793
Ischemia Induces Translocation of the
Insulin-Responsive Glucose Transporter GLUT4
to the Plasma Membrane of Cardiac Myocytes
DaQing Sun, MS; Ngoc Nguyen, BS; Timothy R. DeGrado, PhD;
Markus Schwaiger, MD; Frank C. Brosius III, MD
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Background Acute myocardial ischemia is accompanied by
an increase in glucose uptake and metabolism, which appears
to be important in protecting myocardial cells from irreversible
ischemic injury. Because insulin augments myocardial glucose
uptake by inducing the translocation of glucose transporters
from an intracellular compartment to the plasma membrane,
we hypothesized that acute ischemia would trigger a similar
translocation.
Methods and Results We used a subcellular fractionation
method to separate intracellular membranes and plasma membranes from control, ischemic, and hypoxic Langendorff-isolated perfused rat hearts and determined the expression of the
major myocardial glucose transporter, GLUT4, in these separated membrane fractions. We found that translocation of
GLUT4 molecules occurred in ischemic, hypoxic, and insulintreated hearts and in hearts that underwent ischemia plus
insulin treatment. The percentages of GLUT4 molecules
I schemia induces multiple changes in myocardial cell
metabolism. Prominent among these changes is a
marked increase in glucose uptake and use that
occurs shortly after the onset of mild to moderate
ischemia.1 Multiple clinical and experimental trials have
shown that increased glucose uptake and metabolism
during acute myocardial ischemia are associated with
preserved diastolic and systolic myocardial function,
decreased release of myocardial enzymes, improved
recovery during reperfusion, and prevention of ischemic
contracture.1-6 However, the mechanism by which ischemia stimulates glucose uptake remains unknown.
Over the past several years, cDNAs have been isolated for the proteins that transport glucose into myocardial cells.7-18 The predominant transporter in cardiac
muscle is the insulin-responsive glucose transporter
GLUT4.19 It appears that very few GLUT4 molecules
are located on the plasma membrane of myocardial cells
under basal conditions. However, when exposed to
maximal insulin levels, approximately 40% of the myocardial GLUT4 molecules are translocated to the
plasma membrane.20 This increase in plasma membrane
GLUT4 molecules appears to be the major mechanism
by which insulin acutely stimulates glucose uptake in
Received July 7, 1993; revision accepted October 23, 1993.
From the Divisions of Nuclear Medicine and Nephrology,
Department of Internal Medicine, University of Michigan Medical
School, Ann Arbor.
Correspondence to Frank C. Brosius, University of Michigan,
1150 W Medical Center Dr, 1560 MSRBII, Ann Arbor, MI
48109-0676.
present on the plasma membrane in the different conditions
were as follows: control. 18.0+2.8%; ischemia, 41.3±9.4%;
hypoxia, 31.1±2.9%; insulin, 61.1±2.6%; and ischemia plus
insulin, 66.8±5.7%. Among the statistically significant differences in these values were the difference between control and
ischemia and the difference between ischemia alone and
insulin plus ischemia.
Conclusions Ischemia causes substantial translocation of
GLUT4 molecules to the plasma membrane of cardiac myocytes. A combination of insulin plus ischemia stimulates an
even greater degree of GLUT4 translocation. GLUT4 translocation is likely to mediate at least part of the increased
glucose uptake of ischemic myocardium and may be a mechanism for the cardioprotective effect of insulin during acute
myocardial ischemia. (Circulation. 1994;89:793-798.)
Key Words * coronary disease * metabolism
insulin-sensitive tissues.21,22 To determine whether ischemia stimulates glucose uptake through a similar mechanism, we used a subcellular membrane fractionation
method to separate rat myocardial plasma membranes
from intracellular membranes. We have found that
acute ischemia causes substantial translocation of
GLUT4 molecules and that a combination of insulin
plus ischemia stimulates an even greater degree of
GLUT4 translocation.
Methods
Animal Models
Sprague-Dawley rats (weight, 200 to 250 g) were used for all
experiments. Control animals and animals used for ischemic
and hypoxic studies were fasted overnight. Insulin-treated
animals were not fasted and were given intraperitoneal injections of insulin (8 U/kg) plus D-glucose (1 g/kg) 30 minutes
before they were killed. All animals were anesthetized with
intraperitoneal injection of sodium pentobarbital. The hearts
were removed and perfused via the Langendorff method.
Aortic perfusion pressure was maintained at 60 mm Hg
throughout. Control hearts were perfused for 30 minutes at
37°C with Krebs-Henseleit buffer containing 5 mmol/L glucose
and gassed with 95% 02/5% C02. Hypoxia hearts were
perfused identically for 5 minutes, then changed to buffer with
95% air/5% CO2 for an additional 30 minutes. Ischemia hearts
were perfused identically to the control hearts for 15 minutes,
then had perfusion halted for 15 minutes. Insulin-stimulated
hearts were perfused identically to the control hearts, but the
perfusate also contained 100 nmol/L regular insulin. Insulin
plus ischemia hearts were perfused identically to the insulinstimulated hearts for 15 minutes, then had perfusion stopped
794
Circulation Vol 89, No 2 February 1994
for an additional 15 minutes. Only hearts that were still
beating at the end of the experimental period were used for
further assessment. At the end of the experimental period, the
hearts were drained of excess perfusate and snap-frozen in
liquid nitrogen.
Subcellular Membrane Fractionation
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Membrane fractionation was performed by use of a modification of the method of Zaninetti et al.23 For the initial
experiments, three hearts exposed to a single condition (control, ischemia, or hypoxia) were combined together. For subsequent experiments, the protocol was performed with individual hearts (other details of the fractionation protocol were
identical). The hearts were thawed on ice, and each heart was
trimmed of fat and washed in cold phosphate-buffered saline,
pH 7.4. The heart was then transferred to an ice-cold Petri
dish containing 4 mL of 10 mmol/L sodium bicarbonate and 5
mmol/L sodium azide, pH 7.0, and immediately minced with a
razor blade. The resultant slurry was then homogenized with a
Brinkmann Polytron tissue homogenizer at setting 5 for 20
seconds. The homogenate was transferred to a handheld 7-mL
glass/glass homogenizer and subjected to 10 pestle strokes. A
100-mL aliquot of the homogenate was removed for later
analysis (crude homogenate); the remainder was centrifuged
at 7000g for 20 minutes. The resultant supernatant was reserved for preparing the microsomal membrane (MM) fraction, containing intracellular membranes, and the pellet was
used for preparing the plasma membrane (PM) fraction. The
pellet was resuspended in 4 mL of 10 mmol/L Tris-HCl, pH
7.4, and centrifuged at 200g for 20 minutes. The pellet was
discarded, and supernatant was evenly divided into 0.8-mL
portions, each of which was gently layered on top of a 20%
(vol/vol) Percoll gradient in buffer C (255 mmol/L sucrose; 10
mmol/L Tris-HCl, pH 7.4; 2 mmol/L EDTA) in a 4-mL tube
and centrifuged at 55 00g for 1 hour. The band at density
1.030 as determined by density marker beads (Pharmacia) was
aspirated and pelleted by centrifugation at 170 000g for 1 hour
and resuspended in 0.2 mL of buffer C as the PM fraction. The
supernatant from the original 7000g centrifugation was centrifuged at 48 000g for 20 minutes. The membranes in the
resultant supernatant were then pelleted by centrifugation at
170 OOOg for 1 hour and resuspended in 0.2 mL of buffer C as
the MM fraction. Protein concentrations for each fraction
were determined by Coomassie protein assay (Pierce).
Relative enrichments of the crude homogenates and PM
and MM fractions were determined by analysis of marker
enzymes. NADPH-cytochrome c reductase is a mitochondrial
enzyme that is enriched in the microsomal fraction,23 and
Na+,K+-ATPase al and a2 subunits are enriched in the PM
fraction. NADPH-cytochrome c reductase activity was determined by a modification of the protocol of Williams and
Kamin.24,25 Relative levels of Na+ ,K+-ATPase al or a2 subunit
were determined by immunoblotting using rabbit anti-rat
polyclonal antisera specific for each isotype (UBI). Antibodies
to either subunit gave identical results in initial experiments.
However, blots with the al antibody were more intensely
labeled, so this antiserum was used exclusively in later
experiments.
Immunoblot Analysis
GLUT4 levels were semiquantified by immunoblotting. Aliquots of 50 jig of the MM and PM fractions and crude
homogenate from either fasted and insulin-treated hearts or
control, ischemia, hypoxia, and insulin plus ischemia hearts
were electrophoresed on individual 10% SDS-polyacrylamide
gels. Immunoblotting and GLUT4 determinations were performed as previously described.26 The contents of the gels
were transferred electrophoretically to nitrocellulose membranes, which were then stained briefly with Ponceau S to
determine uniformity of electrophoretic transfer. The membranes were washed in Tris-buffered saline (TBS) (20 mmol/L
Tris-HCl, pH 7.5; 150 mmol/L NaCl; 1% Nonidet-P-40 [Sigma]) and blocked in 5% dry milk in TBS. The membranes
were then incubated with a 1:800 dilution of a rabbit polyclonal antiserum directed against a peptide encoding the 16
carboxy-terminal amino acids of rat GLUT426 (gift of Dr
Maureen Charron). The filters were then washed three times
in TBS, blocked again, and incubated with '251-donkey antirabbit IgG (ICN). Finally, the filters were washed three times
in TBS, dried, and exposed to film. Semiquantification of the
1251 signal was determined either by direct scintillation counting of the excised GLUT4 membrane bands or by videodensitometry using the National Institutes of Health IMAGE 1.43
program. For densitometric analysis, several exposures of each
blot were performed on preflashed Xomat-AR film, and only
those films were used for which the resultant autoradiographic
signals were within the linear response range of the film.
Data Analysis
GLUT4 data were expressed as the relative number of
GLUT4 molecules in 50 mg protein from the various fractions
from paired sets of control, ischemia, and hypoxia hearts. For
all groups, the data were also expressed as the percentage of
total GLUT4 protein in either of the two membrane fractions
as follows:
PM GLUT4=
OD PM GLUT4x% of Total Protein Recovery in PM Fraction
(OD PM GLUT4x% Protein in PM)+(OD MM GLUT4x % Protein in MM)
MM GLUT4=
OD MM GLUT4x % of Total Protein Recovery in MM Fraction
(OD PM GLUT4x % Protein in PM)+(OD MM GLUT4x % Protein in MM)
where OD is the relative densitometric value of GLUT4 in 50
mg of membranes and % of total protein recovery in MM (or
PM) fraction is the amount of protein recovered in final PM or
MM fraction divided by the amount of protein in the original
crude homogenate. Expression of the GLUT4 data in this way
allows comparison with immunohistochemical findings from
other studies in which GLUT4 translocation has been analyzed.20 The equations also correct for variabilities in the
percentage of total cardiac protein recovered in each fraction.
Statistical analysis of GLUT4 translocation was performed by
ANOVA, and differences between groups were determined by
the Fisher least significant difference test. Differences were
considered significant at 95% confidence intervals.
Results
MM fractions derived from the subcellular fractionation method were substantially enriched in NADPHcytochrome c reductase activity, and the PM fractions
were substantially enriched in the a subunits of the
Na+,K+-ATPase (Table). None of the experimental
treatments changed the relative enrichments of the
marker enzymes in either of the fractions. Along with
substantial marker enrichment, the fractionation protocol resulted in a recovery of approximately 60% of the
total GLUT4 molecules in the MM and PM fractions
(Table). Total GLUT4 recovery was not affected by the
various experimental conditions.
Compared with control animals, maximal insulin
stimulation produced a 1.9-fold increase in the number
of GLUT4 molecules detected in the myocardial PM
fraction and a 62% decrease in the number of MM
GLUT4 molecules. When these data were corrected for
variations in protein recovery and expressed as the
percentage of total GLUT4 molecules found in the PM
fraction, insulin increased PM GLUT4 expression from
18+2.8% to 61.1±2.6%, indicating translocation of
Sun et al GLUT4 Translocation by Myocardial Ischemia
795
Characterization of Myocardial Membrane Fractions
Fractions
Protein recovery, % of crude homogenate
a,-NaXKI-ATPase enrichment relative to crude homogenate
NADPH: cytochrome c reductase activity, mmol/min mg
Total GLUT4 recovery, % of crude homogenate
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approximately 43% of total GLUT4 molecules from an
intracellular compartment to the plasma membrane
(Figs 1 and 2). Comparable to insulin stimulation,
hypoxia and ischemia caused 2.4-fold and 3.1-fold increases in PM GLUT4 molecules and 47% and 40%
decreases in MM GLUT4 numbers. Expressed as percentages of total GLUT4 molecules, hypoxia increased
PM GLUT4 to 31.1+2.3% and ischemia increased PM
GLUT4 to 41.3±9.3%, indicating substantial translocation of GLUT4 molecules by acute hypoxia and ischemia (Figs 2 and 3). Ischemia plus insulin increased PM
GLUT4 molecules to 66.8 ±5.7%, which was significantly greater than that caused by ischemia alone and
was slightly greater than that caused by insulin alone
(Figs 1 and 2).
Discussion
Glucose uptake by cardiac myocytes is mediated by
a facilitative diffusion glucose transport process in
which glucose moves down its concentration gradient
into cells. Over the past several years, the proteins that
transport glucose across the plasma membrane have
been identified by molecular cloning.7-18 At present,
six related proteins have been identified that can
conduct glucose across biological membranes.7 18 The
members of this family differ in terms of their affinity
for glucose and other hexoses, their regulation by
hormonal and other factors, and their distribution in
tissue.7-18,27 In accordance with previous functional
Insulin
CH
PM
200
Crude
Homogenate
100
1.0
1.1 ±0.1
100
2.7-0.2
59.3±5.1
data, it has been determined that the major glucose
transporter in heart is an insulin-responsive transporter known as GLUT4.19,27 GLUT4 is expressed
primarily in fat and in skeletal and cardiac muscle.
Under fasting conditions, in each of these tissues, the
majority of GLUT4 molecules reside in an intracellular membrane compartment and therefore do not
participate in cellular glucose uptake. However, when
the cells are stimulated by insulin, GLUT4 molecules
are rapidly translocated to the plasma membrane and
augment cellular glucose uptake up to 20-fold.'9-23 28-3'
After insulin levels drop, the transporters are recycled
to the intracellular compartment. Stimuli such as
exercise, hypoxia, and other hormones have also been
shown to cause translocation of GLUT4 molecules in
fat or muscle.27-31 Because ischemia is known to cause
a rapid increase in myocardial glucose extraction from
the blood,' the present study investigated the hypothesis that acute myocardial ischemia induces GLUT4
translocation to the plasma membrane of cardiac
myocytes.
To test our hypothesis, we used a modified subcellular
membrane fractionation technique for rat myocardium
that yields reliable separation of intracellular membranes from plasma membranes independent of experimental conditions. The method produces enrichments
of marker enzymes that are comparable to those obtained with other reported myocardial subcellular fractionation methods.23,28-30 Furthermore, this method
yields a recovery of approximately 60% of myocardial
Insulin plus ischemia
CH
PM
Mm
Mm
Plasma Membrane
Fraction
4.3+0.5
6.5+0.6
Microsomal Membrane
Fraction
2.5+0.2
1.4+0.2
5.6+0.3
100 -
E
80-
E
60-
9769-
'U
T
0.
0.
...
V.Q
20-
,
30-
21-
FIG 1. GLUT4 immunoblots of 50 mg of crude homogenates
(CH) and plasma membrane (PM) and microsomal membrane
(MM) fractions from insulin-treated and insulin plus ischemia rat
hearts, demonstrating substantial GLUT4 expression in PM
fractions. Positions of molecular weight markers are indicated in
kilodaltons to the left of the blots.
e
-
_
control
(8)
hypoxia
(5)
isehemia
(5)
insulin
(6)
insulin +
ischemia
(6)
FIG 2. Bar graph showing summary of plasma membrane
GLUT4 expression (mean±SEM) in hearts from control, ischemia, hypoxia, insulin-stimulated, and insulin plus ischemia
hearts. The number of animals in each group is indicated in
parentheses below the condition. *Conditions in which plasma
membrane (PM) GLUT4 expression was significantly greater
than control; #conditions in which PM GLUT4 expression was
significantly greater than ischemia.
Circulation Vol 89, No 2 February 1994
796
Crude Homogenate
c
I
Plasma Membrane
H
97.
.t
46-
Microsomal Membrane
C
a
.E'tyEyS
f-';'
I
.....
H
..a
...4..
:0* Ll-#
l W-' 'X' ' ' l...F"''-.,',...' *V0d'"'w' ~ ~ ~ ~ . . . . .
...
FIG 3. GLUT4 immunoblot of 50 mg of crude homogenates and
plasma membrane and microsomal membrane fractions from
control (C), ischemic (1), and hypoxic (H) rat hearts. Plasma
membrane GLUT4 expression was increased and microsomal
membrane GLUT4 expression was decreased in both the ischemic and hypoxic hearts. Positions of molecular weight markers
are indicated in kilodaltons to the left of the blot.
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GLUT4 molecules in the two final fractions. This relatively quantitative GLUT4 recovery, which has not
previously been documented in earlier membrane fractionation studies, ensures that translocation results will
not be affected spuriously by losses of GLUT4 in
discarded fractions. Our results show that the major
cardiac glucose transporter, GLUT4, is translocated to
the plasma membrane after myocardial cells are subjected to an episode of acute ischemia or hypoxia,
although the magnitude of this translocation is less than
that stimulated by pharmacological doses of insulin. To
the best of our knowledge, this is the first report of the
effects of myocardial ischemia on glucose transporter
translocation. The conditions or stimuli that are now
known to induce translocation of glucose transporters in
heart include insulin,20,23,28'29 catecholamines,30 increased workload,3' hypoxia,29 and ischemia.
Partly because of the difficulties inherent in subcellular membrane fractionation of cardiac tissue, there have
been only a few previous studies of the translocation of
myocardial glucose transporters. Watanabe et a128 were
the first to develop a subcellular fractionation method
that documented translocation of rat myocardial glucose transporters by insulin. Wheeler,29 using a modification of the Watanabe protocol, subsequently showed
that hypoxia caused translocation of myocardial glucose
transporters. Both of these studies were performed
before the molecular cloning of GLUT4 and its identification as the major myocardial glucose transporter.
Therefore, in Wheeler's study, the degree of translocation was evaluated by comparing three properties of the
plasma and microsomal membrane fractions: reconstituted glucose transport activity; binding of the ligand
cytochalasin B, which recognizes all glucose transporter
isoforms to some extent; and immunoblotting with an
antibody that recognizes GLUT1, a transporter that is
expressed only at low levels in myocardium.'9 In both
studies, insulin caused a 60% to 100% increase in PM
glucose transporters and a 16% to 38% decrease in MM
fractions (called "high-speed pellet" fractions).28'29
Hypoxia caused similar changes and induced a 20% to
70% increase in PM glucose transporters and a 20% to
30% decrease in glucose transporters in the microsomal
fraction.29 Using the same membrane fractionation procedure, Rattigan et a130 documented GLUT4 translocation after catecholamine stimulation, with a 7% increase
in GLUT4 molecules in the fractions enriched in plasma
membranes, and a 34% to 38% decrease in GLUT4
molecules in the fraction enriched in microsomal membranes. Zaninetti et a123 used a much different fractionation method yielding PM protein recoveries approximately 5 times greater than those from the Watanabe
and Wheeler protocols. Using cytochalasin B binding
determinations, they also documented a twofold increase in PM glucose transporters with insulin.23 More
recently, Slot et a120 studied the extent of GLUT4
translocation in rat hearts induced by a combination of
maximal insulin and exercise stimulation using immunohistochemical localization of GLUT4. In contrast to
results found with subcellular fractionation, these investigators demonstrated minimal (<1%) PM expression
of GLUT4 under basal, fasting conditions and showed
that insulin plus exercise induced translocation of approximately 42% of the GLUT4 molecules to the PM.
By thus demonstrating a much greater degree of glucose
transporter translocation than shown by the other studies, these findings called into question the ability of
subcellular fractionation techniques to adequately separate plasma membranes from other intracellular
membranes.
With minor modifications of the membrane fractionation protocol of Zaninetti et al, we found that approximately 18% of GLUT4 molecules were expressed in the
PM fraction in fasted animals and that insulin caused
translocation of another 43% of the total GLUT4
molecules. The differences between our results and
those of the earlier fractionation studies are most likely
a result of our use of a different fractionation procedure
that allowed us to capture approximately 60% of
GLUT4 molecules within our final two fractions and
produced relatively small amounts of cross-contamination. Our results, however, also differ to some extent
from those of Zaninetti et al.23 These differences may be
a result of our use of semiquantitative immunoblotting
with a specific GLUT4 antibody rather than the more
difficult and less specific cytochalasin B binding assays.
Another potential factor is that the cytochalasin B
binding experiments in the previous studies would have
detected GLUT1 molecules as well as GLUT4. However, GLUT1 appears to be expressed at much lower
levels than GLUT4 in rat myocardium.19 Therefore, this
difference in technique is unlikely to have contributed
significantly to the higher basal PM GLUT expression
and diminished translocation found in the earlier studies. Of all the fractionation studies, our results are most
consistent with the immunolocalization findings of Slot
et al.20 The higher PM GLUT4 expression found under
basal conditions in our study is most likely a result of
contamination of the PM fraction with intracellular
membranes, as evidenced by enrichment of a mitochondrial enzyme, NADPH-cytochrome c reductase, in the
PM fraction. The relative contamination was not substantial, however, and although both basal and insulinstimulated PM GLUT4 expression were somewhat overrepresented with this method, the absolute degree of
translocation was nearly identical to that detected by
the immunolocalization study. Therefore, myocardial
membrane fractionation accompanied by specific immunoblotting appears to produce reliable data on subcellular glucose transporter localization and can be used as
a complement to immunohistochemical studies for this
purpose. Furthermore, this method possesses several
distinct advantages over immunohistochemical studies.
Sun et al GLUT4 Translocation by Myocardial Ischemia
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For example, quantification of specific proteins is easier
and more reproducible, and multiple samples can be
more reliably assayed and compared.
Our data suggest that much of the increase in glucose
metabolism of acutely ischemic myocardial cells is
caused by the translocation of GLUT4 molecules. An
increase in plasma membrane transporters leads to
more rapid glucose uptake, and because myocardial
glucose uptake and phosphorylation normally appear to
be rate limiting for glucose metabolism,32-35 the number
of transporters on the cell surface at least partly determines the rate of myocardial glucose metabolism. Indeed, conditions or stimuli that acutely augment glucose
uptake and metabolism in skeletal muscle and in other
tissues do so through the translocation of glucose
transporters.27
It is interesting that global ischemia stimulates
GLUT4 translocation less than does maximal insulin
treatment. Ischemia alone increased PM GLUT4 molecules from 18% to 41%, whereas insulin treatment
alone and ischemia plus insulin treatment resulted in
the plasma membrane expression of 61% and 67% of
total myocardial GLUT4 molecules, respectively. The
reason for the difference in the degree of GLUT4
translocation induced by either ischemia or insulin is
not known. It is conceivable that separate intracellular
pools of GLUT4 molecules exist, one of which is
responsive to insulin and the other responsive to ischemia. This conclusion seems somewhat unlikely, since
insulin plus ischemia failed to increase plasma membrane GLUT4 expression in an additive manner. It is
also conceivable that the severe global ischemia produced in our model could have been associated with the
accumulation of toxic metabolites (eg, lactate) that may
have suppressed maximal GLUT4 translocation. Indeed, clinical and experimental studies have found that
severely ischemic myocardium loses its ability to extract
increased amounts of glucose from the blood.1"36 However, toxic suppression of translocation seems unlikely
to have occurred in our ischemic model, since hypoxia,
which produced a degree of myocardial cell GLUT4
translocation similar to that of severe ischemia, does not
cause the buildup of toxic metabolites and consistently
increases myocardial glucose uptake and utilization.1"36
Therefore, decreased glucose metabolism in severely
ischemic myocardium is probably caused by depressed
glycolytic enzyme activity and not by decreased transporter expression. It seems most likely that the difference between the degree of translocation induced by
ischemia and hypoxia on the one hand and insulin on
the other is a result of the stimulation of different
intracellular signaling pathways. The elucidation of
these complex pathways is now being pursued energetically by a number of investigators.
Despite these unanswered questions, there appears
to be a significant clinical correlate to the finding that
insulin causes a greater degree of GLUT4 translocation
than ischemia alone. Numerous clinical studies have
shown that administration of insulin, alone or in combination with glucose and potassium, improves cardiac
function and results in less extensive myocardial injury
in patients with ischemia, infarction, or coronary bypass.5'37-4' We propose that insulin infusion is protective
in these conditions because it stimulates additional
translocation of GLUT4 molecules to the plasma mem-
797
brane of ischemic cardiac myocytes, thus enhancing the
protective increase in glucose metabolism. Our biochemical findings may therefore give added impetus to
clinical efforts to enhance glucose uptake and utilization
by myocardium at risk.
We studied the effects of ischemia on the translocation of the GLUT4 isoform only, and it is possible that
myocardial ischemia also causes translocation of the
other cardiac transporter, GLUT1, to the plasma membrane. Under acute conditions, such as those of the
present study, GLUT1 translocation is unlikely to contribute substantially to myocardial glucose uptake, because GLUT1 appears to be expressed at much lower
levels than GLUT4 in heart.19 However, under more
chronic ischemic or hypoxic conditions, GLUT1 expression could be induced to physiologically significant
levels. Indeed, it was shown recently that chronic hypobaric hypoxia resulted in a modest increase in
GLUT1, but not GLUT4, mRNA and protein expression in rat hearts.42 Interestingly, GLUT4 expression
actually decreased in the right ventricle of hypobaric
hypoxic rats.42 Thus, many alterations in glucose transporter expression may accompany chronic hypoxic and
ischemic conditions. It would be intriguing to determine
the exact role that glucose transporter expression plays
in mediating myocardial glucose metabolism under
these chronic conditions and to determine whether
modulating this response alters myocardial viability.
Molecular reagents and techniques with which these
important issues may be addressed are now available.
Acknowledgments
This work was supported by a Grant-in-Aid from the
American Heart Association of Michigan, by National Institutes of Health grant RO1-HL-41047, and by American Heart
Association Established Investigator Award 890219 (to M.S.).
We thank Jin Liu for valuable technical assistance, Dr Heinrich Taegtmeyer for helpful discussions, Cindy Hogston for
secretarial support, and Dr Maureen Charron for her gift of
the GLUT4 antiserum.
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D Sun, N Nguyen, T R DeGrado, M Schwaiger and F C Brosius, 3rd
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Circulation. 1994;89:793-798
doi: 10.1161/01.CIR.89.2.793
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