Is the secretory K channel in the rat CCT ROMK?

The biophysical properties of low-conductance secretory K (SK) channels in the apical membrane of the rat cortical collecting tubule were examined to compare these properties with those of the cloned renal K channels of the ROMK family expressed in oocytes. At room temperature, with the tubule superfused with 140 mM K and 110 mM cation in the pipette, the inward single-channel conductance of the SK channels was 36 +/- 1 pS for K, 41 +/- 2 pS for NH4, and 22 +/- 3 pS for Tl. The reversal potential was nearly the same for K and Tl in the pipette but was shifted by -60 mV for NH4. The kinetics of the channel when K was the permeant ion could be described by a single open state (mean open time, 24 ms) and two closed states (mean closed times, 1.6 and 65 ms). The kinetics of SK changed when Tl was the permeant ion (mean open times of 6.6 ms and no long closed state) and when NH4 was the permeant ion (mean open time of 3.0 ms and a more prevalent long closed state). Thus the gating kinetics of the channel depend strongly on the nature of the conducted ion. The properties of SK channels were quite similar to those of ROMK2 expressed in Xenopus oocytes and measured under similar conditions, suggesting that these channels are identical.