Vestibular hair bundles control pH with (Na+, K+)/H+ exchangers NHE6 and NHE9

In hair cells of the inner ear, robust Ca (2+)/H (+) exchange mediated by plasma-membrane Ca (2+) -ATPase would rapidly acidify mechanically sensitive hair bundles without efficient removal of H (+). We found that, whereas the basolateral membrane of vestibular hair cells from the frog saccule extrudes H (+) via an Na (+)-dependent mechanism, bundles rapidly remove H (+) in the absence of Na (+) and HCO3 (-), even when the soma is acidified. K (+) was fully effective and sufficient for H (+) removal; in contrast, Rb (+) failed to support pH recovery. Na (+)/ H (+) -exchanger isoform 1 (NHE1) was present on hair-cell soma membranes and was likely responsible for Na (+) -dependent H (+) extrusion. NHE6 and NHE9 are organellar isoforms that can appear transiently on plasma membranes and have been proposed to mediate K (+)/ H (+) exchange. We identified NHE6 in a subset of hair bundles; NHE9 was present in all bundles. Heterologous expression of these isoforms in yeast strains lacking endogenous exchangers conferred pH-dependent tolerance to high levels of KCl and NaCl. NHE9 preferred cations in the order K (+), Na (+) >> Rb (+) consistent with the relative efficacies of these ions in promoting pH recovery in hair bundles. Electroneutral K (+)/ H (+) exchange, which we propose is performed by NHE9 in hair bundles, exploits the high-K (+) endolymph, responds only to pH imbalance across the bundle membrane, is unaffected by the + 80mV endocochlear potential, and uses mechanisms already present in the ear for K (+) recycling. This mechanism allows the hair cell to remove H (+) generated by Ca (2+) pumping without ATP hydrolysis in the cell.