Bio

Academic Appointments


Publications

Journal Articles


  • How Close Should the Outer Hair Cell RC Roll-Off Frequency Be to the Characteristic Frequency? BIOPHYSICAL JOURNAL Ospeck, M., Iwasa, K. H. 2012; 102 (8): 1767-1774

    Abstract

    Recent experiments have shown a much larger conductance in outer hair cells, the central components of the mammalian cochlear amplifier. The report used only the cell's linear capacitance, which together with increased conductance, raised the cell's RC corner frequency so that voltage-dependent motility was better able to amplify high-frequency sounds. We construct transfer functions for a simple model of a high characteristic frequency (CF) local cochlear resonance. These show that voltage roll-off does not occur above the RC corner. Instead, it is countered by high-pass filtering that is intrinsic to the mammal's electromechanical resonance. Thus, the RC corner of a short outer hair cell used for high-frequency amplification does not have to be close to the CF, but depending on the drag, raised only above 0.1 CF. This high-pass filter, built in to the mammalian amplifier, allows for sharp frequency selectivity at very high CF.

    View details for DOI 10.1016/j.bpj.2012.02.049

    View details for Web of Science ID 000303003300010

    View details for PubMedID 22768932

  • The Evolution of Prestin: Examination with Membrane Thickness Sensitivity WHAT FIRE IS IN MINE EARS: PROGRESS IN AUDITORY BIOMECHANICS Izumi, C., Bird, J., Schaechinger, T., Oliver, D., Iwasa, K. H. 2011; 1403

    View details for DOI 10.1063/1.3658079

    View details for Web of Science ID 000301945200028

  • Gating of Two Mechanoelectrical Transducer Channels Associated with a Single Tip Link BIOPHYSICAL JOURNAL Sul, B., Iwasa, K. H. 2010; 99 (4): 1027-1033

    Abstract

    Although gating of mechanoelectrical transducer (MET) channels has been successfully described by assuming that one channel is associated with a tip link in the hair bundle, recent reports indicate that a single tip link is associated with more than one channel. To address the consistency of the model with the observations, gating of MET channels is described here by assuming that each tip link is associated with two identical MET channels, which are connected either in series or in parallel. We found that series connection does not lead to a single minimum of stiffness with respect to hair bundle displacement unless the minimum is above a certain positive value. Thus, negative stiffness must appear in pairs in the displacement axis. In contrast, parallel connection of the two channels predicts gating compliance similar to that predicted by the one-channel-per-tip-link model of channel gating, within the physiological range of parameters. Parallel connection of MET channels is, therefore, a reasonable assumption to explain most experimental observations. However, the compatibility with series connection cannot be ruled out for experimental data on turtle hair cells.

    View details for DOI 10.1016/j.bpj.2010.05.029

    View details for Web of Science ID 000281103200005

    View details for PubMedID 20712985

  • Sensitivity of Prestin-Based Membrane Motor to Membrane Thickness BIOPHYSICAL JOURNAL Fang, J., Izumi, C., Iwasa, K. H. 2010; 98 (12): 2831-2838

    Abstract

    Prestin is the membrane protein in outer hair cells that harnesses electrical energy by changing its membrane area in response to changes in the membrane potential. To examine the effect of membrane thickness on this protein, phosphatidylcholine (PC) with various acyl-chain lengths were incorporated into the plasma membrane by using gamma-cyclodextrin. Incorporation of short chain PCs increased the linear capacitance and positively shifted the voltage dependence of prestin, up to 120 mV, in cultured cells. PCs with long acyl chains had the opposite effects. Because the linear capacitance is inversely related to the membrane thickness, these voltage shifts are attributable to membrane thickness. The corresponding voltage shifts of electromotility were observed in outer hair cells. These results demonstrate that electromotility is extremely sensitive to the thickness of the plasma membrane, presumably involving hydrophobic mismatch. These observations indicate that the extended state of the motor molecule, which is associated with the elongation of outer hair cells, has a conformation with a shorter hydrophobic height in the lipid bilayer.

    View details for DOI 10.1016/j.bpj.2010.03.034

    View details for Web of Science ID 000278913500011

    View details for PubMedID 20550895

  • Organization of membrane motor in outer hair cells: an atomic force microscopic study PFLUGERS ARCHIV-EUROPEAN JOURNAL OF PHYSIOLOGY Sinha, G. P., Sabri, F., Dimitriadis, E. K., Iwasa, K. H. 2010; 459 (3): 427-439

    Abstract

    Using atomic force microscopy, we imaged the cytosolic surface of the lateral plasma membrane of outer hair cells from guinea pigs' inner ear. We used a "cell-free" preparation, in which a patch of plasma membrane was firmly attached to a substrate and the cytoplasmic face was exposed. The membrane patches contained densely packed particles whose diameter, after correcting for the geometry of the probing tip, was approximately 10 nm. The particles were predominantly aligned unidirectionally with spacing of approximately 36 nm. The density of the particle was approximately 850 microm(-2), which could be an underestimate presumably due to the method of sample preparation. Antibody-labeled specimens showed particles more elevated than unlabeled preparation indicative of primary and secondary antibody complexes. The corrected diameters of these particles labeled with anti-actin were approximately 12 nm while that with antiprestin were approximately 8 nm. The alignment pattern in antiprestin-labeled specimens resembled that of the unlabeled preparation. Specimens labeled with actin antibodies did not show such alignment. We interpret that the particles observed in the unlabeled membranes correspond to the 10-nm particles reported by electron microscopy and that these particles contain prestin, a member of the SLC26 family, which is essential for electromotility.

    View details for DOI 10.1007/s00424-009-0742-3

    View details for Web of Science ID 000274586900008

    View details for PubMedID 19809831

  • AFM Images of Outer Hair Cells' Lateral Plasma Membrane: An Auto-correlation Function Analysis BIOPHYSICAL JOURNAL Sinha, G., Dimitriadis, E. K., Iwasa, K. H. 2010; 98 (3): 508A-508A
  • Effect of Having Two Identical Channels Per Tip-Link in Hair Bundles BIOPHYSICAL JOURNAL Sul, B., Iwasa, K. H. 2010; 98 (3): 509A-509A
  • Effectiveness of Hair Bundle Motility as the Cochlear Amplifier BIOPHYSICAL JOURNAL Sul, B., Iwasa, K. H. 2009; 97 (10): 2653-2663

    Abstract

    The effectiveness of hair bundle motility in mammalian and avian ears is studied by examining energy balance for a small sinusoidal displacement of the hair bundle. The condition that the energy generated by a hair bundle must be greater than energy loss due to the shear in the subtectorial gap per hair bundle leads to a limiting frequency that can be supported by hair-bundle motility. Limiting frequencies are obtained for two motile mechanisms for fast adaptation, the channel re-closure model and a model that assumes that fast adaptation is an interplay between gating of the channel and the myosin motor. The limiting frequency obtained for each of these models is an increasing function of a factor that is determined by the morphology of hair bundles and the cochlea. Primarily due to the higher density of hair cells in the avian inner ear, this factor is approximately 10-fold greater for the avian ear than the mammalian ear, which has much higher auditory frequency limit. This result is consistent with a much greater significance of hair bundle motility in the avian ear than that in the mammalian ear.

    View details for DOI 10.1016/j.bpj.2009.08.039

    View details for Web of Science ID 000271917700002

    View details for PubMedID 19917218

  • Amplifying effect of a release mechanism for fast adaptation in the hair bundle JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA Sul, B., Iwasa, K. H. 2009; 126 (1): 4-6

    Abstract

    A "release" mechanism, which has been experimentally observed as the fast component in the hair bundle's response to mechanical stimulation, appears similar to common mechanical relaxation with a damping effect. This observation is puzzling because such a response is expected to have an amplifying role in the mechanoelectrical transduction process in hair cells. Here it is shown that a release mechanism can indeed have a role in amplification, if it is associated with negative stiffness due to the gating of the mechonoelectric transducer channel.

    View details for DOI 10.1121/1.3143782

    View details for Web of Science ID 000268065100009

    View details for PubMedID 19603855

  • CELLULAR BASIS OF THE COCHLEAR AMPLIFIER CONCEPTS AND CHALLENGES IN THE BIOPHYSICS OF HEARING Iwasa, K. H., Sul, B., Fang, J., Sinha, G. P. 2009: 303-309
  • BIG AND POWERFUL: A MODEL OF THE CONTRIBUTION OF BUNDLE MOTILITY TO MECHANICAL AMPLIFICATION IN HAIR CELLS OF THE BIRD BASILAR PAPILLA CONCEPTS AND CHALLENGES IN THE BIOPHYSICS OF HEARING Koeppl, C., Iwasa, K. H., Sul, B. 2009: 444-450
  • Probing conformational changes of prestin with thiol-reactive optical switches BIOPHYSICAL JOURNAL Fang, J., Sakata, T., Marriott, G., Iwasa, K. H. 2008; 95 (6): 3036-3042

    Abstract

    Thiol-reactive optical switch probes were used to examine conformational changes of prestin-based membrane motor. Because this motor is based on mechanoelectric coupling similar to piezoelectricity, the motile activity can be monitored by charge movements across the plasma membrane, which appears as nonlinear capacitance. When the plasma membrane is conjugated with the probes, optically induced spiro-merocyanine transition positively shifted nonlinear capacitance of outer hair cells and prestin-transfected cells by approximately 10 mV. These shifts were reversible and were eliminated by pretreatment with iodoacetamide. However, they were little affected by pretreatment with biotin maleimide, which cannot reach the cytoplasmic surface. Our results showed that merocyanine states, with a larger dipole moment, interact with the motor's extended conformation stronger than with the compact conformation by 1.6 x 10(-21) J/molecule. The interaction sites are near the cytoplasmic side of the motor protein.

    View details for DOI 10.1529/biophysj.108.132878

    View details for Web of Science ID 000258826900039

    View details for PubMedID 18556757

  • Effect of the cochlear microphonic on the limiting frequency of the mammalian ear JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA Iwasa, K. H., Sul, B. 2008; 124 (3): 1607-1612

    Abstract

    Electromotility is a basis for cochlear amplifier, which controls the sensitivity of the mammalian ear and contributes to its frequency selectivity. Because it is driven by the receptor potential, its frequency characteristics are determined by the low-pass RC filter intrinsic to the cell, which has a corner frequency about 1/10th of the operating frequency. This filter significantly decreases the efficiency of electromotility as an amplifier. The present paper examines a proposal that the cochlear microphonic, the voltage drop across the extracellular medium by the receptor current, contributes to overcome this problem. It is found that this effect can improve frequency dependence. However, this effect alone is too small to enhance the effectiveness of electromotility beyond 10 kHz in the mammalian ear.

    View details for DOI 10.1121/1.2953317

    View details for Web of Science ID 000259790500023

    View details for PubMedID 19045652

  • Effects of chlorpromazine and trinitrophenol on the membrane motor of outer hair cells BIOPHYSICAL JOURNAL Fang, J., Iwasa, K. H. 2007; 93 (5): 1809-1817

    Abstract

    The motile activity of outer hair cells' cell body is associated with large nonlinear capacitance due to a membrane motor that couples electric displacement with changes in the membrane area, analogous to piezoelectricity. This motor is based on prestin, a member of the SLC26 family of anion transporters and utilizes the electric energy available at the plasma membrane associated with the sensory function of these cells. To understand detailed mechanism of this motile activity, we examined the effect of amphipathic ions, cationic chlorpromazine and anionic trinitrophenol, which are thought to change the curvature of the membrane in opposite directions. We found that both chemicals reduced cell length at the holding potential of -75 mV and induced positive shifts in the cells' voltage dependence. The shift observed was approximately 10 mV for 500 microM trinitrophenol and 20 mV for 100 microM cationic chlorpromazine. Length reduction at the holding potential and voltage shifts of the motile activity were well correlated. The voltage shifts of nonlinear capacitance were not diminished by eliminating the cells' turgor pressure or by digesting the cortical cytoskeleton. These observations suggest that the membrane motor undergoes conformational transitions that involve changes not only in membrane area but also in bending stiffness.

    View details for DOI 10.1529/biophysj.106.100834

    View details for Web of Science ID 000248722200036

    View details for PubMedID 17483184

  • Cytosolic surface of outer hair cells' plasma membrane: Search of membrane motor by atomic force microscopy BIOPHYSICAL JOURNAL Sinha, G., Sabri, F., Dimitriadis, E., Iwasa, K. 2007: 348A-348A
  • Effects of tarantula toxin GsMTx4 on the membrane motor of outer hair cells NEUROSCIENCE LETTERS Fang, J., Iwasa, K. H. 2006; 404 (1-2): 213-216

    Abstract

    GsMTx4, a cationic hydrophobic peptide isolated from tarantula venom, is a specific inhibitor of stretch-activated channels (SACs). Here, we show that the toxin also affects the membrane motor of outer hair cells at low doses. The membrane motor of outer hair cells is based on prestin, a member of the SLC26 family of membrane proteins, and directly uses electrical energy available at the plasma membrane. It is considered to be an essential part of the "cochlear amplifier," which increases the sensitivity, tuning, and dynamic range of the mammalian ear. The toxin shifts the operating point of the motor. The saturating value of the voltage shift is (26 +/- 1) mV, capable of significantly reducing the performance of the cochlear amplifier. The dissociation constant is (3.1 +/- 0.6) microM, about five-fold higher than that for SACs.

    View details for DOI 10.1016/j.neulet.2006.05.059

    View details for Web of Science ID 000239764200043

    View details for PubMedID 16797839

  • Electromotility in outer hair cells: A supporting role for fast potassium conductance ORL-JOURNAL FOR OTO-RHINO-LARYNGOLOGY AND ITS RELATED SPECIALTIES Ospeck, M., Dong, X., Fang, J., Iwasa, K. H. 2006; 68 (6): 373-377

    Abstract

    Motility of outer hair cells underlies the cochlear amplifier, which is critical for the ear's sensitivity and fine tuning. Of the two motile mechanisms present in these cells, electromotility at the lateral wall depends on the receptor potential and thus depends on currents through the cell body. We found that, in the guinea pig cochlea, basal turn outer hair cells have a fast-activating ion current (tau < 0.3 ms at 23 degrees C), which is absent in apical turn cells. This finding is consistent with our previous theoretical analysis that a fast-activating potassium current is required only in the basal turn to counteract the capacitive current and thereby to enhance the effectiveness of electromotility. Thus, our finding is consistent with the functional significance of electromotility. We conjecture therefore that the current reduces the capacitance of the outer hair cell in order to increase hearing bandwidth.

    View details for DOI 10.1159/000095280

    View details for Web of Science ID 000241664500013

    View details for PubMedID 17065832

  • "Area change paradox" in outer hair cells' membrane motor AUDITORY MECHANISMS: PROCESSES AND MODELS Iwasa, K. H. 2006: 155-161
  • Effects of tarantula toxin GsMTx4 and amphipathic ions that affect mechanosensitive channels on the membrane motor of outer hair cells BIOPHYSICAL JOURNAL Iwasa, K. H., Fang, J. 2005; 88 (1): 505A-506A
  • Tension sensitivity of prestin: Comparison with the membrane motor in outer hair cells BIOPHYSICAL JOURNAL Dong, X. X., Iwasa, K. H. 2004; 86 (2): 1201-1208

    Abstract

    The membrane motor in outer hair cells undergoes conformational transitions involving charge displacement of approximately 0.8 e across the membrane and changes of approximately 4 nm(2) in its membrane area. Previous reports have established that the charge transfer in the membrane motor and that in prestin, a membrane protein in the plasma membrane of outer hair cells, are approximately equal. Here, we determine the membrane area changes based on its sensitivity to membrane tension. We found that prestin does undergo area changes and that the magnitude is approximately 1 nm(2), smaller than the value 4 nm(2) for outer hair cell motor. This result confirms that prestin is a protein that functions as a membrane motor based on piezoelectricity. The discrepancy in the magnitude could suggest a prestin-containing complex in outer hair cells.

    View details for Web of Science ID 000188437200049

    View details for PubMedID 14747354

  • Gating compliance revisited BIOPHYSICAL JOURNAL Iwasa, K. H. 2004; 86 (1): 547A-547A
  • Limiting frequency of the cochlear amplifier based on electromotility of outer hair cells BIOPHYSICAL JOURNAL Ospeck, M., Dong, X. X., Iwasa, K. H. 2003; 84 (2): 739-749

    Abstract

    Outer hair cells are the critical element for the sensitivity and sharpness of frequency selectivity of the ear. It is believed that fast motility (electromotility) of these cells is essential for this function. Indeed, force produced by outer hair cells follows their membrane potential very closely at least up to 60 kHz. However, it has been pointed out that the cell's receptor potential is attenuated by a low-pass RC circuit inherent to these cells, with the RC roll-off frequencies significantly lower than their operating frequencies. This would render electromotility ineffective in producing force. To address this issue, we assume that multiple degrees of freedom and vibrational modes due to the complex structure of the organ of Corti provide optimal phases for outer hair cells' force to cancel viscous drag. Our derived frequency limit depends on the drag-capacitance product, not directly on the RC time constant. With a reasonable assumption for the viscous drag, the estimated limit is 10-13 kHz, exceeding the RC corner frequency. Our analysis shows that a fast-activating potassium current can substantially extend the frequency limit by counteracting the capacitive current.

    View details for Web of Science ID 000183123700002

    View details for PubMedID 12547758

  • Motor noise in outer hair cells FLUCTUATIONS AND NOISE IN BIOLOGICAL, BIOPHYSICAL, AND BIOMEDICAL SYSTEMS Iwasa, K. H., Dong, X. X. 2003; 5110: 151-160
  • Fast nonlinear currents in outer hair cells from the basal turn of the cochlea BIOPHYSICS OF THE COCHLEA: FROM MOLECULES TO MODELS Dong, X. X., Ospeck, M., Iwasa, K. H. 2003: 161-168
  • Noise in the outer hair cell and gain control in the ear UNSOLVED PROBLEMS OF NOISE AND FLUCTUATIONS Ospeck, M., Iwasa, K. H. 2003; 665: 116-124
  • Cooperative interaction as the physical basis of the negative stiffness in hair cell stereocilia (vol 111, pg 2208, 2002) JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA Iwasa, K. H., Ehrenstein, G. 2002; 112 (5): 2193-2193

    View details for DOI 10.1121/1.1489690

    View details for Web of Science ID 000178870100045

  • Cooperative interaction as the physical basis of the negative stiffness in hair cell stereocilia JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA Iwasa, K. H., Ehrenstein, G. 2002; 111 (5): 2208-2212

    Abstract

    A recent report confirmed that stiffness of the stereocilia can be negative, as predicted by the Howard-Hudspeth model. According to this model, the mechanotransducer channel's gating not only reduces the stereociliary stiffness, but can alter its sign as well. The basic assumptions of this model do not include cooperativity in channel gating. Here we consider two possible explanations for the observed negative stiffness. If the stereocilia have a special structure so that microscopic displacement can be imposed on each channel by controlling the bending of the bundle, negative stiffness can occur without channel cooperativity. If such a microscopic condition cannot be imposed by a macroscopic manipulation, an additional physical process, such as cooperativity in channel gating, is required to explain negative stiffness.

    View details for DOI 10.1121/1.1466864

    View details for Web of Science ID 000175632500026

    View details for PubMedID 12051440

  • Piezoelectric reciprocal relationship of the membrane motor in the cochlear outer hair cell BIOPHYSICAL JOURNAL Dong, X. X., Ospeck, M., Iwasa, K. H. 2002; 82 (3): 1254-1259

    Abstract

    It has been shown that the membrane motor in the outer hair cell is driven by the membrane potential. Here we examine whether the motility satisfies the reciprocal relationship, the characteristic of piezoelectricity, by measuring charge displacement induced by stretching the cell with known force. The efficiency of inducing charge displacement was membrane potential dependent. The maximum efficiency of inducing charge displacement by force was approximately 20 fC/nN for 50-microm-long lateral membrane. The efficiency per cell stretching was 0.1 pC/microm. We found that these values are consistent with the reciprocal relationship based on the voltage sensitivity of approximately 20 nm/mV for 50-microm-long cell and force production of 0.1 nN/mV by the cell. We can thus conclude that the membrane motor in the outer hair cell satisfies a necessary condition for piezoelectricity and that the hair cell's piezoelectric coefficient of 20 fC/nN is four orders of magnitude greater than the best man-made material.

    View details for Web of Science ID 000174170700012

    View details for PubMedID 11867442

  • RC time constant paradox of outer hair cells BIOPHYSICAL JOURNAL Ospeck, M. C., Dong, X. X., Iwasa, K. H. 2002; 82 (1): 251A-251A
  • Outer hair cell motility as two-state piezoelectricity BIOPHYSICAL JOURNAL Iwasa, K. H. 2001; 80 (1): 157A-158A
  • Fluctuation of motor charge in the lateral membrane of the cochlear outer hair cell BIOPHYSICAL JOURNAL Dong, X. X., Ehrenstein, D., Iwasa, K. H. 2000; 79 (4): 1876-1882

    Abstract

    Functioning of the membrane motor of the outer hair cell is tightly associated with transfer of charge across the membrane. To obtain further insights into the motor mechanism, we examined kinetics of charge transfer across the membrane in two different modes. One is to monitor charge transfer induced by changes in the membrane potential as an excess membrane capacitance. The other is to measure spontaneous flip-flops of charges across the membrane under voltage-clamp conditions as current noise. The noise spectrum of current was inverse Lorentzian, and the capacitance was Lorentzian, as theoretically expected. The characteristic frequency of the capacitance was approximately 10 kHz, and that for current noise was approximately 30 kHz. The difference in the characteristic frequencies seems to reflect the difference in the modes of mechanical movement associated with the two physical quantities.

    View details for Web of Science ID 000089688000018

    View details for PubMedID 11023893

  • Effect of membrane motor on the axial stiffness of the cochlear outer hair cell JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA Iwasa, K. H. 2000; 107 (5): 2764-2766

    View details for Web of Science ID 000086967900048

    View details for PubMedID 10830400

  • Frequency dependence of the membrane capacitance and current noise spectrum PROCEEDINGS OF THE INTERNATIONAL SYMPOSIUM ON RECENT DEVELOPMENTS IN AUDITORY MECHANICS Dong, X. X., Ehrenstein, D., Iwasa, K. H. 2000: 337-343
  • Membrane motor of the outer hair cell PROCEEDINGS OF THE INTERNATIONAL SYMPOSIUM ON RECENT DEVELOPMENTS IN AUDITORY MECHANICS Iwasa, K. H., Adachi, M. 2000: 275-280
  • Electrically driven motor in the outer hair cell: Effect of a mechanical constraint PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Adachi, M., Iwasa, K. H. 1999; 96 (13): 7244-7249

    Abstract

    The outer hair cell has a unique voltage-dependent motility associated with charge transfer across the plasma membrane. To examine mechanical changes in the membrane that are coupled with such charge movements, we digested the undercoating of the membrane with trypsin. We inflated the cell into a sphere and constrained the surface area by not allowing volume changes. We found that this constraint on the membrane area sharply reduced motor-associated charge movement across the membrane, demonstrating that charge transfer is directly coupled with membrane area change. This electromechanical coupling in the plasma membrane must be the key element for the motile mechanism of the outer hair cell.

    View details for Web of Science ID 000081056000025

    View details for PubMedID 10377399

  • Effect of mechanical constraint on the membrane capacitance in trypsin-treated outer hair cells BIOPHYSICAL JOURNAL Iwasa, K. H., Adachi, M. 1999; 76 (1): A46-A46
  • Electromechanical coupling in the outer hair cell: a statistical thermodynamic examination BIOPHYSICAL JOURNAL Iwasa, K. H. 1998; 74 (2): A86-A86
  • Prenatal expression of inwardly rectifying potassium channel mRNA (Kir4.1) in rat brain NEUROREPORT Ma, W., Zhang, L., Xing, G., Hu, Z., Iwasa, K. H., Clay, J. R. 1998; 9 (2): 223-227

    Abstract

    The levels and cellular localization of the mRNA encoding the inwardly rectifying potassium ion channel Kir4.1 were investigated in the embryonic rat brain by Northern blots and in situ hybridization. This transcript was absent at embryonic day 13 (E13), whereas it was clearly present in E14-15 preparations, principally in the neuroepithelium of the cerebral cortex, thalamus, and hypothalamus. At later embryonic stages (E17-20), Kir4.1 mRNA levels increased and expanded to the mantle zone, such as the cortical plate, hippocampus, thalamus, and hypothalamus. The early appearance of Kir4.1 mRNA in various brain regions suggests an involvement of the channel in cell proliferation, migration and differentiation in the rat CNS.

    View details for Web of Science ID 000071865300009

    View details for PubMedID 9507959

  • Does nonlinear capacitance of the outer hair cell really improve its high-frequency response? JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA Iwasa, K. H., Chadwick, R. S. 1997; 102 (5): 3055-3056
  • Effect of diamide on force generation and axial stiffness of the cochlear outer hair cell BIOPHYSICAL JOURNAL Adachi, M., Iwasa, K. H. 1997; 73 (5): 2809-2818

    Abstract

    We found that diamide, which affects spectrin, reduces the axial stiffness of the cochlear outer hair cell, the cylindrically shaped mechanoreceptor cell with a unique voltage-sensitive motility. This effect thus provides a means of examining the relationship between the stiffness and the motility of the cell. For measuring axial stiffness and force production, we used an experimental configuration in which an elastic probe was attached to the cell near the cuticular plate and the other end of the cell was held with a patch pipette in the whole-cell recording mode. Diamide at concentrations of up to 5 mM reduced the axial stiffness in a dose-dependent manner to 165 nN per unit strain from 502 nN for untreated cells. The isometric force elicited by voltage pulses under whole-cell voltage clamp was also reduced to 35 pN/mV from 105 pN/mV for untreated cells. Thus the isometric force was approximately proportional to the axial stiffness. Our observations suggest a series connection between the motor and cytoskeletal elements and can be explained by the area motor model previously proposed for the outer hair cell.

    View details for Web of Science ID A1997YD84800055

    View details for PubMedID 9370475

  • Force generation of the cochlear outer hair cell MOLECULAR BIOLOGY OF THE CELL Adachi, M., Iwasa, K. H. 1997; 8: 2249-2249
  • Force generation in the outer hair cell of the cochlea BIOPHYSICAL JOURNAL Iwasa, K. H., Adachi, M. 1997; 73 (1): 546-555

    Abstract

    The outer hair cell of the mammalian cochlea has a unique motility directly dependent on the membrane potential. Examination of the force generated by the cell is an important step in clarifying the detailed mechanism as well as the biological importance of this motility. We performed a series of experiments to measure force in which an elastic probe was attached to the cell near the cuticular plate and the cell was driven with voltage pulses delivered from a patch pipette under whole-cell voltage clamp. The axial stiffness was also determined with the same cell by stretching it with the patch pipette. The isometric force generated by the cell is around 0.1 nN/mV, somewhat smaller than 0.15 nN/mV, predicted by an area motor model based on mechanical isotropy, but larger than in earlier reports in which the membrane potential was not controlled. The axial stiffness obtained, however, was, on average, 510 nN per unit strain, about half of the value expected from the mechanical isotropy of the membrane. We extended the area motor theory incorporating mechanical orthotropy to accommodate the axial stiffness determined. The force expected from the orthotropic model was within experimental uncertainties.

    View details for Web of Science ID A1997XF77600051

    View details for PubMedID 9199816

  • Ultrastructural localization of the Na-K-Cl cotransporter in the lateral wall of the rabbit cochlear duct HEARING RESEARCH Mizuta, K., Adachi, M., Iwasa, K. H. 1997; 106 (1-2): 154-162

    Abstract

    Localization of the immunoreactivity in the lateral wall of the rabbit cochlear duct was examined using a post-embedding immunogold method with a polyclonal antiserum raised against the rabbit parotid Na-K-Cl cotransporter. In the stria vascularis, the labeling was significant on the basolateral membrane infolding of marginal cells, whereas no labeling was seen on the luminal membrane of these cells. Immunoreactivity was also detected on the cell membranes of various other cells. These include fibrocytes of the spiral ligament and the spiral prominence, and vascular endothelial cells in the stria vascularis and the spiral ligament. In contrast, virtually no gold particles were seen on the membrane of intermediate cells, basal cells of the stria vascularis, the epithelial cells of the spiral prominence, or Reissner's membrane. Our result on the localization of the Na-K-Cl cotransporter in marginal cells is consistent with electrophysiological studies (Wangemann et al. (1995) Hear. Res. 84, 19-29). Our result on fibrocytes is discussed in relation to K+ circulation into endolymph from perilymph (Schulte and Steel (1994) Hear. Res. 78, 65-76).

    View details for Web of Science ID A1997WT37800014

    View details for PubMedID 9112115

  • Force generation in the cochlear outer hair cell under voltage clamp. BIOPHYSICAL JOURNAL Iwasa, K. H., Adachi, M. 1997; 72 (2): WAMJ5-WAMJ5
  • Current noise power spectrum of the membrane motor of the cochlear outer hair cell. BIOPHYSICAL JOURNAL Ehrenstein, D., Iwasa, K. H. 1997; 72 (2): TH119-TH119
  • Amiloride-sensitive Na+ channel-like immunoreactivity in the luminal membrane of some non-sensory epithelia of the inner ear (vol 88, pg 199, 1995) HEARING RESEARCH Mizuta, K., Iwasa, K. H., Tachibana, M., Benos, D. J., Lim, D. J. 1996; 98 (1-2): 180-180
  • Viscoelastic relaxation in the membrane of the auditory outer hair cell. Biophysical journal Ehrenstein, D., Iwasa, K. H. 1996; 71 (2): 1087-1094

    Abstract

    The outer hair cell (OHC) in the mammalian ear has a unique membrane potential-dependent motility, which is considered to be important for frequency discrimination (tuning). The OHC motile mechanism is located at the cell membrane and is strongly influenced by its passive mechanical properties. To study the viscoelastic properties of OHCs, we exposed cells to a hypoosmotic solution for varying durations and then punctured them, to immediately release the osmotic stress. Using video records of the cells, we determined both the imposed strain and the strain after puncturing, when stress was reset to zero. The strain data were described by a simple rheological model consisting of two springs and a dashpot, and the fit to this model gave a time constant of 40 +/- 19 s for the relaxation (reduction) of tension during prolonged strain. For time scales much shorter or longer than this, we would expect essentially elastic behavior. This relaxation process affects the membrane tension of the cell, and because it has been shown that membrane tension has a modulatory role in the OHC's motility, this relaxation process could be part of an adaptation mechanism, with which the motility system of the OHC can adjust to changing conditions and maintain optimum membrane tension.

    View details for PubMedID 8842245

  • Ultrastructural localization of G-protein G(s) in the lateral wall of the guinea pig cochlear duct HEARING RESEARCH Mizuta, K., Iwasa, K. H., Simonds, W. F., Tachibana, M. 1996; 93 (1-2): 111-119

    Abstract

    Immunocytochemical localization of a GTP-binding protein, Gs, in the various cells of the lateral wall of guinea pig cochlear duct was investigated using a post-embedding immunogold method with antibody raised against a synthetic decapeptide (RMHLRQYELL) encoding the C-terminus of the alpha-subunit of Gs. In the stria vascularis, labeling was observed on the basolateral membrane infoldings of marginal cells, on the juxtaposed membrane of intermediate cells, and on the cell membrane of basal cell. In contrast, no significant labeling was observed on the luminal membrane of marginal cells. Immunoreactivity also was detected on the cell membranes of various other cells. These include spiral prominence epithelial cells, fibrocytes of spiral ligament, external sulcus cells, and epithelial and mesothelial cells of Reissner's membrane. Adenylylcyclase has been functionally implicated in some of the cell types with membranes labeled in this study. The significance of these findings is briefly discussed.

    View details for Web of Science ID A1996UL65500007

    View details for PubMedID 8735072

  • Active control of waves in a cochlear model with subpartitions PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Chadwick, R. S., Dimitriadis, E. K., Iwasa, K. H. 1996; 93 (6): 2564-2569

    Abstract

    Multiscale asymptotic methods developed previously to study macromechanical wave propagation in cochlear models are generalized here to include active control of a cochlear partition having three subpartitions, the basilar membrane, the reticular lamina, and the tectorial membrane. Activation of outer hair cells by stereocilia displacement and/or by lateral wall stretching result in a frequency-dependent force acting between the reticular lamina and basilar membrane. Wavelength-dependent fluid loads are estimated by using the unsteady Stokes' equations, except in the narrow gap between the tectorial membrane and reticular lamina, where lubrication theory is appropriate. The local wavenumber and subpartition amplitude ratios are determined from the zeroth order equations of motion. A solvability relation for the first order equations of motion determines the subpartition amplitudes. The main findings are as follows: The reticular lamina and tectorial membrane move in unison with essentially no squeezing of the gap; an active force level consistent with measurements on isolated outer hair cells can provide a 35-dB amplification and sharpening of subpartition waveforms by delaying dissipation and allowing a greater structural resonance to occur before the wave is cut off; however, previously postulated activity mechanisms for single partition models cannot achieve sharp enough tuning in subpartitioned models.

    View details for Web of Science ID A1996UB12100059

    View details for PubMedID 8637914

  • Mechanisms for the fast motility of the outer hair cell BIOPHYSICAL JOURNAL Iwasa, K. H. 1996; 70 (2): SUAM1-SUAM1
  • Spontaneous firing of NG108-15 cells induced by transient exposure to ammonium chloride CELLULAR AND MOLECULAR NEUROBIOLOGY Kowtha, V. C., Bryant, H. J., Krauthamer, V., Iwasa, K. H., Stenger, D. A. 1996; 16 (1): 1-9

    Abstract

    1. We report that NG108-15 (neuroblastoma x glioma) cells differentiated in defined serum-free media are capable of exhibiting stable automaticity (the spontaneous occurrence of regenerative action potentials) following exposure to extracellular perfusates containing NH4Cl. 2. Membrane depolarization (4-5 mV) concomitant with an increased pHi during NH4Cl exposure are followed by hyperpolarization (5-7 mV), sub-threshold oscillations, and spontaneous firing after the removal of NH4Cl. 3. Cells cultured in 10% serum did not exhibit automaticity. Cells cultured in serum-free media are twice as likely to show automaticity as those cultured in reduced (1.5%) serum media. 4. We have examined factors that contribute to the events following NH4Cl exposure, namely, membrane depolarization and hyperpolarization, subthreshold oscillations, and automaticity. The inward currents activated at more negative potentials and the ionic currents associated with pronounced afterhyperpolarization in NG108-15 cells cultured in serum-free media provide a basis for the repetitive activity in general and automaticity in particular.

    View details for Web of Science ID A1996TV60800001

    View details for PubMedID 8714555

  • Viscoelastic relaxation in the membrane of the auditory outer hair cell BIOPHYSICAL JOURNAL Ehrenstein, D., Iwasa, K. H. 1996; 70 (2): TU421-TU421
  • Ultrastructural localization of G-protein G(s) in the organ of Corti NEUROSCIENCE LETTERS Mizuta, K., Iwasa, K. H., Simonds, W. F., Tachibana, M. 1995; 201 (2): 147-150

    Abstract

    Immunocytochemical localization of a stimulatory GTP-binding protein Gs in the organ of Corti in the inner ear was examined with a post-embedding immunogold technique, using antibodies raised against a synthetic decapeptide (RMHLRQYELL) of the C-terminus of the alpha subunit of Gs. Immunoreactivity was strong on the membranes of supporting cells in the reticular lamina, including inner and outer pillar cells and the phalangeal process of Deiters' cells. Immunolabeling also was seen on the membranes of cell bodies of those cells which surround nerve fibers, basilar fibers, outer spiral fibers and afferent nerve endings at outer hair cells. Gold particles also labeled the membrane of inner phalangeal cells and border cells. In contrast, outer and inner hair cells were not labeled. Possible roles of Gs in the organ of Corti are discussed.

    View details for Web of Science ID A1995TK80900013

    View details for PubMedID 8848239

  • AMILORIDE-SENSITIVE NA+ CHANNEL-LIKE IMMUNOREACTIVITY IN THE LUMINAL MEMBRANE OF SOME NONSENSORY EPITHELIA OF THE INNER-EAR HEARING RESEARCH Mizuta, K., Iwasa, K. H., Tachibana, M., Benos, D. J., Lim, D. J. 1995; 88 (1-2): 199-205

    Abstract

    Some non-sensory epithelia of the inner ear were examined for the localization of immunoreactivity to polyclonal antibodies raised against amiloride-sensitive Na+ channels from the bovine kidney. The pre-embedding immunogold technique was used for this purpose. Labelings were found on the membrane of the endolymphatic surface of strial marginal cells, epithelial cells of spiral prominence and Reissner's membrane, and ampullar dark cells. In contrast, no labeling was found on the luminal membrane of mesothelial cells of Reissner's membrane, the cells lining the supra-strial perilymphatic space, transitional cells and ampullar ceiling cells. Since the antibodies used may also label non-selective cation channels and non-functional sodium channel precursors as suggested by others, it was not possible to determine the labelings are solely due to amiloride-sensitive Na+ channels. However, the observed result was consistent with the previous studies that amiloride blocks ion transport in strial marginal cells and the semicircular canal. It is therefore likely that the observed labeling includes amiloride-sensitive Na+ channels. These labeled ion channels in a variety of epithelial cells lining the endolymphatic space could be important in the inner ear fluid regulation.

    View details for Web of Science ID A1995TF52300021

    View details for PubMedID 8575996

  • CAN MEMBRANE-PROTEINS DRIVE A CELL BIOPHYSICAL JOURNAL Iwasa, K. H., Li, M., Jia, M. 1995; 68 (4): S214-S214
  • A MEMBRANE MOTOR MODEL FOR THE FAST MOTILITY OF THE OUTER HAIR CELL JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA Iwasa, K. H. 1994; 96 (4): 2216-2224

    Abstract

    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.

    View details for Web of Science ID A1994PL70800026

    View details for PubMedID 7963034

  • AMILORIDE-SENSITIVE CHANNELS IN MARGINAL CELLS IN THE STRIA VASCULARIS OF THE GUINEA-PIG COCHLEA NEUROSCIENCE LETTERS Iwasa, K. H., Mizuta, K., Lim, D. J., Benos, D. J., Tachibana, M. 1994; 172 (1-2): 163-166

    Abstract

    We examined marginal cells in stria vascularis for the presence of amiloride-sensitive Na+ channels, a possible pathway for maintaining a low Na+ concentration in the endolymph. Whole-cell voltage-clamp experiment shows that amiloride at 1 microM concentration reversibly reduces inward current more than outward current. Immunogold-labeling method shows that the luminal and lateral membrane have antigenic sites for these antibodies. These observations indicate the presence of amiloride-sensitive channels in the marginal cell. If amiloride-sensitive channels in the luminal membrane are highly selective to Na+, they could be an efficient pathway for Na+ uptake from the endolymph. In the basolateral membrane, amiloride-sensitive Na+ channels may make a relatively small contribution to the unusual resting potential.

    View details for Web of Science ID A1994NM24100040

    View details for PubMedID 8084526

  • ELASTICITY AND ACTIVE FORCE GENERATION OF COCHLEAR OUTER HAIR-CELLS JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA Iwasa, K. H., Chadwick, R. S. 1992; 92 (6): 3169-3173

    Abstract

    The cochlear outer hair cell is described by a cylindrical membrane model, characterized by area and shear moduli for a passive elastic element and an active tension element dependent on the membrane potential. In passive experiments, these moduli are determined from the pressure-strain relations. The area modulus obtained is 0.07 N m-1, similar to a lipid bilayer and the shear modulus is 0.007 N m-1. These moduli combined with previous active experiments show that the active tension is nearly isotropic and is about 1.6 x 10(-2) N m-1 V-1, resulting in a 0.5 nN/mV force per cell. This implies that the receptor potential for acoustical stimulation produces an active force comparable to the acoustic force applied to the basilar membrane per outer hair cell. This finding supports the hypothesis that the outer hair cell acts as feedback motor in the fine tuning mechanism of the mammalian ear.

    View details for Web of Science ID A1992KB22300013

    View details for PubMedID 1474231

  • EVIDENCE FOR A MODEL OF EXOCYTOSIS THAT INVOLVES CALCIUM-ACTIVATED CHANNELS ANNALS OF THE NEW YORK ACADEMY OF SCIENCES Ehrenstein, G., Stanley, E. F., POCOTTE, S. L., Iwasa, K. H., KREBS, K. E. 1991; 635: 297-306

    View details for Web of Science ID A1991JM20600026

    View details for PubMedID 1720603

  • EVIDENCE FOR A MODEL OF EXOCYTOSIS THAT INVOLVES CALCIUM-ACTIVATED CHANNELS CALCIUM ENTRY AND ACTION AT THE PRESYNAPTIC NERVE TERMINAL Ehrenstein, G., Stanley, E. F., POCOTTE, S. L., Iwasa, K. H., KREBS, K. E. 1991; 635: 297-306
  • HIGH-CONCENTRATION OF INOSITOL 1,4,5-TRISPHOSPHATE IN SEA-URCHIN SPERM BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS Iwasa, K. H., Ehrenstein, G., Defelice, L. J., Russell, J. T. 1990; 172 (2): 932-938

    Abstract

    We measured inositol 1,4,5-trisphosphate (InsP3) content of sea urchin gametes by using a specific protein binding assay, and found that a spermatozoon contains 4 x 10(-19) to 1 x 10(-18) moles of InsP3 before the acrosome reaction. Since the acrosome reaction has previously been shown to increase the InsP3 content of sperm severalfold, our measurement indicates that a spermatozoon contains at least 2 x 10(-18) moles of InsP3 at fertilization, corresponding to a concentration in the spermatozoon of about 1 mM. The threshold for activation of eggs by injection of InsP3 dissolved in a much larger volume of solution has been found to be about 3 x 10(-18) moles, corresponding to a concentration in the injectate of 1 microM. This suggests that sea urchin sperm may contain enough InsP3 to activate eggs. With an electroporation method, we also showed that sperm extract acts on eggs only from inside, consistent with a primary messenger role for InsP3.

    View details for Web of Science ID A1990EG08500084

    View details for PubMedID 2241981

  • POTASSIUM CHANNELS IN MOTOR CELLS OF SAMANEA-SAMAN - A PATCH-CLAMP STUDY PLANT PHYSIOLOGY Moran, N., Ehrenstein, G., Iwasa, K., Mischke, C., Bare, C., SATTER, R. L. 1988; 88 (3): 643-648

    Abstract

    Leaflet movements in Samanea saman are driven by the shrinking and swelling of cells in opposing (extensor and flexor) regions of the motor organ (pulvinus). Changes in cell volume, in turn, depend upon large changes in motor cell content of K(+), Cl(-) and other ions. We performed patch-clamp experiments on extensor and flexor protoplasts, to determine whether their plasma membranes contain channels capable of carrying the large K(+) currents that flow during leaflet movement. Recordings in the "whole-cell" mode reveal depolarization-activated K(+) currents in extensor and flexor cells that increase slowly (t((1/2)) = ca. 2 seconds) and remain active for minutes. Recordings from excised patches reveal a single channel conductance of ca. 20 picosiemens in both cell types. The magnitude of the K(+) currents is adequate to account quantitatively for K(+) loss, previously measured in vivo during cell shrinkage. The K(+) channel blockers tetraethylammonium (5 millimolar) or quinine (1 millimolar) blocked channel opening and decreased light- and dark-promoted movements of excised leaflets. These results provide evidence for the role of potassium channels in leaflet movement.

    View details for Web of Science ID A1988Q947400027

    View details for PubMedID 16666362

  • UNUSUAL CALCIUM-ACTIVATED POTASSIUM CHANNELS OF BOVINE PARATHYROID CELLS PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Jia, M., Ehrenstein, G., Iwasa, K. 1988; 85 (19): 7236-7239

    Abstract

    Parathyroid cells have unusual responses to an increase in the external calcium concentration--a reduction in secretion and a depolarization. In an attempt to explain this behavior, we have studied voltage-clamped inside-out patches from bovine parathyroid cells. We found a potassium-selective channel that requires internal calcium to open and has a 175-pS conductance. This channel differs from calcium-activated potassium channels that have been found in other cells in that it closes when the internal calcium concentration is increased above about 160 nM. This channel can account for the depolarization that occurs in parathyroid cells in high-calcium solutions. It may also account for the reduced secretion in high-calcium solutions.

    View details for Web of Science ID A1988Q358500041

    View details for PubMedID 2459700

  • EVIDENCE FOR INTERACTIONS BETWEEN BATRACHOTOXIN-MODIFIED CHANNELS IN HYBRID NEUROBLASTOMA-CELLS BIOPHYSICAL JOURNAL Iwasa, K., Ehrenstein, G., Moran, N., Jia, M. 1986; 50 (3): 531-537

    Abstract

    Current records from voltage-clamped membrane patches containing two batrachotoxin-modified sodium channels were analyzed to determine whether these channels are identical and independent. In most two-channel patches, the experimentally observed probabilities that zero, one, or two channels are open differ from the binomial distribution, demonstrating that the two channels are nonidentical or nonindependent or both. From the same current records, we also determined the rate for the transition from two open channels to one open channel and for the transition from one open channel to zero open channels. These data are consistent with closing rates for the two channels that are equal and independent. Both probability and closing rate data can be fit by a model wherein the channels are identical, the closing rates are independent, and the opening rate is greater when the other channel is closed than when it is open. The implications of this model for analyzing noise spectra and current variance are examined.

    View details for Web of Science ID A1986E006400018

    View details for PubMedID 2428414

  • ION CHANNELS IN PLASMALEMMA OF WHEAT PROTOPLASTS SCIENCE Moran, N., Ehrenstein, G., Iwasa, K., Bare, C., Mischke, C. 1984; 226 (4676): 835-838

    Abstract

    The patch-clamp technique was used to study passive movements of ions through the plasmalemma of wheat leaf protoplasts. This method overcomes the problems inherent in conventional electrophysiological study of plant cells. Changes in conductance were recorded in patches excised from the plasmalemma. Two types of patches were observed: (i) regions of low channel density, where discrete single-channel currents could be resolved and conductance ranged from 10 to 200 picosiemens and (ii) regions of high channel density, where single-channel currents could not be resolved and conductance was on the order of a few nanosiemens. The results indicate a striking similarity between animal and plant cell membranes in the basic phenomena of transport. Moreover, the approach used constitutes a new degree of refinement in the study of processes of regulation, pathology, and toxicity in plants.

    View details for Web of Science ID A1984TR34000033

    View details for PubMedID 6093255

Stanford Medicine Resources: