Plasma Membrane Ca2+-ATPase Extrudes Ca2+from Hair Cell Stereocilia

Abstract

Mechanically sensitive hair cells of the auditory and vestibular systems use Ca²⁺to control adaptation of mechanical transduction, to effect frequency tuning, to trigger neurotransmitter release, and to mediate efferent synaptic signaling. To determine the role that pumps play in regulation of Ca²⁺in the hair bundle, the organelle responsible for mechanoelectrical transduction, we localized and quantified the plasma membrane Ca²⁺-ATPase (PMCA) of the bundle. We found that each hair bundle contains ∼10⁶PMCA molecules or ∼2000 per square micrometer of bundle membrane and that PMCA is the principal calmodulin binding protein of the bundle. Consistent with biochemical estimates of PMCA density, we measured with extracellular Ca²⁺-selective electrodes a substantial Ca²⁺efflux from bundles. The number of bundle Ca²⁺pumps and magnitude of resting Ca²⁺efflux suggested that PMCA should generate a substantial membrane current as bundles expel Ca²⁺. Measurement of whole-cell currents revealed a transduction-dependent outward current that was consistent with the activity of PMCA. Finally, dialysis of hair cells with PMCA inhibitors led to a large increase in the concentration of Ca²⁺in bundles, which suggests that PMCA plays a major role in regulating bundle Ca²⁺concentration. Our data further indicate that PMCA could elevate the extracellular Ca²⁺concentration close to hair bundles above the low level found in bulk endolymph.