Auditory outer hair cells can elongate and shorten at acoustic frequencies in response to changes of plasma membrane potential. We show that this fast bidirectional contractile activity consists of an electromechanical transduction process that occurs at the lateral plasma membrane and can be activated and analyzed independently in small membrane patches inside a patch electrode. Bidirectional forces are generated by increases and decreases in membrane area in response to hyperpolarization and depolarization, respectively. We suggest that the force generation mechanism is driven by voltage-dependent conformational changes within a dense array of large transmembrane proteins associated with the site of electromechanical transduction.

A model is presented for describing the membrane potential-dependent motility of the outer hair cell. This model assumes that the motility is due to conformational changes of motor molecules in the plasma membrane. Two kinds of experimental observations, elasticity of the cell and stretch dependence of the motor molecule, are important for characterize this motility by the model. The motor molecule can be described by a two-state model which has electrical and mechanical components in the free energy. The electrical component is due to the charge transferred across the membrane and the mechanical component is due to a change in membrane area in the two states. It can be shown that the elastic element and the motor element are connected in series. Thus the apparent strain of the cell is represented by the sum of true elastic strain and changes due to motor molecules. This model predicts the amplitude of the movement and the force produced by the motility. The model predicts the force produced under isometric condition is about 0.1 nN/mV, in agreement with values estimated from in vivo conditions. The effect of an elastic load attached to the cell is also discussed.