1 Department of Biomedical Engineering, Johns Hopkins University, Baltimore, Maryland 21205; and 2 Department of Chemical Engineering, Michigan State University, East Lansing, Michigan 48824
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ABSTRACT |
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Jerry, Rocco A. and Ashim Dutta. Molecular motor and electrokinetic contributions to outer hair cell electromotility. J. Neurophysiol. 79: 471-473, 1998. The outer hair cell of the inner ear is believed to be responsible for the high sensitivity and selectivity of mammalian hearing. Molecular motors are generally believed to cause the electrically-driven length change (electromotility) of the outer hair cell. It has been suggested that electrokinetic effects might also play a significant role in electromotility, along with the molecular motors. This paper describes a new technique that can be used to experimentally determine the percentage of the electromotile response that is caused by electrokinetic effects. The technique is based on the novel idea that molecular motor activity cannot in itself generate a net force on the cell, but that electrokinetic effects can. Our method is the first that can separate molecular motor behavior from electrokinetic behavior, during experiments on the outer hair cell.
The cylindrically shaped outer hair cell can change its length when it is electrically stimulated. This phenomenon is known as electromotility, and was first observed by Brownell et al. (1985)
INTRODUCTION
Abstract
Introduction
References
. Electromotility is believed to be responsible for the high sensitivity and the sharp tuning of mammalian hearing.
and Jerry and Dutta (1998a
,b
), among others, have included molecular motors in their models for the outer hair cell.
, electrokinetic forces on the cell wall can be classified into two categories: electrophoretic forces and electro-osmotic drag forces. Electrophoretic forces are simply forces that act on stationary charge sites, because of the presence of an electric field. The wall of the outer hair cell is expected to possess many of these charge sites, and thus electrophoretic forces may be present (Jerry et al. 1995b
). Electro-osmotic drag forces also can be experienced by the cell wall. An electric field can cause the fluid near the cell wall to move (electro-osmosis) and this fluid motion can cause drag forces on the cell wall. Thus it is possible that part of the cell's length change could be produced by both these electrokinetic effects, in addition to the action of the molecular motors.
have shown that the electrokinetic effect could be significant in electromotility of the outer hair cell and should not be ignored. In contrast, certain experiments suggest that electrokinetic effects could be relatively small. For example, see the experiments of Kakehata and Santos-Sacchi (1996)
that treat the outer hair cell with salicylate and lanthanides and also the experiments of Kakehata and Santos-Sacchi (1995)
that show the influence of intracellular pressure on electromotility. However, in the experiments that were performed thus far, there is no obvious way of quantitatively evaluating how much of the electromotile response is produced by electrokinetic effects and how much is produced by molecular motors. Below, we describe a method that can be used to determine the percentage of the electromotile response, which is produced by electrokinetic effects.
, Kakehata and Santos-Sacchi (1995
, 1996)
and others, the voltage difference across the cell wall is controlled; it is assumed that the electric field is directed normal to the cell wall. The second type of electrical stimulation may be called transcellular stimulation and was used in the original electromotility experiments of Brownell et al. (1985)
. Transcellular stimulation simply means that the cell is situated between two electrodes. The electrodes are positioned along the axis of the cell, but far from the cell. One end of the cell is attached to a stationary support. Thus applying a voltage across the electrodes causes the cell to change its length.
, who found that there can be a significant electrokinetic force on the nonstationary end of the cell; they also found that the net force on the cylindrical wall of the cell was negligible, because of a balancing of the electrophoretic and electro-osmotic drag forces. Thus the electrokinetic force simply acts like an axially-directed load on the nonstationary end of the cell. [Transcellular stimulation can produce electro-osmosis inside the cell, but the forces produced by intracellular electro-osmosis were shown to be negligibly small in Jerry et al. (1995a
, 1996)
.]
. In his experiments, a flexible glass fiber was attached to one end of the cell, while the other end of the cell was attached to a stationary support. The glass fiber allows the experimenter to apply an axially directed force to one end of the cell. Because the fiber is flexible, Hallworth was able to estimate the magnitude of the applied force, by measuring how much the fiber bends. (The applied force is linearly related to the amount of bending and the fiber is calibrated in advance.) Starting with a cell having an unloaded length L, Hallworth was able to apply different loads to the cell and measure both the applied force f as well as the new length of the cell L2. From this information, he was able to estimate the axial stiffness of the cell.
and the Brownell et al. (1985)
experiments can be utilized to develop a novel method for estimating the importance of electrokinetic effects. We describe the way to combine these experiments below.
glass fiber is sufficiently sensitive to measure the net electrically generated force on the cell. In the process of measuring the cell's axial stiffness, Hallworth had accurately measured the force required to shorten the cell by a few microns. In general, electrical stimulation usually produces a cell length change that is also approximately a few microns (Brownell et al., 1985
). If electrokinetics were entirely responsible for this length change, then one would expect to measure a net force that is approximately the same magnitude as that measured by Hallworth. Thus his glass fiber is suitable for our purposes.
, the glass fiber can easily be detached from the cell, because of very weak bonding between the cell and the glass fiber. In our proposed experiment, however, it is recommended that the glass fiber be firmly attached to the cell. Thus it may be necessary to pretreat the surface of the glass fiber with a material that can adhere to the cell wall. Alternatively, one end of the cell could be pretreated with an adhesive agent. The experimenter would be wise to select a surface treatment that possesses little surface charge of its own. There is an additional consideration that we point out. Our analysis ignores the surface charges on the end of the cell that is attached to the glass fiber. These particular surface charges could contribute to the net electrokinetic force on the wall. If this end of the cell were completely covered by the glass rod, then the surface charge of this cell end may be ignored. Even if the glass rod does not completely cover the cell end, then the surface charge may still be ignored if the cell end is pretreated with an adhesive having a low surface charge.
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ACKNOWLEDGEMENTS |
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We thank S. McLaughlin for a very helpful initial discussion and the Frederick National Cancer Institute for use of computer facilities. The discussion on lanthanides, salicylate, and intracellular pressure was added on request of a referee.
This work was partially supported by the Gesotti Research Fund.
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FOOTNOTES |
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Present address and address for reprint requests: R. A. Jerry, 808 Gabriel Court, Apt. 371; Frederick, MD 21702.
Received 18 September 1997; accepted in final form 8 October 1997.
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REFERENCES |
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