Naltrexone/Bupropion extended release-induced weight loss is independent of nausea in subjects without diabetes

Naltrexone/bupropion extended release (NB) is indicated as an adjunct to a reduced-calorie diet and increased physical activity for chronic weight management in adults with an initial body mass index of ≥30 or ≥27 kg m(-2) and ≥1 weight-related comorbidity (e.g. hypertension, type 2 diabetes and dyslipidaemia). In phase 3 clinical studies, nausea occurred in significantly higher proportions of subjects randomized to NB vs. placebo (PBO). In this pooled analysis of three phase 3, 56-week, PBO-controlled studies, we characterized nausea and weight loss in NB- and PBO-treated subjects without diabetes. Subjects receiving NB (n = 1778) lost significantly more weight than those receiving PBO (n = 1160). Weight change was not significantly different between subjects reporting and not reporting nausea in either treatment arm. Severity of nausea was mild to moderate in ≥95% of all cases. In the NB arm, the highest incidence of nausea onset (9%) was reported during week 1. The median duration of mild, moderate and severe nausea in subjects receiving NB was 14, 9 and 13 days, respectively. Our results demonstrate that nausea associated with NB is rarely severe, primarily occurs early in treatment and is not a contributor to weight loss.

Enhanced weight loss following coadministration of pramlintide with sibutramine or phentermine in a multicenter trial

Preclinical evidence suggests that pharmacotherapy for obesity using combinations of agents targeted at distinct regulatory pathways may produce robust additive or synergistic effects on weight loss. This randomized placebo-controlled trial examined the safety and efficacy of the amylin analogue pramlintide alone or in combination with either phentermine or sibutramine. All patients also received lifestyle intervention. Following a 1-week placebo lead-in, 244 obese or overweight, nondiabetic subjects (88% female; 41 +/- 11 years; BMI 37.7 +/- 5.4 kg/m(2); weight 103 +/- 19 kg; mean +/- s.d.) received placebo subcutaneously (sc) t.i.d., pramlintide sc (120 microg t.i.d.), pramlintide sc (120 microg t.i.d.) + oral sibutramine (10 mg q.a.m.), or pramlintide sc (120 microg t.i.d.) + oral phentermine (37.5 mg q.a.m.) for 24 weeks. Treatment was single-blind for subjects receiving subcutaneous medication only and open-label for subjects in the combination arms. Weight loss achieved at week 24 with either combination treatment was greater than with pramlintide alone or placebo (P < 0.001; 11.1 +/- 1.1% with pramlintide + sibutramine, 11.3 +/- 0.9% with pramlintide + phentermine, -3.7 +/- 0.7% with pramlintide; -2.2 +/- 0.7% with placebo; mean +/- s.e.). Elevations from baseline in heart rate and diastolic blood pressure were demonstrated with both pramlintide + sibutramine (3.1 +/- 1.2 beats/min, P < 0.05; 2.7 +/- 0.9 mm Hg, P < 0.01) and pramlintide + phentermine (4.5 +/- 1.3 beats/min, P < 0.01; 3.5 +/- 1.2 mm Hg, P < 0.001) using 24-h ambulatory monitoring. However, the majority of subjects receiving these treatments remained within normal blood pressure ranges. These results support the potential of pramlintide-containing combination treatments for obesity.

Sustained weight loss following 12-month pramlintide treatment as an adjunct to lifestyle intervention in obesity

OBJECTIVE

To assess long-term weight loss efficacy and safety of pramlintide used at different dosing regimens and in conjunction with lifestyle intervention (LSI).

RESEARCH DESIGN AND METHODS

In a 4-month, double-blind, placebo-controlled, dose-ranging study, 411 obese subjects were randomized to receive pramlintide (six arms: 120, 240, and 360 microg b.i.d. and t.i.d.) or placebo in conjunction with a structured LSI program geared toward weight loss. Of the 4-month evaluable subjects (n = 270), 77% opted to continue preexisting treatment during an 8-month single-blind extension (LSI geared toward weight maintenance).

RESULTS

At month 4, mean weight loss from baseline in the pramlintide arms ranged from 3.8 +/- 0.7 to 6.1 +/- 0.8 kg (2.8 +/- 0.8 kg with placebo). By month 12, initial 4-month weight loss was regained in the placebo group but was maintained in all but the 120-microg b.i.d. group. Placebo-corrected weight loss with 120 microg t.i.d. and 360 microg b.i.d. averaged 3.2 +/- 1.2 kg (3.1 +/- 1.1% body wt) and 3.3 +/- 1.1 kg (3.1 +/- 1.0% body wt), respectively, at month 4 (both P < 0.01; 4-month evaluable n = 270) and 6.1 +/- 2.1 kg (5.6 +/- 2.1% body wt) and 7.2 +/- 2.3 kg (6.8 +/- 2.3% body wt), respectively, at month 12 (both P < 0.01; 12-month evaluable n = 146). At month 12, 40 and 43% of subjects treated with 120 microg t.i.d. and 360 microg b.i.d., respectively, achieved >or=10% weight loss (vs. 12% for placebo). Nausea, the most common adverse event with pramlintide in the 4-month study (9-29% pramlintide vs. 2% placebo), was generally mild to moderate and occurred in <10% of subjects during the extension.

CONCLUSIONS

When used over 12 months as an adjunct to LSI, pramlintide treatment, with low-dose three-times-daily or higher-dose two-times-daily regimens, helped obese subjects achieve greater initial weight loss and enhanced long-term maintenance of weight loss.

Pramlintide treatment reduces 24-h caloric intake and meal sizes and improves control of eating in obese subjects: a 6-wk translational research study

Evidence from rodent studies indicates that the beta-cell-derived neurohormone amylin exerts multiple effects on eating behavior, including reductions in meal size, intake of highly palatable foods, and stress-induced sucrose consumption. To assess the effect of amylin agonism on human eating behavior we conducted a randomized, blinded, placebo-controlled, multicenter study investigating the effects of the amylin analog pramlintide on body weight, 24-h caloric intake, portion sizes, "fast food" intake, and perceived control of eating in 88 obese subjects. After a 2-day placebo lead-in, subjects self-administered pramlintide (180 microg) or placebo by subcutaneous injection 15 min before meals for 6 wk without concomitant lifestyle modifications. Compared with placebo, pramlintide treatment elicited significant mean reductions from baseline in body weight on day 44 (-2.1 +/- 0.3 vs. +0.1 +/- 0.4%, P < 0.001), 24-h caloric intake (-990 +/- 94 vs. -243 +/- 126 kcal on day 3, P < 0.0001; -680 +/- 86 vs. -191 +/- 161 kcal on day 43, P < 0.01), portion sizes, and caloric intake at a "fast food challenge" (-385 +/- 61 vs. -109 +/- 88 kcal on day 44, P < 0.05). Pramlintide treatment also improved perceived control of eating, as demonstrated by a 45% placebo-corrected reduction in binge eating scores (P < 0.01). The results of this translational research study confirm in humans various preclinical effects of amylin agonism, demonstrating that pramlintide-mediated weight loss in obese subjects is accompanied by sustained reductions in 24-h food intake, portion sizes, fast food intake, and binge eating tendencies.

Exenatide augments first- and second-phase insulin secretion in response to intravenous glucose in subjects with type 2 diabetes

CONTEXT

First-phase insulin secretion (within 10 min after a sudden rise in plasma glucose) is reduced in type 2 diabetes mellitus (DM2). The incretin mimetic exenatide has glucoregulatory activities in DM2, including glucose-dependent enhancement of insulin secretion.

OBJECTIVE

The objective of the study was to determine whether exenatide can restore a more normal pattern of insulin secretion in subjects with DM2.

DESIGN

Fasted subjects received iv insulin infusion to reach plasma glucose 4.4-5.6 mmol/liter. Subjects received iv exenatide (DM2) or saline (DM2 and healthy volunteers), followed by iv glucose challenge.

PATIENTS

Thirteen evaluable DM2 subjects were included in the study: 11 males, two females; age, 56 +/- 7 yr; body mass index, 31.7 +/- 2.4 kg/m2; hemoglobin A1c, 6.6 +/- 0.7% (mean +/- sd) treated with diet/exercise (n = 1), metformin (n = 10), or acarbose (n = 2). Controls included 12 healthy, weight-matched subjects with normal glucose tolerance: nine males, three females; age, 57 +/- 9 yr; and body mass index, 32.0 +/- 3.0 kg/m2.

SETTING

The study was conducted at an academic hospital.

MAIN OUTCOME MEASURES

Plasma insulin, plasma C-peptide, insulin secretion rate (derived by deconvolution), and plasma glucagon were the main outcome measures.

RESULTS

DM2 subjects administered saline had diminished first-phase insulin secretion, compared with healthy control subjects. Exenatide-treated DM2 subjects had an insulin secretory pattern similar to healthy subjects in both first (0-10 min) and second (10-180 min) phases after glucose challenge, in contrast to saline-treated DM2 subjects. In exenatide-treated DM2 subjects, the most common adverse event was moderate nausea (two of 13 subjects).

CONCLUSIONS

Short-term exposure to exenatide can restore the insulin secretory pattern in response to acute rises in glucose concentrations in DM2 patients who, in the absence of exenatide, do not display a first phase of insulin secretion. Loss of first-phase insulin secretion in DM2 patients may be restored by treatment with exenatide.

Role of PYK2 in the development of obesity and insulin resistance

Non-receptor proline-rich tyrosine kinase-2 (PYK2), which is activated by phosphorylation of one or more of its tyrosine residues, has been implicated in the regulation of GLUT4 glucose transporter translocation and glucose transport. Some data favor a positive role of PYK2 in stimulating glucose transport, whereas other studies suggest that PYK2 may participate in the induction of insulin resistance. To ascertain the importance of PYK2 in the setting of obesity and insulin resistance, we (1) evaluated the regulation of PYK2 in mice fed a high-fat diet and (2) characterized body and glucose homeostasis in wild type (WT) and PYK2(-/-) mice on different diets. We found that both PYK2 expression and phosphorylation were significantly increased in liver and adipose tissues harvested from high-fat diet fed mice. Wild type and PYK2(-/-) mice were fed a high-fat diet for 8 weeks to induce insulin resistance/obesity. Surprisingly, in response to this diet PYK2(-/-) mice gained significantly more weight than WT mice (18.7+/-1.2g vs. 9.5+/-0.6g). Fasting serum leptin and insulin and blood glucose levels were significantly increased in high-fat diet fed mice irrespective of the presence of PYK2 protein. There was a close correlation between serum leptin and body weight. Intraperitoneal glucose tolerance tests revealed that as expected, the high-fat diet resulted in increased blood glucose levels following glucose administration in wild type mice compared to those fed normal chow. An even greater increase in blood glucose levels was observed in PYK2(-/-) mice compared to wild type mice. These results demonstrate that a lack of PYK2 exacerbates weight gain and development of glucose intolerance/insulin resistance induced by a high-fat diet, suggesting that PYK2 may play a role in slowing the development of obesity, insulin resistance, and/or frank diabetes.

Fiber type-specific determinants of Vmax for insulin-stimulated muscle glucose uptake in vivo

The aim of this study was to determine barriers limiting muscle glucose uptake (MGU) during increased glucose flux created by raising blood glucose in the presence of fixed insulin. The determinants of the maximal velocity (V(max)) of MGU in muscles of different fiber types were defined. Conscious rats were studied during a 4 mU x kg(-1) x min(-1) insulin clamp with plasma glucose at 2.5, 5.5, and 8.5 mM. [U-(14)C]mannitol and 3-O-methyl-[(3)H]glucose ([(3)H]MG) were infused to steady-state levels (t = -180 to 0 min). These isotope infusions were continued from 0 to 40 min with the addition of a 2-deoxy-[(3)H]glucose ([(3)H]DG) infusion. Muscles were excised at t = 40 min. Glucose metabolic index (R(g)) was calculated from muscle-phosphorylated [(3)H]DG. [U-(14)C]mannitol was used to determine extracellular (EC) H(2)O. Glucose at the outer ([G](om)) and inner ([G](im)) sarcolemmal surfaces was determined by the ratio of [(3)H]MG in intracellular to EC H(2)O and muscle glucose. R(g) was comparable at the two higher glucose concentrations, suggesting that rates of uptake near V(max) were reached. In summary, by defining the relationship of arterial glucose to [G](om) and [G](im) in the presence of fixed hyperinsulinemia, it is concluded that 1) V(max) for MGU is limited by extracellular and intracellular barriers in type I fibers, as the sarcolemma is freely permeable to glucose; 2) V(max) is limited in muscles with predominantly type IIb fibers by extracellular resistance and transport resistance; and 3) limits to R(g) are determined by resistance at multiple steps and are better defined by distributed control rather than by a single rate-limiting step.

Acute and chronic treatment of ob/ob and db/db mice with AICAR decreases blood glucose concentrations

The enzyme 5'AMP-activated protein kinase (AMPK) is activated by increases in intracellular AMP concentration through a complex interaction of phosphorylation and allosteric regulation. Actions of AMPK elucidated thus far suggest that AMPK may be a viable target for pharmacologic intervention in type II diabetes. Activation of AMPK is believed to mediate both the acute increase in skeletal muscle glucose uptake during exercise, as well as the adaptive responses to chronic exercise such as regulation of expression of components of the muscle glucose uptake system. In addition, AMPK is known to inhibit key enzymes involved in lipid and cholesterol synthesis, suggesting that activation of this kinase may also ameliorate dyslipidemia. To investigate the effects of AMPK activation in animal models of type II diabetes, db/db and ob/ob mice were administered 5-aminoimidazole-4-carboxamide 1-beta-ribofuranoside (AICAR) subcutaneously either acutely (single injection) or twice per day for 8 days (chronic treatment). Blood glucose was lowered transiently in both db/db and ob/ob mice by acute AICAR treatment, returning to basal levels approximately 3 h after AICAR administration. In response to chronic treatment, blood glucose (measured 18 h post-AICAR administration) was significantly decreased in both mouse models when compared to vehicle control groups, with morning blood glucose values on Day 8 being decreased approximately 30-35% in both mouse models. Chronic AICAR administration also resulted in an elevation of total Glut4 concentration in skeletal muscle from ob/ob mice, but not db/db mice. In contrast to the beneficial effects on glucose metabolism, AICAR treatment of db/db and ob/ob mice led to approximately a 2.5-3-fold increase in serum triglyceride levels compared to vehicle-treated controls. These data suggest that pharmacological activation of AMPK may enhance glucose uptake in individuals with type II diabetes, however, this benefit may be offset by the concomitant elevation in triglycerides.

Functional limitations to glucose uptake in muscles comprised of different fiber types

Skeletal muscle glucose uptake requires delivery of glucose to the sarcolemma, transport across the sarcolemma, and the irreversible phosphorylation of glucose by hexokinase (HK) inside the cell. Here, a novel method was used in the conscious rat to address the roles of these three steps in controlling the rate of glucose uptake in soleus, a muscle comprised of type I fibers, and two muscles comprised of type II fibers. Experiments were performed on conscious rats under basal conditions or during hyperinsulinemic euglycemic clamps. Rats received primed, constant infusions of 3-O-methyl-[3H]glucose (3-O-MG) and [1-14C]mannitol. Total muscle glucose concentration and the steady-state ratio of intracellular to extracellular 3-O-MG concentration, which distributes based on the transsarcolemmal glucose gradient (TSGG), were used to calculate glucose concentrations at the inner and outer sarcolemmal surfaces ([G](im) and [G](om), respectively) in muscle. Muscle glucose uptake was much lower in muscle comprised of type II fibers than in soleus under both basal and insulin-stimulated conditions. Under all conditions, the TSGG in type II muscle exceeded that in soleus, indicating that glucose transport plays a more important role to limit glucose uptake in type II muscle. Although hyperinsulinemia increased [G](im) in soleus, indicating that phosphorylation was a limiting factor, type II muscle was limited primarily by glucose delivery and glucose transport. In conclusion, the relative importance of glucose delivery, transport, and phosphorylation in controlling the rate of insulin-stimulated muscle glucose uptake varies between muscle fiber types, with glucose delivery and transport being the primary limiting factors in type II muscle.

Limitations to basal and insulin-stimulated skeletal muscle glucose uptake in the high-fat-fed rat

Rats fed a high-fat diet display blunted insulin-stimulated skeletal muscle glucose uptake. It is not clear whether this is due solely to a defect in glucose transport, or if glucose delivery and phosphorylation are also impaired. To determine this, rats were fed standard chow (control rats) or a high-fat diet (HF rats) for 4 wk. Experiments were then performed on conscious rats under basal conditions or during hyperinsulinemic euglycemic clamps. Rats received primed constant infusions of 3-O-methyl-[(3)H]glucose (3-O-MG) and [1-(14)C]mannitol. Total muscle glucose concentration and the steady-state ratio of intracellular to extracellular 3-O-MG concentration [which distributes based on the transsarcolemmal glucose gradient (TSGG)] were used to calculate glucose concentrations at the inner and outer sarcolemmal surfaces ([G](im) and [G](om), respectively) in soleus. Total muscle glucose was also measured in two fast-twitch muscles. Muscle glucose uptake was markedly decreased in HF rats. In control rats, hyperinsulinemia resulted in a decrease in soleus TSGG compared with basal, due to increased [G](im). In HF rats during hyperinsulinemia, [G](im) also exceeded zero. Hyperinsulinemia also decreased muscle glucose in HF rats, implicating impaired glucose delivery. In conclusion, defects in extracellular and intracellular components of muscle glucose uptake are of major functional significance in this model of insulin resistance.

Glucagon response to exercise is critical for accelerated hepatic glutamine metabolism and nitrogen disposal

The aim of this study was to determine the role of glucagon in hepatic glutamine (Gln) metabolism during exercise. Sampling (artery, portal vein, and hepatic vein) and infusion (vena cava) catheters and flow probes (portal vein, hepatic artery) were implanted in anesthetized dogs. At least 16 days after surgery, an experiment, consisting of a 120-min equilibration period, a 30-min basal sampling period, and a 150-min exercise period, was performed in these animals. [5-(15)N]Gln was infused throughout experiments to measure gut and liver Gln kinetics and the incorporation of Gln amide nitrogen into urea. Somatostatin was infused throughout the study. Glucagon was infused at a basal rate until the beginning of exercise, when the rate was either 1) gradually increased to simulate the glucagon response to exercise (n = 5) or 2) unchanged to maintain basal glucagon (n = 5). Insulin was infused during the equilibration and basal periods at rates designed to achieve stable euglycemia. The insulin infusion was reduced in both protocols to simulate the exercise-induced insulin decrement. These studies show that the exercise-induced increase in glucagon is 1) essential for the increase in hepatic Gln uptake and fractional extraction, 2) required for the full increment in ureagenesis, 3) required for the specific transfer of the Gln amide nitrogen to urea, and 4) unrelated to the increase in gut fractional Gln extraction. These data show, by use of the physiological perturbation of exercise, that glucagon is a physiological regulator of hepatic Gln metabolism in vivo.

Role of Ca(2+) fluctuations in L6 myotubes in the regulation of the hexokinase II gene

Expression of the hexokinase (HK) II gene in skeletal muscle is upregulated by electrically stimulated muscle contraction and moderate-intensity exercise. However, the molecular mechanism by which this occurs is unknown. Alterations in intracellular Ca(2+) homeostasis accompany contraction and regulate gene expression in contracting skeletal muscle. Therefore, as a first step in understanding the exercise-induced increase in HK II, the ability of Ca(2+) to increase HK II mRNA was investigated in cultured skeletal muscle cells, namely L6 myotubes. Exposure of cells to the ionophore A-23187 resulted in an approximately threefold increase in HK II mRNA. Treatment of cells with the extracellular Ca(2+) chelator EGTA did not alter HK II mRNA, nor was it able to prevent the A-23187-induced increase. Treatment of cells with the intracellular Ca(2+) chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra(acetoxymethyl) ester (BAPTA-AM) also resulted in an approximately threefold increase in HK II mRNA in the absence of ionophore, which was similar to the increase in HK II mRNA induced by the combination of BAPTA-AM and A-23187. In summary, a rise in intracellular Ca(2+) is not necessary for the A-23187-induced increase in HK II mRNA, and increases in HK II mRNA occur in response to treatments that decrease intracellular Ca(2+) stores. Depletion of intracellular Ca(2+) stores may be one mechanism by which muscle contraction increases HK II mRNA.

Overexpression of hexokinase II increases insulinand exercise-stimulated muscle glucose uptake in vivo

The hypothesis of this investigation was that glucose uptake would be increased in skeletal muscle of transgenic mice (TG) overexpressing hexokinase II (HK II) compared with their nontransgenic littermates (NTG) during euglycemic hyperinsulinemia and treadmill exercise. For insulin experiments, catheters were surgically implanted in the jugular vein and carotid artery for infusions and sampling, respectively. Conscious mice underwent experiments approximately 5 days later in which 4 mU. kg-1. min-1 insulin and variable glucose (n = 7 TG and n = 7 NTG) or saline (n = 5 TG and n = 4 NTG) was infused for 140 min. Over the last 40 min of the experiments, 2-deoxy-[3H]glucose ([2-3H]DG) was infused, after which muscles were removed. For the exercise experiments, jugular vein catheters were surgically implanted. Five days later, mice received a bolus of [2-3H]DG and then remained sedentary (n = 6 TG and n = 8 NTG) or ran on a motorized treadmill (n = 12 TG and n = 8 NTG) for 30 min. TG and NTG had similar muscle [2-3H]DG 6-phosphate ([2-3H]DGP) accumulation in the basal state (P > 0.05). In the hyperinsulinemic experiments, TG required approximately 25% more glucose to maintain euglycemia (P < 0.05), and muscle [2-3H]DGP accumulation normalized to infusate [2-3H]DG was similarly increased (P < 0.05). In the exercise experiments, muscle [2-3H]DGP accumulation was significantly greater in TG than NTG (P < 0.05). In conclusion, we did not detect an effect of HK II overexpression on muscle [2-3H]DGP accumulation under basal conditions. Hyperinsulinemia and exercise shift the control of muscle glucose uptake so that phosphorylation is a more important determinant of the rate of this process.

Limitations to exercise- and maximal insulin-stimulated muscle glucose uptake

The hypothesis of this investigation was that insulin and muscle contraction, by increasing the rate of skeletal muscle glucose transport, would bias control so that glucose delivery to the sarcolemma (and t tubule) and phosphorylation of glucose intracellularly would exert more influence over glucose uptake. Because of the substantial increases in blood flow (and hence glucose delivery) that accompany exercise, we predicted that glucose phosphorylation would become more rate determining during exercise. The transsarcolemmal glucose gradient (TSGG; the glucose concentration difference across the membrane) is inversely related to the degree to which glucose transport determines the rate of glucose uptake. The TSGG was determined by using isotopic methods in conscious rats during euglycemic hyperinsulinemia [Ins; 20 mU/(kg. min); n = 7], during treadmill exercise (Ex, n = 6), and in sedentary, saline-infused rats (Bas, n = 13). Rats received primed, constant intravenous infusions of trace 3-O-[3H]methyl-D-glucose and [U-14C]mannitol. Then 2-deoxy-[3H]glucose was infused for the calculation of a glucose metabolic index (Rg). At the end of experiments, rats were anesthetized, and soleus muscles were excised. Total soleus glucose concentration and the steady-state ratio of intracellular to extracellular 3-O-[3H]methyl-D-glucose (which distributes on the basis of the TSGG) were used to calculate ranges of possible glucose concentrations ([G]) at the inner and outer sarcolemmal surfaces ([G]im and [G]om, respectively). Soleus Rg was increased in Ins and further increased in Ex. In Ins, total soleus glucose, [G]om, and the TSGG were decreased compared with Bas, while [G]im remained near 0. In Ex, total soleus glucose and [G]im were increased compared with Bas, and there was not a decrease in [G]om as was observed in Ins. In addition, accumulation of intracellular free 2-deoxy-[3H]glucose occurred in soleus in both Ex and Ins. Taken together, these data indicate that, in Ex, glucose phosphorylation becomes an important limitation to soleus glucose uptake. In Ins, both glucose delivery and glucose phosphorylation influence the rate of soleus glucose uptake more than under basal conditions.

An overview of muscle glucose uptake during exercise. Sites of regulation

The uptake of blood glucose by skeletal muscle is a complex process. In order to be metabolized, glucose must travel the path from blood to interstitium to intracellular space and then be phosphorylated to glucose 6-phosphate (G6P). Movement of glucose from blood to interstitium is determined by skeletal muscle blood flow, capillary recruitment and the endothelial permeability to glucose. The influx of glucose from the interstitium to intracellular space is determined by the number of glucose transporters in the sarcolemma and the glucose gradient across the sarcolemma. The capacity to phosphorylate glucose is determined by the amount of skeletal muscle hexokinase II, hexokinase II compartmentalization within the cell, and the concentration of the hexokinase II inhibitor G6P. Any change in glucose uptake occurs due to an alteration in one or more of these steps. Based on the low calculated intracellular glucose levels and the higher affinity of glucose for phosphorylation relative to transport, glucose transport is generally considered rate-determining for basal muscle glucose uptake. Exercise increases both the movement of glucose from blood to sarcolemma and the permeability of the sarcolemma to glucose. Whether the ability to phosphorylate glucose is increased in the working muscle remains to be clearly shown. It is possible that the accelerated glucose delivery and transport rates during exercise bias regulation so that muscle glucose phosphorylation exerts more control on muscle glucose uptake. Conditions that alter glucose uptake during exercise, such as increased NEFA concentrations, decreased oxygen availability and adrenergic stimulation, must work by altering one or more of the three steps involved in glucose uptake. This review describes the regulation of glucose uptake during exercise at each of these sites under a number of conditions, as well as describing muscle glucose uptake in the post-exercise state.

Regulation of hepatic glutamine metabolism during exercise in the dog

The goal of this study was to determine how liver glutamine (Gln) metabolism adapts to acute exercise in the 18-h-fasted dogs (n = 7) and in dogs that were glycogen depleted by a 42-h fast (n = 8). For this purpose, sampling (carotid artery, portal vein, and hepatic vein) and infusion (vena cava) catheters and Doppler flow probes (portal vein, hepatic artery) were implanted under general anesthesia. At least 16 days later an experiment, consisting of a 120-min equilibration period, a 30-min basal sampling period, and a 150-min exercise period was performed. At the start of the equilibration period, a constant-rate infusion of [5-15N]Gln was initiated. Arterial Gln flux was determined by isotope dilution. Gut and liver Gln release into and uptake from the blood were calculated by combining stable isotopic and arteriovenous difference methods. The results of this study show that 1) in the 18-h-fasted dog, approximately 10% and approximately 35% of the basal Gln appearance in arterial blood is due to Gln release from the gut and liver, respectively, whereas approximately 30% and approximately 25% of the basal Gln disappearance is due to removal by these tissues; 2) extending the fast to 42 h does not affect basal arterial Gln flux or the contribution of the gut to arterial Gln fluxes but decreases hepatic Gln release, causing a greater retention of gluconeogenic carbon by the liver; 3) moderate-intensity exercise increases hepatic Gln removal from the blood regardless of fast duration but does not affect the hepatic release of Gln; and 4) Gln plays an important role in channeling nitrogen into the ureagenic pathway in the basal state, and this role is increased by approximately 80% in response to exercise. These studies illustrate the quantitative importance of the splanchnic bed contribution to arterial Gln flux during exercise and the ability of the liver to acutely adapt to changes in metabolic requirements induced by the combined effects of fasting and exercise.

Analysis of insulin-stimulated skeletal muscle glucose uptake in conscious rat using isotopic glucose analogs

An isotopic method was used in conscious rats to determine the roles of glucose transport and the transsarcolemmal glucose gradient (TSGG) in control of basal and insulin-stimulated muscle glucose uptake. Rats received an intravenous 3-O-[3H]methylglucose (3-O-[3H]MG) infusion from -100 to 40 min and a 2-deoxy-[3H]glucose infusion from 0 to 40 min to calculate a glucose metabolic index (Rg). Insulin was infused from -100 to 40 min at rates of 0.0, 0.6, 1.0, and 4.0 mU.kg-1.min-1, and glucose was clamped at basal concentrations. The ratios of soleus intracellular to extracellular 3-O-[3H]MG concentration and soleus glucose concentrations were used to estimate the TSGG using principles of glucose counter-transport. Tissue glucose concentrations were compared in well-perfused, slow-twitch muscle (soleus) and poorly perfused, fast-twitch muscle (vastus lateralis, gastrocnemius). Data show that 1) small increases in insulin increase soleus Rg without decreasing TSGG, suggesting that muscle glucose delivery and phosphorylation can accommodate the increased flux; 2) due to a limitation in soleus glucose phosphorylation and possibly delivery, insulin at high physiological levels decreases TSGG, and at supraphysiological insulin levels the TSGG is not significantly different from 0; 3) maximum Rg is maintained even though TSGG decreases with increasing insulin levels, indicating that glucose transport continues to increase and is not rate limiting for maximal insulin-stimulated glucose uptake; and 4) muscle consisting of fast-twitch fibers that are poorly perfused exhibits a 35-45% fall in tissue glucose with insulin, suggesting that glucose delivery is a major limitation in sustaining the TSGG. In conclusion, control of glucose uptake is distributed between glucose transport and factors that determine the TSGG. Insulin stimulation of glucose transport increases the demands on the factors that maintain glucose delivery to the muscle membrane and glucose phosphorylation inside the muscle.

Effect of physical activity and fasting on gut and liver proteolysis in the dog

The aim of this study was to determine how gut and liver protein kinetics adapt to acute exercise in the 18-h-fasted dog (n = 7) and in dogs glycogen depleted by a 42-h fast (n = 8). For this purpose, sampling (artery and portal and hepatic veins) and infusion (vena cava) catheters and Doppler flow probes (portal vein and hepatic artery) were implanted with animals under general anesthesia. At least 16 days later, an experiment, consisting of a 120-min equilibration period, a 30-min basal sampling period, and a 150-min exercise period, was performed. At the start of the equilibration period, a constant rate infusion of [1-13C]leucine was initiated. Gut and liver leucine appearance and disappearance rates were calculated in these studies by combining a novel stable isotopic method and arteriovenous difference methods. In the determination of tissue leucine kinetics the tissue inflow of both alpha-[13C]ketoisocaproic acid and [13C]leucine was taken into account. The results of this study show that 1) the splanchnic bed (liver plus gut) contributes approximately 40% to the whole body proteolytic rate in the basal state and during exercise in dogs fasted for either 18 or 42 h, 2) the contributions of the gut and liver to splanchnic bed proteolysis is about equal in the basal state in both 18- and 42-h-fasted dogs, and 3) exercise in the 18-h-fasted dog leads to a greater emphasis on gut proteolysis and a lesser emphasis on hepatic proteolysis. These studies highlight the important contribution of gut and hepatic proteolysis to whole body proteolysis and the ability of the gut to acutely adapt to changes in physical activity.

Metabolic responses of rat respiratory muscles to voluntary exercise training

Voluntary wheel running for 4 or 8 wk was used to assess whether a volitional training stimulus would induce adaptations in the oxidative capacity [citrate synthase activity (CS)], glucose phosphorylation capacity [hexokinase activity (HK)], and glucose transporter protein level (GLUT-4) of rat respiratory muscles. Running distances averaged approximately 10-13 km/day over the final 5 wk of training. Peak oxygen consumption by the trained animals was 17% greater (P < 0.05) than by age-matched sedentary control animals after 8 wk. CS, HK, and GLUT-4 in soleus and plantaris muscles all increased because of exercise training. CS increased in the rectus abdominis (+17%), external oblique (+28%), and internal oblique (+17%) but not in the costal or crural diaphragm after 4 wk of training. However, after 8 wk, CS in the costal diaphragm was 39% greater than control but was unchanged in the crural diaphragm. Whereas HK was significantly greater than control in the costal diaphragm (+18%) and rectus abdominis (+54%) after 4 wk, 8 wk of running were required for increases in HK in the external oblique (+17%) and internal oblique (+14%). HK in the crural diaphragm was not significantly altered by the exercise training. GLUT-4 did not change significantly in any of the respiratory muscles studied. These results indicate that significant adaptations in the glucose phosphorylation capacity and oxidative capacity of both inspiratory and expiratory muscles can take place in response to voluntary exercise. However, this same stimulus is not sufficient to cause an adaptive response in GLUT-4 protein level in these respiratory muscles.

Adaptive responses of GLUT-4 and citrate synthase in fast-twitch muscle of voluntary running rats

Glucose transporter (GLUT-4) protein, hexokinase, and citrate synthase (proteins involved in oxidative energy production from blood glucose catabolism) increase in response to chronically elevated neuromuscular activity. It is currently unclear whether these proteins increase in a coordinated manner in response to this stimulus. Therefore, voluntary wheel running (WR) was used to chronically overload the fast-twitch rat plantaris muscle and the myocardium, and the early time courses of adaptative responses of GLUT-4 protein and the activities of hexokinase and citrate synthase were characterized and compared. Plantaris hexokinase activity increased 51% after just 1 wk of WR, whereas GLUT-4 and citrate synthase were increased by 51 and 40%, respectively, only after 2 wk of WR. All three variables remained comparably elevated (+50-64%) through 4 wk of WR. Despite the overload of the myocardium with this protocol, no substantial elevations in these variables were observed. These findings are consistent with a coordinated upregulation of GLUT-4 and citrate synthase in the fast-twitch plantaris, but not in the myocardium, in response to this increased neuromuscular activity. Regulation of hexokinase in fast-twitch muscle appears to be uncoupled from regulation of GLUT-4 and citrate synthase, as increases in the former are detectable well before increases in the latter.

Early alterations in soleus GLUT-4, glucose transport, and glycogen in voluntary running rats

Voluntary wheel running (WR) by juvenile female rats was used as a noninterventional model of soleus muscle functional overload to study the regulation of insulin-stimulated glucose transport activity by the glucose transporter (GLUT-4 isoform) protein level and glycogen concentration. Soleus total protein content was significantly greater (+18%; P < 0.05) than in age-matched controls after 1 wk of WR, and this hypertrophic response continued in weeks 2-4 (+24-32%). GLUT-4 protein was 39% greater than in controls in 1-wk WR soleus, and this adaptation was accompanied by a similar increase in in vitro insulin-stimulated glucose transport activity (+29%). After 2 and 4 wk of WR, however, insulin-stimulated glucose transport activity had returned to control levels, despite a continued elevation (+25-28%) of GLUT-4 protein. At these two time points, glycogen concentration was significantly enhanced in WR soleus (+21-42%), which coincided with significant reductions in glycogen synthase activity ratios (-23 to -41%). These results indicate that, in this model of soleus muscle functional overload, the GLUT-4 protein level may initially regulate insulin-stimulated glucose transport activity in the absence of changes in other modifying factors. However, this regulation of glucose transport activity by GLUT-4 protein may be subsequently overridden by elevated glycogen concentration.