I(sK) Channel in Strial Marginal Cells. Voltage-Dependence, Ion-Selectivity, Inhibition by 293B and Sensitivity to Clofilium.

Strial marginal cells (SMC) and vestibular dark cells (VDC) are known to secrete K(+) into endolymph. Slowly-activating, voltage-dependent K(+) channels (KCNQ1/KCNE1; IsK; min K) have been identified in the apical membrane of these cells. Several experimental maneuvers known to increase or decrease transepithelial K(+) secretion have been found in VDC to change the current through these channels in the same ways. In both SMC and VDC the kinetics of activation and deactivation resemble those of the I(sK) channel exogenously expressed in Xenopus oocytes and endogenous to heart myocytes. The present study sought evidence that this current is indeed carried by I(sK) channels and that this current is the basis for transepithelial K(+) secretion. Both on-cell macro-patch recordings of the apical membrane and perforated-patch whole-cell recordings were made on SMC from gerbil in order to measure macroscopic cell currents. The on-cell current was found to 1) be K(+)-selective, 2) have a cation permeability sequence of K(+) ~ Rb(+) > Cs(+) >> Li(+) = Na(+), 3) be activated with a time constant of 1764 ± 413 ms by voltage steps from 0 to +40 mV, 4) be deactivated with a time constant of 324 ± 57 ms by voltage steps from 0 to -40 mV and 5) be reduced 84 ± 5% by bumetanide (10(-5) M), an inhibitor of K(+) secretion. The single-channel conductance of the apical currents in the homologous VDC was estimated by fluctuation analysis to be 1.6 pS. The potent inhibitor of I(sK) channels, chromanol 293B (10(-5) M), reduced the whole-cell current in SMC by 72 ± 10 %. Clofilium (10(-4) M), a putative I(sK) channel inhibitor known to have additional non-specific effects, led to a stimulation of both on-cell (by 598 ± 177%) and whole-cell (by 162 ± 18%) currents in gerbil SMC but to a decrease of whole-cell currents (by 39 ± 12%) in rat SMC. Taken together with other findings reviewed here, these results strongly argue that the slowly-activating, voltage-dependent conductance in the apical membrane of SMC is the I(sK) channel and provide additional evidence for the poor specificity of clofilium.