Membrane potential measurement in isolated outer hair cells of the guinea pig cochlea using conventional microelectrodes

A model of ionic transport and osmotic volume control in cochlear outer hair cells

A computational model of the outer hair cell (OHC) of the mammalian cochlea is presented. It addresses the way in which movement of ions controls the cell shape and regulates pH. The model takes into account the possible chloride-bicarbonate exchange function of prestin, a protein highly expressed in the plasma membrane of OHCs. A model of intracellular pH transients following the imposition of a rapid acid load upon the cell has been adapted from the squid axon literature and further extended in order to investigate the effects of ion transport upon the osmotic flux of water into the cell. The model predicts the slow length changes of OHCs reported in the literature a feature which may control the contribution of OHCs to cochlear amplification.

Three-dimensional culture of newborn rat utricle using an extracellular matrix promotes formation of a cyst

The vestibule is the end organ devoted to sensing of head movements in space. To function properly, its mechano-receptors require the presence of a unique apical extracellular medium, the endolymph. Numerous studies have elucidated the mechanisms involved in the production and homeostasis of this unique medium and the responses of sensory cells to stimulation. However, anatomical constraints have prevented direct and simultaneous studies of their relationships. The aim of this study was the development of an in vitro model that would allow concomitant investigations on maturation and physiological properties of both the hair cells and their endolymphatic compartment. A three-dimensional (3D) culture of newborn rat utricles using an extracellular matrix sustaining 3D cellular growth was developed during 3, 6, or 10 days in vitro (DIV). Using morphological and electrophysiological techniques, we describe the de novo formation of a cyst. It was composed of the sensory epithelium and non-sensory cells-canalar, dark and intermediate cells-that polarized so that their apical surface faced its lumen. During the time of culture, the utricular potential (UP) was steady (-1.1+/-5.0 mV) in oxygenated condition, while in anoxia, the UP significantly decreased to -8.4+/-1.0 mV at 8 DIV. Over the same period, the K+ concentration in the cyst increased up to 86.1+/-33.9 mM (versus 5.6+/-1.5 mM in the bath). These observations indicated that the mechanisms generating the UP and the K-secretory activity were functional at this stage. Concomitantly, the hair cells acquired mature and functional properties: the type 1 and type 2 phenotypes, a mean resting membrane potential of -68.1+/-4.6 mV and typical electrophysiological responses. This preparation provides a powerful means to simultaneous access the hair cells and their endolymphatic compartment, with the possibility to use multi-technical approaches to investigate their interdependent relationships.

A biophysical model of an inner hair cell

Whole-cell patch-clamp recordings on isolated inner hair cells (IHCs) of guinea pig cochleae have revealed the presence of voltage-gated potassium channels. A biophysical model of an IHC is presented that indicates activation of slow voltage-gated potassium channels may lead to receptor potentials whose dc component decreases during the stimulus, and membrane potential hyperpolarizes when the stimulus is turned off. Both the decreasing dc and the hyperpolarization are, respectively, consistent with rapid adaptation and suppression of spontaneous rate in the auditory nerve. Receptor potentials recorded in vivo do not show these features, and when a nonspecific leak is included in the model to simulate microelectrode impalement, the model's receptor potentials become similar to those in vivo. The nonspecific leak creates an electrical shunt that masks slow channel activity and allows the cell to depolarize. Both the decreasing dc and the hyperpolarization are sensitive to the resting potential. Because the reported resting potentials in vivo and in vitro differ greatly, the model is used to investigate homeostatic mechanisms responsible for the resting potential. It is found that the voltage-gated potassium channels have the greatest influence on the resting potential, but that the standing transducer current may be sufficient to eliminate the decreasing dc and after-stimulus hyperpolarization.

Simultaneous monitoring of slow cell motility and calcium signals of the guinea pig outer hair cells

'Slow' motility (shape changes over seconds to minutes) of the mammalian cochlear outer hair cell (OHC) could play a protection role from intense sound pressure and is associated with elevation of the cytosolic free Ca(2+) concentration ([Ca(2+)](i)). In the present work, a new approach was elaborated using fluorescent imaging for continuous monitoring of both [Ca(2+)](i) changes and slow motility of OHCs employing the Ca(2+) fluorescent indicator Fura-2. Whole OHC fluorescence and that of cell segments were analyzed to discriminate between fluorescence changes caused by [Ca(2+)](i) rise and those related to change of the cell shape. The reliability of the method was examined by simultaneous monitoring of [Ca(2+)](i) and OHC length changes induced by change of buffer osmolarity or by increase of KCl concentration. The method revealed that the time course of [Ca(2+)](i) increase and rate of cell shortening often do not coincide. It was also observed that [Ca(2+)](i) increased in 70 mM KCl more slowly than the rate of KCl delivery to OHCs. The comparison of the time courses of [Ca(2+)](i) elevation, induced by increase of K(+)/Na(+) ratio and by substitution of Na(+) with N-methyl-D-glucamine(+), indicated that the relatively slow kinetics of [Ca(2+)](i) increase in the OHC is partially attributed to regulation of Ca(2+) homeostasis by the Na(+)/Ca(2+) exchanger.

The Goldman-Hodgkin-Katz equation and graphical 'load-line' analysis of ionic flow through outer hair cells

While the 'membrane potential' of a cell which has a homogeneous membrane and surrounding environment, and which is not pumping ions electrogenically (passing no net current through its membranes), can be estimated from the Goldman voltage equation, this equation is inappropriate for other cells. In the mammalian cochlea such problematic cells include the cells of stria vascularis and the sensory hair cells of the organ of Corti. Not only is the Goldman voltage equation inappropriate, but in asymmetric cells the concept of a single 'membrane potential' is misleading: a different transmembrane voltage is required to define the electrical state of each section of the cell's heterogeneous membrane. This paper presents a graphical 'load-line analysis' of currents through one such asymmetric cell, the outer hair cells of the organ of Corti. The approach is extremely useful in discussing the effects of various cochlear manipulations on the electrical potential within hair cells, even without a detailed knowledge of their membrane conductance. The paper discusses how modified Goldman-Hodgkin-Katz equations can be used to describe stretch-activated channels, voltage-controlled channels, ligand-mediated channels, and how the combination of these channels and the extracellular ionic concentrations should affect the hair cell's resting intracellular potential and resting transcellular current, its receptor current and receptor potential, and the extracellular microphonic potential around these cells. Two other issues discussed are the role of voltage-controlled channels in genetically determining membrane potential, and the insensitivity of hair cells to changes of extracellular potassium concentration under some conditions.

Receptor potential characteristics during direct stereocilia stimulation of isolated outer hair cells from the guinea-pig

The receptor potential as a function of stimulus amplitude and frequency was studied with the patch-clamp technique in isolated outer hair cells (OHCs) with a length ranging from 30 to 87 microm during direct mechanical stimulation of the stereocilia. The amplitude and frequency of the stimulation were varied from 125 nm to 2 microm and from 100 Hz to 2.5 kHz, respectively. The mean resting membrane potential before stimulation was -64.25 +/- 1.4 mV (mean +/- SE, n = 26). Irrespective of the frequency used, stereocilia stimulation produced a combination of AC and DC responses, and both components showed saturation with increasing stimulation. Frequency responses appeared to be a function of intensity and resembled a low-pass filter with a time constant ranging from 0.2 to 2.0 ms. With increasing stereocilia stimulation, the relative contribution of high frequencies to the AC component decreased, suggesting a decrease of the corner frequency. The saturated amplitude of the AC component for low-frequency stimulation (100 Hz) was proportional to cell length and increased with a mean rate of 0.014 mV microm(-1). A relationship between the DC response of the receptor potential and the pre-stimulus membrane potential was found. Recordings with more negative membrane potentials had greater DC components, while more depolarized recordings demonstrated smaller DC components. These fluctuations seemed to be defined by the interaction between the probe and stereocilia bundle and could be in the range of the transfer function for each cell.

Effects of protein kinase C on the Na(+)-H+ exchange in the cochlear outer hair cell

Two routes, associated with protein kinase C (PKC) and cyclic AMP (cAMP), to regulate the activation of Na(+)-H+ exchange (NHE) were investigated in the outer hair cell (OHC). The intracellular pH (pHi) of OHC loaded with the pHi indicator dye was measured by using fluorescence ratio imaging microscopy. The acid load was carried out by an "NH4Cl pre-pulse". The rate of acid extrusion after an acid load was increased as pHi became more acidic. The linear relationship between the rate of acid extrusion and pHi suggests the presence of an internal modifier site for the H+ ion on the NHE. Although addition of phorbol 12-myristate 13-acetate (TPA) had no effect on pHi in unstimulated conditions, the pHi recovery from an acid load was enhanced by exposure to TPA. This finding can be explained by the pHi dependence of the PKC action, namely, that the activating effect of PKC on the NHE is facilitated by internal acidification. In contrast to the PKC effect, addition of dibutyryl cAMP failed to change the rate of acid extrusion. These findings are consistent with the characteristic property of the NHE-1. Thus, intracellular signal transduction to regulate the activity of the NHE in the OHC involves phosphorylation by PKC.