absence of potassium accumulation, slow decay has been attributed to slow inactivation of the K+ conductance. Slow inactivation of calcium-independent potassium currents responds to the following: (i) the inactivation has a time course that may be fitted by a sum of two exponentials; (ii) inactivation is use-dependent (increasing frequency or duration of depolarizing pulses increases the extent of inactivation). Use-dependent inactivation does not appear when short (100 ms) hyperpolarizing prepulses are applied; (iii) at a membrane potential of −60 mV, approximately 20% of the channels are inactivated; (iv) recovery from inactivation is very slow. At a holding potential of −70 mV it takes about 20 s to reach the original value. The rate of inactivation recovery is voltage-dependent. The more hyperpolarized the membrane potential, the faster the recovery. The presence of use-dependent inactivation with slow recovery could be relevant for the stimulus–secretion coupling modulation in chromaffin cells.