Gating of IsK channels expressed in Xenopus oocytes

The channel underlying the slow component of the voltage-dependent delayed outward rectifier K+ current, I-Ks, in heart is composed of the minK and K(v)LQT1 proteins. Expression of the minK protein in Xenopus oocytes results in I-Ks-like currents, I-sK, due to coassembly with the endogenous XK(v)LQT1, The kinetics and voltage-dependent characteristics of I-sK suggest a distinct mechanism for voltage-dependent gating. Currents recorded at 40 mV from holding potentials between -60 and -120 mV showed an unusual "cross-over," with the currents obtained from more depolarized holding potentials activating more slowly and deviating from the Cole-Moore prediction. Analysis of the current traces revealed two components with fast and slow kinetics that were not affected by the holding potential. Rather, the relative contribution of the fast component decreased with depolarized holding potentials. Deactivation and reactivation, after a short period of repolarization (100 ms), was markedly faster than the fast component of activation. These gating properties suggest a physiological mechanism by which cardiac I-Ks may suppress premature action potentials.