Direct activation of cloned KATP channels by intracellular acidosis

ATP-sensitive K+ (K-ATP) channels may be regulated by protons in addition to ATP, phospholipids, and other nucleotides, Such regulation allows a control of cellular excitability in conditions when pH is low but ATP concentration is normal. However, whether the K-ATP changes its activity with pH alterations remains uncertain, In this study we showed that the reconstituted K-ATP was strongly activated during hypercapnia and intracellular acidosis using whole-cell recordings. Further characterizations in excised patches indicated that channel activity increased with a moderate drop in intracellular pH and decreased with strong acidification. The channel activation was produced by a direct action of protons on the Kir6 subunit and relied on a histidine residue that is conserved in all K-ATP. The inhibition appeared to be a result of channel rundown and was not seen in whole-cell recordings. The biphasic response may explain the contradictory pH sensitivity observed in cell-endogenous K-ATP in excised patches. Site-specific mutations of two residues showed that pH and ATP sensitivities were independent of each other. Thus, these results demonstrate that the proton is a potent activator of the K-ATP. The pH-dependent activation may enable the K-ATP to control vascular tones, insulin secretion, and neuronal excitability in several pathophysiologic conditions.