Two distinct Ca²⁺-dependent signaling pathways regulate the motor output of cochlear outer hair cells

The outer hair cells (OHCs) of the cochlea have an electromotility mechanism, based on conformational changes of voltage-sensitive 'motor' proteins in the lateral plasma membrane. The translocation of electrical charges across the membrane that accompanies electromotility imparts a voltage dependency to the membrane capacitance. We used capacitance measurements to investigate whether electromotility may be influenced by different manipulations known to affect intracellular Ca²⁺ or Ca²⁺-dependent protein phosphorylation. Application of acetylcholine (ACh) to the synaptic pole of isolated OHCs evoked a Ca²⁺-activated apamin-sensitive outward K⁺ current. It also enhanced electromotility, probably because of a phosphorylation-dependent decrease of the cell's axial stiffness. However, ACh did not change the voltage-dependent capacitance either in conventional whole-cell experiments or under perforated-patch conditions. The effects produced by the Ca²⁺ ionophore ionomycin mimicked those produced by ACh. Hyperpolarizing shifts of the voltage dependence of capacitance and electromotility were induced by okadaic acid, a promoter of protein phosphorylation, whereas trifluoperazine and W-7, antagonists of calmodulin, caused opposite depolarizing shifts. Components of the protein phosphorylation cascade - IP₃ receptors and calmodulin-dependent protein kinase type IV - were immunolocalized to the lateral wall of the OHC. Our results suggest that two different Ca²⁺-dependent pathways may control the OHC motor output. The first pathway modulates cytoskeletal stiffness and can be activated by ACh. The second pathway shifts the voltage sensitivity of the OHC electromotile mechanism and may be activated by the release of Ca²⁺ from intracellular stores located in the proximity of the lateral plasma membrane.