Ammonium ion enhances the calcium-dependent gating of a mammalian large conductance, calcium-sensitive K+ channel

We observed that the current amplitude and activation of expressed, mouse b rain large conductance, calcium-sensitive K+ channels (BKCa channels) may b e reversibly enhanced following addition of low concentrations of the weakl y permeant cation NH4+ to the cytoplasmic face of the channel in excised, i nside-out membrane patches from HEK 293 cells. Conductance-voltage relation s were left-shifted along the voltage axis by addition of NH4Cl in a concen tration-dependent manner, with an EC50 of 18.5 mM. Furthermore, this effect was observed in the presence of cytosolic free calcium (similar to1 muM), but was absent in a cytosolic bath solution containing nominally zero free calcium (e.g., 5 mM EGTA only), a condition under which these channels unde rgo largely voltage-dependent gating. Recordings of single BKCa channel eve nts indicated that NH4+ increased the channel open probability of single ch annel activity similar to3-fold, but did not alter the amplitude of single channel currents. These findings suggest that the calcium-sensitive gating of mammalian BKCa channels may be modified by other ions present in cytosol ic solution.