1 Department of Internal
Medicine, To examine effects of free fatty acids (FFA) on
insulin-stimulated glucose fluxes, euglycemic hyperinsulinemic (86 pmol · kg
insulin resistance; glycogen synthesis; glycolysis
THE CONCEPT OF SUBSTRATE competition between glucose
and free fatty acids (FFA) as oxidative fuel sources in muscle
(glucose-fatty acid cycle hypothesis) was introduced more than 30 years
ago by Randle et al. (26). Since then, many investigators have examined the effect of FFA on whole body and/or skeletal muscle glucose metabolism (7, 9, 21). However, it is still controversial whether
increased plasma FFA levels are responsible for insulin resistance
observed in diabetes and obesity.
Many studies in humans have suggested that reduction in glycogen
synthase (GS) activity and glycogen synthesis in skeletal muscle is the
main cause of insulin resistance in non-insulin-dependent diabetes and
obesity (6, 31). Although it is now generally accepted that increased
provision of FFA suppresses whole body glucose oxidation (2, 7, 9, 14),
the effect of FFA on glycogen synthesis or GS activity is not
established. Previous studies have reported decreased (1, 2, 7, 14),
increased (11, 15, 20), or unchanged (12, 28, 32) insulin-stimulated glycogen synthesis or GS activity with elevated plasma FFA levels. The
cause of this discrepancy is presently unknown.
Several recent studies have demonstrated that the effects of FFA on
insulin-stimulated glucose metabolism are time dependent (2, 28).
Increased plasma FFA by lipid-heparin infusion replaced glucose as fuel
for oxidation within 1 h, but glucose uptake did not decrease until
2-4 h of lipid infusion (2). In a recent study, Kim et al. (16)
found a similar pattern of changes in insulin-stimulated glucose fluxes
in high-fat-fed rats; insulin-stimulated glycolysis was suppressed
within 2 days of high-fat feeding, whereas insulin-stimulated glucose
uptake decreased only after a prolonged period (>1 wk) of high-fat
feeding. Insulin-stimulated glycogen synthesis increased significantly
during the initial few days to compensate for the decrease in
glycolysis but subsequently decreased as insulin resistance manifested.
On the basis of these data, we postulated that suppression of
glycolysis and a compensatory increase in glycogen synthesis would
precede a decrease in insulin-stimulated glucose uptake in rats infused
with lipid emulsion and heparin. In this study, we examined effects of
intravenous infusion of lipid and heparin on insulin-stimulated glucose
metabolism in the whole body and in skeletal muscle of conscious rats.
Animals
ABSTRACT
Top
Abstract
Introduction
Methods
Results
Discussion
References
1 · min
1)
clamps were performed for 5 h in conscious rats with
(n = 8) or without
(n = 8) lipid-heparin infusion.
Glucose infusion rate required to maintain euglycemia was not different
between the two groups during the first 2 h of clamps but
became significantly lower with lipid-heparin infusion in the 3rd h and
thereafter. To investigate changes in intracellular glucose metabolism
during lipid-heparin infusion, additional clamps
(n = 8 each) were performed for 1, 2, 3, or 5 h with an infusion of
[3-3H]glucose.
Insulin-stimulated whole body glucose utilization
(Rd), glycolysis, and glycogen synthesis
were estimated on the basis of tracer concentrations in plasma during
the final 40 min of each clamp. Similar to changes in glucose infusion
rate, Rd was not different between the two
groups in the 1st and 2nd h but was significantly lower with
lipid-heparin infusion in the 3rd h and thereafter. Whole body
glycolysis was significantly lower with lipid-heparin infusion in all
time periods, i.e., 1st, 2nd, 3rd, and 5th h of clamps. In contrast,
whole body glycogen synthesis was higher with lipid-heparin infusion in
the 1st and 2nd h but lower in the 5th h. Similarly, accumulation of
[3H]glycogen
radioactivity in muscle glycogen was significantly higher with
lipid-heparin during the 1st and 2nd h but lower during the 3rd and 5th
h. Glucose 6-phosphate (G-6-P)
concentrations in gastrocnemius muscles were significantly higher with
lipid-heparin infusion throughout the clamps. Muscle glycogen synthase
(GS) activity was not altered with lipid-heparin infusion at 1, 2, and
3 h but was significantly lower at 5 h. Thus increased availability of
FFA significantly reduced whole body glycolysis, but compensatory increase in skeletal muscle glycogen synthesis in association with
accumulation of G-6-P masked this
effect, and Rd was not affected in the early
phase (within 2 h) of lipid-heparin infusion. Rd was reduced in the later phase (>2 h) of
lipid-heparin infusion, when glycogen synthesis was reduced in
association with reduced skeletal muscle GS activity.
INTRODUCTION
Top
Abstract
Introduction
Methods
Results
Discussion
References
RESEARCH DESIGN AND METHODS
Top
Abstract
Introduction
Methods
Results
Discussion
References
Table 1.
Age, body weight, plasma insulin, and FFA levels during glucose clamps
among different groups of animals
Catheterization
Five hours before the clamp experiments, food was removed from the cage, and each animal underwent a placement of catheters (PE-10, Intramedic, Clay Adams, Parsippany, NJ) into two tail veins and a tail artery for the purpose of infusion and blood sampling, respectively. Catheters were placed percutaneously during local anesthesia with lidocaine while animals were briefly restrained in a towel. Animals were returned to their cages after catheter placement, with tails secured as described above. Patency of arterial catheter was maintained by a slow (0.015 ml/min) infusion of saline.Euglycemic Hyperinsulinemic Clamps
Experiment A: effects of lipid-heparin infusion on
plasma FFA and glucose infusion rate.
Euglycemic hyperinsulinemic clamps were conducted for 5 h with
(n = 8) or without
(n = 8) an infusion of heparin (40 U/h
with 10 U as a priming bolus) and triglyceride emulsion (Intralipid; 10% wt/vol; 1.2 ml/h) to raise plasma FFA levels. Human insulin (Velosulin, Novo-Nordisk, Gentofte, Denmark) was infused at a rate of
86 pmol · kg1 · min
1
starting at time 0. Blood samples were
taken for glucose measurement at 10-min intervals, and 25% dextrose
was infused at variable rates to clamp plasma glucose at basal levels.
Blood samples for the measurements of insulin and FFA were obtained at
time 0 and at 1, 2, 3, 4, and 5 h. FFA
samples were collected in prechilled tubes containing EDTA and Paraoxon
(diethyl p-nitrophenyl phosphate, Sigma; a lipoprotein lipase inhibitor, 0.275 mg/ml of blood), immediately centrifuged, and stored at
70°C until analysis.
Experiment B: effects of lipid-heparin infusion on
insulin-stimulated whole body glucose fluxes and muscle glycogen
synthesis.
To investigate changes in whole body glucose fluxes during
lipid-heparin infusion, additional clamps were performed for 1, 2, 3, or 5 h with or without lipid-heparin infusion
(n = 8 each). Insulin was infused at a
rate of 86 pmol · kg1 · min
1.
A primed (20 µCi) and continuous (0.2 µCi/min) infusion of
[3-3H]glucose (NEN,
Boston, MA) was initiated at
120 min and continued throughout
the experiments. Blood samples for the determination of
[3H]glucose and
[3H]water specific
activity were obtained every 10 min during the final 40 min of each 1-, 2-, 3-, or 5-h clamp. Blood samples for the measurements of insulin and
FFA were obtained at the end of each 1-, 2-, 3-, or 5-h clamp.
Analytic Procedure
Plasma glucose was measured by the glucose oxidase method using a glucose analyzer (Beckman Instruments, Palo Alto, CA). Plasma FFA was measured by enzymatic assay using a kit from Eiken Chemical (Tokyo, Japan). Plasma insulin was measured by radioimmunoassay using kits for rat (basal insulin; Linco, St. Charles, MO) and human insulin (clamp insulin; Dainabott, Tokyo, Japan). Plasma [3H]glucose radioactivity was measured in duplicate by deproteinizing plasma samples with BaOH2 and ZnSO4, drying to eliminate tritiated water, and counting for 3H in a liquid scintillation spectrophotometer (Beckman). The plasma concentration of [3H]water was determined by the difference between 3H counts with and without drying.Skeletal muscle glucose 6-phosphate (G-6-P) was determined by an enzymatic assay as described by Michal (23). Because muscle G-6-P concentration may be sensitive to plasma glucose, glucose infusion was continued during the muscle sampling procedures to prevent any significant perturbation of plasma glucose concentration. Care was also taken to prevent G-6-P concentration from rising because of glycogenolysis during the procedures. Frozen muscles were crushed in liquid nitrogen and homogenized with 6% perchloric acid at 0°C.
Incorporation of [3H]glucose into muscle glycogen was measured to estimate de novo glycogen synthesis during the glucose clamps (29). Muscle samples were digested in 30% KOH at 100°C for 30 min and then incubated in 60% ethanol and 0.3% lithium bromide for 30 min at 0°C. The precipitates were washed twice with 60% ethanol and digested with amyloglucosidase (4). The digested solution was mixed with scintillation solution, and the radioactivity was measured on a liquid scintillation spectrophotometer. The amount of 3H in muscle glycogen was expressed as disintegrations per minute per gram of tissue wet weight. The rate of [3H]glycogen accumulation in muscle was estimated by the increment of 3H radioactivity in each time period between muscle sampling.
GS activity was measured according to the method of Golden et al. (10) with modifications. We used a superficial part of gastrocnemius muscle, which mainly consists of white muscle fibers, for the determination of GS activity, since we previously observed that superficial parts of gastrocnemius have higher GS activities than deep parts of the muscle (25). GS activity was expressed as the ratio of the activity in the absence of G-6-P to the activity at 10 mM G-6-P (GSI; GS independent of G-6-P) (13) or as the ratio of the activity at 0.1 mM G-6-P to the activity at 10 mM G-6-P (the fractional velocity of GS activity) (18). GSI and the fractional velocity of GS activity are indicators of the active form of GS and are believed to represent GS activity in vivo.
Isotopic Determination of Glucose Fluxes
Rates of total glucose appearance and whole body glucose uptake (Rd) were determined as the ratio of the [3H]glucose infusion rate (dpm/min) to the specific activity of plasma glucose (dpm/mol) during the final 40 min of the clamps (30). Whole body glycolysis was calculated from the rate of increase in plasma 3H2O concentration during the final 40 min of the clamps as previously described (29). The rate of increase in plasma 3H2O was determined by linear regression of the measurements at 10-min intervals during the final 40 min of the clamps. Whole body glycogen synthesis was estimated by subtracting whole body glycolysis from whole body glucose uptake, with the assumption that glycolysis and glycogen synthesis account for the majority of insulin-stimulated glucose uptake (29).Statistical Analysis
Data are presented as means ± SE. Statistical analysis was performed using the PC-SAS program (SAS Institute, Cary, NC). The significance of differences between the two experimental groups was assessed by ANOVA with repeated measures (experiment A) and by two-way ANOVA with the use of group (with 2 levels) and time (with 4 levels: 1st, 2nd, 3rd, and 5th h) as independent factors (experiment B). To test whether the differences between groups were time dependently different, we first checked the significance of interaction effects between group and time. If it turned out to be significant at the 5% level, then various contrasts for the detection of differential between-group effects by time were examined. To handle multiplicity (the total number was 6) in time-pair comparisons, Bonferroni adjustment for significance level was adopted. ![]() |
RESULTS |
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Experiment A: Effects of Lipid-Heparin Infusion on Plasma FFA and Glucose Infusion Rate
Plasma insulin was raised to ~2.1 nM, and plasma glucose was maintained at levels (~6.8 mM) similar to basal levels in both groups (Fig. 1, A and B). Plasma glucose and insulin concentrations were not different between the two groups. Plasma FFA levels decreased from 0.45 ± 0.03 mM in the basal state to 0.15 ± 0.03 mM during the control clamps. In contrast, plasma FFA levels increased threefold (from 0.43 ± 0.03 to 1.53 ± 0.04 mM) during the clamps with lipid-heparin infusion (P < 0.001 vs. control clamp; Fig. 1C).
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Glucose infusion rate (GIR) was almost identical between the two groups during the initial 2 h of the clamps (Fig. 1D). In the control clamps, GIR further increased to reach peak level at 3 h after the start of insulin infusion. In contrast, in the clamps with lipid-heparin infusion, GIR began to decrease at 2 h, resulting in significantly lower rates compared with the control clamps thereafter (P < 0.01).
Experiment B: Effects of Lipid-Heparin Infusion on Insulin-Stimulated Whole Body Glucose Fluxes and Muscle Glycogen Synthesis
Insulin-stimulated whole body glucose fluxes. Plasma insulin levels were raised to similar levels at the end of each 1-, 2-, 3-, or 5-h clamp (Table 1). There was no statistical difference among these values for each clamp and between the control and treatment groups. Plasma FFA levels at the end of the clamps with lipid-heparin infusion were all significantly higher than those in the control clamps (P < 0.001).
Similar to the changes of GIR, Rd (measured during the final 40 min of each clamp) was almost identical between the two groups in the 1st and 2nd h of clamps (Fig. 2A). In contrast, Rd was 40% lower with lipid-heparin infusion in the 3rd and 5th h (P < 0.01). Two-way ANOVA analysis revealed that contrasts of 1st vs. 3rd h, 1st vs. 5th h, 2nd vs. 3rd h, and 2nd vs. 5th h were significant (P < 0.001 each), which means that the lipid-heparin group had significantly lower mean Rd levels at later hours (4th and 5th h) compared with early hours (1st and 2nd h).
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Accumulation of [3H]glycogen in gastrocnemius muscle. The radioactivity of 3H in muscle glycogen was significantly higher with lipid-heparin infusion at 1 and 2 h (35,284 ± 2,450 vs. 9,924 ± 540 and 97,420 ± 3,520 vs. 39,499 ± 2,706 dpm/g wet wt; P < 0.05, respectively). The radioactivity of 3H in muscle glycogen was similar in the two groups at 3 and 5 h (Fig. 3D). The differences between groups were significantly higher at 1 and 2 h compared with 3 and 5 h (P < 0.001).
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Skeletal muscle G-6-P and GS activity. G-6-P concentrations in gastrocnemius muscle taken at 1, 2, 3, and 5 h of clamps were all significantly higher with lipid-heparin infusion than those in the control group (P < 0.05 at 1 h, P < 0.01 at 2, 3, and 5 h; Fig. 3A). GSI ratio and the fractional velocity of GS activity were not altered at 1, 2, and 3 h but were significantly lower at 5 h with lipid-heparin infusion (GSI ratio: 0.26 ± 0.03 vs. 0.32 ± 0.06; fractional velocity: 0.37 ± 0.04 vs. 0.44 ± 0.07; P < 0.05 for both) (Fig. 3, B and C). The differences in GSI ratio and fractional velocity between groups were significant between 1 and 2 h vs. 5 h (P < 0.01), respectively.
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DISCUSSION |
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In agreement with a previous study in humans (2), the present study showed that the GIR required to maintain euglycemia was not different between the lipid-heparin group and the control group during the first 2 h of the clamps but became significantly lower with lipid-heparin infusion in the 3rd h and thereafter. Additional experiments to investigate the changes in whole body glucose fluxes during lipid-heparin infusion disclosed that FFA had multiple effects on glucose metabolism that were time dependent (2, 17, 28). Although increased provision of FFA by intravenous lipid-heparin infusion rapidly (within 1 h) suppressed whole body glycolysis, insulin-stimulated whole body glucose uptake did not change until 3 h of lipid-heparin infusion. Of note in our study is that during an early period of lipid-heparin infusion, compensatory increase in skeletal muscle glycogen synthesis in association with accumulation of G-6-P counterbalanced the decrease in glycolysis, resulting in the lack of FFA effect on glucose uptake.
Profound inhibition of insulin-stimulated glycolysis with elevated plasma FFA levels is consistent with the glucose-fatty acid cycle (26). According to this hypothesis, elevated FFA oxidation increases intracellular acetyl-CoA and citrate concentrations. Acetyl-CoA suppresses glucose oxidation by inhibiting the activity of pyruvate dehydrogenase, the entry point of pyruvate into oxidative metabolism. Citrate decreases glycolysis by inhibiting phosphofructokinase (24), a key enzyme for glycolysis. Many studies in vivo confirmed the inhibition of glucose oxidation by elevated plasma FFA (2, 7, 9, 14). However, other studies have shown that inhibition of glucose oxidation by FFA does not necessarily lead to a decrease in glucose uptake. One explanation for this phenomenon would be shunting of glucose carbons from oxidation to production of lactate or alanine (14, 34). However, our data showing a reduced rate of whole body glycolysis, assessed by measuring [3H]water production from [3-3H]glucose, suggest that glycolysis is also suppressed by FFA at a step(s) proximal to triose isomerization between dihydroxyacetone phosphate and glyceraldehyde 3-phosphate, presumably the reaction catalyzed by phosphofructokinase (15).
Our results showed that insulin-stimulated glycogen synthesis was significantly increased in the early phase (within 2 h) of lipid-heparin infusion. This was consistently demonstrated with two independent measurements: rate of whole body glycogen synthesis, calculated as the difference between whole body glucose uptake and glycolysis, and incorporation of [3H]glucose into skeletal muscle glycogen. However, GS activity, assessed in vitro, was not different between the two groups in this early period. On the other hand, muscle G-6-P levels were significantly higher with lipid-heparin infusion. Because GS activity is dependent on the concentration of G-6-P, the increases in skeletal muscle and whole body glycogen synthesis appear to be due to increases in muscle G-6-P levels. Taken together, these results suggest that inhibition of glucose oxidation and glycolysis by FFA results in accumulation of G-6-P in skeletal muscle, which in turn leads to a stimulation of skeletal muscle glycogen synthesis. This compensatory increase in skeletal muscle glycogen synthesis in the early phase of lipid-heparin infusion appears to mask the effect of FFA on overall glucose metabolism.
In contrast to the effects in the early period, lipid-heparin infusion for a longer period (>2 h) induced a state of insulin resistance characterized by a decrease in both whole body glycolysis and glycogen synthesis. In agreement with previous studies (1, 2, 14), this was associated with a decrease in skeletal muscle GS activity. Delayed occurrence of these effects suggests that FFA do not inhibit GS activity directly but indirectly through secondary metabolic changes in the cells. The mechanism underlying FFA-induced inhibition of GS activity is presently not settled, but accumulation of certain metabolites, such as glycogen (8), long-chain acyl-CoA (33), or glucosamine pathway metabolites (5), has been implicated.
In the present study, muscle G-6-P levels were increased with lipid-heparin infusion throughout the hyperinsulinemic clamps. This may indicate that the impairments of glucose metabolism at sites distal to G-6-P (i.e., glycolysis or glycogen synthesis) are quantitatively more important than the impairments of more proximal fluxes such as glucose transport and phosphorylation. However, this study is limited in that FFA concentrations used in the present study were very high. Boden et al. (1) reported that, in humans, the mechanisms underlying the FFA effect to decrease insulin-stimulated glucose uptake were dependent on the concentration of FFA studied. An impairment of muscle GS activity was seen after 4-6 h of high FFA concentrations (~750 µM) and was associated with an increase in muscle G-6-P level. On the other hand, at lower FFA concentrations (~550 µM), a transport or phosphorylation defect, associated with a decrease in muscle G-6-P level, was the major defect and preceded changes in GS activity. Roden et al. (28) also indicated, on the basis of the nuclear magnetic resonance technology, that skeletal muscle G-6-P level was lower in human subjects given lipid-heparin infusion during hyperinsulinemic euglycemic clamps, suggesting that FFA may induce insulin resistance by inhibiting glucose transport and/or phosphorylation. However, in that study, plasma FFA levels were raised to a concentration of 2 mM, higher than those in the present study and that of Boden et al. The reasons for the discrepancy among these studies are presently unclear.
Another limitation of our study is that we only examined the effects of FFA on maximal insulin-stimulated glucose fluxes. It is now well established that insulin actions on different tissues are quite different according to insulin levels (19, 27). Thus hepatic glucose production is completely suppressed at plasma insulin levels much lower than the level which stimulates peripheral glucose utilization maximally. Similarly, FFA actions on different tissues are dependent on insulin levels. Elevated plasma FFA levels lead to the suppression of peripheral glucose utilization at higher insulin levels but lead to the enhancement of hepatic glucose production at lower insulin levels (9, 21). Because the plasma insulin levels during the clamps were very high, we could not evaluate FFA actions on tissues other than skeletal muscle.
In conclusion, the present study clearly demonstrates time-dependent effects of FFA on insulin-stimulated glucose metabolism. Increased provision of FFA rapidly inhibited glucose oxidation and glycolysis in skeletal muscle and in the whole body. Consequent increase in intracellular G-6-P stimulated glycogen synthesis in skeletal muscle during the early phase of lipid-heparin infusion, and insulin-stimulated glucose uptake did not change significantly because of this compensatory increase in glycogen synthesis. On the other hand, insulin-stimulated glucose uptake was reduced in the later phase (>2 h) of lipid-heparin infusion, when glycogen synthesis was reduced in association with reduced GS activity.
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ACKNOWLEDGEMENTS |
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We thank Dr. M. H. Huh (Dept. of Statistics, Koryo Univ., Seoul, Korea) for assistance in statistical analysis and Dr. J. H. Youn (Dept. of Physiology and Biophysics, Univ. of Southern California) for critically reading the manuscript. We also thank H. S. Park, S. H. Shin, and N. M. Kim for technical assistance.
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FOOTNOTES |
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This work was supported by a grant from Asan Institute for Life Sciences (97-006) and a grant from BIOTECH 2000 of National R & D Program (96-NB-02-09-A-03), Ministry of Science and Technology, Seoul, Korea.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Address for reprint requests: K.-U. Lee, Dept. of Internal Medicine, Asan Medical Center, Univ. of Ulsan College of Medicine, 388-1 Poong-Nap Dong, Song-Pa Ku, Seoul 138-736, Korea.
Received 3 February 1998; accepted in final form 29 April 1998.
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REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Boden, G.,
X. Chen,
J. Ruiz,
J. V. White,
and
L. Rossetti.
Mechanism of fatty acid-induced inhibition of glucose uptake.
J. Clin. Invest.
93:
2438-2446,
1994[Medline].
2.
Boden, G.,
F. Jadali,
J. White,
Y. Liang,
M. Mozzoli,
X. Chen,
E. Coleman,
and
C. Smith.
Effects of fat on insulin-stimulated carbohydrate metabolism in normal men.
J. Clin. Invest.
88:
960-966,
1991[Medline].
3.
Buchanan, T. A.,
G. F. Sipos,
M. Madrilejo,
C. Liu,
and
V. M. Campese.
Hypertension without peripheral insulin resistance in spontaneously hypertensive rats.
Am. J. Physiol.
262 (Endocrinol. Metab. 25):
E14-E19,
1992
4.
Chan, T. M.,
and
J. H. Exton.
A rapid method for the determination of glycogen content and radioactivity in small quantities of tissue or isolated hepatocytes.
Anal. Biochem.
71:
96-105,
1976[Medline].
5.
Crook, E. D.,
J. Zhou,
M. Daniels,
J. L. Neidigh,
and
D. A. McClain.
Regulation of glycogen synthase by glucose, glucosamine, and glutamine: fructose-6-phosphate aminotransferase.
Diabetes
44:
314-320,
1995[Abstract].
6.
Damsbo, P.,
A. Vaag,
O. Hother-Nielsen,
and
H. Beck-Nielsen.
Reduced glycogen synthase activity in skeletal muscle from obese patients with and without type 2 (non-insulin-dependent) diabetes mellitus.
Diabetologia
34:
239-245,
1991[Medline].
7.
Felber, J. P.,
E. Ferrannini,
A. Golay,
H. U. Meyer,
D. Theibaud,
B. Curchod,
E. Maeder,
E. Jequier,
and
R. A. DeFronzo.
Role of lipid oxidation in pathogenesis of insulin resistance of obesity and type II diabetes.
Diabetes
36:
1341-1350,
1987[Abstract].
8.
Felber, J. P.,
E. Haesler,
and
E. Jequier.
Metabolic origin of insulin resistance in obesity with and without type 2 (non-insulin-dependent) diabetes mellitus.
Diabetologia
36:
1221-1229,
1993[Medline].
9.
Ferrannini, E.,
E. J. Barrett,
S. Bevilacqua,
and
R. A. DeFronzo.
Effect of fatty acids on glucose production and utilization in man.
J. Clin. Invest.
72:
1737-1747,
1983[Medline].
10.
Golden, S.,
P. A. Wals,
and
J. Katz.
An improved procedure for the assay of glycogen synthase and phosphorylase in rat liver homogenates.
Anal. Biochem.
77:
436-445,
1977[Medline].
11.
Jenkins, A. B.,
L. H. Storlien,
D. J. Chisholm,
and
E. W. Kraegen.
Effects of nonesterified fatty acid availability on tissue-specific glucose utilization in rats in vivo.
J. Clin. Invest.
82:
293-299,
1988[Medline].
12.
Johnson, A. B.,
M. Argyraki,
J. C. Thow,
B. G. Cooper,
G. Fulcher,
and
R. Taylor.
Effect of increased free fatty acid supply on glucose metabolism and skeletal muscle glycogen synthase activity in normal man.
Clin. Sci. (Colch.)
82:
219-226,
1992[Medline].
13.
Johnson, A. B.,
M. Argyraki,
J. C. Thow,
I. R. Jones,
D. Broughton,
M. Miller,
and
R. Taylor.
Impaired activation of skeletal muscle glycogen synthase in non-insulin-dependent diabetes mellitus is unrelated to the degree of obesity.
Metabolism
40:
252-260,
1991[Medline].
14.
Kelley, D. E.,
M. Mokan,
J.-A. Simoneau,
and
L. J. Mandarino.
Interaction between glucose and free fatty acid metabolism in human skeletal muscle.
J. Clin. Invest.
92:
91-98,
1993[Medline].
15.
Kim, J. K.,
J. K. Wi,
and
J. H. Youn.
Plasma free fatty acids decrease insulin-stimulated skeletal muscle glucose uptake by suppressing glycolysis in conscious rats.
Diabetes
45:
446-453,
1996[Abstract].
16.
Kim, J. K.,
J. K. Wi,
and
J. H. Youn.
Metabolic impairment precedes insulin resistance in skeletal muscle during high-fat feeding in rats.
Diabetes
45:
651-658,
1996[Abstract].
17.
Kim, J. K.,
and
J. H. Youn.
Prolonged suppression of glucose metabolism causes insulin resistance in rat skeletal muscle.
Am. J. Physiol.
272 (Endocrinol. Metab. 35):
E288-E296,
1997
18.
Kochan, R. G.,
D. R. Lamb,
S. A. Lutz,
C. V. Perrill,
E. M. Reimann,
and
K. K. Schlender.
Glycogen synthase activation in human skeletal muscle: effects of diet and exercise.
Am. J. Physiol.
236 (Endocrinol. Metab. Gastrointest. Physiol. 5):
E660-E666,
1979
19.
Kraegen, E. W.,
D. E. James,
A. B. Jenkins,
and
D. J. Chisholm.
Dose-response curves for in vivo insulin sensitivity in individual tissues in rats.
Am. J. Physiol.
248 (Endocrinol. Metab. 11):
E353-E362,
1985
20.
Kruszynska, Y. T.,
J. G. McCormack,
and
N. McIntyre.
Effects of non-esterified fatty acid availability on insulin-stimulated glucose utilisation and tissue pyruvate dehydrogenase activity in the rat.
Diabetologia
33:
396-402,
1990[Medline].
21.
Lee, K. U.,
H. K. Lee,
C. S. Koh,
and
H. K. Min.
Artificial induction of intravascular lipolysis by lipid-heparin infusion leads to insulin resistance in man.
Diabetologia
31:
285-290,
1988[Medline].
22.
Lee, K. U.,
J. Y. Park,
C. H. Kim,
S. K. Hong,
K. I. Suh,
K. S. Park,
and
S. W. Park.
Effect of decreasing plasma free fatty acid by acipimox on hepatic glucose metabolism in normal rats.
Metabolism
45:
1408-1414,
1996[Medline].
23.
Michal, G.
D-Glucose 6-phosphate and D-fructose 6-phosphate.
In: Methods of Enzymatic Analysis, edited by H. U. Bergmeyer. Wenheim, Germany: VCH Verlagsgesellschaft, 1985, vol. 6, p. p.191-198.
24.
Newsholme, E. A.,
P. H. Sugden,
and
T. Williams.
Effects of citrate on the activities of phosphofructokinase from nervous and muscle tissue from different animals and its relationship to the regulation of glycolysis.
Biochem. J.
166:
123-129,
1977[Medline].
25.
Park, S. W.,
K. I. Suh,
J. H. Kim,
H. S. Park,
Y. J. Jang,
and
K. U. Lee.
The effect of insulin on glycogen synthase activity in individual skeletal muscle in rat.
J. Korean Diabetes Ass.
15:
35-40,
1991.
26.
Randle, P. J.,
P. B. Garland,
C. N. Hales,
and
E. A. Newsholme.
The glucose fatty acid cycle: its role in insulin sensitivity and the metabolic disturbances of diabetes mellitus.
Lancet
I:
785-789,
1963.
27.
Rizza, R. A.,
L. J. Mandarino,
and
J. E. Gerich.
Dose-response characteristics for effects of insulin on production and utilization of glucose in man.
Am. J. Physiol.
240 (Endocrinol. Metab. 3):
E630-E639,
1981
28.
Roden, M.,
T. B. Price,
G. Perseghin,
K. F. Petersen,
D. L. Rothman,
G. W. Cline,
and
G. I. Shulman.
Mechanism of free fatty acid-induced insulin resistance in humans.
J. Clin. Invest.
97:
2859-2865,
1996
29.
Rossetti, L.,
Y. T. Lee,
J. Ruiz,
S. C. Aldridge,
H. Shamoon,
and
G. Boden.
Quantitation of glycolysis and skeletal muscle glycogen synthesis in humans.
Am. J. Physiol.
265 (Endocrinol. Metab. 28):
E761-E769,
1993
30.
Steele, R.
Influence of glucose loading and of injected insulin on hepatic glucose output.
Ann. NY Acad. Sci.
82:
420-430,
1959.
31.
Thorburn, A. W.,
B. Gumber,
F. Bulacan,
P. Wallace,
and
R. R. Henry.
Intracellular glucose oxidation and glycogen synthase activity are reduced in non-insulin-dependent (type II) diabetes independent of impaired glucose uptake.
J. Clin. Invest.
85:
522-529,
1990[Medline].
32.
Walker, M.,
G. R. Fulcher,
C. F. Sum,
H. Orskov,
and
K. G. M. M. Alberti.
Effect of glycemia and nonesterified fatty acids on forearm glucose uptake in normal humans.
Am. J. Physiol.
261 (Endocrinol. Metab. 24):
E304-E311,
1991
33.
Wititsuwannakul, D.,
and
K. H. Kim.
Mechanism of palmityl coenzyme A inhibition of liver glycogen synthase.
J. Biol. Chem.
252:
7812-7817,
1977[Abstract].
34.
Yki-Jarvinen, H.,
I. Puhakainen,
and
V. A. Koivisto.
Effect of free fatty acids on glucose uptake and non-oxidative glycolysis across human forearm tissues in the basal state and during insulin stimulation.
J. Clin. Endocrinol. Metab.
72:
1268-1277,
1991[Abstract].