Single-channel properties of I-Ks potassium channels

Expressed in Xenopus oocytes, KvLQT1 channel subunits yield a small, rapidl y activating, voltage-dependent potassium conductance. When coexpressed wit h the minK gene product, a slowly activating and much larger potassium curr ent results. Using fluctuation analysis and single-channel recordings, we h ave studied the currents formed by human KvLQT1 subunits alone and in conju nction with human or rat minK subunits. With low external K+, the single-ch annel conductances of these three channel types are estimated to be 0.7, 4. 5, and 6.5 pS, respectively, based on noise analysis at 20 kHz bandwidth of currents at +50 mV. Power spectra computed over the range 0.1 Hz-20 kHz sh ow a weak frequency dependence, consistent with current interruptions occur ring on a broad range of time scales. The broad spectrum causes the apparen t single-channel current value to depend on the bandwidth of the recording, and is mirrored in very "flickery" single-channel events of the channels f rom coexpressed KvLQT1 and human minK subunits. The increase in macroscopic current due to the presence of the minK subunit is accounted for by the in creased apparent single-channel conductance it confers on the expressed cha nnels. The rat minK subunit also confers the property that the outward sing le-channel current is increased by external potassium ions.