proposed to be an endolymphatic potential generated by the stria vascularis and is small or absent

22 Pages·2005·2.63 MB·English

J. Physiol. (1978), 281,pp. 445-465 445
With 1plateand 7 text-flgure8
Printedin Great Britain
THE GENERATION OF RESTING MEMBRANE POTENTIALS
IN AN INNER EAR HAIR CELL SYSTEM
BY H. BRACHO* AND R. BUDELLI
From the Department ofHead and Neck Surgery,
School ofMedicine, University ofCalifornia, Los Angeles,
California 90024, U.S.A.
(Received 12 December 1977)
SUMMARY
1. The macula sacculi in the mudpuppy is an inner ear sensory area accessible
for intracellular recordings in vitro and in vivo.
2. The restingpotentials recorded in vitro canbe explained by the electrodiffusion
theory assuming a uniform ionic selectivity in the membranes ofthe neuroepithelial
cells.
3. The resting potentials recorded in vivo are significantly larger than predicted
by the electrodiffusion theory, probably because ofan electrogenic metabolic process
present in the neuroepithelial cells.
4. An equivalent circuit is proposed to explain the resting electrogenesis in the
neuroepithelial cells present in the sensory area.
INTRODUCTION
In the inner ear ofvertebrates, the mechanoreceptors responsible for transforming
mechanical disturbances into electrical signals are the hair cells, which are epithelial
cells with a ciliated membrane. Davis (1965) proposed that the transduction process
consists of the modulation of a resting current by a mechanically induced change
in membrane impedance which takes place on the ciliated side of the hair cells.
This hypothesis, known as the mechano-electrical model of hair cell function, is
consistent with experimental results obtained with the use ofextracellular electrodes
to measure inner ear electrogenesis (Butler, 1965; Kurokawa, 1965; Honrubia,
Strelioff & Ward, 1971; Honrubia, Strelioff & Sitko, 1976).
The source of the resting current in the inner ear hair cell system has been
proposed to be an endolymphatic potential generated by the stria vascularis and
the transmembrane potential of the hair cells, the latter being the most important
source (Davis, 1965). In the vestibular system where the endolymphatic potential
is small or absent (Smith, Davis, Deatherage & Gessert, 1958; Schmidt & Fernandez,
1962), the source of electrical current is mainly the resting membrane potentials
of the neuroepithelial cells (hair and supporting cells). Hair resting membrane
potentials havebeenfound inhair cellsfromdifferentpreparations (Harris,Frishkopf
& Flock, 1970; Flock, Jorgensen & Russell, 1973; Mulroy, Altmann, Weiss & Peake,
1974). To account for a steady current an asymmetry must exist in the mechanism
* Present address: 4832 Coolidge Av., Culver City, California 90230, U.S.A.
446 H. BRACHO AND R. BUDELLI
formembranepotentialgeneration.Thisasymmetrymayoriginateinanon-uniformity
ofthe cell membrane's electrogenetic properties as postulated for the photoreceptors
(Zuckerman, 1973) and the transporting epithelia (Nagel, 1977), or in different
ionic gradients in different regions ofthe cell membrane.
The mechanism for generation of resting membrane potentials in neuroepithelial
cells has not been investigated. In other excitable cells the membrane potentials are
generated by two mechanisms operating simultaneously: the passive diffusion of
ions across a highly potassium-selective membrane (Adrian, 1956; Baker, Hodgkin
& Shaw, 1962; Brown, 1976) ansl the active exchange ofsodium bypotassium across
the cellmembrane mediatedbyanadenosinetriphosphatase (Thomas, 1972; Marmor,
1975). The elucidation of the ionic mechanism involved in hair cell electrogenesis
must take into account the peculiar ionic environment faced by these receptors.
On their ciliated side they are bathed by a fluid (the endolymph) with an extremely
high potassium concentration, and on their innervated side by a fluid (perilymph)
oflow potassium concentration.
To study the mechanism for membrane potential generation in neuroepithelial
cells, one must find a preparation where these cells are large enough to allow stable
intracellular recording oftheir membrane potentials under a variety ofexperimental
conditions. In this paper we introduce such a preparation. A preliminary account
ofour results was published elsewhere (Bracho, 1977).
METHODS
Preparation. The experiments were performed in vivo and in vitro on the macula sacculi
from the mudpuppy (Necturuw maculo8tm). For the in vivo preparation, the animal was anaes-
thetized by immersion in a solution of tricaine methensulphonate (0025%). The roof of the
otic capsule was removed, exposing the sacculus which fills the volume ofthe capsule almost
completely. The otolith is seen inside the sacculus through the transparent free saccular mem-
brane (Text-fig. 1A). The in vitro preparation was obtained by decapitating the animal and
ablating the membranous labyrinth through the hard palate; the labyrinth was placed in a
petri dish containingartificialperilymph (see Perfusion ofsolutions) forfurther dissection. The
remaining medial saccular wall containing the macula was either placed in a single bath with
a Sylgardg-covered bottom or clamped between two chambers in such a way that it formed
the boundary between them, its endolymphatic side facing upwards (see Text-fig. 1B). The
latter arrangement allowed independent perfusion of the upper (endolymphatic) and lower
(perilymphatic) chambers.
Recording. The membrane potential ofthe neuroepithelial cellswas recordedwith a 3 M-KCl
glass micro-electrode (10-20MQ and 0-5 um tip diameter) which was connected to an oscillo-
scope (Tektronix 502) and a chart recorder (Brush 220) through a pre-amplifier ofhigh input
impedance (Biodyne). The reference electrode consisted of a 3M-KCl-agar bridge which con-
nected the solution in the bath with a small container filled with a 3M-KCl solution. The
fluid from the container was connected to the amplifier through an Ag-AgCl pellet (WPI). The
reference electrode was placed in the upper chamber when the experiment was performed
in vitro and immersed in the solution surrounding the preparation (artificial perilymph) when
the experiment was performed in vivo. Those potentials measured with micro-electrodes with
atippotentiallargerthan 5 mVwerediscarded (Adrian, 1956). Thepotentialdifferencebetween
the reference electrode and the micro-electrode in artificial perilymph was less than 1 mV. No
attempt was made to correct for junction potentials when the ionic content of the solution
was modified. In a typical experiment the membrane potentials from ten to fifteen cells were
recorded in each experimental condition (described later) and the mean and the standard
deviation ofthis sample calculated. In the Results section, the size ofthe sample, n, refers to
the number ofanimals studied and not to thenumber ofcellsimpaled. To identify the impaled
RESTING POTENTIALS OF HAIR CELLS 447
cells we used a micro-electrode filled with Chicago blue 6b(ACB) at a concentration of 40%.
After the membrane potential was recorded, the dye was injected ionophoretically (Kaneko,
1970). Thepreparationwasfixedinglutaraldehyde (2%)andablackandwhitephotomicrograph
of the endolymphatic surface of the macula was taken through the phase contrast microscope
(PI. 1A and B).
Perfunion of Solutiof. Perfusion of solutions through the in vitro and in vivo preparations
was accomplished by means of a teflon valve (Chromatronix) connected to six 60ml. plastic
syringes containing different ionic solutions. In thein vitro experiments the output ofthe valve
was connected to a glass capillary tube (0.5 mm diameter), the end ofwhich was placed 3 mm
away from the epithelialsurface ofthe macula. The perilymphatic side was perfused ina similar
way except that the micropipette was connected directly to the syringe. The perfusion rate
was fixed at 3 ml./min. A glass capillary tube in each chamber, with tips approximately 1 cm
away from the epithelial surface, was connected to a vacuum pump in order to drain solutions.
Perfusion in the in vivo experiments was accomplished in a similar fashion, except that both
sides ofthe epithelium were perfused with the same solution after the free saccular membrane
and theotolith were removed. Since the otic capsule has a volume ofabout 0-3ml., its contents
were replaced ten times in 1 min. To insure a complete washout of the space between the
perilymphaticsurface andtheoticcapsule, weperfusedeachsolutionfor 20minbeforerecording
the membrane potentials. In the in vitro preparation the washout time was estimated by
observing the change in coloration of the solutions in the chambers when they were replaced
by Chicago blue (40%) and then washed out with artificial perilymph. In less than 10sec the
chambers were devoid ofany blue coloration. In these experiments the period before the effects
of a new solution were tested was 10 min. To evaluate the integrity of the saccular wall, the
movements of a 40% Chicago blue solution were observed. This solution was placed in one of
the chambers and if the blue coloration was observed in the other chamber, the experiment
was discarded. The solutions used were: artificial perilymph (NaCl, 115; KCl, 3; CaCl2, 2;
MgCl2, 1; Tris malate buffer, 5; pH, 7-3), artificial endolymph (NaCl, 20; KCl, 100; CaCl2, 2;
MgCl2, 1; Tris malate buffer, 5; pH, 7.3) and solutions with 10 and 30mM ofKCl prepared by
substituting potassium for sodium in equimolar amounts in the artificial perilymph. The
experiments were carried out at room temperature (23 'C).
The following is a list ofthe symbols used in the present paper.
V membrane potential
Ve membranepotentialrecordedwhen theneuroepithelialcellsweresurroundedby artificial
endolymph
Vp membrane potential recorded when the neuroepithelial cells were surrounded by peri-
lymph
Vt transepithelial potential (endolymphatic potential)
Re fraction ofthe input resistance given by the endolymphatic membrane
Rp fraction ofthe input resistance given by the perilymphatic membrane
R. shunt resistance
I resting current
K, concentration ofpotassium in the endolymph
Kp concentration ofpotassium in the perilymph
Nap concentration ofsodium in the perilymph
Na0 concentration ofsodium in the endolymph
PK membrane permeability to potassium
PN& membrane permeability to sodium
b P.a/PK
MX flux ofthe ionxacross a membrane
MK flux ofpotassium across the endolymphatic membrane
MNS flux ofsodium across the endolymphatic membrane
M'K flux ofpotassium across the perilymphatic membrane
M'N. flux ofsodium across the perilymphatic membrane
R universal gas constant
F Faraday constant
e base ofnatural logarithms
T absolute temperature
448 H. BRACHO AND R. BUDELLI
RESULTS
The macula sacculi in the mudpuppy
Anatomy. The macula sacculi in the mudpuppy is located in the medial saccular
wall, slightly off its centre. It is seen under the dissecting microscope, after the
otolith has been removed, as a kidney-shaped structure protruding from the epi-
thelial surface, about 0*7 mm in its longest diameter. In the macula, capillary blood
vessels form a dense network through which the circulation is slower than elsewhere
in the epithelium. In the non-sensory region the density of capillaries is relatively
low. The medial wall ofthe sacculus is usually covered on its perilymphatic surface
by melanocytes which obstruct the view ofthe neuroepithelial cells.
A B
Text-fig. 1. Macula sacculi preparations. A, experimental arrangement for the in vivo
recordings. B, experimentalarrangementforintracellularrecordinginthetwochamber
bath.
Under the phase contrast microscope, the surface of the saccular epithelium has
a honeycomb appearance. The epithelial cells surrounding the macula have aregular
polygonal shape, their width ranging from 10 to 15/m, among which are scattered
a few circular cells with a diameter ranging from 20 to 25sam. There are two types
of cells in the macula: one with a regular circular shape and a diameter from 10 to
15Itm, and the other with an irregular polygonal shape, its longest diameter ranging
from 10 to 15,am (PI. 1A and B). The circular cells have been identified as hair
cells whilst polygonal cells in the macula have been considered to be supporting
cells (Lindemann, 1967; Lewis & Nemanic, 1972; Watanuki & Schuknecht, 1976).
The number of hair cells counted on a photomicrograph taken with a scanning
electron microscope was 1300 (Lewis, E. R. & Nemanic, P.,unpublishedobservation).
Intracellular recordings. As a test of the feasibility of impaling neuroepithelial
cells with glass micro-electrodes, the medial saccular wall was placed in a single
bath of artificial perilymph with the endolymphatic surface facing upwards. When
the micro-electrode was advanced towards the epithelial cells, the penetration of
a cell was signalled by a sudden change in d.c. potential. This potential had a value
RESTING POTENTIALS OF HAIR CELLS 449
of about -70 mV and disappeared if the micro-electrode, after impaling the cell,
was moved a few microns upwards or downwards. No d.c. changes were recorded
when the micro-electrode used had a low resistance (less than 5 MQ). From one
preparation to another the potentials obtained from the sensory area ranged from
-60 to -80 mV. In the same preparation the potentials obtained from the macula
and the surrounding epithelium fluctuated within 5 mV. To identify the impaled
cells we injected them ionophoretically with Chicago blue after recording their
resting potential. P1. 1B shows two stained cells from the edges of the macula,
where the melanocyte network is less dense. In this particular instance, two types
of cells were identified: circular and polygonal. A similar picture was obtained from
three other preparations where the strained cells were in the centre of the macula.
The average potential from the four preparations was - 70 7+5.5 (S.D.) for the
circular cells and -73.4 + 5.1 mV (S.D.) for the polygonal cells. These experiments
show that the two types of cells are accessible for micro-electrode penetration and
that they both have similar resting membrane potentials. In the subsequent experi-
ments no attempt was made to identify the impaled cells.
When the macula was left in perilymph without continuous perfusion, the resting
potentials of the neuroepithelial cells gradually decreased to a value of about
50% of the initial control in about 30 min but potentials were restored to their
control values after perfusing the preparation with artificial perilymph. This de-
polarization may be caused by a substance secreted by the neuroepithelial cells in
the sacculus (Smith, 1970). To avoid this effect in the subsequent experiments we
used continuous perfusion.
The mechanism responsible for the generation of membrane resting potentials
continues its operation for relatively long periods of time after the sensory area is
excised from the animal. If the macula sacculi is kept in the refrigerator overnight,
high resting potentials can still be recorded when the surrounding solution is
placed by artificial perilymph at room temperature. The resting potential does not
start to decay until at least 36h after the sensory area is removed from the animal;
after 48 h no trans-membrane potentials are detected.
The potentialprofile in the intact sacculu8
In the in vivo preparation with the labyrinth intact, the recording micro-electrode
penetrated the free saccular membrane and the otolith before reaching the neuro-
epithelial cells in the macula. Text-fig. 2 (first three records on the first line) shows
the potential recorded when the micro-electrode was advanced towards the macula
from the top of the otic capsule. The first potential shift was recorded when the
micro-electrode impinged on epithelial cells in the free saccular membrane; the
potential change was negative and had an absolute value of 15-5+ 6-2 mV (S.D.,
n = 7). Since the potential disappeared when the micro-electrode was driven down-
wards to penetrate the saccular cavity, its most likely source was the membrane
oftheepithelial cellsinthefree saccularmembrane. The lack ofapotentialdifference
between the endo- and perilymphatic compartments (Vt) is consistent with previous
reports in which the endolymphatic potential recorded from vestibular organs has
been found to be small or absent (Smith et al. 1958; Schmidt & Fernandez, 1962).
The penetration of the otolith did not produce any d.c. potential change but the
15 PHY 281
450 H. BRACHO AND R. BUDELLI
base line became slightly irregular. When the micro-electrode penetrated the neuro-
epithelial cells in the macula, a sudden negative d.c. change was recorded. This
potential had an absolute value of 70 0+ 7.3 mV (s.D., n = 19). Further advance-
ment ofthe micro-electrode resulted in the loss ofthe d.c. potential and eventually
the micro-electrode broke andits resistance dropped. Thepenetration ofthesaccuwar
walls with a low-resistance micro-electrode (less than 5 MQ) did not produce any
d.c. changes.
50mV1 10sec
t -- r-
I u u VLi VuV - V
E -40
I
*-0
r -80 _-
_
(0U0
E
I I 1L -1
0
Time (min)
50r ,_ l
I I
I I
I I
F
40 I I
I I
x=12-1
III III S.=56
30 F I I Sv=1*3
I I
I I P<00005
I I
I I
20 -
I I
II I
I
10- I I
I I
I L.i
I I
iI ~ I I
%vF -60 -80 -100 0 20
Membrane potential (mV) AMembrane potential (mV)
Text-fig. 2. For legend see facing page.
RESTING POTENTIALS OF HAIR CELLS 451
When the experiments areperformedwith theintactlabyrinth, theneuroepithelial
cells face on their ciliated side a solution ofhigh potassium concentration: the endo-
lymph (Rauch & Rauch, 1974). If the endolymphatic membrane of the neuro-
epithelial cells is permeable to potassium, the endolymph should have a depolarizing
action on these cells. To test this possibility the free saccular membrane and the
otolith were removed and the endolymph was replaced by artificial perilymph in
the in vivo preparation. Under this condition the membrane potentials of the cells
in the macula and the surrounding epithelium reached a value of -82-1 + 5.0 mV
(S.D., n = 19). The average difference between the potentials recorded in the intact
sacculus and those recorded after replacing the endolymph by perilymph was
statistically significant (Student's test, P < 0.0005) and had a value of 12-1 + 5-6 mV
(S.D., n = 19) (Text-fig. 2). This result demonstrates that the endolymph has a
depolarizing action on the neuroepithelial cells in the macula and onthe surrounding
epithelial cells when the experiment is carried out in vivo. The in vivo preparation
yielded high membrane potentials from the neuroepithelial cells for several hours
after the saccular membrane had been removed provided an adequate circulation
of blood was maintained. Under these conditions the drop in membrane potential
after 3 h was less than 20 %.
Potasium effects
Symmetricalperfusion
Applicability of the electrodiffusion equations. In other excitable cells such as
nerve and muscle fibres, the resting membrane potential is generated mainly by
the potassium concentration gradient across the membrane (Adrian, 1956; Baker
et al. 1962). It is possible that the depolarization produced by endolymph is a result
of the high concentration ofpotassium present in the fluid. To test the dependence
Text-fig. 2. Potential profile in the sacculus. At the top left, a recording micro-
electrode's pathway (dashedline) through thesacculus toward the cellsinthe macula,
with the resultant polygraph tracings below. From left to right: first, penetration
through the cell membranes in the free saccular membrane; secondly, through the
otolith,whenonly aslightirregularityinthebaselinewasrecorded. Thirdly, asudden
change in d.c. potential recorded when the micro-electrode reached the saccular epi-
thelium. and penetrated the cellular membranes. Three potentials corresponding to
three different lateral positions ofthe electrode are shown. The following four groups
of records were obtained at different times after the free saccular membrane was
removed and the endolymph replaced by artificial perilymph. The graph below the
records shows the time course of the membrane potentials' value. At time 0 the
saccular membrane was opened and the endolymph replaced by perilymph (dashed
line). Thepotentials before time 0wereobtained with thelabyrinth intact and include
membranepotentials obtainedfromthemacula andthenon-sensory epithelium ofthe
same animal, and those from the macula ofa different animal. Each point represents
theaverageof10-15membranepotentialsrecordedfromdifferentcells.Thehistograms
below the graph represent the distribution of the mean membrane potentials (left)
when the recordings were made in the intact sacculus (continuous lines) and after
removal ofthe free saccular membrane and the otolith (dashed line). The histogram
at the right corresponds to the difference of means of membrane potentials before
and after the sacculus was opened and perfused. x = mean, 8a = S.D., 81 = 5.E. of
the mean, P = the level ofsignificance at which the null hypothesis ofno difference
was rejected.
I5-2
452 H. BRACHO AND R. BUDELLJ
A
0
0
/
/
/
° 1 min /
/
/
-20 _ /
/
/
/
/
1E- /
/
/
a-) /
00 -401- //
/
CL /
a) /
/
.E0 //
a) S //
-60 _ //
/
/
/
/
0 /
/
/
/
-80 I I
3 10 30 100
B
0
a)
20 40 60 830
External potassium concentration (mM)
Text-fig 3. For legend see facing page.
RESTING POTENTIALS OF HAIR CELLS 453
of the resting membrane potentials on the extracellular potassium concentration
we used an in vitro preparation clamped between two chambers. The change in
potassium concentration was accomplished by perfusing both chambers with
solutions containing 3, 10, 30, and 100 mM-KCl. The membrane potentials of ten
to fifteen cells were recorded in each solution. The results are shown in Text-fig. 3A
where the membrane potential was plotted against the logarithm of the external
potassium concentration. The experimental points failed to fit the theoretical line
traced using the Nernst equation for a potassium electrode. The potentials at 3 and
10 mM-KCl were smaller than the ones expected from a purely potassium-
selective membrane. This departure from potassium electrode behaviour has been
found in nerve fibres (Baker et al. 1962; Huxley & StAmpfli, 1951), muscle fibres
(Hodgkin & Horowicz, 1959) and photoreceptors (Brown, 1977) and has been
explained on the basis of a resting sodium permeability of the membrane. In this
case the constant field equation (Goldman, 1943; Hodgkin & Katz, 1949) fits the
experimental results (Hodgkin & Horowicz, 1959). In the Goldman-Hodgkin-Katz
equation the Na:K permeability ratio is included as a coefficient (b) ofthe external
and internal sodium concentration. This equation states
V = RTIn[K]o+b[Na]O+(PcI/PR)[Cl]i
F [K]i+b[Na]i+(PcL/PK)[Cl]o
Assuming a low permeability to chloride and a low intracellular sodium, Moreton
(1969) applied a modified version of the Goldman-Hodgkin-Katz equation to the
neurones in the central nervous system ofthe snail. The arrangement by Moreton is
eVF/RT [K]o+b [Na]0
-
[K]1 [K]1
which provides a simple way to estimate b and the internal potassium concentration,
ifb is assumed to be constant.
In this equation the dependent variable (exp (VF/RT)) is a linear function of
the external potassium concentration and K1 and b can be estimated by measuring
the slope (1/K1,) and the y intercept (bNao/Ki). We applied this analysis to the
results obtained from the saccular neuroepithelium to determine whether its charac-
teristics are similar to those of other excitable membranes. In our experimental
methods, changes in external potassium concentration imply changes in external
Text-fig. 3. A, the membrane potentials were plotted against the logarithm of the
externalpotassium concentration. The interrupted linerepresents theNernstequation
for a potassium electrode at an internal potassium concentration of 136mm. In the
inset, showing intracellular recordings from the macula, the first eight potentials are
eight different cells. The external potassium concentration was changed from 3 to
100mm (first arrow) (the micro-electrode remained inside the cell) and from 100
to 3mm(themicro-electrodeleftthecell 5timesanda different cellhadtobeimpaled).
B, the data from A were plotted using the modified Moreton equation (see text). The
valueexp (VFIRT) was plotted against the potassium concentration inalinearscale.
The straight line is the least-square fitting to the experimental results. The internal
potassium and the sodium:potassium permeability ratio were calculated using this
fitting. n = 15. s.E. ofmean included within the points.
454 H. BRACHO AND R. BUDELLI
sodium concentration, butthe total amount ofmonovalent cations is constant (118).
Taking this into consideration we modified the Moreton equation obtaining
eVF/RT = (-b) [K] b+118 .
Usinjgointihnegthaebove equation replotte~d~~t~h~e[Kd]a1t+a1f1ro8m-Text-fig. 3A in B. The line
we
joining the points calculated from the experimental values of V is a straight line
as expected from a membrane whose potential is given by the passive movements
of sodium and potassium. The values of Ki and b estimated from the graph were
136 mm and 005 respectively. The value of Ki is similar to that found in frog
muscle (Adrian, 1956) and the value of b is within the range obtained in other
excitable cells (Marmor, 1975). We do not know ifthe assumptions included in the
electrodiffusion equation are valid for this preparation, since we did not study the
effects of chloride and sodium independently. At this point it is reasonable to say
that the effects of symmetrical perfusion with different potassium concentrations
on the neuroepithelial cells' membrane potential are similar to the potassium effects
in other excitable cells, indicating that in both cases the membrane potentials are
generated in the same fashion.
Asymmetricalperfusion
Applicability of the electrodiffu8ion equation8. Under symmetrical conditions the
membrane potential of the neuroepithelial cells is -7 6+ 3*2 (s.D., n = 13) when
the bathing fluid is artificial endolymph and -702 + 7.9 (S.D., n = 84) when the
bathing fluid is artificial perilymph. The latter potential is similar to that recorded
in vivo (-70*0mV)withthesacculusintact. Sincewehaveshownthattheendolymph
has a depolarizing action in vivo, the similarity of these membrane potentials
cannot be interpreted to mean that the endolymphatic membrane is impermeable.
Another alternative is that the two different potassium gradients present in the
peri- and endolymphatic membranes interact and the measured membrane potential
istheresultofthisinteraction. Totestthispossibilityweusedtheinvitropreparation
to measure the membrane potential ofthe neuroepithelial cells under asymmetrical
conditions. This was accomplished by changing the potassium concentration in the
endolymphatic chamber while keeping the perilymphatic potassium constant at
3 mmandviceversa. Theresults ofthistypeofexperiment areshowninText-fig. 4A
where the membrane potentials are plotted against the logarithm of the external
potassium concentration whenthe changetookplacein theendolymphatic chamber,
intheperilymphatic chamber, andinbothchambers. Inallthreecasesthemembrane
potential depends on the external potassium concentrations. The slope ofthe best-
fittedlineisdifferentforeachcase,thesteepestbeingthatforsymmetricalperfusion,
followed by that for the change in potassium concentration in the perilymph. These
results indicate that the plasma membrane ofthe neuroepithelial cells is potassium-
selective on both epithelial sides. We applied the electrodiffusion equations to our
experimental results obtained under asymmetrical perfusion, assuming a uniform
ionic selectivity of the epithelial membrane and taking into consideration the fact
that there is no transepithelial potential. This implies that the perilymphatic and