K+-induced stimulation of K+ secretion involves activation of the I-sK channel in vestibular dark cells

Vestibular dark cells in the inner ear secrete K+ from perilymph containing 4 mM K+ to endolymph containing 145 mM K+. Sensory transduction causes K+ to flow from endolymph to perilymph, thus threatening the homeostasis of the perilymphatic K+ concentration which is crucial for maintaining sensory transduction since the basolateral membranes of the sensory cells and adjacent neuronal elements need to be protected from K+-induced depolarization. The present study addresses the questions (1) whether increases in the perilymphatic K+ concentration by as little as 1 mM are sufficient to stimulate KCl uptake across the basolateral membrane of vestibular dark cells, (2) whether K+-induced stimulation of KCl uptake causes stimulation of the I-sK channel in the apical membrane, and (3) whether the rate of transepithelial K+ secretion depends on the perilymphatic (basolateral) K+ concentration when the apical side of the epithelium is bathed with a solution containing 145 mM K+, as in vivo. Uptake of KCl was monitored by measuring cell height as an indicator for cell volume. The current (I-IsK), conductance (g(IsK)) and inactivation time constant (tau(IsK)) of the I-sK channel as well as the apparent reversal potential of the apical membrane (V-r) were obtained with the cell-attached macro-patch technique. V-r was corrected for the membrane voltage previously measured with microelectrodes. The rate of transepithelial K+ secretion J(K) was obtained as equivalent short circuit current from measurements of the transepithelial voltage (V-t) and resistance (R(t)) measured in the micro-Ussing chamber. Cell height of vestibular dark cells was 7.2 mu M (average). Elevations of the extracellular K+ concentration from 3.5 to 4.5 mM caused cell swelling with an initial rate of cell height change of 11 mm/s. With 3.6 mM K+ in the pipette I-IsK was outwardly directed and elevation of the extracellular K+ concentration from 3.6 to 25 mM caused an increase of I-IsK from 12 to 65 pA, g(IsK) from 152 to 950 pS and tau(IsK) from 278 to 583 ms as well as a hyperpolarization of V-r from -50 to -60 mV. With 150 mM K+ in the pipette I-IsK was inwardly directed and the elevation of the extracellular K+ concentration caused an increase of I-IsK from -1 to -143 pA, g(IsK) from 141 to 1833 pS and tau(IsK) from 248 to 729 ms. V-r remained within+/-10 mV from zero. J(K) was 4.8 nmolxcm(-2)xs(-1) when the both the apical side and the basolateral side of the epithelium were perfused with a solution containing 3.5 mM K+. Elevation of the basolateral K+ concentration by 1 mM caused J(K) to increase by 1.1 nmolxcm(-2)xs(-1) or 23%. When the basolateral side of the epithelium was perfused with a solution containing 3.5 mM K+ and the apical side with a solution containing 145 mM K+, as in vivo, J(K) was 0.8 nmolxcm(-2)xs(-1) and elevation of the basolateral K+ concentration by 1 mM caused J(K) to increase by 0.8 nmolxcm(-2)xs(-1) or 100%. These data suggest that physiologically relevant increases in the perilymphatic K+ concentration increase J(K) by increasing KCl uptake across the basolateral membrane and activation of K+ release via the I-sK channel in the apical membrane. Thus, the data demonstrate that vestibular dark cells adjust the rate of K+ secretion into endolymph according to the perilymphatic K+ concentration.