From the * Department of Physiological Science and Department of Ophthalmology, University of California, Los Angeles, Los Angeles, California 90095;
Physiological Laboratory, University of Cambridge, Cambridge, United Kingdom CB2 3EG; and § Department of
Physiology, Boston University Medical School, Boston, Massachusetts 02118
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ABSTRACT |
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Simultaneous measurements of photocurrent and outer segment Ca2+ were made from isolated salamander cone photoreceptors. While recording the photocurrent from the inner segment, which was drawn into a suction pipette, a laser spot confocal technique was employed to evoke fluorescence from the outer segment of a cone loaded with the Ca2+ indicator fluo-3. When a dark-adapted cone was exposed to the intense illumination of the laser, the circulating current was completely suppressed and fluo-3 fluorescence rapidly declined. In the more numerous red-sensitive cones this light-induced decay in fluo-3 fluorescence was best fitted as the sum of two decaying exponentials with time constants of 43 ± 2.4 and 640 ± 55 ms (mean ± SEM, n = 25) and unequal amplitudes: the faster component was 1.7-fold larger than the slower. In blue-sensitive cones, the decay in fluorescence was slower, with time constants of 140 ± 30 and 1,400 ± 300 ms, and nearly equal amplitudes. Calibration of fluo-3 fluorescence in situ from red-sensitive cones allowed the calculation of the free-Ca2+ concentration, yielding values of 410 ± 37 nM in the dark-adapted outer segment and 5.5 ± 2.4 nM after saturating illumination (mean ± SEM, n = 8). Photopigment bleaching by the laser resulted in a considerable reduction in light sensitivity and a maintained decrease in outer segment Ca2+ concentration. When the photopigment was regenerated by applying exogenous 11-cis-retinal, both the light sensitivity and fluo-3 fluorescence recovered rapidly to near dark-adapted levels. Regeneration of the photopigment allowed repeated measurements of fluo-3 fluorescence to be made from a single red-sensitive cone during adaptation to steady light over a range of intensities. These measurements demonstrated that the outer segment Ca2+ concentration declines in a graded manner during adaptation to background light, varying linearly with the magnitude of the circulating current.
Key words: phototransduction; retina; bleaching adaptation; light adaptation; calcium feedback ![]() |
INTRODUCTION |
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The concentration of Ca2+ in the outer segment of vertebrate photoreceptors is believed to be controlled by a
dynamic balance of Ca2+ fluxes across the outer segment membrane. Ca2+ enters the outer segment
through the cGMP-gated conductance as part of the
circulating current (Yau and Nakatani, 1984a; Hodgkin et al., 1985
), and is extruded by a Na+/Ca2+, K+ exchanger (Yau and Nakatani, 1984b
; Cervetto et al.,
1989
). The activation of the photopigment rhodopsin
by light initiates a series of biochemical events that culminates in the closure of cGMP-gated channels (Fesenko et al., 1985
), thereby reducing Ca2+ influx; however, Ca2+ efflux through the exchanger continues
(Yau and Nakatani, 1985
). The resulting fall in outer
segment Ca2+ concentration has been shown to play an
important role in the regulation of sensitivity in both
rod and cone photoreceptors. If this fall in Ca2+ concentration is prevented during exposure to background
light, then light adaptation is also prevented (Matthews
et al., 1988
, 1990
; Nakatani and Yau, 1988a
). Furthermore, if outer segment Ca2+ is prevented from changing during exposure of a cone to light sufficiently intense to bleach a significant fraction of the photopigment, then bleaching adaptation is also prevented
(Matthews et al., 1996
). These results strongly suggest
that both background adaptation and bleaching adaptation require a reduction in outer segment Ca2+ concentration. A number of Ca2+-sensitive mechanisms have
been identified that may collectively contribute to adaptation (Koch and Stryer, 1988
; Kawamura and Murakami, 1991
; Hsu and Molday, 1993
; Gorczyca et al.,
1994
; Lagnado and Baylor, 1994
). To understand how
these mechanisms might contribute to adaptation, it is
necessary to determine the magnitude and time course
of light-induced changes in outer-segment Ca2+ concentration.
Most of our understanding of the role of Ca2+ in
modulating phototransduction is based on studies in
rod photoreceptors (McNaughton et al., 1986; Ratto
et al., 1988
; Korenbrot and Miller, 1989
; Gray-Keller
and Detwiler, 1994
; McCarthy et al., 1994
; Sampath et al.,
1998
), from which direct measurements of the magnitude and time course of the fall in outer segment Ca2+
concentration are available. In cones, although several
studies have characterized the time course of the decaying current associated with the electrogenic Na+/Ca2+,
K+ exchanger at the onset of saturating light (Nakatani
and Yau, 1989
; Perry and McNaughton, 1991
), which is
thought to reflect the decline in Ca2+ concentration
adjacent to the plasma membrane, no direct measurements have been made of light-induced changes in
outer-segment Ca2+ concentration.
In this study, we report the first determination of
outer segment Ca2+ concentration from cone photoreceptors with a laser spot confocal technique (Escobar
et al., 1994; Sampath et al., 1998
), which provides the
spatial precision and sensitivity required to measure the change in fluorescence of the Ca2+-sensitive dye
fluo-3 within the small volume of the cone outer segment, which reflects the decline in Ca2+ induced by
stimulation of the cone by the intense laser spot. In addition, we have used the rapid pigment regeneration
that occurs in the presence of 11-cis-retinal to investigate the role of Ca2+ in cone-bleaching adaptation, by
measuring outer segment Ca2+ concentration during
the photopigment cycle. Repeated regeneration of the
photopigment allowed multiple Ca2+ measurements to
be made from a single cone, enabling the relationship between the circulating current and outer segment
Ca2+ concentration to be studied.
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METHODS |
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Preparation
Aquatic tiger salamanders (Ambystoma tigrinum) purchased from
Charles Sullivan were kept between 7° and 10°C on a 12-h light- dark cycle. Animals were dark-adapted overnight and killed by decapitation, followed by rostral and caudal pithing, in accordance with protocols approved by the Animal Research Council
of the University of California, Los Angeles. Retinae were removed and dissociated under infrared illumination to yield isolated photoreceptors, as described previously (Sampath et al.,
1998). All experiments were carried out at room temperature
(23°C). A fixed volume of the resulting cell suspension was collected with a transfer pipette and injected into the recording
chamber. The dissociated cells were continuously superfused
with Ringer solution (111 mM NaCl, 2.5 mM KCl, 1.0 mM CaCl2,
1.6 mM MgCl2, 3.0 mM HEPES-NaOH, 10 µM EDTA-NaOH, 10 mM glucose, 0.1 mg/ml bovine serum albumin, pH 7.7). During
in situ fluorescence calibrations, the entire recording chamber
was superfused with nominal zero and isotonic Ca2+ solutions
containing ionomycin (see below). Unless otherwise specified,
all reagents were purchased from Sigma Chemical Co.
Ca2+ Measurements
Localized measurements of intracellular Ca2+ were made from
the outer segments of isolated red- and blue-sensitive salamander cones. This method, which has been used previously on salamander rod photoreceptors (Sampath et al., 1998), is based on a
laser spot confocal technique developed by Escobar et al. (1994)
.
In brief, dissociated salamander retinal tissue was loaded with the Ca2+ indicator fluo-3 (Molecular Probes, Inc.) by incubating the
cells in Ringer solution containing 10 µM fluo-3 acetoxymethyl
ester for 30 min. The inner segment of an isolated cone was
drawn into a suction pipette, leaving the outer segment protruding into the recording chamber. This configuration provided an
unobstructed optical path for fluorescence measurements from
the cone outer segment and allowed it to be exposed to solutions
of different compositions. An argon ion laser (
max = 514 nm, or
488 nm in a minority of experiments) was used to illuminate a
pinhole, which was imaged by a high numerical aperture microscope objective to illuminate a spot ~8 µm in diameter on the
cone outer segment. The suction pipette was positioned so that
the laser spot would largely cover the cone outer segment without extending onto the ellipsoid region of the inner segment. Although not all of the outer segment was illuminated by the laser,
in most cases we estimate that 90% or more was exposed to the laser.
The fluorescence evoked by the laser was collected by the objective and detected by a PIN photodiode mounted on the headstage of a patch clamp amplifier (Axopatch 200A; Axon Instruments). The analogue signal from the photodiode was low-pass
filtered at 250 Hz with an active eight-pole Bessel filter (Frequency Devices Inc.) and digitized at 1,000 Hz (PClamp 6; Axon
Instruments). The fluorescence signals measured from the cone
outer segment seem unlikely to have been retarded by the kinetics of Ca2+ binding to fluo-3, since previous studies have shown
that the rate of dissociation of Ca2+ from fluo-3 is considerably
faster (
off ~ 7 ms) (Escobar et al., 1997
) than the changes in fluorescence that we recorded. In addition, fluo-3 loading is unlikely to have contributed significantly to the buffering capacity
of the cone, because photoresponses recorded in the absence of
fluo-3 have identical response waveforms to those of fluo-3-
loaded cones.
The intensity of the laser was attenuated by reflective neutral
density filters to yield a flux of 1.2 × 1011 photons per µm2 s
1 at
514 nm, and 1.3 × 1011 photons per µm
2 s
1 at 488 nm at the
plane of the pinhole image. Bleaching of fluo-3 by even the longest laser exposures used in these experiments is likely to have
been negligible since these laser intensities are ~25% lower than
that which was used in our previous study of rod photoreceptors
(Sampath et al., 1998
), during which little or no dye bleaching
was observed. In addition to exciting fluo-3 fluorescence, the laser also powerfully stimulated the cone, bleaching a substantial
fraction of the photopigment (see below). Several laser illumination protocols were used, depending on the nature of the experiment. In experiments designed to measure the time course of
the fall in fluo-3 fluorescence produced by saturating illumination, a continuous 3-s laser exposure was used. In other experiments, laser exposure was minimized by instead presenting multiple brief laser pulses. In experiments in which the fluorescence was calibrated to determine the outer segment Ca2+ concentration, or in which the photopigment was regenerated, four laser
pulses each of 20 ms were delivered over the course of 5 s. For experiments in which the relationship between steady state current
and outer segment Ca2+ concentration was determined, a single
20-ms laser pulse was delivered.
To calculate the free-Ca2+ concentration in the dark-adapted
cone and after saturating illumination, the fluo-3 fluorescence signal was calibrated in situ. This was accomplished by measuring the minimum (Fmin) and maximum (Fmax) fluorescence that
could be elicited from each cell during superfusion with solutions designed to raise or lower the cytoplasmic Ca2+ concentration (Minta et al., 1989; Sampath et al., 1998
). The cone outer
segment was first exposed to a low Ca2+ solution (111 mM NaCl,
2.5 mM KCl, 2.05 mM MgCl2, 3.0 mM HEPES-NaOH, 2 mM
EGTA, pH 7.7), followed by a high Ca2+ solution (76.6 mM
CaCl2, 2.5 mM KCl, 3.0 mM HEPES-NaOH, pH 7.7). These solutions also contained 25 µM of the Ca2+ ionophore ionomycin
(Sigma Chemical Co. and Calbiochem Corp.). The cone outer
segment was exposed to these solutions for a time sufficiently
long (typically 1 min in each solution) for the laser-evoked fluorescence to reach a steady value. The free-Ca2+ concentration in
the cone outer segment was calculated on a cell-by-cell basis from
the Michaelis-Menten equation with a value of the dissociation constant for the binding of Ca2+ to fluo-3 of 400 nM, as previously determined (Sampath et al., 1998
). The initial peak of
evoked fluorescence at the opening of the laser shutter was taken
as the dark-adapted value, and the pedestal fluorescence was
taken as the light saturated value of Ca2+.
Electrical Recording and Light Stimuli
Light responses from salamander cones were recorded from the
inner segment, which was drawn into the suction pipette (Baylor et al., 1979; Yau et al., 1981
). The suction pipette current signal was filtered at 25 Hz with an active eight-pole Bessel filter. Unpolarized light stimuli were delivered from a DC-driven tungsten halogen lamp through a 570-nm interference filter for red-sensitive cones, or a 460-nm interference filter for blue-sensitive
cones. Cone types were distinguished based on their responses to
flashes of light at these wavelengths. The absolute light intensity
of the photostimulator was calibrated daily at 570 nm with a silicon photodiode (United Detector Technologies) and adjusted
with calibrated neutral density filters (Fish-Schurman Corp.). In
experiments to determine the relationship between red-sensitive
cone outer segment Ca2+ concentration and the circulating current, light stimuli were instead delivered through a 500-nm interference filter to prevent their detection by the PIN photodiode,
which is located after the 530-nm long-pass fluorescence emission filter. Brief flashes were of 20-ms duration.
Pigment Bleaching and Regeneration
The percentage of red-sensitive cone photopigment bleached by
the light from the argon ion laser, which was polarized transversely to the outer segment of the cone when held in the suction
pipette, was estimated from the photosensitivity measured spectrophotometrically for transverse illumination of isolated salamander cones (6.0 × 109 µm2) (Jones et al., 1993
) and the ratio
of absorbance at 514 and 610 nm predicted from the pigment
nomogram (Dartnall, 1972
). Both the continuous and the short-pulse laser exposures that were used to measure the outer segment Ca2+ concentration can be calculated to have bleached
>99% of the photopigment within the area of the laser spot.
However, in practice the total percentage bleach will have been
reduced by two factors. First, the laser spot may not have covered
the entire outer segment, especially at its basal end where it was
important not to allow the spot to extend onto the strongly fluorescing ellipsoid region of the inner segment. Second, even
within the area illuminated by the laser spot, the fractional
bleach may have been reduced somewhat by photoregeneration
from bleaching intermediates (Williams, 1964
; see also Pugh,
1975
). This seems likely to have been a relatively modest effect,
since even the shortest 20-ms laser pulses were considerably
longer than the lifetimes that have been reported for intermediates from which substantial photoregeneration can take place in
vertebrate photoreceptors.
Pigment regeneration was accomplished by bath application
of 11-cis-retinal, delivered either as a 0.1% ethanolic solution in
Ringer, or in phosphatidylcholine vesicles (Type V; Sigma Chemical Co.), which were prepared by sonication as described previously (Jones et al., 1989; Sampath et al., 1998
). The concentration of 11-cis-retinal was assessed spectrophotometrically (UV-2101PC spectrophotometer; Shimadzu Scientific Instruments)
from these solutions diluted 10-fold in absolute ethanol, using a
molar extinction coefficient of 25,000 M
1 cm
1 at 380 nm (Hubbard et al., 1971
). To regenerate the photopigment of a
bleached cone, a transfer pipette was used to add to the recording chamber either 1 ml of an ethanolic solution containing 50-
75 µM 11-cis-retinal, or the appropriate quantity of vesicle suspension to achieve a 11-cis-retinal concentration of 100 µM. Regeneration of the photopigment was assessed by measuring the electrical responses of the cone to repeated flashes of light.
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RESULTS |
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Kinetics of the Light-induced Decline in Ca2+ Concentration
Fig. 1 shows an example of a simultaneous measurement of fluo-3 fluorescence and suction pipette current
from an isolated red-sensitive cone previously loaded
with fluo-3 acetoxymethyl ester. First, the dark-adapted
cone was exposed to the laser for 3 s, an exposure that
will have bleached a considerable fraction of the photopigment (see METHODS), and which resulted in complete suppression of the circulating current (Fig. 1 B,
1). This was accompanied by a rapid decline in fluo-3
fluorescence (Fig. 1 A, 1), corresponding to a fall in the
intracellular free-Ca2+ concentration. The measured
fluorescence originated in the cone outer segment,
since movement of the cone off the laser spot produced no detectable fluorescence when the laser shutter was opened. A second laser exposure 10-s later
showed that at that time the circulating current remained almost completely suppressed (Fig. 1 B, 2) and
that fluo-3 fluorescence had reached a minimum level (Fig. 1 A, 2). It can be seen from Fig. 1 A that fluo-3 fluorescence did not fall to zero during saturating illumination, but instead declined to a maintained pedestal
level that was insensitive to further stimulation of the
cone by laser light, much as is the case for rod photoreceptors. This light-insensitive component of fluorescence is unlikely to be due to compartmentalization of
fluo-3 within the outer segment during loading, since
the cone outer segment contains no closed compartments. Instead, this pedestal, which is greater than the
minimum fluorescence evoked from fluo-3 under Ca2+-free conditions in vitro, may be attributable to a nonspecific interaction of fluo-3 with other molecules in
the cone outer segment (Sampath et al., 1998). The decline in fluo-3 fluorescence during the first 2 s of exposure to saturating laser light could be fitted with the
sum of two decaying exponentials, the shorter of which was of greater amplitude than the longer. Measurements from 25 red-sensitive cones yielded mean values
for the two time constants of 43 ± 2.4 and 640 ± 55 ms
(mean ± SEM), while the ratio of their amplitudes was
1.7 ± 0.11.
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Occasionally in the course of these experiments,
blue-sensitive cones were encountered; these are
much less numerous than red-sensitive cones in the
salamander retina (Perry and McNaughton, 1991).
On these occasions, the opportunity was taken to determine the kinetics of the light-induced decline in
fluo-3 fluorescence from the outer segments. An example of such a measurement is shown in Fig. 2. It can
be seen that the time course of the decay in fluorescence is slower than that in red-sensitive cones, approximately matching the slower kinetics of the blue-sensitive cone photoresponse (Perry and McNaughton, 1991
). The fall in fluo-3 fluorescence was again
fitted as the sum of two decaying exponentials, but in
this case with nearly equal amplitudes. Measurements
from four blue-sensitive cones yielded mean values for
the two time constants of 140 ± 30 and 1,400 ± 300 ms (mean ± SEM).
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Magnitude of the Light-induced Decline in Ca2+ Concentration
To estimate the absolute values of free Ca2+ in darkness
and during steady light, in situ calibrations of fluo-3 fluorescence were performed by artificially elevating or
depressing the Ca2+ concentration in the cone outer
segment to levels that would maximize or minimize its
binding to the dye. A typical result is shown in Fig. 3.
First, a continuous laser exposure was used to measure
the initial level of fluo-3 fluorescence evoked from a
dark-adapted red-sensitive cone (Fig. 3 A, top, 1). This
laser spot was sufficiently intense to ensure that the circulating current remained close to complete suppression until the light-induced decline in fluo-3 fluorescence was completed (Fig. 3 A, bottom, 1). Then, a second laser exposure was delivered while the response remained close to saturation (Fig. 3 A, bottom, 2) in order to measure the fluorescence once the circulating
current was suppressed (Fig. 3 A, top, 2). These signals
were then calibrated by determining the minimum and
maximum levels of fluorescence that could be evoked by repeated 20-ms laser pulses during exposure to low
and high Ca2+ solutions in the presence of 25 µM of
the Ca2+ ionophore ionomycin. 20-ms pulses were used
in preference to continuous laser exposures as a precaution against photodynamic damage and cell death,
which was observed during a number of calibrations. Low Ca2+ solution (Fig. 3 B) evoked a substantial elevation in the circulating current and fluo-3 fluorescence
was reduced to a constant low level, while high Ca2+ solution (Fig. 3 C) abolished the circulating current and
led to a large increase in fluorescence. These changes
in circulating current and fluorescence suggest that
these manipulations successfully lowered and raised
the outer segment Ca2+ concentration from the normal
levels in saturating light and darkness. The values for
the minimum (Fmin) and maximum (Fmax) fluorescence
derived from these measurements were used in conjunction with the previously determined dissociation
constant of fluo-3 for Ca2+ (400 nM; Sampath et al.,
1998) to calculate the free-Ca2+ concentration in the
cone outer segment, yielding values of 410 ± 37 nM in
the dark-adapted red-sensitive cone, and 5.5 ± 2.4 nM
during suppression of the circulating current by saturating light (mean ± SEM, n = 8).
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This estimate of free Ca2+ may have been influenced
by several sources of error. Incomplete solution
changes into zero and isotonic Ca2+ solutions would
have produced discrepancies in the estimation of Fmin
and Fmax, though the rate of solution perfusion was sufficiently rapid to produce a complete change of solution. This difficulty is especially relevant for the solution change into the isotonic Ca2+ solution to increase
outer segment Ca2+, since the photoreceptors contain
a powerful Na+/Ca2+, K+ exchanger that extrudes
Ca2+. However, simultaneous measurements of circulating current indicate that these solution changes may
have been complete. When presented with this zero
Ca2+ solution, an increase in circulating current was
observed (Fig. 3 B, bottom), and in the isotonic Ca2+
solution this current was abolished (Fig. 3 C, bottom).
These changes in circulating current are consistent
with a Ca2+-dependent modulation of the guanylyl cyclase that was observed in previous experiments on rods
(see, for example, Dizhoor et al., 1994; Sampath et al.,
1998
). In addition, small errors in the estimation of the
Kd of fluo-3 for binding Ca2+ would be expected to
change the calculated values of Ca2+. It is possible that
the intracellular environment may marginally change
this parameter (Schnetkamp et al., 1991b
).
Changes in Outer Segment Ca2+ Concentration during the Photopigment Cycle
In rods, the changes in light sensitivity that occur after
photopigment bleaching have been shown to be accompanied by changes in outer segment Ca2+ concentration (Sampath et al., 1998). A primary objective of this study was to investigate whether similar changes
also occur in cones. To this end, Ca2+ measurements
were made from isolated red-sensitive cones during the
photopigment cycle. An example of such an experiment is shown in Fig. 4. First, a sequence of flashes of
progressively increasing intensity was delivered to determine the response-intensity relation in darkness
(Fig. 4 D, Dark-adapted), and then a measurement of fluo-3 fluorescence was made from the dark-adapted
cone (Fig. 4 A). To minimize the total laser exposure,
fluorescence was excited by four 20-ms laser pulses presented over a 5-s period. Nevertheless, this relatively
brief exposure to the laser spot will have bleached a
substantial fraction of the photopigment (see METHODS). Once the sensitivity of the cone had stabilized after this bleach, the response-intensity relation was
again determined (Fig. 4 D, Bleached) and a second series of laser pulses was delivered (Fig. 4 B). After
bleaching, the response-intensity relation was shifted to
higher intensities, corresponding to a fall in sensitivity (Jones et al., 1993
). This bleach-induced desensitization was accompanied by a reduction in the initial level
of fluo-3 fluorescence evoked by the first laser pulse to
55% of its dark-adapted value in this cell. After this second laser exposure, the photopigment was regenerated
by exposing the outer segment to an ethanolic solution
of 11-cis-retinal. Once the sensitivity of the cone to dim
flashes returned to approximately its dark-adapted
value, superfusion with Ringer solution recommenced
and the response-intensity relation was determined a final time (Fig. 4 D, Regeneration). A final series of laser
pulses revealed that the recovery of sensitivity in the
presence of 11-cis-retinal was accompanied by the restoration of fluo-3 fluorescence to 93% of its original dark-adapted value.
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Similar results were obtained in a total of 13 experiments, in 7 of which an ethanolic solution and in 6 of which a vesicular solution of 11-cis-retinal was used to regenerate the photopigment. To compensate for differences in dye loading between cells, and for the possibility of slight movement of the outer segment during each experiment, the data were normalized by dividing the peak fluo-3 fluorescence for each cell by the light- and Ca2+-insensitive pedestal of fluorescence that remained after complete suppression of the circulating current. When analyzed in this way, the normalized peak fluorescence fell from 2.7 ± 0.2 (mean ± SEM) times the pedestal value in the dark-adapted cone at the start of the experiment to 1.5 ± 0.1 after bleaching by the laser spot, a difference that is significant at the 1% level (paired t test, t = 7.8). However, after regeneration of the photopigment with 11-cis-retinal, the normalized peak fluorescence rose again to 2.6 ± 0.2, a value not significantly different at the 5% level from the original level in darkness (paired t test, t = 1.1).
The regeneration of photopigment in a cone could be repeated reproducibly many times in the same cell. To demonstrate this, a cone was exposed to an ethanolic solution of 11-cis-retinal, and fluo-3 fluorescence was measured every 2-3 min with a 20-ms laser pulse. Before each fluorescence measurement, the responses to just-saturating flashes were recorded to ensure the complete recovery of the response amplitude and waveform. Five measurements were made in each of four cells, and the fluorescence evoked by each of these laser pulses fell within 15% of the original dark-adapted value (data not shown). These results indicate that, when cone sensitivity was completely restored after bleaching, the peak fluo-3 fluorescence also returned reproducibly to its original dark-adapted level. They thus suggest that there is little adverse effect of prolonged exposure to an ethanolic solution of 11-cis-retinal on the physiology of the cone. In contrast, prolonged exposure to vesicular solutions of 11-cis-retinal might be expected to have deleterious effects on the highly invaginated cone outer segment membrane. We therefore used ethanolic solutions in preference to vesicular solutions in experiments in which the outer segment was exposed continuously to 11-cis-retinal.
The Relationship between Outer Segment Ca2+ Concentration and Circulating Current
We have made use of the rapid and reproducible pigment regeneration that occurs in the continuous presence of an ethanolic solution of 11-cis-retinal to investigate the relationship between the circulating current and outer segment Ca2+ concentration in red-sensitive cones. An example of such an experiment in the presence of 11-cis-retinal is shown in Fig. 5 A. First, the cone was exposed to steady light that suppressed a proportion of the circulating current. After 10 s of background illumination, a period sufficient to allow the circulating current to decline to a new steady level, the cell was exposed to a 20-ms laser pulse and fluo-3 fluorescence was measured. After each laser pulse, which will have bleached the great majority of the photopigment (see METHODS), the suction pipette current responses to saturating flashes were used to monitor the recovery of both the circulating current and the waveform of the light response in the continuous presence of 11-cis-retinal. Either four or five fluorescence measurements were made from a given cone, each following adaptation to a different intensity of background light, and the cone was allowed to recover its dark-adapted sensitivity after each measurement. Fig. 5 B shows fluo-3 fluorescence measured from the same cone, plotted against the steady state circulating current at the time at which the laser pulse was delivered. It can be seen that as the circulating current was progressively suppressed by backgrounds of increasing intensity, the fluo-3 fluorescence decreased linearly, ultimately falling to a pedestal level once the response to the background saturated.
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Collected results from nine such experiments are shown in Fig. 6. To allow a direct comparison, the data have been normalized to compensate for variation between cells in the absolute level of fluorescence and the magnitude of the circulating current. Normalization of fluo-3 fluorescence also compensates for the variability of the calibration procedure. The data have been fitted with a straight line constrained to pass through unity for both the circulating current and fluorescence. The progressive decline in fluorescence that was seen as the circulating current was suppressed by steady light is consistent with a proportional variation between the outer segment Ca2+ concentration and the magnitude of the circulating current during background adaptation.
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DISCUSSION |
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Light-induced Decline in Cone Outer Segment Ca2+ Concentration
The decline in fluo-3 fluorescence that accompanies
the stimulation of a dark-adapted cone by the laser spot
is consistent with a light-induced reduction in outer
segment Ca2+ concentration, as previously shown to
take place during the rod photoresponse (McNaughton et al., 1986; Ratto et al., 1988
; Korenbrot and
Miller, 1989
; Gray-Keller and Detwiler, 1994
; McCarthy
et al., 1994
; Sampath et al., 1998
). While such a fall in
Ca2+ concentration has also been proposed to take
place in cones (Nakatani and Yau, 1988a
; Perry and McNaughton, 1991
), this study represents the first direct
measurement of its magnitude and kinetics. Our measurement relies upon the spatial precision, temporal
resolution, and sensitivity of the spot confocal technique (Escobar et al., 1994
; Sampath et al., 1998
),
which allows fluo-3 fluorescence to be excited and collected from the small tissue volume of the cone outer
segment, while excluding contributions from neighboring structures, such as the ellipsoid, which also load
heavily with the dye. The changes in fluo-3 fluorescence that we have observed can therefore be attributed to changes in Ca2+ concentration in the cone
outer segment.
In this study, we found that both the kinetics and the
magnitude of the light-induced decline in Ca2+ concentration in cone outer segments are considerably different from those previously measured in rods. We
therefore confirm and extend previous indirect measurements that suggested that, for similar changes in
outer segment current, changes in outer segment Ca2+
concentration are larger and faster in cones than rods
(Miller and Korenbrot, 1994). The initial light-induced
decline in fluo-3 fluorescence from the dark-adapted
cone outer segment could best be fitted as the sum of
two exponentials, as is the case for rods. However, the
time constants of the red-sensitive cone of 43 ± 2.4 and
640 ± 55 ms are three- to sixfold faster than those previously observed in rods with the same technique (Sampath et al., 1998
). Consequently, the fall of fluo-3 fluorescence to a baseline value after exposure to bleaching illumination normally requires only a few seconds
in the red-sensitive cone, whereas it takes much longer
in the rod. Furthermore, in red-sensitive cones the
faster of these two exponential components is dominant, contributing nearly twice the amplitude of the
slower component to the light-induced decline in Ca2+
concentration. This contrasts with the situation in rods,
for which the two components are of approximately
equal amplitude. The rapidity of the decline in red-sensitive cone Ca2+ concentration matches the similarly
rapid kinetics of their responses to light, ensuring that
the feedback actions of Ca2+ on the transduction mechanism take place on an appropriate time scale (Nakatani and Yau, 1989
; Matthews et al., 1990
). However, the photoresponse of a red-sensitive cone appears to be
intrinsically more rapid than that of a rod, since this
difference in response kinetics remains even when the
light-induced fall in outer segment Ca2+ concentration
is prevented (Nakatani and Yau, 1988b
; Matthews et al.,
1990
).
Although the explanation for multiple exponential
components of Ca2+ decline is uncertain, it is noteworthy that the time constant of the faster component in
cones approximates to that of previous measurements
of the time course of Na+/Ca2+, K+ exchange (Nakatani and Yau, 1989; Perry and McNaughton, 1991
). Since the invaginated structure of the amphibian cone
outer segment places most of the small intracellular
volume adjacent to the plasma membrane, it seems
possible that in red-sensitive cones the volume-averaged fall in outer segment Ca2+ concentration measured by the dye may be dominated by the kinetics of
Na+/Ca2+, K+ exchange to a greater degree than in
rods. Alternatively, the difference in the relative amplitudes of the two components may reflect differences in
low and high affinity buffering capacity between rod
and cone outer segments. The light-induced decline in
outer segment Ca2+ concentration is two- or threefold
slower in blue- than red-sensitive cones, broadly matching the slower time course of their photoresponses (Perry and McNaughton, 1991
). These kinetic differences between the two cone types may reflect differences in Ca2+ buffering capacity, in the ratio of surface
area to volume of the outer segment, or in the density
of Na+/Ca2+, K+ exchanger in the outer segment membrane (Nakatani and Yau, 1989
).
Calibration of these fluo-3 fluorescence signals revealed that Ca2+ concentration falls from 410 ± 37 nM
in the dark-adapted red-sensitive cone outer segment
to 5.5 ± 2.4 nM when the outer segment conductance is completely suppressed by light. In contrast, in a salamander rod the outer segment Ca2+ concentration is
higher both in darkness and during saturating light
(Sampath et al., 1998). Previous studies have shown
that during rod saturation the outer segment Ca2+ concentration falls to ~30-50 nM (Gray-Keller and Detwiler, 1994
; McCarthy et al., 1994
; Sampath et al.,
1998
), a value considerably greater than the subnanomolar values predicted from the Na+ and K+ gradients
if the Na+/Ca2+, K+ exchanger were to attain thermodynamic equilibrium (Cervetto et al., 1989
; Schnetkamp et al., 1991a
). This contrasts with the value for
the outer segment Ca2+ concentration in a saturated
red-sensitive cone of <6 nM. This observation suggests
that some process in both rod and cone outer segments
may prevent the Ca2+ concentration from attaining its
equilibrium value after complete suppression of the circulating current. Possible mechanisms include an inactivation of the Na+/Ca2+, K+ exchanger at low Ca2+
concentration (Schnetkamp et al., 1991a
; Schnetkamp
and Szerencsei, 1993
), or (for rods) a slow release of
Ca2+ from a store or buffer within the outer segment
(Schröder and Fain, 1984
).
A consequence of the very low Ca2+ concentration in
the cone outer segment when the circulating current is
completely suppressed is that the Ca2+ concentration
varies over a 75-fold dynamic range in red-sensitive cones, a value more than three times greater than in
the rod, despite the somewhat lower outer segment
Ca2+ concentration in darkness. The greater relative
variation in outer segment Ca2+ concentration in the
cone implies that Ca2+-sensitive sites within phototransduction are likely to be influenced more profoundly in
cones during exposure to steady light than in rods. For
example, the unbinding of Ca2+ from guanylyl cyclase-
activating protein results in an increase in the rate of
cGMP production (Gorczyca et al., 1994) that is believed to be crucial in setting the operating range of
transduction during steady light. Biochemical studies
in vitro have shown that retGC1, which is present in the
photoreceptor outer segment, is inhibited at a half-maximal Ca2+ concentration of 200 nM with a cooperativity of 1.7 (Dizhoor et al., 1994
). These values imply
that during saturating illumination the very low outer
segment Ca2+ concentration attained by cones would
induce an almost 20% greater increase in cyclase activity than in rods. It therefore seems possible that the
shallow cone response-intensity relation for steady illumination (Matthews et al., 1990
; Jones et al., 1993
) may
reflect a more powerful modulation of this and other
Ca2+-sensitive processes in cones than in rods. Compared with the rod photoreceptor, which saturates at a
relatively low background light intensity, this more
powerful modulation of the transduction mechanism
by Ca2+ in cones may extend the range of the cone to
brighter background intensities by preventing saturation of the photocurrent during bright steady light.
Measurements of Ca2+ Concentration during the Photopigment Cycle
After bleaching, fluo-3 fluorescence was significantly
depressed, but it recovered to a level not significantly
different from the original dark-adapted value after regeneration of the photopigment with 11-cis-retinal,
which also restores circulating current, response kinetics, and sensitivity (Jones et al., 1993). It is difficult
from our experiments to make a quantitative comparison between the magnitude of the fall in outer segment
Ca2+ concentration and the degree of suppression of
the circulating current by bleaching, since it is likely
that the laser spot did not excite fluorescence and
bleach pigment within the entirety of the cone outer
segment (see METHODS). Since the suppression of current is believed to be closely localized to the site of illumination or bleaching (Lamb et al., 1981
; Baylor and
Lamb, 1982
; Cornwall et al., 1990
), it seems likely that
the laser-induced bleach will have resulted in a nonuniform suppression of the circulating current. Nevertheless, these systematic changes in fluorescence intensity
indicate that in cones, as in rods (Sampath et al., 1998
),
the bleach-induced suppression of circulating current
is accompanied by a persistent decrease in outer segment Ca2+ concentration that is restored when the
photopigment is regenerated. This is consistent with
the notion that the persistent activation of the transduction cascade that follows cone bleaching (Cornwall et al., 1995
) leads, through partial suppression of the
circulating current, to a reduction in Ca2+ influx and a
corresponding fall in the outer segment Ca2+ concentration. Since bleach-induced changes in photoresponse sensitivity and kinetics are abolished in cones if
this fall in Ca2+i is prevented (Matthews et al., 1996
), it
would appear that the reduction in Ca2+i after bleaching is necessary for the adaptation that ensues, and that
restoration of Ca2+ concentration after the regeneration of the photopigment is required for the recovery
of dark-adapted sensitivity.
The Relationship between Outer Segment Ca2+ Concentration and the Circulating Current
The characteristic rapidity of photopigment regeneration in cones, which enables recovery of dark-adapted
sensitivity within just a few minutes after bleaching in
the presence of exogenous 11-cis-retinal, allowed multiple measurements of outer segment Ca2+ concentration
to be made from an individual cone by repeatedly regenerating the photopigment after successive bleaches
by the laser spot. It would be difficult to carry out this
same procedure in a rod, because the time required after bleaching to reach steady state and to regenerate
the photopigment with exogenous 11-cis-retinal can exceed 1 h (Jones et al., 1989). This approach was used to investigate the relationship between circulating current
and outer segment Ca2+ concentration in cones during
adaptation to steady background light. In these experiments, the background illumination was uniform over
the entire cone outer segment; therefore, the resulting
suppression of circulating current is likely also to have
been uniform. In addition, the background illumination was presented for a time period that allowed the
circulating current to reach steady state. Even though
the laser spot did not cover the entire outer segment, this steady state condition ensured that Ca2+ measurements from a portion of the outer segment were representative of the entire outer segment.
Since the outer segment Ca2+ concentration is approximately equal to the dissociation constant of fluo-3
in darkness and declines from this value during illumination, the relationship between dye fluorescence and
Ca2+ concentration will have deviated little (<10%)
from a linear relation. Consequently, the approximately linear variation of fluo-3 fluorescence with circulating current that we have observed suggests that
outer segment Ca2+ concentration may also vary linearly with circulating current. A similar conclusion has
been reached for rod photoreceptors from measurements from the entire retina (Younger et al., 1996; but see also Gray-Keller and Detwiler, 1994
). If the relative
permeability of the outer segment conductance to Ca2+
remains unchanged during light adaptation, the influx
of Ca2+ would be directly proportional to the magnitude of the circulating current (Nakatani and Yau,
1988b
). Similarly, the rate of Ca2+ extrusion should
vary linearly with the outer segment Ca2+ concentration,
since the Ca2+ concentration throughout the physiological range is considerably below the Ca2+ affinity at
the internal face of the Na+/Ca2+, K+ exchanger
(Hodgkin and Nunn, 1987
; Lagnado et al., 1988
;
Schnetkamp et al., 1991b
). The linear variation of fluo-3 fluorescence with circulating current that we have observed therefore supports the view that cone outer segment Ca2+ concentration at steady state is principally
determined by the dynamic balance between Ca2+ influx through the outer segment conductance, and by
Ca2+ efflux through the Na+/Ca2+, K+ exchanger, that
the fraction of the circulating current carried by Ca2+
remains constant during the light response, and that
the exchanger operates well below saturation.
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FOOTNOTES |
---|
Address correspondence to A.P. Sampath, Department of Physiological Science, Life Sciences 3836, UCLA, Los Angeles, CA 90095. Fax: 310-825-4667; E-mail: apsampat{at}ucla.edu
Original version received 15 October 1998 and accepted version received 18 November 1998.
Portions of this work were previously published in abstract form (Sampath, A.P., H.R. Matthews, M.C. Cornwall, J. Bandarchi, and G.L. Fain. 1998. J. Physiol. (Camb.). 506:89P; Matthews, H.R., A.P. Sampath, M.C. Cornwall, and G.L. Fain. 1998. Investig. Ophthalmol. Vis. Sci. 39:1124).This work was supported by a Hyde Fellowship from the Department of Physiological Science, UCLA (A.P. Sampath), grants EY-01844 (G.L. Fain) and EY-01157 (M.C. Cornwall) from the National Eye Institute of the National Institutes of Health, United States Public Health Service T32 NS 07101 (J. Bandarchi), and a grant from the Wellcome Trust (H.R. Matthews).
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REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1. | Barnes, S., and B. Hille. 1989. Ionic channels of the inner segment of tiger salamander cone photoreceptors. J. Gen. Physiol. 94: 719-743 [Abstract]. |
2. | Baylor, D.A., and T.D. Lamb. 1982. Local effects of bleaching in retinal rods of the toad. J. Physiol. (Lond.). 328: 49-71 [Abstract]. |
3. | Baylor, D.A., T.D. Lamb, and K.-W. Yau. 1979. The membrane current of single rod outer segments. J. Physiol. (Camb.). 288: 589-611 [Abstract]. |
4. | Cervetto, L., L. Lagnado, R.J. Perry, D.W. Robinson, and P.A. McNaughton. 1989. Extrusion of calcium from rod outer segments is driven by both sodium and potassium gradients. Nature. 337: 740-743 [Medline]. |
5. | Cornwall, M.C., A. Fein, and E.F. MacNichol. 1990. Cellular mechanisms that underlie bleaching and background adaptation. J. Gen. Physiol. 96: 345-372 [Abstract]. |
6. | Cornwall, M.C., H.R. Matthews, R.K. Crouch, and G.L. Fain. 1995. Bleached pigment activates transduction in salamander cones. J. Gen. Physiol. 106: 543-557 [Abstract]. |
7. | Dartnall, H.J.A. 1972. Photosensitivity. In Handbook of Sensory Physiology. H.J.A. Dartnall, editor. Springer-Verlag, Berlin. 122-145. |
8. | Dizhoor, A.M., D.G. Lowe, E.V. Olshevskaya, R.P. Laura, and J.B. Hurley. 1994. The human photoreceptor membrane guanylyl cyclase, RetGC, is present in outer segments and is regulated by calcium and a soluble activator. Neuron. 12: 1345-1352 [Medline]. |
9. | Escobar, A.L., J.R. Monck, J.M. Fernandez, and J.L. Vergara. 1994. Localization of the site of Ca2+ release at the level of a single sarcomere in skeletal-muscle fibers. Nature 367: 739-741 [Medline]. |
10. | Escobar, A.L., P. Velez, A.M. Kim, F. Cifuentes, M. Fill, and J.L. Vergara. 1997. Kinetic properties of DM-nitrophen and calcium indicators: rapid transient response to flash photolysis. Pflügers Arch. 434: 615-631 [Medline]. |
11. | Fesenko, E.E., S.S. Kolesnikov, and A.L. Lyubarsky. 1985. Induction by cyclic GMP of cationic conductance in plasma membrane of retinal rod outer segment. Nature. 313: 310-313 [Medline]. |
12. | Gorczyca, W.A., M.P. Gray-Keller, P.B. Detwiler, and K. Palczewski. 1994. Purification and physiological evaluation of a guanylate cyclase activating protein from retinal rods. Proc. Natl. Acad. Sci. USA. 91: 4014-4018 [Abstract]. |
13. | Gray-Keller, M.P., and P.B. Detwiler. 1994. The calcium feedback signal in the phototransduction cascade of vertebrate rods. Neuron. 13: 849-861 [Medline]. |
14. | Hodgkin, A.L., P.A. McNaughton, and B.J. Nunn. 1985. The ionic selectivity and calcium dependence of the light-sensitive pathway in toad rods. J. Physiol. (Camb.) 358: 447-468 [Abstract]. |
15. | Hodgkin, A.L., and B.J. Nunn. 1987. The effect of ions on sodium calcium exchange in salamander rods. J. Physiol. (Camb.). 391: 371-398 [Abstract]. |
16. | Hsu, Y.-T., and R.S. Molday. 1993. Modulation of the cGMP-gated channel of rod photoreceptor cells by calmodulin. Nature. 361: 76-79 [Medline]. |
17. | Hubbard, R., P.K. Brown, and D. Bownds. 1971. Methodology of vitamin A and visual pigments. Methods Enzymol. 18C:615-653. |
18. | Jones, G.J., R.K. Crouch, B. Wiggert, M.C. Cornwall, and G.J. Chader. 1989. Retinoid requirements for recovery of sensitivity after visual-pigment bleaching in isolated photoreceptors. Proc. Natl. Acad. Sci. USA. 86: 9606-9610 [Abstract]. |
19. | Jones, G.J., A. Fein, E.F.J. MacNichol, and M.C. Cornwall. 1993. Visual pigment bleaching in isolated salamander retinal cones. Microspectrophotometry and light adaptation. J. Gen. Physiol. 102: 483-502 [Abstract]. |
20. | Kawamura, S., and M. Murakami. 1991. Calcium-dependent regulation of cyclic GMP phosphodiesterase by a protein from frog retinal rods. Nature. 349: 420-423 [Medline]. |
21. | Koch, K.-W., and L. Stryer. 1988. Highly cooperative feedback control of retinal rod guanylate cyclase by calcium ions. Nature. 334: 64-66 [Medline]. |
22. | Korenbrot, J.I., and D.L. Miller. 1989. Cytoplasmic free calcium concentration in dark-adapted retinal rod outer segments. Vision Res. 29: 939-948 [Medline]. |
23. | Lagnado, L., L. Cervetto, and P.A. McNaughton. 1988. Ion transport by the Na-Ca exchange in isolated rod outer segments. Proc. Natl. Acad. Sci. USA. 85: 4548-4552 [Abstract]. |
24. | Lagnado, L., and D.A. Baylor. 1994. Calcium controls light-triggered formation of catalytically active rhodopsin. Nature. 367: 273-277 [Medline]. |
25. | Lamb, T.D., P.A. McNaughton, and K.-W. Yau. 1981. Spatial spread of activation and background desensitization in toad rod outer segments. J. Physiol. (Camb.). 319: 463-496 [Medline]. |
26. | Matthews, H.R., G.L. Fain, and M.C. Cornwall. 1996. Role of cytoplasmic calcium concentration in the bleaching adaptation of salamander cone photoreceptors. J. Physiol. (Camb.). 490: 293-303 [Abstract]. |
27. | Matthews, H.R., G.L. Fain, R.L.W. Murphy, and T.D. Lamb. 1990. Light adaptation in cone photoreceptors of the salamander: a role for cytoplasmic calcium. J. Physiol. (Camb.). 420: 447-469 [Abstract]. |
28. | Matthews, H.R., R.L.W. Murphy, G.L. Fain, and T.D. Lamb. 1988. Photoreceptor light adaptation is mediated by cytoplasmic calcium concentration. Nature. 334: 67-69 [Medline]. |
29. | McCarthy, S.T., J.P. Younger, and W.G. Owen. 1994. Free calcium concentrations in bullfrog rods determined in the presence of multiple forms of fura-2. Biophys. J. 67: 2076-2089 [Abstract]. |
30. | McNaughton, P.A., L. Cervetto, and B.J. Nunn. 1986. Measurement of the intracellular free calcium concentration in salamander rods. Nature. 322: 261-263 . |
31. | Miller, J.L., and J.I. Korenbrot. 1994. Differences in calcium homeostasis between retinal rod and cone photoreceptors revealed by the effects of voltage on the cGMP-gated conductance in intact cells. J. Gen. Physiol. 104: 909-940 [Abstract]. |
32. |
Minta, A.,
J.P. Kao, and
R.Y. Tsien.
1989.
Fluorescent indicators for
cytosolic calcium based on rhodamine and fluorescein chromophores.
J. Biol. Chem.
264:
8171-8178
|
33. | Nakatani, K., and K.-W. Yau. 1988a. Calcium and light adaptation in retinal rods and cones. Nature. 334: 69-71 [Medline]. |
34. | Nakatani, K., and K.-W. Yau. 1988b. Calcium and magnesium fluxes across the plasma membrane of the toad rod outer segment. J. Physiol. (Camb.). 395: 695-729 [Abstract]. |
35. | Nakatani, K., and K.-W. Yau. 1989. Sodium-dependent calcium extrusion and sensitivity regulation in retinal cones of the salamander. J. Physiol. (Camb.). 409: 525-548 [Abstract]. |
36. | Perry, R.J., and P.A. McNaughton. 1991. Response properties of cones from the retina of the tiger salamander. J. Physiol. (Camb.). 433: 561-587 [Abstract]. |
37. | Pugh, E.N.. 1975. Rhodopsin flash photolysis in man. J. Physiol. (Camb.). 248: 393-412 [Abstract]. |
38. | Ratto, G.M., R. Payne, W.G. Owen, and R.Y. Tsien. 1988. The concentration of cytosolic free calcium in vertebrate rod outer segments measured with fura-2. J. Neurosci. 8: 3240-3246 [Abstract]. |
39. |
Sampath, A.P.,
H.R. Matthews,
M.C. Cornwall, and
G.L. Fain.
1998.
Bleached pigment produces a maintained decrease in outer segment Ca2+ in salamander rods.
J. Gen. Physiol.
111:
53-64
|
40. |
Schnetkamp, P.P.,
D.K. Basu,
X.B. Li, and
R.T. Szerencsei.
1991a.
Regulation of intracellular free Ca2+ concentration in the outer
segments of bovine retinal rods by Na-Ca-K exchange measured
with fluo-3. II. Thermodynamic competence of transmembrane
Na+ and K+ gradients and inactivation of Na+-dependent Ca2+
extrusion.
J. Biol. Chem
266:
22983-22990
|
41. |
Schnetkamp, P.P.,
X.B. Li,
D.K. Basu, and
R.T. Szerencsei.
1991b.
Regulation of free cytosolic Ca2+ concentration in the outer segments of bovine retinal rods by Na-Ca-K exchange measured with
fluo-3. I. Efficiency of transport and interactions between cations.
J. Biol. Chem.
266:
22975-22982
|
42. |
Schnetkamp, P.P., and
R.T. Szerencsei.
1993.
Intracellular Ca2+ sequestration and release in intact bovine retinal rod outer segments. Role in inactivation of Na-Ca-K exchange.
J. Biol. Chem.
268:
12449-12457
|
43. | Schröder, W.H., and G.L. Fain. 1984. Light-dependent calcium release from photoreceptors measured by laser micro-mass analysis. Nature. 309: 268-270 [Medline]. |
44. |
Williams, T.P..
1964.
Photoreversal of rhodopsin bleaching.
J. Gen.
Physiol.
47:
679-689
|
45. | Yau, K.-W., P.A. McNaughton, and A.L. Hodgkin. 1981. Effect of ions on the light-sensitive current in retinal rods. Nature. 292: 502-505 [Medline]. |
46. | Yau, K.-W., and K. Nakatani. 1984a. Cation selectivity of light-sensitive conductance in retinal rods. Nature. 309: 352-354 [Medline]. |
47. | Yau, K.-W., and K. Nakatani. 1984b. Electrogenic Na-Ca exchange in retinal rod outer segment. Nature. 311: 661-663 [Medline]. |
48. | Yau, K.-W., and K. Nakatani. 1985. Light-induced reduction of cytoplasmic free calcium in retinal rod outer segment. Nature. 313: 579-582 [Medline]. |
49. |
Younger, J.P.,
S.T. McCarthy, and
W.G. Owen.
1996.
Light-dependent control of calcium in intact rods of the bullfrog rana-catesbeiana.
J. Neurophysiol.
75:
354-366
|