rapid communication
Monocarboxylate transporter MCT1 is located in the apical
membrane and MCT3 in the basal membrane of rat RPE
NANCY J. PHILP,1 HEEYONG YOON,1 AND EVELYN F. GROLLMAN2
College of Optometry, Philadelphia, Pennsylvania 19141-3399; and 2Laboratory
of Biochemistry and Metabolism, National Institutes of Health, National Institute of Diabetes,
Digestive Diseases, and Kidney Diseases, Bethesda, Maryland 20892-1812
1Pennsylvania
retinal pigment epithelium; lactate; anion; pH; water homeostasis
THE RETINAL PIGMENT epithelium (RPE) forms the outer
blood-retinal barrier and mediates the transport of
metabolites, ions, and fluid between the choroidal blood
supply and the neural retina (4, 5). The basolateral
surface of the RPE is in contact with the blood plasma,
which filters through the porous capillaries in the
choroid. The apical surface of the RPE is in intimate
contact with the neural retina and extends processes
that protrude between the photoreceptor cell outer
segments. Tight junctions at their apical-lateral borders link the RPE cells. These junctions impede the
movement between cells of even small water-soluble
molecules.
There are no anatomic junctions between the RPE
and the neural retina. Although the two tissues are
closely apposed, they are separated by the subretinal
space (SRS). This space is the embryonic remnant of
the optic vesicle and forms when the optic vesicle
R1824
invaginates to form the optic cup. The RPE actively
regulates the volume and chemical composition of the
SRS in much the same way that the choroid plexus
maintains the composition of the cerebrospinal fluid
(5). This is accomplished through specific proteins in
the apical and basolateral membranes (4, 5).
The maintenance of visual cell function depends on
glycolysis (1–3, 19, 24). The neural retina, with its high
rate of metabolism, uses glucose and produces substantial quantities of lactate, both in the light and in the
dark (3, 19, 24). In the retina, lactate is not simply an
end product of glycolysis but is an important metabolic
intermediate. Glucose is transported across the RPE by
GLUT-1 transporters that are found in both the apical
and basolateral membranes (14, 21). Glucose is used
primarily by Müller cells that have few mitochondria
and rely on glycolysis for ATP production. Lactate
produced in the Müller cells by glycolysis is transported
out of the cells and is used by the photoreceptor cells to
fuel oxidative phosphorylation (19). In studies with
isolated photoreceptor cells, lactate was a better substrate for mitochondrial oxidative metabolism than
glucose (19). The production of lactate by one tissue and
use by another is reported in muscle (11) and brain (23)
and is referred to as the ‘‘lactate shuttle.’’
Excess lactate not used by the photoreceptor cells is
transported out of the subretinal space to the choroidal
circulation by the RPE (1, 2, 22). Physiological studies
on RPE explants and cultured RPE cells have documented the presence of H1-lactate cotransport mechanisms (13, 15, 16). Recently a specific H1-lactate transporter protein has been identified in RPE. MCT3 was
originally cloned from an embryonic chick library
screened with an RPE-specific monoclonal antibody
and is the third member of the monocarboxylate transporter family to be cloned (17, 25).
Studies presented in this paper clearly establish that
RPE expresses two members of the monocarboxylate
transporter family, MCT1 and MCT3. We demonstrate
that these two transporters are polarized to distinct
membrane domains in RPE cells: MCT1 in the apical
membrane and MCT3 in the basolateral membrane. We
further correlate the presence of these transporters
with the recently described proton-lactate-water symporter activity in the apical membrane of RPE (26).
Downloaded from http://ajpregu.physiology.org/ by 10.220.33.3 on October 2, 2016
Philp, Nancy J., Heeyong Yoon, and Evelyn F. Grollman. Monocarboxylate transporter MCT1 is located in the
apical membrane and MCT3 in the basal membrane of rat
RPE. Am. J. Physiol. 274 (Regulatory Integrative Comp.
Physiol. 43): R1824–R1828, 1998.—The retinal pigment epithelium (RPE) forms the outer blood-retinal barrier and
regulates the movement of nutrients, water, and ions between
the choroidal blood supply and the retina. The transport
properties of the RPE maintain retinal adhesion and regulate
the pH and osmolarity in the space surrounding the photoreceptor cell outer segments. In this report we identify two
monocarboxylate transporters, MCT1 and MCT3, expressed
in rat RPE. On the basis of Northern and Western blot
analyses, MCT1 is expressed in both the neural retina and
the RPE, whereas the expression of MCT3 is restricted to the
RPE. Using indirect immunolocalization we show that the
two transporters are polarized to distinct membrane domains. MCT1 antibody labels the apical surface and the
apical processes of the RPE. A polyclonal antibody produced
against the carboxy terminus of rat MCT3 labels only the
basolateral membrane of the RPE. The demonstration of
MCT1 on the apical membrane and MCT3 on the basal
membrane identifies specific proteins involved in the discriminate and critical regulation of water and lactate transport
from the retina to the choroid.
MCT1 AND MCT3 IN RPE
MATERIALS AND METHODS
were bled at 0, 4, and 8 wk. The 8-wk bleed was used in these
studies.
The production of the rat MCT1 polyclonal antibody described by Gerhart and co-workers (8) was purchased from
Chemicon (Temecula, CA).
Western analysis of protein levels. Detergent-soluble lysates were prepared from rat RPE and retina as previously
described (17). Protein was measured using bicinchoninic
acid reagent (Sigma), and samples were diluted with two
times Laemmli sample buffer. Samples, adjusted for equal
protein (15 µg/lane), were separated on 4–12% SDS-polyacrylamide gradient gels (Novex, San Diego, CA) and transferred
to Immobilon-P membrane (Millipore, Bedford, MA). Membranes were incubated for 1 h at room temperature in
blocking buffer (20 mM Tris, 137 mM NaCl, pH 7.5, 5% BSA)
followed by 1 h incubation with primary antibodies, polyclonal MCT1 antibody diluted 1:5,000 (Chemicon), or polyclonal MCT3 antibody diluted 1:500. The secondary antibody
diluted 1:5,000 was rabbit anti-chicken IgY or goat antirabbit IgG conjugated to horseradish peroxidase. Chemiluminescence (Amersham) was used for detection. To demonstrate
the specificity of the MCT3 antibody, blots were also probed
with MCT3 antibody that was preabsorbed for 30 min with 1
µg/ml MCT3 peptide.
Immunocytochemistry. Paraffin sections (8 µm) of adult rat
eye and brain were purchased from Novagen (Madison, WI).
Sections were dewaxed in xylene and rehydrated using a
graded series of ethanol. Sections were labeled with antibodies following standard protocols as previously described (17,
18). Sections were incubated 1 h in blocking solution (5%
BSA-0.1% Tween 20 in PBS), then incubated for 1 h in
anti-MCT1 antibody diluted 1:200 or anti-MCT3 diluted 1:50
in 1% BSA-0.1% Tween 20 in PBS. After four washes in
PBS-0.1% Tween 20, slices were incubated 1 h with rhodamine conjugated rabbit anti-chicken IgY (diluted 1:100) or
goat anti-rabbit IgG (diluted 1:100). Sections were washed,
and coverslips were mounted using Fluormount-G (Southern
Biotechnology, Birmingham, AL). Sections were examined,
and images were captured using a Nikon microscope equipped
with Metamorph Imaging System (Universal Imaging, West
Chester, PA).
RESULTS
Expression of MCT1 and MCT3 mRNA in rat RPE. To
identify MCTs expressed by the RPE, total RNA was
extracted from microdissected rat RPE and neural
retina. Equal amounts of RNA (5 µg) were subjected to
Northern analysis and hybridized using probes specific
for MCT1 and MCT3 as detailed in MATERIALS AND
METHODS. The MCT1 probe hybridized with a single
3.7-kb transcript in both the RPE and neural retina
(Fig. 1). The amount of MCT1 transcript in RPE and
neural retina preparations was similar. MCT1 was also
detected in RNA from rat heart, liver, and skeletal
muscle (data not shown), confirming published reports
(6, 7).
A parallel blot was hybridized with an MCT3 riboprobe that was prepared as detailed in MATERIALS AND
METHODS. A 2.2-kb transcript was detected only in total
RNA prepared from the RPE but not from the neural
retina (Fig. 1). The MCT3 probe did not hybridize with
RNA prepared from rat heart, liver, kidney, or thyroid
(not shown). The limited expression of MCT3 in RPE
and not in other tissues was reported previously in the
chick (17).
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Animals. Male Sprague-Dawley rats weighing 125–150 g
were purchased from Charles River Laboratories (Wilmington, MA) and housed with a 12:12-h light-dark cycle for 1 wk.
Rats were euthanized during the light cycle using pentobarbital sodium (150 mg/kg body wt) before tissue collection. All
procedures followed a protocol approved by the Pennsylvania
College of Optometry Animal Care Committee.
Microdissection of eyes. Enucleated eyes were separated
into anterior and posterior segments with a razor blade. The
posterior eye cup was placed in PBS containing 10 mM EDTA
and 3% sucrose, the neural retina was removed, and the RPE
was peeled off the choroid with fine forceps under a dissecting
microscope (18). RPE and neural retina were used to prepare
total RNA and membrane extracts described below.
Preparation of probes. The MCT1 coding sequence was
RT-PCR amplified from total rat RPE RNA using 58aag atg
cca tcc tgc gat tgg38 as a forward primer and 58aga cag ggc tct
cct cct ct38 as a reverse primer. The PCR product (1.4 kb in
length) was cloned into pCR2.1 vector (Invitrogen, Carlsbad,
CA), and an EcoR I fragment containing MCT1 cDNA was
recloned in pBluescript SK(2) plasmid (Stratagene, La Jolla,
CA). pBSSK(2)MCT1 was linearized with EcoR V enzyme,
and digoxigenin-UTP-labeled riboprobe was prepared using
T3 RNA polymerase following the manufacturer’s instruction
(Boehringer Mannheim, Indianapolis, IN).
The MCT3 probe was synthesized using a 600-bp fragment
of the 38 end of mouse MCT3 (GenBank Accession Number
AF019111) that was cloned into the pCR2.1 vector (Invitrogen). The pCR2.1 MCT3 was linearized with BamH I, and
digoxigenin-UTP-labeled riboprope was prepared using T7
RNA polymerase. The 38 end of the rat MCT3 sequence was
obtained by 38 rapid amplification of cDNA ends (RACE) and
varied only in one nucleotide from the mouse sequence.
Preparation of total RNA and Northern blot analysis. Total
RNA was prepared from various rat tissues using triZOL
Reagent (Life Technologies, Gaithersburg, MD) following the
manufacturer’s instructions. RNA (5 µg) was denatured with
0.5 M glyoxal and 50% dimethylsulfoxide and separated on a
1% agarose gel in 10 mM sodium-phosphate buffer (5 mM
Na2HPO4, 5 mM NaH2PO4, pH 6.5). RNA was transferred and
crosslinked to Hybond N1 nylon membrane (Amersham). The
membrane was prehybridized 4 h and hybridized overnight at
65°C with MCT1 or MCT3 riboprobe (5 ng/ml) in hybridization solution. Blots were washed twice for 5 min each in 23
SSC (13 SSC is 0.15 M NaCl and 0.015 M sodium citrate, pH
7.0)/0.1% SDS at room temperature, twice for 20 min each in
0.53 SSC/0.1% SDS at 65°C, and rinsed in maleate buffer (0.1
M maleic acid, pH 7.5, 150 mM NaCl). After 1 h in blocking
buffer (Boehringer Mannheim), blots were incubated 1 h with
an alkaline phosphatase conjugated anti-digoxigenin antibody (1:5,000 in blocking buffer). Unbound antibody was
removed by washing twice for 20 min each in maleate buffer
and 5 min in GB3 (0.1 M Tris, pH 9.5, 0.1 M NaCl, 50 mM
MgCl2 ). Lumi-Phos 530 was applied, and the signal was
detected using BIOMAX MR X-ray film (Kodak, Rochester,
NY).
MCT antibodies. A polyclonal antibody was produced
against a 21-mer synthetic peptide. The peptide corresponds
to the carboxy terminal amino acids of rat MCT3 deduced
from the cloned cDNA and was synthesized with an aminoterminal cysteine conjugated to keyhole limpet hemocyanin
(KLH) (CAVPELDHESIGGHEARGQKA). The KLH-peptide
(0.1 mg/injection) was emulsified by mixing with an equal
volume of Freund’s adjuvant for injection into New Zealand
White rabbits. Boosts were at weeks 2, 6, and 8, and animals
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MCT1 AND MCT3 IN RPE
Differential expression of MCT1 and MCT3 proteins
in the RPE and neural retina. Detergent-soluble extracts were prepared from microdissected rat RPE and
neural retina as detailed in MATERIALS AND METHODS.
Protein (15 µg) was separated using a 4–12% gradient
SDS-polyacrylamide gel and transferred to PVDF membrane. The blot was incubated with affinity-purified
polyclonal antibody to MCT1, and labeling was detected by chemiluminescence as shown in Fig. 2. A
45-kDa immunoreactive protein was found in both RPE
and neural retina, but the amount in RPE exceeded
that found in neural retina. The amount of RNA does
not correlate with the amount of expressed protein (Fig.
Fig. 2. Expression of MCT1 in RPE and neural retina as determined
by immunoblotting. Detergent extracts were prepared from microdissected RPE and neural retina as detailed in MATERIALS AND METHODS.
Protein (15 µg/lane) was separated on 4–12% gradient gels and
transferred to Immobilon-P membranes. Blot was probed with MCT1
antibody (Ab) and detected with a horseradish peroxidase conjugated
secondary antibody with an enhanced chemiluminescence system.
Fig. 3. Expression of MCT3 in RPE and neural retina as determined
by immunoblotting. Detergent extracts were prepared from microdissected RPE and neural retina as detailed in MATERIALS AND METHODS.
Protein (15 µg/lane) was separated on 4–12% gradient gels and
transferred to Immobilon-P membranes. Blots were probed with
MCT3 antibody alone (left) or MCT3 antibody with 1 µg/ml synthetic
peptide (right). Horseradish peroxidase conjugated secondary antibody and enhanced chemiluminescence were used for detection.
1 compared with Fig. 2). Unlike RPE that is composed
of one cell type, the neural retina lysate is made up of
several cell types.
A polyclonal antibody was made to the COOHterminus of mouse MCT3. This region is not conserved
between different members of the MCT family, so the
antibody is specific for MCT3. As shown in Fig. 3, a
single band of ,43-kDa was detected in RPE. When the
immunizing peptide was present at a concentration of 1
µg/ml, antibody binding was inhibited (Fig. 3). MCT3
immunoreactivity was not detected in lysates from
neural retina (Fig. 3) nor in tissue extracts of rat liver,
heart, and skeletal muscle (not shown).
Immunolocalization of MCT1 to the apical membrane
of rat RPE. Indirect immunofluorescence was used to
determine the subcellular localization of MCT1 in rat
RPE. Paraffin sections through the posterior region of
rat eye were prepared from formaldehyde-fixed tissue
as detailed in MATERIALS AND METHODS and in Ref. 17.
Sections were labeled with a polyclonal antibody directed against the COOH-terminus peptide of MCT1
and a rhodamine conjugated secondary antibody. The
phase contrast (green) and fluorescent (red) images
were captured and superimposed to clarify localization.
The immunostaining (orange) of the apical membrane,
with its extensive processes, is clearly seen in Fig. 4.
The large amount of MCT1 detected in RPE on Western
blot (Fig. 2) is consistent with the intense labeling of
the apical membrane.
Immunolocalization of MCT3 to the basal membrane
of rat RPE. Indirect immunofluorescence was used to
localize MCT3 in rat tissues. Sagittal sections through
adult rat eye showed RPE was the only tissue that
stained with MCT3 antibody. When viewed at low
magnification, labeling was only observed in the RPE,
but not in other ocular tissue or in the extraocular
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Fig. 1. Differential expression of MCT1 and MCT3 mRNA in the
retinal pigment epithelium (RPE) and neural retina (NR). Total RNA
(5 µg) was prepared from rat RPE and neural retina using triZOL
reagent, separated on a 1% agarose gel, and transferred to a nylon
membrane. Blot was hybridized with digoxigenin-UTP-labeled
MCT1 (left) or MCT3 (right) riboprobes. Detection was with alkaline
phosphatase conjugated anti-digoxigenin antibody and chemiluminescence.
MCT1 AND MCT3 IN RPE
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Fig. 4. Immunolocalization of MCT1 and MCT3 in rat
RPE. Paraffin sections of rat eye were prepared from
formaldehyde-fixed tissue as described in MATERIALS
AND METHODS. Sections were incubated with an MCT1
antipeptide antibody followed by a rhodamine conjugated rabbit anti-chicken secondary antibody or an
MCT3 anti-peptide antibody followed by a rhodamine
conjugated goat anti-rabbit antibody. Phase image
(green) and immunofluorescence (red) were superimposed using the Metamorph imaging system. OS: outer
segments.
DISCUSSION
In vivo and in vitro studies have shown that both
lactate and water are transported from the retina to the
choroid by the RPE (2, 13, 15, 16, 22). Transepithelial
movement of lactate and water requires transport
proteins on the apical and basolateral membranes of
the RPE. Monocarboxylate transporters through the
cotransport of proton, lactate, and H2O could regulate
these transport activities of the RPE (26). The data
presented in these studies show that the RPE cells
express two monocarboxylate transporters, MCT1 and
MCT3.
The monocarboxylate transporter MCT1 is found
throughout the apical membrane of the RPE (Figs. 2
and 4). The apical membrane and the tight junctions of
the RPE form the outer boundary of the subretinal
space. The transport proteins in the apical membrane
regulate the composition of the ‘‘extracellular’’ fluid of
the subretinal space (5). One of the constituents of the
subretinal space is lactate, which is both a substrate
and product of retinal metabolism. Lactate concentrations in the subretinal space are estimated to be
between 7 and 13 mM (1, 10, 15) and may fluctuate
during visual activity (19). The cytosolic concentration
of lactate is unknown, but blood plasma is usually ,1
mM. A chemical gradient for lactate would therefore
exist from the subretinal space to the choroidal blood
plasma. Monocarboxylate transporters mediate the electroneutral symport of a proton and lactate. The finding
of MCT1 on the apical membrane (Fig. 4) and MCT3 on
the basolateral membrane (Fig. 4) identifies two proteins, acting in concert, that can mediate the transepithelial transfer of H1-lactate from the subretinal space
to the systemic blood system.
MCT1 also may have an important role in water
homeostasis. Studies by Zeuthen and co-workers (26)
elegantly demonstrate that water is cotransported with
H1-lactate across the apical membrane of frog RPE.
Whether MCT1 is responsible for this symport of
H1-lactate and water has not been shown, but MCT1 is
abundant and correctly located in the apical membrane. The Michaelis constant (Km ) of MCT1 in rat RPE
is unknown, but the reported Km of MCT1 in other
tissues is 2–11 mM (11) and in agreement with the Km
(,4–7 mM) reported for H1-lactate-water symport
across the apical membrane in frog RPE (26).
For water to continue to enter across the apical
membrane with the downhill influx of lactate and
against an osmotic gradient, the cytosolic concentration of lactate in the RPE cannot exceed that in the
subretinal space. MCT3 on the basolateral membrane
provides a mechanism for lactate efflux and thereby
maintains intracellular lactate levels below 7 mM (26).
Whether proton-lactate transport by MCT3 is also
accompanied by water has not been demonstrated, but
aquaporins (9, 20) and ion transport proteins, for
2
example the HCO2
3 / Cl exchanger (12), may contribute
to the transepithelial movement of water.
In addition to water homeostasis, MCT1 and MCT3
in RPE may have a role in maintaining intracellular
pH. Intracellular pH of RPE is estimated to be 7.38, a
value more alkaline than most mammalian cells and
well above the calculated value for equilibrium pH. As
proposed by Kenyon et al. (12), H1-lactate transport is
only one of many overlapping mechanisms available to
RPE for maintaining pH.
Similar to the role of the choroid plexus in regulating
the chemical composition of the fluid available to brain
cells, the RPE plays a critical role in maintaining and
regulating the environment required for retinal function. The demonstration of MCT1 on the apical membrane and MCT3 in the basal membrane, identifies
specific proteins involved in the discriminate and critical regulation of water and lactate transport from the
retina to the choroid.
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muscles. Higher magnification revealed that the labeling was restricted to the basolateral membrane of the
RPE (Fig. 4). Additionally, MCT3 immunostaining was
not detected in sagittal sections through the brain that
contained choroid plexus, a tissue with several barrier
functions in common with RPE.
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MCT1 AND MCT3 IN RPE
We are grateful to Dr. Brian Sauer for helpful discussions.
This study was supported in part by a grant from Henry and
Corinne Bower Laboratory for Macular Degeneration, Wills Eye
Hospital.
Address for reprint requests: N. J. Philp, Pennsylvania College of
Optometry, 1200 W. Godfrey Ave., Philadelphia, PA 19141–3399.
Received 30 December 1997; accepted in final form 9 March 1998.
REFERENCES
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1. Adler, A. J., and R. E. Southwick. Distribution of glucose and
lactate in the interphotoreceptor matrix. Ophthalmic Res. 24:
243–252, 1992.
2. Alm, A., and P. Tornquist. Lactate transport through the
blood-retinal and the blood-brain barrier in rats. Ophthalmic
Res. 17: 181–184, 1985.
3. Ames, A., Y. Y. Li, E. C. Heher, and C. R. Kimble. Energy
metabolism of rabbit retina as related to function: high cost of
Na1 transport. J. Neurosci. 12: 840–853, 1992.
4. Bok, D. The retinal pigment epithelium: a versatile partner in
vision. J. Cell Sci. Suppl. 17: 189–195, 1993.
5. Gallemore, R. P., B. A. Hughes, and S. S. Miller. Retinal
epithelial transport mechanisms and their contributions to the
electroretinogram. Prog. Retinal Res. 16: 509–566, 1997.
6. Garcia, C., J. L. Goldstein, R. K. Pathak, R. G. W. Anderson,
and M. S. Brown. Molecular characterization of a membrane
transporter for lactate, pyruvate and other monocarboxylates:
implications for the cori cycle. Cell 76: 865–873, 1994.
7. Garcia, C. K., M. S. Brown, R. K. Pathak, and J. L.
Goldstein. cDNA cloning of MCT2, a second monocarboxylate
transporter expressed in different cells than MCT1. J. Biol.
Chem. 270: 1843–1849, 1995.
8. Gerhart, D. Z., B. E. Enerson, O. Y. Zhdankina, R. L. Leino,
and L. R. Drewes. Expression of monocarboxylate transporter
MCT1 by brain endothelium and glia in adult and suckling rats.
Am. J. Physiol. 273 (Endocrinol. Metab. 36): E207–E213, 1997.
9. Hamann, S., M. la Cour, T. Zeuthen, G. M. Lui, P. Agre, and
S. Nielsen. Localization of aquaporin 1–4 in ocular epithelia.
Proc. XII Int. Cong. Eye Research, Yokohama, Japan, 1997.
10. Heath, H., S. S. Kang, and D. Philoppou. Glucose, glucose-6phosphate, lactate and pyruvate content of the retina, blood and
liver of streptozotocin-diabetic rats fed sucrose- or starch-rich
diets. Diabetologia 11: 57–62, 1975.
11. Juel, C. Lactate-proton cotransport in skeletal muscle. Physiol.
Rev. 77: 321–358, 1997.
12. Kenyon, E., A. Maminishkis, D. P. Joseph, and S. S. Miller.
Apical and basolateral membrane mechanisms that regulate the
pHi in bovine retinal pigment epithelium. Am. J. Physiol. 273
(Cell Physiol. 42): C456–C472, 1997.
13. Kenyon, E., K. Yu, M. la Cour, and S. S. Miller. Lactate
transport mechanisms at apical and basolateral membranes of
bovine retinal pigment epithelium. Am. J. Physiol. 267 (Cell
Physiol. 36): C1561–C1573, 1994.
14. Kumagai, A. K., B. J. Glasgow, and W. M. Pardridge. GLUT1
glucose transporter expression in the diabetic and nondiabetic
human eye. Invest. Ophthalmol. Vis. Sci. 35: 2887–2894, 1994.
15. La Cour, M., H. Lin, E. Kenyon, and S. S. Miller. Lactate
transport in freshly isolated human fetal retinal pigment epithelium. Invest. Ophthalmol. Vis. Sci. 35: 434–442, 1994.
16. Lin, H., M. la Cour, M. V. Andersen, and S. S. Miller.
Proton-lactate cotransport in the apical membrane of frog retinal
pigment epithelium. Exp. Eye Res. 59: 679–688, 1994.
17. Philp, N. J., P. Chu, T. C. Pan, M. L. Chu, K. Stark, D.
Boettiger, H. Yoon, and T. Kieber-Emmons. Developmental
expression and molecular cloning of REMP, a novel retinal
epithelial membrane protein. Exp. Cell Res. 219: 64–73, 1995.
18. Philp, N. J., and V. T. Nachmias. Polarized distribution of
integrin and fibronectin in retinal pigmented epithelial cells.
Invest. Ophthalmol. Vis. Sci. 28: 1275–1280, 1987.
19. Poitry-Yamate, C. L., S. Poitry, and M. Tsacopoulos. Lactate
released by Müller glial cells is metabolized by photoreceptors
from mammalian retina. J. Neurosci. 15: 5179–5191, 1995.
20. Ruiz, A., and D. Bok. Characterization of the 38 UTR sequence
encoded by the AQP-1 gene in human retinal pigment epithelium. Biochim. Biophys. Acta 1282: 174–178, 1996.
21. Sugasawa, K., J. Deguchi, T. Okami, A. Yamamoto, K.
Omori, M. Uyama, and Y. Tashiro. Immunocytochemical
analyses of distributions of Na,K-ATPase and GLUT1, insulin
and transferrin receptors in the developing retinal pigment
epithelial cells. Cell Struct. Funct. 19: 21–28, 1994.
22. Tornquist, P., and A. Alm. Retinal and choroidal contribution to
retinal metabolism in vivo. A study in pigs. Acta Physiol. Scand.
106: 351–357, 1979.
23. Tsacopoulos, M., and P. J. Magistretti. Metabolic coupling
between glia and neurons. J. Neurosci. 16: 877–885, 1996.
24. Winkler, B. S., M. J. Arnold, M. A. Brassell, and D. R. Sliter.
Glucose dependence of glycolysis, hexose monophosphate shunt
activity, energy status, and the polyol pathway in retinas isolated
from normal (nondiabetic) rats. Invest. Ophthalmol. Vis. Sci. 38:
62–71, 1997.
25. Yoon, H., A. Fanelli, E. F. Grollman, and N. J. Philp.
Identification of a unique monocarboxylate transporter (MCT3)
in retinal pigment epithelium. Biochem. Biophys. Res. Commun.
234: 90–94, 1997.
26. Zeuthen, T., S. Hamann, and M. la Cour. Cotransport of H1,
lactate and H2O by membrane proteins in retinal pigment
epithelium of bullfrog. J. Physiol. (Lond.) 497: 3–17, 1996.