Plasma sphingolipids are biomarkers of metabolic syndrome in non-human primates maintained on a Western-style diet

BACKGROUND

The intake of a Western diet enriched in animal fat has been shown to be a major risk factor for Type 2 diabetes and obesity. Previous rodent studies have indicated that these conditions may be triggered by the accumulation of the sphingolipid ceramide in insulin-sensitive tissues. However, data are lacking in this regard from both humans and non-human primates.

OBJECTIVE

Here we have investigated the relationship between plasma ceramides and metabolic syndrome in Rhesus macaques fed a high-fat and high-fructose (HFFD) 'western' diet.

METHODS

We investigated this relationship in cohorts of monkeys fed a HFFD for a period of 8 months to 5 years. Animals were classified as control, pre-diabetic or diabetic based on fasting plasma parameters and insulin sensitivity.

RESULTS

HFFD treatment produced significant increases in body weight and body fat and also resulted in a decline in insulin sensitivity. In parallel to the reduction in insulin sensitivity, significant increases in both plasma ceramide and dihydroceramide levels were observed, which further increased as animals progressed to the diabetic state. Plasma levels of the rare sphingolipid C18:0 deoxysphinganine, a marker of increased metabolic flux through serine palmitoyl transferase (SPT), were also elevated in both pre- and diabetic animals. Furthermore, plasma serine levels were significantly elevated in diabetic monkeys, which may indicate a shift in SPT substrate selectivity from serine to alanine or glycine. In contrast, branch chain amino acids were unchanged in pre-diabetic non-human primates, and only plasma valine levels were elevated in diabetic animals.

CONCLUSION

Together, these data indicate that HFFD induces de novo synthesis of ceramides in non-human primates, and that increased production of plasma ceramides is significantly correlated with the decline in insulin sensitivity.

Metabolic and anthropometric factors related to skeletal muscle UCP3 gene expression in healthy human adults

This cross-sectional investigation sought to determine the relationship between selected metabolic, endocrine, and anthropometric factors and skeletal muscle UCP3 mRNA in healthy adult humans. Twenty-four healthy adults (13 male and 11 female) across a range of aerobic capacity, age, and body composition were studied. Muscle biopsies were obtained from the vastus lateralis, from which UCP3 mRNA was quantified by Northern blot, and fiber type was determined by use of the myosin ATPase staining procedure. In addition, resting energy expenditure and maximum rate of oxygen consumption were determined by indirect calorimetry, body composition was determined by dual-energy X-ray absorptiometry, and fasting plasma leptin and insulin were determined by ELISA. UCP3 mRNA was correlated positively with the percent type I fibers (r = 0.842, P < 0.001), plasma leptin (r = 0.454, P = 0.026), and plasma insulin (r = 0.615, P < 0.001) and inversely to age (r = -0.411, P = 0.046). Stepwise multiple regression analysis determined that percent type I muscle fibers was the best predictor of vastus lateralis UCP3 mRNA, and no other variable entered the equation (model r(2) = 0.66). This study suggests that of the variables measured, UCP3 mRNA is primarily related to skeletal muscle fiber type in healthy adults. The factors that contribute to fiber-specific differences in UCP3 mRNA expression will need to be examined in future studies.

A role for AMP-activated protein kinase in contraction- and hypoxia-regulated glucose transport in skeletal muscle

Eukaryotic cells possess systems for sensing nutritional stress and inducing compensatory mechanisms that minimize the consumption of ATP while utilizing alternative energy sources. Such stress can also be imposed by increased energy needs, such as in skeletal muscle of exercising animals. In these studies, we consider the role of the metabolic sensor, AMP-activated protein kinase (AMPK), in the regulation of glucose transport in skeletal muscle. Expression in mouse muscle of a dominant inhibitory mutant of AMPK completely blocked the ability of hypoxia or AICAR to activate hexose uptake, while only partially reducing contraction-stimulated hexose uptake. These data indicate that AMPK transmits a portion of the signal by which muscle contraction increases glucose uptake, but other AMPK-independent pathways also contribute to the response.

GLUT4 overexpression in db/db mice dose-dependently ameliorates diabetes but is not a lifelong cure

We previously reported that overexpression of GLUT4 in lean, nondiabetic C57BL/KsJ-lepr(db/+) (db/+) mice resulted in improved glucose tolerance associated with increased basal and insulin-stimulated glucose transport in isolated skeletal muscle. We used the diabetic (db/db) litter mates of these mice to examine the effects of GLUT4 overexpression on in vivo glucose utilization and on in vitro glucose transport and GLUT4 translocation in diabetic mice. We examined in vivo glucose disposal by oral glucose challenge and hyperinsulinemic-hyperglycemic clamps. We also evaluated the in vitro relationship between glucose transport activity and cell surface GLUT4 levels as assessed by photolabeling with the membrane-impermeant reagent 2-N-(4-(1-azi-2,2,2-trifluoroethyl)benzoyl)-1,3-bis(D-mannose-4-yloxy)-2-propylamine in extensor digitorum longus (EDL) muscles. All parameters were examined as functions of animal age and the level of GLUT4 overexpression. In young mice (age 10-12 weeks), both lower (two- to threefold) and higher (four- to fivefold) levels of GLUT4 overexpression were associated with improved glucose tolerance compared to age-matched nontransgenic (NTG) mice. However, glucose tolerance deteriorated with age in db/db mice, although less rapidly in transgenic mice expressing the higher level of GLUT4. Glucose infusion rates during hyperinsulinemic-hyperglycemic clamps were increased with GLUT4 overexpression, compared with NTG mice in both lower and higher levels of GLUT4 overexpression, even in the older mice. Surprisingly, isolated EDL muscles from diabetic db/db mice did not exhibit alterations in either basal or insulin-stimulated glucose transport activity or cell surface GLUT4 compared to nondiabetic db/+ mice. Furthermore, both GLUT4 overexpression levels and animal age are associated with increased basal and insulin-stimulated glucose transport activities and cell surface GLUT4. However, the observed increased glucose transport activity in older db/db mice was not accompanied by an equivalent increase in cell surface GLUT4 compared to younger animals. Thus, although in vivo glucose tolerance is improved with GLUT4 overexpression in young animals, it deteriorates with age; in contrast, insulin responsiveness as assessed by the clamp technique remains improved with GLUT4 overexpression, as does in vitro insulin action. In summary, despite an impairment in whole-body glucose tolerance, skeletal muscle of the old transgenic GLUT4 db/db mice is still insulin responsive in vitro and in vivo.

Transient enhancement of GLUT-4 levels in rat epitrochlearis muscle after exercise training

The purpose of the present study was to examine the effect of detraining on the glucose transport system after short-term swim training (5 days), long-term swim training (5 wk), and treadmill run training (5 wk). Skeletal muscles were isolated from female Wistar rats at 24 or 48 h posttraining. SST produces a 48% increase in GLUT-4 mRNA, a 30% increase in GLUT-4 protein, and a 60% increase in insulin-stimulated glucose transport activity at 24 h posttraining but not at 48 h posttraining. Similar to SST, long-term swim training produces a 60% increase in GLUT-4 mRNA and a 30% increase in GLUT-4 protein content at 24 h posttraining but not at 48 h posttraining. Finally, treadmill run training produces a transient 35% increase in GLUT-4 protein content that is completely reversed at 48 h after the last bout of exercise. These results demonstrate that the increase in GLUT-4 mRNA and GLUT-4 protein occurs during the first week of exercise training and is rapidly lost after training cessation. We believe that the transient enhancement in GLUT-4 protein after exercise training is due to a short GLUT-4 half-life, a process that is primarily regulated by pretranslational mechanisms.

1-[N, O-bis-(5-isoquinolinesulphonyl)-N-methyl-L-tyrosyl]-4- phenylpiperazine (KN-62), an inhibitor of calcium-dependent camodulin protein kinase II, inhibits both insulin- and hypoxia-stimulated glucose transport in skeletal muscle

Previous studies have indicated a role for calmodulin in hypoxia-and insulin-stimulated glucose transport. However, since calmodulin interacts with multiple protein targets, it is unknown which of these targets is involved in the regulation of glucose transport. In the present study, we have used the calcium-dependent calmodulin protein kinase II (CAMKII) inhibitor 1-[N, O-bis-(5-isoquinolinesulphonyl) -N-methyl-L-tyrosyl]-4-phenylpiperazine (KN-62) to investigate the possible role of this enzyme in the regulation of glucose transport in isolated rat soleus and epitrochlearis muscles. KN-62 did not affect basal 2-deoxyglucose transport, but it did inhibit both insulin- and hypoxia-stimulated glucose transport activity by 46 and 40% respectively. 1-[N,O-Bis-(1, 5-isoquinolinesulphonyl)-N-methyl-l-tyrosyl]-4-phenylpiperazine (KN-04), a structural analogue of KN-62 that does not inhibit CAMKII, had no effect on hypoxia-or insulin-stimulated glucose transport. Accordingly, KN-62 decreased the stimulated cell-surface GLUT4 labelling by a similar extent as the inhibition of glucose transport (insulin, 49% and hypoxia, 54%). Additional experiments showed that KN-62 also inhibited insulin- and hypoxia-stimulated transport by 37 and 40% respectively in isolated rat epitrochlearis (a fast-twitch muscle), indicating that the effect of KN-62 was not limited to the slow-twitch fibres of the soleus. The inhibitory effect of KN-62 on hypoxia-stimulated glucose transport appears to be specific to CAMKII, since KN-62 did not inhibit hypoxia-stimulated 45Ca efflux from muscles pre-loaded with 45Ca, or hypoxia-stimulated glycogen breakdown. Additionally, KN-62 affected neither insulin-stimulated phosphoinositide 3-kinase nor Akt activity, suggesting that the effects of KN-62 are not due to non-specific effects of this inhibitor on these regions of the insulin-signalling cascade. The results of the present study suggest that CAMKII might have a distinct role in insulin- and hypoxia-stimulated glucose transport, possibly in the vesicular trafficking of GLUT4.

Calorie restriction increases cell surface GLUT-4 in insulin-stimulated skeletal muscle

Reduced calorie intake [calorie restriction (CR); 60% of ad libitum (AL)] leads to enhanced glucose transport without altering total GLUT-4 glucose transporter abundance in skeletal muscle. Therefore, we tested the hypothesis that CR (20 days) alters the subcellular distribution of GLUT-4. Cell surface GLUT-4 content was higher in insulin-stimulated epitrochlearis muscles from CR vs. AL rats. The magnitude of this increase was similar to the CR-induced increase in glucose transport, and GLUT-4 activity (glucose transport rate divided by cell surface GLUT-4) was unaffected by diet. The CR effect was specific to the insulin-mediated pathway, as evidenced by the observations that basal glucose transport and cell surface GLUT-4 content, as well as hypoxia-stimulated glucose transport, were unchanged by diet. CR did not alter insulin's stimulation of insulin receptor substrate (IRS)-1-associated phosphatidylinositol 3-kinase (PI3K) activity. Muscle abundance of IRS-2 and p85 subunit of PI3K were unaltered by diet, but IRS-1 content was lower in CR vs. AL. These data demonstrate that, despite IRS-1-PI3K activity similar to AL, CR specifically increases insulin's activation of glucose transport by enhancing the steady-state proportion of GLUT-4 residing on the cell surface.

Short-term exercise enhances insulin-stimulated GLUT-4 translocation and glucose transport in adipose cells

This investigation examined the effects of short-term exercise training on insulin-stimulated GLUT-4 glucose transporter translocation and glucose transport activity in rat adipose cells. Male Wistar rats were randomly assigned to a sedentary (Sed) or swim training group (Sw, 4 days; final 3 days: 2 x 3 h/day). Adipose cell size decreased significantly but minimally (approximately 20%), whereas total GLUT-4 increased by 30% in Sw vs. Sed rats. Basal 3-O-methyl-D-[14C]glucose transport was reduced by 62%, whereas maximally insulin-stimulated (MIS) glucose transport was increased by 36% in Sw vs. Sed rats. MIS cell surface GLUT-4 photolabeling was 44% higher in the Sw vs. Sed animals, similar to the increases observed in MIS glucose transport activity and total GLUT-4. These results suggest that increases in total GLUT-4 and GLUT-4 translocation to the cell surface contribute to the increase in MIS glucose transport with short-term exercise training. In addition, the results suggest that the exercise training-induced adaptations in glucose transport occur more rapidly than previously thought and with minimal changes in adipose cell size.

Insulin, but not contraction, activates Akt/PKB in isolated rat skeletal muscle

Insulin and muscle contraction potently stimulate glucose uptake in mammalian skeletal muscle. Studies in muscle and adipose tissue have shown that insulin induces its receptor-dependent phosphorylation of insulin receptor substrates 1 and 2, which leads to activation of polyphosphatidylinositol (PI) 3'-kinase. In contrast, muscle contraction stimulates glucose transport via a mechanism that is independent of insulin, but the two pathways may converge downstream at the level of stimulation of GLUT4 translocation. In the present study, we have examined the role of Akt, an insulin-activated serine threonine kinase that has previously been shown to increase glucose transport in adipocytes. Either insulin or in vitro muscle contraction significantly elevated glucose transport in isolated rat epitrochlearis and soleus muscles. However, Akt kinase activity was significantly stimulated by insulin and not contraction. Moreover, wortmannin, an inhibitor of PI 3'-kinase, completely blocked the insulin-stimulated increase in Akt activity and glucose transport but did not alter either of these parameters in contracting muscles. The increases in Akt activity were paralleled by a decrease in the electrophoretic mobility of Akt, indicative of phosphorylation of Akt by an upstream kinase. These changes in Akt mobility appeared to be at least partially because of phosphorylation of Akt on serine 473. A putative downstream target of Akt, p70 S6 kinase, showed similar changes in mobility in response to insulin but not contraction. These data support the view that Akt is a downstream target of PI 3'-kinase and is involved in the signaling pathways involved in insulin but not contraction stimulation of glucose transport in skeletal muscle. These data provide further evidence that two distinct pathways exist for the stimulation of glucose transport in mammalian skeletal muscle.

Mechanism of hypoxia-stimulated glucose transport in rat skeletal muscle: potential role of glycogen

We have previously reported that exercise training is associated with enhanced insulin-stimulated glucose transport activity and inhibited hypoxia-stimulated glucose transport activity in rat epitrochlearis muscle. Here we examine the potential role of muscle glycogen in the inhibited glucose transport response to hypoxia. Three days of swim training (2 x 3 h/day) produce a 100% increase in glycogen and a 70% increase in GLUT-4 in epitrochlearis muscle. Glucose transport after 1 h of hypoxia in muscles from fed exercise-trained (ET) rats is not significantly elevated above basal and is 40% lower than that in muscles from fed sedentary (SED) rats. Glycogen levels after 1 h of hypoxia are reduced by 27 and 64% in muscles from fed ET and fed SED rats, respectively. After 2 h of hypoxia, glucose transport is significantly increased above basal in muscles from fed ET rats, but this response is still 55% lower than that in muscles from fed SED rats. After 2 h of hypoxia, glycogen is reduced by 50 and 83% in muscles from fed ET and fed SED rats, respectively. After a modified overnight fast (approximately 4.5 g of chow), the glucose transport and glycogen responses to 1 h of hypoxia are not significantly different between muscles from ET and SED rats. These findings demonstrate a strong inverse relationship between glycogen and hypoxia-stimulated glucose transport activity and that high levels of glycogen contribute to the inhibited glucose transport response to hypoxia. Furthermore, failure of the overexpression of GLUT-4 after exercise training to enhance the glucose transport response to contraction/hypoxia suggests selective targeting of the additional GLUT-4 to the insulin-responsive pool.

Effects of exercise training on glucose transport and cell surface GLUT-4 in isolated rat epitrochlearis muscle

The effects of exercise training on maximal glucose transport activity and cell surface GLUT-4 were examined in rat epitrochlearis muscle. Five days of swim training (2 x 3 h/day) produce a significant increase in citrate synthase activity (24.5 +/- 0.6 vs. 20.1 +/- 0.7 micromol x min(-1) x g(-1)), GLUT-4 content (22.9 +/- 0.8 vs. 17.4 +/- 0.4% GLUT-4 standard), and glycogen levels (54.3 +/- 9.4 vs. 28.6 +/- 9.4 micromol/g). Maximally, insulin-stimulated glucose transport activity and cell surface GLUT-4 are increased by 55 (1.50 +/- 0.11 vs. 0.97 +/- 0.10 micromol x ml(-1) x 20 min(-1)) and 48% [12.0 +/- 0.8 vs. 8.1 +/- 0.9 disintegrations x min(-1) (dpm) x mg(-1)], respectively, in exercise-trained epitrochlearis muscles. In contrast, hypoxia-stimulated glucose transport activity and cell surface GLUT-4 are reduced by 38 (0.78 +/- 0.08 vs.1.25 +/- 0.14 micromol x ml(-1) x 20 min(-1)) and 40% (5.7 +/- 0.9 vs. 9.4 +/- 1.2 dpm/mg), respectively, in exercise-trained epitrochlearis muscles. These results demonstrate that changes in insulin- and hypoxia-stimulated glucose transport activity after exercise training are fully accounted for by the appearance of cell surface GLUT-4 and support the concept of two intracellular pools of GLUT-4. Finally, we propose that high levels of muscle glycogen with exercise training may contribute to the decrease in hypoxia-stimulated glucose transport activity.

Regulation of cell surface GLUT4 in skeletal muscle of transgenic mice

Marked overexpression of the glucose transporter GLUT4 in skeletal muscle membrane fractions of GLUT4 transgenic (TG) mice is accompanied by disproportionately small increases in basal and insulin-stimulated glucose transport activity. Thus we have assessed cell surface GLUT4 by photolabelling with the membrane-impermeant reagent 2-N-[4-(1-azi-2,2,2-trifluoroethyl)benzoyl]-1, 3-bis(D-mannos-4-yloxy)-2-propylamine (ATB-BMPA) and measured the corresponding glucose transport activity using 2-deoxyglucose in isolated extensor digitorum longus (EDL) muscles from non-transgenic (NTG) and GLUT4 TG mice in the absence and presence of 13.3 nM (2000 mu units/ml) insulin, without or with hypoxia as a model of muscle contraction. TG mice displayed elevated rates of glucose transport activity under basal and insulin-stimulated conditions, and in the presence of insulin plus hypoxia, compared with NTG mice. Photoaffinity labelling of cell surface GLUT4 indicated corresponding elevations in plasma membrane GLUT4 in the basal and insulin-stimulated states, and with insulin plus hypoxia, but no difference in cell surface GLUT4 during hypoxia stimulation. Subcellular fractionation of hindlimb muscles confirmed the previously observed 3-fold overexpression of GLUT4 in the TG compared with the NTG mice. These results suggest that: (1) alterations in glucose transport activity which occur with GLUT4 overexpression in EDL muscles are directly related to cell surface GLUT4 content, regardless of the levels observed in the corresponding subcellular membrane fractions, (2) while overexpression of GLUT4 influences both basal and insulin-stimulated glucose transport activity, the response to hypoxia/ contraction-stimulated glucose transport is unchanged, and (3) subcellular fractionation provides little insight into the subcellular trafficking of GLUT4, and whatever relationship is demonstrated in EDL muscles from NTG mice is disrupted on GLUT4 overexpression.

Glucose transport and GLUT4 protein distribution in skeletal muscle of GLUT4 transgenic mice

The aim of the present investigation was to determine whether the subcellular distribution and insulin-stimulated translocation of the GLUT4 isoform of the glucose transporter are affected when GLUT4 is overexpressed in mouse skeletal muscle, and if the overexpression of GLUT4 alters maximal insulin-stimulated glucose transport and metabolism. Rates of glucose transport and metabolism were assessed by hind-limb perfusion in GLUT4 transgenic (TG) mice and non-transgenic (NTG) controls. Glucose-transport activity was determined under basal (no insulin), submaximal (0.2 m-unit/ml) and maximal (10 m-units/ml) insulin conditions using a perfusate containing 8 mM 3-O-methyl-D-glucose. Glucose metabolism was quantified by perfusing the hind limbs for 25 min with a perfusate containing 8 mM glucose and 10 m-units/ml insulin. Under basal conditions, there was no difference in muscle glucose transport between TG (1.10 +/- 0.10 mumol/h per g; mean +/- S.E.M.) and NTG (0.93 +/- 0.16 mumol/h per g) mice. However, TG mice displayed significantly greater glucose-transport activity during submaximal (4.42 +/- 0.49 compared with 2.69 +/- 0.33 mumol/h per g) and maximal (11.68 +/- 1.13 compared with 7.53 +/- 0.80 mumol/h per g) insulin stimulation. Nevertheless, overexpression of the GLUT4 protein did not alter maximal rates of glucose metabolism. Membrane purification revealed that, under basal conditions, plasma-membrane (approximately 12-fold) and intracellular-membrane (approximately 4-fold) GLUT4 protein concentrations were greater in TG than NTG mice. Submaximal insulin stimulation did not increase plasma-membrane GLUT4 protein concentration whereas maximal insulin stimulation increased this protein in both NTG (4.1-fold) and TG (2.6-fold) mice. These results suggest that the increase in insulin-stimulated glucose transport following overexpression of the GLUT4 protein is limited by factors other than the plasma-membrane GLUT4 protein concentration. Furthermore, GLUT4 overexpression is not coupled to glucose-metabolic capacity.

Glucose uptake and GLUT-4 protein distribution in skeletal muscle of the obese Zucker rat

The rates of muscle glucose uptake of lean and obese Zucker rats were assessed by hindlimb perfusion under basal conditions (no insulin), in the presence of a maximally stimulating concentration of insulin (10 mU/ml), and after muscle contraction elicited by electrical stimulation of the sciatic nerve. After perfusion, plasma and microsomal membranes were isolated from selected hindlimb muscles for determination of GLUT-4 protein distribution. Under basal conditions, rates of glucose uptake were similar for lean and obese rats despite plasma membranes from lean rats containing 82% more GLUT-4 protein than obese rats. Insulin stimulation resulted in significant increases in plasma membrane GLUT-4 protein concentration in lean but not obese rats. Glucose uptake of lean rats (35.3 +/- 4.7 mumol.h-1.g-1) in the presence of insulin was approximately fourfold greater than that of obese rats (8.8 +/- 1.3 mumol.h-1.g-1), but this difference in glucose uptake could not be completely accounted for by the difference in plasma membrane GLUT-4 protein concentration. Stimulation by contraction resulted in significant increases in plasma membrane GLUT-4 protein concentration in both lean and obese rats and similar rates of glucose uptake. These results suggest that the muscle insulin resistance of the obese Zucker rat is due to 1) a reduced plasma membrane GLUT-4 protein concentration, which results in part from an impairment in the insulin-stimulated GLUT-4 protein translocation process, and 2) a defect in the insulin-stimulated activation of this protein. However, contraction-stimulated glucose uptake, GLUT-4 protein translocation, and activation are normal in the obese Zucker rat.

The effects of muscle contraction and insulin on glucose-transporter translocation in rat skeletal muscle

The effect of electrically induced muscle contraction, insulin (10 m-units/ml) and electrically-induced muscle contraction in the presence of insulin on insulin-regulatable glucose-transporter (GLUT-4) protein distribution was studied in female Sprague-Dawley rats during hindlimb perfusion. Plasma-membrane cytochalasin B binding increased approximately 2-fold, whereas GLUT-4 protein concentration increased approximately 1.5-fold above control with contractions, insulin, or insulin + contraction. Microsomal-membrane cytochalasin B binding and GLUT-4 protein concentration decreased by approx. 30% with insulin or insulin + contraction, but did not significantly decrease with contraction alone. The rate of muscle glucose uptake was assessed by determining the rate of 2-deoxy[3H]glucose accumulation in the soleus, plantaris, and red and white portions of the gastrocnemius. Both contraction and insulin increased glucose uptake significantly and to the same degree in the muscles examined. Insulin + contraction increased glucose uptake above that of insulin or contraction alone, but this effect was only statistically significant in the soleus, plantaris and white gastrocnemius. The combined effects of insulin + contraction of glucose uptake were not fully additive in any of the muscles investigated. These results suggest that (1) insulin and muscle contraction are mobilizing two separate pools of GLUT-4 protein, and (2) the increase in skeletal-muscle glucose uptake due to insulin + contraction is not due to an increase in plasma-membrane GLUT-4 protein concentration above that observed for insulin or contraction alone.

Interaction of aerobic exercise training and clenbuterol: effects on insulin-resistant muscle

The effects of aerobic exercise training, chronic administration of the selective beta 2-adrenergic agonist clenbuterol, and the combination of these two treatments on muscle insulin resistance were compared in female obese (fa/fa) Zucker rats. Rats were randomly assigned to trained, clenbuterol, clenbuterol-trained, or control groups. Training consisted of treadmill running for 2 h/day at 18 m/min up an 8% grade. Clenbuterol was administered by intubation (0.4-0.8 mg.kg body wt-1 x day-1) approximately 30 min before the rats ran each day. After 8 wk of treatment, muscle insulin resistance was assessed via hindlimb perfusion in the presence of 8 mM glucose and a submaximal (500 microU/ml) insulin concentration. Training increased citrate synthase activity (mumol.g wet wt-1 x min-1) by 32-74% and insulin-stimulated glucose uptake by 45%. Clenbuterol ingestion induced a 17-29% increase in muscle mass but decreased citrate synthase activity by 34-42% and had no effect on muscle glucose uptake. Administration of clenbuterol to rats that exercise trained prevented the training-induced improvement in insulin-stimulated glucose uptake and attenuated the increases in citrate synthase activity. In addition, both clenbuterol-treated groups displayed a 42% decrease in beta-adrenergic receptor density. The results indicate that clenbuterol administration, possibly through beta-adrenergic receptor downregulation, attenuated a cellular reaction essential for the exercise training-induced increase in citrate synthase activity and improvement in skeletal muscle insulin resistance of the obese Zucker rat.

Effects of exercise training on muscle GLUT-4 protein content and translocation in obese Zucker rats

The rates of muscle glucose uptake of trained (TR) and untrained (UT) obese Zucker rats were assessed by hindlimb perfusion under basal conditions (no insulin) in the presence of a maximally stimulating concentration of insulin (10 mU/ml) and after muscle contraction elicited by electrical stimulation of the sciatic nerve. Perfusate contained 28 mM glucose and 7.5 microCi/mmol of 2-deoxy-D-[3H]glucose. Muscle GLUT-4 concentration was determined by Western blot analysis and expressed as a percentage of a heart standard. The rates of insulin-stimulated glucose uptake were significantly higher in the plantaris, red gastrocnemius (RG), and white gastrocnemius (WG), but not the soleus or extensor digatorum longus (EDL) of TR compared with UT rats. After muscle contraction the rates of glucose uptake in the TR rats were significantly higher in the soleus, plantaris, and RG. TR rats had significantly higher GLUT-4 protein concentration and citrate synthase activity than the UT rats in the soleus, plantaris, RG, and WG. Basal plasma membrane GLUT-4 protein concentration of TR rats was 144% above UT rats (P < 0.01). Stimulation by insulin and contraction resulted in a significant increase in plasma membrane GLUT-4 protein concentration in UT rats only. However, plasma membrane GLUT-4 protein concentration in insulin- and contraction-stimulated TR rats remained 53% and 30% greater than that of UT rats, respectively (P < 0.05). Exercise training did not alter basal, insulin-, or contraction-stimulated GLUT-4 functional activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Exercise training and clenbuterol reduce insulin resistance of obese Zucker rats

This study compared the effects of aerobic exercise training and chronic administration of the selective beta 2-adrenergic agonist clenbuterol on whole body and skeletal muscle insulin resistance in obese (fa/fa) Zucker rats. Obese rats were randomly assigned to training, clenbuterol, or sedentary control groups. Lean littermates served as a second control group. After 4-5 wk of treatment, an oral glucose tolerance test was performed, followed 1 wk later by hindlimb perfusion, during which time the rates of glucose uptake and 3-O-methyl-D-glucose (3-MG) transport were assessed in the presence of a submaximal (500 microU/ml) insulin concentration. Training resulted in a significant increase in citrate synthase and cytochrome oxidase activity in the recruited muscles. Clenbuterol induced a large increase in muscle mass but provoked a significant decrease in oxidative enzyme activity and beta-adrenergic receptor density. Both treatments increased glucose tolerance and reduced the postglucose insulin response, with the improvements being more pronounced in the clenbuterol group. However, only exercise training improved insulin-stimulated hindlimb muscle glucose uptake (11.37 +/- 0.65, 8.73 +/- 0.77, and 8.27 +/- 0.41 mumol.g-1.h-1 for trained, clenbuterol, and sedentary control groups, respectively) and 3-MG transport. These results suggest that aerobic exercise training attenuated the insulin-resistant condition in the obese Zucker rat by a mechanism other than or in addition to beta 2-adrenergic receptor activation.

Effects of exercise training on skeletal muscle glucose uptake and transport

Exercise training increases the concentration of GLUT-4 protein in skeletal muscle that is associated with an increase in maximal insulin-stimulated glucose transport. The purpose of this study was to determine whether exercise training results in a long-lasting increase in insulin-stimulated glucose transport in rat skeletal muscle. Glucose uptake and skeletal muscle 3-O-methyl-D-glucose (3-MG) transport were determined during hindlimb perfusion in the presence of a maximally stimulating concentration of insulin (10 mU/ml). Hindlimb glucose uptake was approximately 29% above sedentary (Sed) levels in rats examined within 24 h (24H) of their last exercise session. However, when rats were examined 48 h (48H) after their last exercise session, hindlimb glucose uptake was not different from Sed levels. Maximal 3-MG transport was enhanced, above Sed levels, in red (RG; 72% increase) and white (WG; 44% increase) gastrocnemius and plantaris (Plan; 67% increase) muscles, but not soleus (Sol), of 24H rats. GLUT-4 protein content was significantly elevated in those muscles that exhibited enhanced 3-MG transport in 24H rats. GLUT-4 protein content was also elevated in RG, WG, and Plan of 48H rats and was not different from 24H rats. Despite the elevated GLUT-4 protein content, 3-MG transport in 48H rats was only slightly, although statistically not significantly, higher than in Sed rats. These results provide evidence that exercise training does not result in a persistent increase in skeletal muscle glucose uptake or transport, despite an increase in GLUT-4 protein content.

Muscle glucose transport, GLUT-4 content, and degree of exercise training in obese Zucker rats

The effects of high (HI)- and low (LI)-intensity exercise training were examined on insulin-stimulated 3-O-methyl-D-glucose (3-MG) transport and concentration of insulin-regulatable glucose transporter protein (GLUT-4) in the red (fast-twitch oxidative) and white (fast-twitch glycolytic) quadriceps of the obese Zucker rat. Sedentary obese (SED) and lean (LN) Zucker rats were used as controls. 3-MG transport was determined during hindlimb perfusion in the presence of 8 mM 3-MG, 2 mM mannitol, 0.3 mM pyruvate, and 0.5 mU/ml insulin. HI and LI rats displayed greater rates of red quadriceps 3-MG transport and GLUT-4 concentrations than SED rats. No significant differences in rates of 3-MG transport or GLUT-4 concentrations were observed in the red quadriceps of HI and LI rats. There were no differences found in the rates of 3-MG transport in the white quadriceps of HI, LI, and SED rats although the difference between the HI and SED rats approached significance (P < 0.07). The GLUT-4 concentration and citrate synthase activity of HI rats were significantly greater than SED rats. The 3-MG transport rates of LN rats were twofold greater than SED rats regardless of fiber type, but a difference in GLUT-4 content between the LN and SED rats was observed only in the white quadriceps. GLUT-4 content of the obese rats was significantly correlated with citrate synthase activity (r = 0.93) and 3-MG transport (r = 0.82).(ABSTRACT TRUNCATED AT 250 WORDS)

Contraction-activated glucose uptake is normal in insulin-resistant muscle of the obese Zucker rat

The rates of muscle glucose uptake of lean and obese Zucker rats were assessed via hindlimb perfusion under basal conditions (no insulin), in the presence of a maximal insulin concentration (10 mU/ml), and after electrically stimulated muscle contraction in the absence of insulin. The perfusate contained 28 mM glucose and 7.5 microCi/mmol of 2-deoxy-D-[3H-(G)]glucose. Glucose uptake rates in the soleus (slow-twitch oxidative fibers), red gastrocnemius (fast-twitch oxidative-glycolytic fibers), and white gastrocnemius (fast-twitch glycolytic fibers) under basal conditions and after electrically stimulated muscle contraction were not significantly different between the lean and obese rats. However, the rate of glucose uptake during insulin stimulation was significantly lower for obese than for lean rats in all three fiber types. Significant correlations were found for insulin-stimulated glucose uptake and glucose transporter protein isoform (GLUT-4) content of soleus, red gastrocnemius, and white gastrocnemius of lean (r = 0.79) and obese (r = 0.65) rats. In contrast, the relationships between contraction-stimulated glucose uptake and muscle GLUT-4 content of lean and obese rats were negligible because of inordinately low contraction-stimulated glucose uptakes by the solei. These results suggest that maximal skeletal muscle glucose uptake of obese Zucker rats is resistant to stimulation by insulin but not to contractile activity. In addition, the relationship between contraction-stimulated glucose uptake and GLUT-4 content appears to be fiber-type specific.

Insulin resistance of obese Zucker rats exercise trained at two different intensities

The effect of exercise intensity on oral glucose tolerance and hindlimb glucose uptake and transport was studied in 26 female obese Zucker rats after a treadmill training program. The rats were randomly assigned to either a low-intensity (LI) or high-intensity (HI) exercise group, with equal work being performed by the two groups. A third group of rats served as sedentary controls (SED). The trained rats demonstrated a significant improvement in oral glucose tolerance while maintaining significantly lower plasma insulin concentrations when compared with the SED rats. However, no significant differences in plasma glucose or insulin concentrations were observed between the LI and HI exercise-trained groups. During hindlimb perfusion (500 microU/ml insulin, 8 mM glucose), the rate of muscle glucose uptake for the HI rats (13.5 +/- 0.8 mumol.h-1.g-1) was significantly faster than that of the LI rats (11.4 +/- 0.8 mumol.h-1.g-1), which was significantly faster than that of the SED rats (8.3 +/- 0.6 mumol.h-1.g-1). The rates of 3-O-methyl-D-glucose (3-MG) transport were substantially greater in the fast-twitch red fibers of the HI (10.11 +/- 0.49 mumol.h-1.g-1) and LI (9.08 +/- 0.46 mumol.h-1.g-1) rats when compared with those of the SED rats (6.15 +/- 0.41 mumol.h-1.g-1). However, only the HI training resulted in a significant increase in the 3-MG transport of the fast-twitch white fibers (HI, 2.37 +/- 0.27; LI, 1.48 +/- 0.11; SED, 1.31 +/- 0.15 mumol.h-1.g-1). Only muscles with an increased citrate synthase activity demonstrated an improved insulin-stimulated glucose transport.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of pyruvate and dihydroxyacetone consumption on the growth and metabolic state of obese Zucker rats

Female obese Zucker rats (aged 6 wk) were randomly assigned to one of two control or one of three experimental-diet groups. Experimental diets contained 6% pyruvate (Pyr). 6% dihydroxyacetone (Dha), or 3% each pyruvate and dihydroxyacetone (Pyr-Dha). Control-group 1 was fed a normal diet ad libitum and control-group 2 was pair fed according to the experimental group with the lowest consumption. After 5 wk the rats receiving Pyr (357.5 +/- 12.7 g) were significantly lighter than pair-fed (385.9 +/- 4.9 g) and ad libitum-fed (404.3 +/- 10.1) controls. Resting oxygen consumption (mL.min-1.kg0.65) was significantly higher in Pyr-fed rats than in pair-fed controls and food-conversion efficiency was significantly decreased. Rats fed Pyr had a lower resting respiratory-exchange ratio than did ad libitum- and pair-fed controls (0.81 +/- 0.01 vs 0.88 +/- 0.01 and 0.87 +/- 0.01, respectively). Results suggest that pyruvate consumption reduced the weight gain and food-conversion efficiency of obese Zucker rats, in part by increasing resting metabolic rate and fatty acid oxidation.

Muscle glucose uptake of obese Zucker rats trained at two different intensities

Exercise training reduces the muscle insulin resistance of the obese Zucker rat. The purpose of the present study was to determine whether the magnitude of this training response is exercise intensity specific. Obese Zucker rats were randomly divided into sedentary (SED), low-intensity (LI), and high-intensity (HI) exercise groups. For the LI rats, exercise training consisted of running on a rodent treadmill at 18 m/min up an 8% grade for 90 min. Rats in the HI group ran at 24 m/min up an 8% grade for four 17-min bouts with 3 min between bouts. Both exercise groups performed the same amount of work and trained 5 days/wk for 7 wk. To evaluate muscle insulin resistance, rat hindlimbs were perfused for 30 min with perfusate containing 6 mM glucose (0.15 mu Ci of D-[14C(U)] glucose/ml) and either a maximal (10.0 mU/ml) or a submaximal (0.50 mU/ml) insulin concentration. Perfusions were performed 48-56 h after the last exercise bout and a 12-h fast. In the presence of 0.5 mU/ml insulin, the rate of muscle glucose uptake was found to be significantly faster for the HI (9.56 +/- 0.66 mumol.h-1.g-1) than for the LI (7.72 +/- 0.65 mumol.h-1.g-1) and SED (6.64 +/- 0.44 mumol.h-1.g-1) rats. The difference in glucose uptake between the LI and SED rats was not significant. In the presence of 10.0 mU/ml insulin, the rate of glucose uptake was significantly faster for the HI (16.43 +/- 1.02 mumol.h-1.g-1) than for the LI rats (13.76 +/- 0.84 mumol.h-1.g-1) and significantly faster for the LI than for the SED rats (11.02 +/- 0.35 mumol.h-1.g-1).(ABSTRACT TRUNCATED AT 250 WORDS)

Substrate utilization during acute exercise in obese Zucker rats

The purpose of the present study was to compare the carbohydrate use of insulin-resistant obese Zucker rats with that of their lean littermates during steady-state exercise. Obese and lean rats were randomly assigned to a sedentary group or to a run group in which rats ran at 72-73% of their maximal O2 consumption, with the duration of exercise set to require an energy expenditure of 2.1-2.2 kcal. During the run the respiratory exchange ratio was significantly higher in the obese than in the lean rats [0.94 +/- 0.01 (SE) and 0.86 +/- 0.01, respectively], which indicate that the obese rats required 54% more carbohydrate than the lean rats. Total muscle glycogen utilization in the soleus, plantaris, and red and white gastrocnemius was not different between groups. Obese rats had total liver glycogen values five times greater than those of lean rats (833.38 +/- 101.4 and 152.8 +/- 37.5 mg, respectively) and utilized twice as much liver glycogen as their lean littermates (193.5 and 90.4 mg, respectively). The obese rats exhibited higher blood glucose and insulin concentrations than the lean rats during the run. These findings indicate that, despite their characteristic insulin resistance, the obese Zucker rats had a greater dependency on carbohydrate as a substrate during exercise than their lean littermates and that the major source of this carbohydrate was liver glycogen.

Muscle morphological and biochemical adaptations to training in obese Zucker rats

The purposes of the present study were to characterize the histochemical and enzymatic profiles of various hindlimb skeletal muscles, as well as to determine maximal O2 consumption (VO2max) and respiratory exchange ratios (R) during steady-state exercise in the obese Zucker rat. The changes that occurred in these parameters in response to a 6-wk training program were then assessed. Obese rats were randomly assigned to a sedentary or training group. Lean littermates served as a second control. Training consisted of treadmill running at 18 m/min up an 8% grade, 1.5 h/day, 5 day/wk for 6 wk. During week 6, VO2max and R during a steady-state run (74% max) were determined. After 2 days of inactivity, hindlimb muscles were excised, stained for fiber type and capillaries, and assayed for hexokinase, citrate synthase, cytochrome oxidase, and beta-hydroxyacetyl-CoA dehydrogenase. The obese sedentary rats demonstrated greater oxidative enzyme activities per gram of muscle tissue than their lean littermates, greater R values during submaximal exercise of the same relative intensity, and greater absolute VO2max values. Training resulted in a 20-56% increase in oxidative enzymes, a 10% increase in VO2max, and an increase in capillary density in the soleus and plantaris. There was no alteration in R values during exercise at 74% VO2max or in fiber type composition in response to exercise training. Results suggest that the muscle of the obese Zucker rat manifests a greater oxidative capacity than the muscle of its lean littermates. The apparent inability of the obese rat to increase its use of fat during submaximal exercise of the same relative intensity in response to training remains to be elucidated.

Skeletal muscle glucose transport in obese Zucker rats after exercise training

Exercise training has been found to reduce the muscle insulin resistance of the obese Zucker rat (fa/fa). The purpose of the present study was to determine whether this reduction in muscle insulin resistance was associated with an improvement in the glucose transport process and if it was fiber-type specific. Rats were randomly assigned to a sedentary or training group. Training consisted of treadmill running at 18 m/min up an 8% grade, 1.5 h/day, 5 days/wk, for 6-8 wk. The rate of muscle glucose transport was assessed in the absence of insulin and in the presence of a physiological (0.15 mU/ml), a submaximal (1.50 mU/ml), and a maximal (15.0 mU/ml) insulin concentration by determining the rate of 3-O-methyl-D-glucose (3-OMG) accumulation during hindlimb perfusion. The average 3-OMG transport rate of the red gastrocnemii (fast-twitch oxidative-glycolytic fibers) was significantly higher in the trained compared with the sedentary obese rats in the absence of insulin and in the presence of the three insulin concentrations. Significant improvements in 3-OMG transport were also observed in the plantarii (mixed fibers) of trained obese rats in the presence of 0, 0.15, and 15.0 mU/ml insulin. Training appeared to have little effect on the insulin-stimulated 3-OMG transport of the soleus (slow-twitch oxidative fibers) or white gastrocnemius (fast-twitch glycolytic fibers). The results suggest that the improvement in the muscle insulin resistance of the obese Zucker rat after moderate endurance training was associated with an improvement in the glucose transport process but that it was fiber-type specific.

Muscle glycogen storage postexercise: effect of mode of carbohydrate administration

The primary purpose of this study was to determine whether gastric emptying limits the rate of muscle glycogen storage during the initial 4 h after exercise when a carbohydrate supplement is provided. A secondary purpose was to determine whether liquid (L) and solid (S) carbohydrate (CHO) feedings result in different rates of muscle glycogen storage after exercise. Eight subjects cycled for 2 h on three separate occasions to deplete their muscle glycogen stores. After each exercise bout they received 3 g CHO/kg body wt in L (50% glucose polymer) or S (rice/banana cake) form or by intravenous infusion (I; 20% sterile glucose). The L and S supplements were divided into two equal doses and administered immediately after and 120 min after exercise, whereas the I supplement was administered continuously during the first 235 min of the 240-min recovery period. Blood samples were drawn from an antecubital vein before exercise, during exercise, and throughout recovery. Muscle biopsies were taken from the vastus lateralis immediately after and 120 and 240 min after exercise. Blood glucose and insulin declined during exercise and increased significantly above preexercise levels during recovery in all treatments. The increase in blood glucose during the I treatment, however, was three times greater than during the L or S treatments. The average insulin response of the L treatment (61.7 +/- 4.9 microU/ml) was significantly greater than that of the S treatment (47.5 +/- 4.2 microU/ml) but not that of the I (55.3 +/- 4.5 microU/ml) treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Muscle glycogen storage after different amounts of carbohydrate ingestion

The purpose of this study was to determine whether the rate of muscle glycogen storage could be enhanced during the initial 4-h period postexercise by substantially increasing the amount of the carbohydrate consumed. Eight subjects cycled for 2 h on three separate occasions to deplete their muscle glycogen stores. Immediately and 2 h after exercise they consumed either 0 (P), 1.5 (L), or 3.0 g glucose/kg body wt (H) from a 50% glucose polymer solution. Blood samples were drawn from an antecubital vein before exercise, during exercise, and throughout recovery. Muscle biopsies were taken from the vastus lateralis immediately, 2 h, and 4 h after exercise. Blood glucose and insulin declined significantly during exercise in each of the three treatments. They remained below the preexercise concentrations during recovery in the P treatment but increased significantly above the preexercise concentrations during the L and H treatments. By the end of the 4 h-recovery period, blood glucose and insulin were still significantly above the preexercise concentrations in both treatments. Muscle glycogen storage was significantly increased above the basal rate (P, 0.5 mumol.g wet wt-1.h-1) after ingestion of either glucose polymer supplement. The rates of muscle glycogen storage, however, were not different between the L and H treatments during the first 2 h (L, 5.2 +/- 0.9 vs. H, 5.8 +/- 0.7 mumol.g wet wt-1.h-1) or the second 2 h of recovery (L, 4.0 +/- 0.9 vs. H, 4.5 +/- 0.6 mumol.g wet wt-1. h-1).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of fasting and training on pyruvate dehydrogenase activation during exercise

1. The effect of exercise (2 hr treadmill running at 28 m/min) on PDHa (the activity of the active form of pyruvate dehydrogenase) in untrained rats, trained rats (2 hr/d at 25 m/min for 4 wk), and in 24 hr fasted rats was determined. 2. Exercise increased PDHa activity approximately 2 fold in fed-untrained rats. 3. Fasting decreased PDHa activity in sedentary rats to approximately half the activity in fed rats. 4. The increase in PDHa activity during exercise was less in fasted than fed rats. 5. Training did not change the total activity of PDH (phosphorylated plus nonphosphorylated forms) but the percent of PDH in the active form was increased in muscle of trained-rested rats. 6. PDHa activity was unchanged by acute exercise (2.5 hr at 40 m/min) in the trained rats.