It is the prevailing notion that cochlear outer hair cells function as mechanical effectors as well as sensory receptors. Electrically induced changes in the shape of mammalian outer hair cells, studied in vitro, are commonly assumed to represent an aspect of their effector process that may occur in vivo. The nature of the motile process is obscure, even though none of the established cellular motors can be involved. Although it is known that the motile response is under voltage control, it is uncertain whether the stimulus is a drop in the voltage along the long axis of the cell or variation in the transmembrane potential. We have now performed experiments with cells partitioned in differing degrees between two chambers. Applied voltage stimulates the cell membrane segments in opposite polarity to an amount dependent on the partitioning. The findings show, in accordance with previous suggestions, that the driving stimulus is a local transmembrane voltage drop and that the cellular motor consists of many independent elements, distributed along the cell membrane and its associated cortical structures. We further show that the primary action of the motor elements is along the longitudinal dimension of the cell without necessarily involving changes in intracellular hydrostatic pressure. This establishes the outer hair cell motor as unique among mechanisms that control cell shape.

1. A theory of cochlear outer hair cell electromotility is developed and specifically applied to somatic shape changes elicited in a microchamber. The microchamber permits the arbitrary electrical and mechanical partitioning of the outer hair cell along its length. This means that the two partitioned segments are stimulated with different input voltages and undergo different shape changes. Consequently, by imposing more constraints than other methods, experiments in the microchamber are particularly suitable for testing different theories of outer hair cell motility. 2. The present model is based on simple hypotheses. They include a distributed motor associated with the cell membrane or cortex and the assumption that the displacement generated by the motor is related to the transmembrane voltage across the associated membrane element. It is expected that the force generated by the motor is counterbalanced by an elastic restoring force indigenous to the cell membrane and cortex, and a tensile force due to intracellular pressure. It is assumed that all changes take place while total cell volume is conserved. The above elements of the theory taken together permit the development of qualitative and quantitative predictions about the expected motile responses of both partitioned segments of the cell. Only a DC treatment is offered here. 3. Both a linear motor and an expanded treatment that incorporates a stochastic molecular motor model are considered. The latter is represented by a two-state Boltzmann process. We show that the linear motor treatment is an appropriate extrapolation of the stochastic motor theory for the case of small voltage driving signals. Comparison of experimental results with model responses permits the estimation of model parameters. Good match of data is obtained if it is assumed that the molecular motors undergo conformational length changes of 0.7-1.0 nm, that they have an effective displacement vector at approximately -20 degrees with the long axis of the cell, and that their linear density is approximately 80/microns. 4. An effort is made to parcel out motile response components that are a direct consequence of the motor action from those that are mediated by cytoplasmic pressure changes brought about by the concerted action of the motors. We show that pressure effects are of minor importance, and thus rule out models that rely on radial constriction/expansion-mediated internal pressure change as the primary cause of longitudinal motility. 5. As a consequence of the interaction between the Boltzmann process and the mechanical characteristics of the cell, the electromotile response is asymmetric.(ABSTRACT TRUNCATED AT 400 WORDS)

Metabolism and mitochondrial dysfunction are known to be involved in many different disease states. We have employed two-photon fluorescence imaging of intrinsic mitochondrial reduced nicotinamide adenine dinucleotide (NADH) to quantify the metabolic state of several cultured cell lines, multicell tumor spheroids, and the intact mouse organ of Corti. Historically, fluorescence intensity has commonly been used as an indicator of the NADH concentration in cells and tissues. More recently, fluorescence lifetime imaging (FLIM) has revealed that changes in metabolism produce not only changes in fluorescence intensity, but also significant changes in the lifetimes and concentrations of free and enzyme-bound pools of NADH. Since NADH binding changes with metabolic state, this approach presents a new opportunity to track the cellular metabolic state.