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Minerva Foundation Institute for Medical Research and Department of Medicine (H.Y.-J.), Division of Endocrinology and Diabetology, University of Helsinki, Helsinki, Finland; and the Veterans Administration Medical Center (M.C.D.) and Department of Medicine (D.M.), University of Mississippi Medical Center, Jackson, Mississippi 39216
Address all correspondence and requests for reprints to: Dr. Antti Virkamäki, Minerva Foundation Institute for Medical Research, Tukholmankatu 2, Helsinki, SF-00250 Finland. E-mail: virkamak{at}helsinki.fi
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Introduction |
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GFA activity is found in several tissues other than skeletal muscle in the rat (11, 12, 13). These include insulin-sensitive tissues such as the heart, adipose tissue, the liver, and the submandibular gland (13). The possibility that overactivity of the hexosamine pathway also contributes to insulin resistance in these insulin sensitive tissues has not yet been tested.
Studies in adipocytes have suggested that GFA is under rapid transcriptional control and that its activity can be significantly inhibited by exposing adipocytes to glucosamine or glucose, insulin and glutamine (14). Half-maximal inhibition of GFA activity was observed at a glucosamine concentration of 0.21 mmol/liter within 4 h. However, recent studies using an assay specifically measuring GlcN6P by HPLC rather than a variety of hexosamines by the colorimetric technique (7) have suggested that GFA activity in skeletal muscle is unaffected by 4 h of hyperinsulinemia in normal subjects (15) but can be increased by prolonged exposure to glucose and insulin in cultured human muscle cells (16). Whether these discrepant results regarding regulation of GFA are due to species, tissue or methodological differences, is unknown.
The effect on glucosamine induced insulin resistance on the intracellular fate of glucose is also controversial. In rats in vivo, coinfusion of glucosamine as compared with saline with insulin decreases the rate of glycogen synthesis in rat abdominis muscle. On the other hand, in isolated soleus muscle, glucosamine induces insulin resistance of glucose uptake but preferentially channels glucose to glycogen (17). This effect could be attributed to stimulation of glycogen synthase activity by GlcN6P, which accumulated in glucosamine infused rats (data not shown). However, it is unclear whether the different results regarding the effect of glucosamine on glycogen synthesis can be attributed differences in GlcN concentrations or to the type of muscle examined.
The present study was undertaken to answer several of the above questions. First, we wished to determine whether GlcN induces insulin resistance of glucose uptake in tissues other than skeletal muscle. Second, we determined the effect of in vivo hyperinsulinemia on GFA activity in various insulin-sensitive tissues. Finally, we determined the effect of an in vivo GlcN infusion on the rate of insulin-stimulated glycogen synthesis and the glycogen content in multiple insulin sensitive tissues including three different types of skeletal muscle, the heart, epididymal fat, the liver and the submandibular gland, which is both insulin sensitive (18) and has a several-fold higher GFA activity than skeletal muscle (13).
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Study design
Three groups of rats were studied. The first group received only
saline but no insulin or GlcN (basal, BAS, n = 6). Two other
groups were studied under normoglycemic hyperinsulinemic conditions
(insulin infusion rate 18 mU/kg·min). These rats received, in
addition to glucose and insulin, iv infusions of either saline (INS
rats, n = 8) or GlcN at a rate of 30 µmol/kg·min (GLCN rats,
n = 8), as detailed below.
The rats were studied after a 16-h fast. On the study morning (0730 h), the rats were weighed, connected to infusion and blood sampling lines, and placed in metabolic cages, where they were allowed to move freely (19). After 1 h, a primed/continuous infusion of insulin (INS and GLCN rats) or saline (BAS) was started (19, 20, 21). Plasma glucose was measured every 5 min (INS and GLCN rats) or 30 min (BAS) with the glucose oxidase method (22) using Beckman Glucose Analyzer II (Beckman Instruments Corp., Fullerton, CA). In INS and GLCN rats, a variable rate infusion of 20% glucose was started at 4 min and adjusted to maintain the plasma glucose concentration at the fasting concentrations of 5.5 mmol/liter. Serum insulin concentrations at 0, 30, 60, 120, 180, 240, 300, and 360 min were determined using RIA (23).
In all groups, a primed (15 µCi), continuous (0.2 µCi/min) infusion of \[3-3H\]glucose was started at 0 min to measure rates of whole body glucose utilization and glycolysis (20). Plasma samples for determination of glucose ([3H]GSA) and water specific activities (WSA) were withdrawn at 0, 30, 60, 120, 180, 240, 300, and 360 min. At 315 min, a bolus injection of 2-\[1-14C\]-deoxyglucose (14C-DOG, 50 µCi) was given to determine tissue glucose uptake. Samples for determination of 14C-DOG specific activity in plasma (dpm/µmol glucose) were withdrawn at 317, 320, 325, 330, 335, 345, and 360 min. At the end of the study, the rats were anesthetized (pentobarbital, ia) and tissues (soleus, gastrocnemius and abdominis muscles, epididymal fat, liver, heart, and submandibular gland) were freeze-clamped in situ with aluminum tongues precooled in liquid nitrogen for measurement of glycogen and 2-\[1-14C\]deoxyglucose-6-phosphate ([14C]DOG6P) concentrations, rates of liver glycogen synthesis, and GFA activities. GFA activity was measured in tissues obtained from the BAS and INS rats but not in GLCN rats as increases in GlcN6P formed from GlcN in vivo (data not shown) interferes with in vitro measurement of GlcN6P formation by GFA. Tissue samples were stored in liquid nitrogen until analysis. The experimental protocol was approved by the Ethical Committee of the Helsinki University Central Hospital.
Plasma glucose and water specific activities
Aliquots of plasma (50 µl) for the determination of
[14C]DOGSA and [3H]GSA and WSA were
deproteinized with 100 µl Ba(OH)2 and 100 µl
Zn(SO)4 and centrifuged. The protein-free supernatant
(Somoqyi filtrate) was divided into two aliquots, of which one (40
µl) was counted directly to obtain total [3H]- and
[14C]-radioactivity after adding liquid scintillation
fluid (OptiPhase 'HiSafe' 3, Wallac UK, Milton Keynes, UK) in a
Rackbeta 1214 liquid scintillation counter (Wallac, Turku, Finland).
The other aliquot was counted for [3H]- and
[14C]-radioactivity after evaporation to dryness and
reconstitution in water (40 µl). Specific activities were calculated
by dividing [3H]- and [14C]-radioactivities
in the dried reconstituted aliquots by the ambient plasma glucose
concentration. WSA was calculated by subtracting
[3H]-radioactivity of the dried Somoqyi filtrate from
total [3H]-radioactivity (20). Plasma water was assumed
to be 93% of total plasma volume and total body water mass 65% of the
body mass (20).
Glycogen concentration and specific activity
The frozen tissue samples were freeze-dried (Edwards EF4
Modulyo, Edwards High Vacuum, West Sussex, UK), dissected free of blood
and connective tissue, weighed, and extracted for 15 min at +50 C with
1 N KOH (100 µl/mg dry) (24). The alkaline extract was
used for the determination of tissue glycogen concentration and
glycogen specific activity.
Tissue glycogen concentrations were determined after alkaline hydrolysis with amyloglucosidase (25). In brief, an aliquot of the alkaline extract (200 µl) was neutralized with 1 N HCl and its pH was adjusted to 4.9 with 0.15 N sodium acetate buffer. The glucose concentration was determined before and after amyloglucosidase (Sigma Chemicals, St. Louis, MO) treatment (1 h at room temperature) in the acified extract (26). The glycogen concentration (mmol/kg dry) was calculated by dividing the difference in the glucose concentration between the amyloglucosidase treated and untreated samples by tissue dry weight.
For determination of glycogen [3H]- and [14C]-specific activities, glycogen was precipitated in another aliquot (800 µl) of the alkaline extract with 1600 µl of 99.6% ethanol (-20 C, 2 h). After centrifugation (3000 rpm, +4 C, 30 min) (19, 24), the precipitate (ppt) was washed three times with 600 µl of ice-cold 66% ethanol and the supernatants from the ethanol washes were discarded. The washed ppt was dissolved in water, its pH was adjusted to 4.9 using 0.15 M sodium acetate buffer and an aliquot was taken for the determination of the glycogen concentration using the amyloglucosidase method (vide supra). The measurement of the glycogen concentration in the ethanol ppt was necessary because the recovery of glycogen in the ethanol precipitation procedure was less than 100% (data not shown). Another aliquot was counted for [3H]- and [14C]-radioactivity by dual channel counting (Rackbeta 1214, Wallac) and its glucose concentration was measured after amyloglucosidase treatment. Glycogen specific activity was calculated by dividing the [3H]- or [14C]-radioactivity (dpm/ml) by the glycogen concentration (µmol/ml) in the ethanol ppt. The amount of radioactivity incorporated into glycogen per dry muscle (dpmgly) was calculated by multiplying glycogen specific activity (dpm/µmol) times the tissue glycogen concentration (mmol/kg dry).
Tissue preparation for 14C-DOG6P accumulation and GFA
activity
For determination of the tissue concentration of
14C-DOG6P (dpm/ml) and the activities of GFA, frozen tissue
specimens weighing 100–200 mg were homogenized (S25N-8G, Janke &
Kunkel GmbH & Co., Staufen, Germany) for 15 sec in ice-cold 50
mM potassium phosphate buffer (pH 7.4) containing 2
mM dithiotreitol, 20 mM EDTA, 20 mM
sodium fluoride, 10 µg/ml leupeptin, 10 µg/ml soybean trypsin
inhibitor, 20 µg/ml p-aminobenzamidine, 70 µg/ml
Na-p-tosyl-L-lysine chloromethyl ketone; and 170 µg/ml
phenylmethylsulfonyl fluoride (27). The homogenates were centrifuged at
13,000 x g, and aliquots of the supernatant were taken
for determination of the concentrations of protein and
14C-DOG6P and for measurement of GFA activity.
Tissue glucose uptake
For determination of the 14C-DOG6P concentration
(dpm/ml), the homogenate was deproteinized with equal volume of
ice-cold perchloric acid (1 mM, 10 min) and centrifuged
(3000 G, 15 min). The supernatant was neutralized with 0.25 vol 2.2
M KHCO3. An aliquot (500 µl) was passed
through an anion-exchange (Ag 1-X8, acetate form, Bio-Rad, Hercules,
CA) column, which was then washed with 2.5 ml of distilled water to
elute nonanionic compounds such as glucose and glycogen. This eluate
and another aliquot (500 µl, total radioactivity) of the neutralized
sample were then counted for [14C]-radioactivity. The
[14C]DOG6P concentration (dpm/ml) was calculated from the
difference between the radioactivities in the total and neutral eluate
radioactivities.
GFA activity
GFA activity was assayed as previously described (15). Briefly,
after homogenization and centrifugation, 50 µl of the supernatant was
incubated in a sodium phosphate buffered reaction mix (pH 7.4)
containing EDTA and dithiotreitol and excess of F6P and glutamine for
45 min at 37 C in the presence and absence of excess of
UDP-N-acetylglucosamine (UDP-GlcNAc), which is an allosteric
inhibitor of GFA. After termination of the reaction with PCA, the
sample was centrifuged, neutralized with KHCO3, delipidated
and derivatized with o-phtaldialdehyde (OPA) for 1 min
before analysis of the GlcN6P concentration with a reverse-phase
C18 column (25 cm x 4.6 mm Spherisorb ODS, Phase
Separations, Norwalk, CT). The mobile phase consisted of a one-step
gradient made of 2.5% acetonitrile, 2.5% isopropanol in 30
mM sodium phosphate buffer and 12% acetonitrile, 12%
isopropanol in 30 mM sodium phosphate buffer. Absorbance of
the eluent was analyzed using a fluorescence detector and the peak area
was integrated. OPA-derivatized standards were run separately to
determine the retention time and to generate the standard curve to
correlate area to activity. The correlation coefficient between the
concentration of GlcN6P standards and the area under the GlcN6P peak
was over 0.999. Internal and external standards were run in every
assay. The recovery of samples spiked with GlcN6P before derivatization
was 100%. Activity is expressed as pmols of GlcN6P formed per
milligram protein per min. The coefficient of variation of GFA
measurements in two pieces of rat muscle was <2%.
GlcN concentrations
Serum GlcN concentrations were assayed using the same sample
preparation procedure and reverse-phase HPLC setup as in GlcN6P assay
described above. The mobile phase consisted of a two-step gradient made
of identical buffers as in GlcN6P assay. GlcN eluted with a retention
time of 20.6 min. The correlation coefficient between the concentration
of GlcN standards mixed in blank serum and the area under curve was
0.985.
Calculations
The rate of glucose appearance (Ra,
µmol/kg·min) was calculated from the formula: Ra =
(F/GSA)/W, where F denotes the isotope infusion rate (dpm/min), GSA the
mean specific activity of glucose (dpm/µmol) in plasma, and W body
weight (kg). The rate of liver glycogen synthesis (Rs;
µmol/kg dry·min) was calculated from the formula Rs =
[(dpmgly/dt)]/GSA, where dt denotes the time period. The
rate of whole body glycolysis was calculated from the increment in
tritiated water radioactivity (dpm/ml·min) multiplied by total body
water and divided by mean GSA (20). Tissue specific
[14C]DOG uptake was calculated by dividing
[14C]DOG6P and [14C]glycogen radioactivity
(dpm/mg tissue) per mg protein with the total plasma area under the
curve of [14C]DOG specific activity (dpm·min/µmol
plasma glucose). This formula is similar to that validated by Hom
et al. (28) and Jenkins et al. (29), with the
exception that the amount of 14C-radioactivity incorporated
into glycogen from [14C]DOG is also taken into account.
This is necessary since DOG6P is incorporated into glycogen (30, 31)
under insulin-stimulated conditions (32).
Statistical analysis
Data between the study groups were analyzed using the unpaired
Student’s t test. Simple correlation’s between selected
study variables were calculated using Pearson’s correlation
coefficient for variables that were not normally distributed. All
statistical calculations were made using the BMDP statistical software
(BMDP Statistical Software, Los Angeles, CA). All data are expressed as
means ± SE.
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Results |
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Figure 1
shows the rate of glucose uptake as a
function of time, as determined from the glucose infusion rate
necessary to maintain normoglycemia in the INS and GLCN rats. The rate
of glucose uptake was similar in INS (210 ± 7 µmol/kg·min)
and GLCN rats (170 ± 10 µmol/kg·min) between 60–120 min,
but during the last hour of the study the rate of glucose uptake was
49% lower in the GLCN (208 ± 8 µmol/kg·min) than the INS
(107 ± 11 µmol/kg·min, P < 0.001) rats.
During the entire study (0–360 min), the rate of glucose uptake was
30% lower in the GLCN (168 ± 9 µmol/kg·min) than the INS
(239 ± 8 µmol/kg·min), P < 0.001) rats.
The rate of glucose turnover in the BAS rats averaged 49 ± 5
µmol/kg·min. The rate of glucose turnover (Rt) during
60–360 min of hyperinsulinemia measured with isotopic dilution method
(\[3-3H\]glucose) was significantly lower in the GLCN
(151 ± 7 µmol/kg·min) than the INS (192 ± 7
µmol/kg·min, P < 0.05) rats. The greatest
difference between INS and GLCN rats in Rt was observed
during the last hour of the study (300–360 min), when Rt
averaged 133 ± 7 µmol/kg·min in the GLCN and 194 ± 8
µmol/kg·min in the INS rats (P < 0.001).
Rt:s at 60–120 min were comparable between GLCN (169
± 7 µmol/kg·min) and INS (186 ± 6 µmol/kg·min, NS)
rats. These isotopically measured rates of glucose turnover were not
different from the glucose infusion rate needed to maintain euglycemia
confirming that hepatic glucose production was completely suppressed in
both GLCN and INS rats. Rates of whole body glycolysis (60–360 min)
were similar in GLCN (74 ± 5 µmol/kg·min) and INS (74
± 5 µmol/kg·min, NS) rats.
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). Insulin
increased the rate of [14C]DOG uptake 27-fold in the
heart, 10-fold in the gastrocnemius, 3-fold in the soleus, 21-fold in
abdominis muscle, 7-fold in epididymal fat, and by 3-fold in the
submandibular gland. Insulin-stimulated glucose uptake decreased
significantly in all tissues by the GlcN infusion (Fig. 2). GlcN
induced decreases in [14C]DOG uptake averaged 41% in the
heart, 52% in the gastrocnemius, 38% in the soleus, 31% in abdominis
muscle, 35% in epididymal fat and 34% in the submandibular gland.
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Glycogen concentrations and rates of glycogen synthesis
Tissue glycogen concentrations were significantly higher in INS
than in BAS rats in soleus, abdominis and gastrocnemius (Table 1). Infusion of GlcN abolished glycogen synthesis in
the liver (228 ± 40 vs. 1 ± 5 µmol/kg
dry·min, Fig. 2
) and significantly decreased glycogen content in
abdominis muscle (Table 1). In contrast, the glycogen content increased
significantly in the heart and was unaltered in soleus and
gastrocnemius muscles (Table 1).
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).
Increases in GFA activity by insulin were also observed in soleus
muscle and epididymal fat, but they did not reach statistical
significance. |
Discussion |
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The dose of GlcN and the 30% decrease in whole body glucose uptake induced by GlcN in the present study are similar to those described by Rossetti et al (30% decrease over 5 h) (9). In keeping with both the latter data (9) and those of Giaccari et al (8), the decrease in whole body glucose uptake was associated with a 31% decrease in glucose uptake in abdominis muscle. Abdominis muscle consists mostly of glycolytic, white or type 2a fibers (33) and has lower rates of insulin-sensitive glucose uptake than muscles such as the soleus which consists of insulin-sensitive, type 1 fibers (34). These characteristics were confirmed in the present study as both the absolute rates of glucose uptake (6.4 ± 0.7 vs. 2.2 ± 0.3 nmol/mg protein·min, soleus vs. abdominis muscle, P < 0.05) and the insulin induced increases in glucose uptake above basal (4.6 ± 0.5 vs. 2.1 ± 0.3 nmol/mg protein·min, respectively, P < 0.05) were significantly higher in soleus than abdominis muscles. Insulin-stimulated rates of glucose uptake were also inhibited to a significantly greater extent in soleus (by 2.4 nmol/mg protein·min) than in abdominis (by 0.7 nmol/mg protein·min) muscle, suggesting that the sensitivity of inhibition of glucose uptake by the hexosamine pathway depends on muscle fiber composition or insulin sensitivity.
In addition to skeletal muscle, we observed a significant decrease in
glucose uptake by GlcN in adipose tissue, the heart, liver and the
submandibular gland. Both the absolute rates of insulin stimulated
glucose uptake, expressed per mg of protein, and the percent decrease
by GlcN were comparable in the submandibular gland and in hindlimb
muscle (Figs. 2 and 3). These data imply that basal GFA activity, which
was 10-fold higher in the submandibular gland than in hindlimb muscle,
does not determine the extent to which a tissue is susceptible to
inhibition of glucose uptake via activation of the hexosamine pathway.
The assumption that the proportion of glucose metabolized via the
hexosamine pathway is determined by GFA is supported by several
observations. First, GFA is the rate-limiting enzyme of the pathway
(4). Second, accumulation of hexosamine metabolites seems to parallel
GFA activity, as a 2-fold overexpression of the enzyme in transgenic
mice (5) doubles the concentration of
UDP-N-acetyl-hexosamines. Third,
UDP-N-acetyl-hexosamine concentrations parallel GFA
activities between tissues (7). The dissociation between the extent of
insulin resistance of glucose uptake vs. GFA activity of a
particular tissue could be due to tissue specific differences in the
intracellular channeling of products of the hexosamine pathway,
particularly UDP-GlcNAc to pathways mediating insulin resistance and to
those not involved in glucose uptake regulation. The former could
involve changes in O-linked glycosylation of intracellular proteins,
which is known to regulate the function of several proteins and is
often reciprocal to protein phosphorylation (35). The pathways not
involved in regulation of insulin sensitivity would then be those
utilizing UDP-GlcNAc for synthesis of glycolipids and extracellular
N-linked glycoproteins (36, 37). Consistent with the idea that O-linked
glycosylation might mediate insulin resistance, the submandibular gland
uses a major proportion of UDP-GlcNAc for production of extracellular
glycoproteins and mucins (38), whereas O-linked glycosylation
predominates in skeletal muscle (39). Also, we have recently
demonstrated that the activity of specific UDP-GlcNAc transferase,
which catalyzes final O-linked attachment of UDP-GlcNAc to serine and
threonine residues on intracellular proteins, is equally active in the
submandibular gland and in gastrocnemius muscles although GFA activity
was markedly higher in the submandibular gland than in gastrocnemius
muscle (13).
The absolute GlcN-induced decrease in whole body glucose uptake became
significant after 2 h and reached a maximum of 101
µmol/kg·min between 5 and 6 h (Fig. 1). Because the
magnitude of the maximal decrease in glucose uptake was more than three
times greater than the infusion rate of GlcN (30 µmol/kg·min),
simple competition between glucose and GlcN cannot explain the
GlcN-induced insulin resistance. These data are consistent with those
of Rossetti et al. (9). In addition, the time delay in the
ability of GlcN to induce insulin resistance and the modest inhibition
of muscle hexokinase by GlcN (9) also argues against simple competition
as the mechanism underlying GlcN induced insulin resistance. Because
skeletal muscles consume the majority of glucose under normoglycemic
hyperinsulinemic conditions in the rat (40), simple competition is also
unlikely to explain insulin resistance in skeletal muscle. On the other
hand, this may not apply to tissues such as the liver where glucose
phosphorylation by glucokinase is rate-limiting for glucose uptake and
where intracellular glucose concentrations equal extracellular glucose
concentrations (41). Under such conditions in the liver, inhibition of
glucokinase by GlcN may not be trivial. Clearly, further studies are
needed to determine the exact cause(s) for abolition of glycogen
synthesis by GlcN in the liver.
Consistent with the data of Rossetti et al. (9) and those of Giaccari et al. (8), we found a significant decrease in the rate of glycogen synthesis in abdominal muscle. On the other hand, the glycogen concentrations in soleus and hindlimb muscles were unchanged by GlcN. The latter data are in keeping with those of Fürnsinn et al. (17), who observed diminished total glucose uptake in the face of inhibition of glycolysis and increased incorporation of glucose into glycogen in the presence of insulin and GlcN compared to insulin alone in the isolated soleus muscle in vitro. Other studies have shown GlcN to increase the fractional velocity and total activity of glycogen synthase activity in Rat-1 fibroblasts overexpressing the human insulin receptor (10), and an increase in basal glycogen synthase activity in GFA-transfected fibroblasts (6). The apparent increase in glycogen synthase activity coupled with increased channeling of glucose to glycogen in the heart, and two of the three skeletal muscle examined, implies that GlcN does not perfectly mimic insulin resistance caused by chronic hyperglycemia, which has invariably been associated with diminished rates of glycogen synthesis in both rats (20) and patients with IDDM (42) and NIDDM (15). One possibility to explain the increased rates of glycogen synthesis is that GlcN induces a large increase in GlcN6P concentrations, which stimulate glycogen synthase activity and consequently glucose incorporation into glycogen (17).
The tissue distribution of GFA activities found in the BAS rats in the present study contrasts earlier failure of Kaufman et al. (12) to detect significant GFA activity in heart or skeletal muscle using a colorimetric assay for detection of multiple acetylated hexosamines but is consistent with more recent data obtained using a sensitive and specific HPLC method for measurement of GFA activity (7, 13, 15, 16). Regarding the effect of insulin on GFA activity, we found no change in skeletal muscle GFA activity during 6 h of hyperinsulinemia in rats. This result is in keeping with our previous data showing no change in GFA activity in human skeletal muscle in normal subjects or patients with NIDDM during 4 h of hyperinsulinemia (15), or in control, insulin-deficient diabetic or insulin treated rats 1 or 2 h after a glucose injection (7). Long-term exposure of cultured human skeletal muscle cells to hyperinsulinemia (16), hyperglycemia (16) does, however, increase GFA activity (16), and an increase in GFA activity also characterizes chronically hyperglycemic patients with NIDDM (15). Stimulation of GFA activity in the heart and in the submandibular gland by insulin has not been previously reported but is in line with the data demonstrating insulin induced increases in skeletal muscle GFA activity. These data, together with the lack of an effect of increased GlcN concentrations on GFA activity in adipose tissue in the present study, differs from the data by Marshall et al., who found decreases in GFA activity within a few hours both by low concentrations GlcN, and by the combination of glucose, insulin and glutamine in primary cultures of rat adipocytes (14).
In conclusion, the present data demonstrate that infusion of GlcN to normal rats induces insulin resistance in several insulin sensitive tissues including fast- and slow-twitch skeletal muscles, the heart, liver, adipose tissue, and the submandibular gland. The magnitude of insulin resistance exhibited tissue specificity and was greatest in the liver and lowest in the fast-twitch glycolytic abdominis muscle. These data raise the possibility that overactivity of the hexosamine pathway may contribute to glucose toxicity not only in skeletal muscle but also in the heart and in the liver.
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Footnotes |
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Received November 8, 1996.
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