Kir4.1 K+ channels are regulated by external cations

The inwardly rectifying potassium channel (Kir), Kir4.1 mediates spatial K+-buffering in the CNS. In this process the channel is potentially exposed to a large range of extracellular K+ concentrations ([K+](o)). We found that Kir4.1 is regulated by K-o(+). Increased [K+] o leads to a slow (mins) increase in the whole-cell currents of Xenopus oocytes expressing Kir4.1. Conversely, removing K+ from the bath solution results in a slow decrease of the currents. This regulation is not coupled to the pHi-sensitive gate of the channel, nor does it require the presence of K67, a residue necessary for K-o(+)-dependent regulation of Kir1.1. The voltage-dependent blockers Cs+ and Ba2+ substitute for K+ and prevent deactivation of the channel in the absence of K-o(+). Cs+ blocks and regulates the channel with similar affinity, consistent with the regulatory sites being in the selectivity-filter of the channel. Although both Rb+ and NH4+ permeate Kir4.1, only Rb+ is able to regulate the channel. We conclude that Kir4.1 is regulated by ions interacting with specific sites in the selectivity filter. Using a kinetic model of the permeation process we show the plausibility of the channel's sensing the extracellular ionic environment through changes in the selectivity occupancy pattern, and that it is feasible for an ion with the selectivity properties of NH4+ to permeate the channel without inducing these changes.