Cloned neuronal IK(A) channels reopen during recovery from inactivation

The kinetic behaviour and functional role of potassium ion (K+) channels mediating a fast-inactivating K+ current (IK(A)) has been widely discussed. Activating in the subthreshold range of excitation, IK(A) channels are assumed to reduce the excitatory effect of depolarizing membrane currents in a time-dependent manner. Here we report that IK(A) channels not only open in response to a depolarization but open again after repolarization of the membrane. Although the current in response to the depolarization is rapidly inactivating, the current elicited by repolarization declines slowly and produces long-lasting afterhyperpolarizations under current-clamp conditions. This implies an additional physiological role for IK(A) channels, particularly those that activate positive to the threshold of excitation. The underlying biophysical mechanism was studied by fast-application of peptides corresponding to the N-terminal end of the IK(A) channel proteins. It was found to be a voltage-dependent release of the inactivation gate.