Permeant Cations and Blockers Modulate pH Gating of ROMK Channels

External potassium (K) activates the inward rectifier ROMK (K ir1.1) by altering the pH gating of the channel. The present study examines this link between external K and internal pH sensitivity using both the two-electrode voltage clamp and the perfused, cut-open Xenopus oocyte preparation. Elevating extracellular K from 1 mM to 10 mM to 100 mM activated ROMK channels by shifting their apparent pK a from 7.2 ± 0.1 ( n = 6) in 1 mM K, to 6.9 ± 0.02 ( n = 5) in 10 mM K, and to 6.6 ± 0.03 ( n = 5) in 100 mM K. At any given internal pH, the number of active ROMK channels is a saturating function of external [ K]. Extracellular Cs (which blocks almost all inward K current) also stimulated outward ROMK conductance (at constant 1 mM external K) by shifting the apparent pK a of ROMK from 7.2 ± 0.1 ( n = 6) in 1 mM K to 6.8 ± 0.01 ( n = 4) in 1 mM K + 104 mM Cs. Surprisingly, the binding and washout of the specific blocker, Tertiapin-Q, also activated ROMK in 1 mM K and caused a comparable shift in apparent pK a. These results are interpreted in terms of both a three-state kinetic model and a two-gate structural model that is based on results with KcsA in which the selectivity filter can assume either a high or low K conformation. In this context, external K, Cs, and Tertiapin-Q activate ROMK by destabilizing the low-K (collapsed) configuration of the selectivity filter.