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

Insulin, Muscle Glucose Uptake, and Hexokinase: Revisiting the Road Not Taken

Research conducted over the last 50 yr has provided insight into the mechanisms by which insulin stimulates glucose transport across the skeletal muscle cell membrane Transport alone, however, does not result in net glucose uptake as free glucose equilibrates across the cell membrane and is not metabolized. Glucose uptake requires that glucose is phosphorylated by hexokinases. Phosphorylated glucose cannot leave the cell and is the substrate for metabolism. It is indisputable that glucose phosphorylation is essential for glucose uptake. Major advances have been made in defining the regulation of the insulin-stimulated glucose transporter (GLUT4) in skeletal muscle. By contrast, the insulin-regulated hexokinase (hexokinase II) parallels Robert Frost's "The Road Not Taken." Here the case is made that an understanding of glucose phosphorylation by hexokinase II is necessary to define the regulation of skeletal muscle glucose uptake in health and insulin resistance. Results of studies from different physiological disciplines that have elegantly described how hexokinase II can be regulated are summarized to provide a framework for potential application to skeletal muscle. Mechanisms by which hexokinase II is regulated in skeletal muscle await rigorous examination.

Blood glucose regulation in context of infection

The immune and endocrine systems collectively control homeostasis in the body. The endocrine system ensures that values of essential factors and nutrients such as glucose, electrolytes and vitamins are maintained within threshold values. The immune system resolves local disruptions in tissue homeostasis, caused by pathogens or malfunctioning cells. The immediate goals of these two systems do not always align. The immune system benefits from optimal access to nutrients for itself and restriction of nutrient availability to all other organs to limit pathogen replication. The endocrine system aims to ensure optimal nutrient access for all organs, limited only by the nutrients stores that the body has available. The actual state of homeostatic parameters such as blood glucose levels represents a careful balance based on regulatory signals from the immune and endocrine systems. This state is not static but continuously adjusted in response to changes in the current metabolic needs of the body, the amount of resources it has available and the level of threats it encounters. This balance is maintained by the ability of the immune and endocrine systems to interact and co-regulate systemic metabolism. In context of metabolic disease, this system is disrupted, which impairs functionality of both systems. The failure of the endocrine system to retain levels of nutrients such as glucose within threshold values impairs functionality of the immune system. In addition, metabolic stress of organs in context of obesity is perceived by the immune system as a disruption in local homeostasis, which it tries to resolve by the excretion of factors which further disrupt normal metabolic control. In this chapter, we will discuss how the immune and endocrine systems interact under homeostatic conditions and during infection with a focus on blood glucose regulation. In addition, we will discuss how this system fails in the context of metabolic disease.

Alleles in metabolic and oxygen-sensing genes are associated with antagonistic pleiotropic effects on life history traits and population fitness in an ecological model insect

Genes with opposing effects on fitness at different life stages are the mechanistic basis for evolutionary theories of aging and life history. Examples come from studies of mutations in model organisms, but there is little knowledge of genetic bases of life history tradeoffs in natural populations. Here, we test the hypothesis that alleles affecting oxygen sensing in Glanville fritillary butterflies have opposing effects on larval versus adult fitness-related traits. Intermediate-frequency alleles in Succinate dehydrogenase d, and to a lesser extent Hypoxia inducible factor 1α, are associated in larvae with variation in metabolic rate and activation of the hypoxia inducible factor (HIF) pathway, which affects tracheal development and delivery of oxygen to adult flight muscles. A dominant Sdhd allele is likely to cause antagonistic pleiotropy for fitness through its opposing effects on larval metabolic and growth rate versus adult flight and dispersal, and may have additional effects arising from sensitivity to low-iron host plants. Prior results in Glanville fritillaries indicate that fitness of alleles in Sdhd and another antagonistically pleiotropic metabolic gene, Phosphoglucose isomerase, depend strongly on the size and distribution of host plant patches. Hence, these intermediate-frequency alleles are involved in ecoevolutionary dynamics involving life history tradeoffs.

'Beauty and the beast' in infection: How immune-endocrine interactions regulate systemic metabolism in the context of infection

The immune and endocrine systems ensure two vital functions in the body. The immune system protects us from lethal pathogens, whereas the endocrine system ensures proper metabolic function of peripheral organs by regulating systemic homeostasis. These two systems were long thought to operate independently. The immune system uses cytokines and immune receptors, whereas the endocrine system uses hormones to regulate metabolism. However, recent findings show that the immune and endocrine systems closely interact, especially regarding regulation of glucose metabolism. In response to pathogen encounter, cytokines modify responsiveness of peripheral organs to endocrine signals, resulting in altered levels of blood hormones such as insulin, which promotes the ability of the body to fight infection. Here we provide an overview of recent literature describing various mechanisms, which the immune system utilizes to modify endocrine regulation of systemic metabolism. Moreover, we will describe how these immune-endocrine interactions derail in the context of obesity. From a clinical perspective we will elaborate how infection and obesity aggravate the development of metabolic diseases such as diabetes mellitus type 2 in humans. In summary, this review provides a comprehensive overview of immune-induced changes in systemic metabolism following infection, with a focus on regulation of glucose metabolism.

Effects of alpha-melanocyte-stimulating hormone on mitochondrial energy metabolism in rats of different age-groups

Hypothalamic alpha-melanocyte-stimulating hormone (α-MSH) is a key catabolic mediator of energy homeostasis. Its anorexigenic and hypermetabolic effects show characteristic age-related alterations that may be part of the mechanism of middle-aged obesity and geriatric anorexia/cachexia seen in humans and other mammals. We aimed to investigate the role of α-MSH in mitochondrial energy metabolism during the course of aging in a rodent model. To determine the role of α-MSH in mitochondrial energy metabolism in muscle, we administered intracerebroventricular (ICV) infusions of α-MSH for 7-days to different age-groups of male Wistar rats. The activities of oxidative phosphorylation complexes I to V and citrate synthase were determined and compared to those of age-matched controls. We also quantified mitochondrial DNA (mtDNA) copy number and measured the expression of the master regulators of mitochondrial biogenesis, peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) and peroxisome proliferator-activated receptor gamma (PPARγ). The peptide reduced weight gain in juvenile rats to one fifth of that of controls and increased the weight loss in older animals by about five fold. Mitochondrial DNA copy number inversely correlated with changes in body weight in controls, but not in α-MSH-treated animals. The strong increase in body weight in young rats was associated with a low mtDNA copy number and high PPARγ mRNA levels in controls. Expression of PGC-1α and PPARγ declined with age, whereas OXPHOS and citrate synthase enzyme activities were unchanged. In contrast, α-MSH treatment suppressed OXPHOS enzyme and citrate synthase activity. In conclusion, our results showed age-related differences in the metabolic effects of α-MSH. In addition, administration of α-MSH suppressed citrate synthase and OXPHOS activities independent of age. These findings suggest that α-MSH exposure may inhibit mitochondrial biogenesis.

Performance effects and metabolic consequences of caffeine and caffeinated energy drink consumption on glucose disposal

This review documents two opposing effects of caffeine and caffeine-containing energy drinks, i.e., their positive effects on athletic performance and their negative impacts on glucose tolerance in the sedentary state. Analysis of studies examining caffeine administration prior to performance-based exercise showed caffeine improved completion time by 3.6%. Similar analyses following consumption of caffeine-containing energy drinks yielded positive, but more varied, benefits, which were likely due to the diverse nature of the studies performed, the highly variable composition of the beverages consumed, and the range of caffeine doses administered. Conversely, analyses of studies administering caffeine prior to either an oral glucose tolerance test or insulin clamp showed a decline in whole-body glucose disposal of ~30%. The consequences of this resistance are unknown, but there may be implications for the development of a number of chronic diseases. Both caffeine-induced performance enhancement and insulin resistance converge with the primary actions of caffeine on skeletal muscle.

Effect of H1- and H2-histamine receptor blockade on postexercise insulin sensitivity

Following a bout of dynamic exercise, humans experience sustained postexercise vasodilatation in the previously exercised skeletal muscle which is mediated by activation of histamine (H1 and H2) receptors. Skeletal muscle glucose uptake is also enhanced following dynamic exercise. Our aim was to determine if blunting the vasodilatation during recovery from exercise would have an adverse effect on blood glucose regulation. Thus, we tested the hypothesis that insulin sensitivity following exercise would be reduced with H1- and H2-receptor blockade versus control (no blockade). We studied 20 healthy young subjects (12 exercise; eight nonexercise sham) on randomized control and H1- and H2-receptor blockade (fexofenadine and ranitidine) days. Following 60 min of upright cycling at 60% VO2 peak or nonexercise sham, subjects consumed an oral glucose tolerance beverage (1.0 g/kg). Blood glucose was determined from "arterialized" blood samples (heated hand vein). Postexercise whole-body insulin sensitivity (Matsuda insulin sensitivity index) was reduced 25% with H1- and H2-receptor blockade (P < 0.05), whereas insulin sensitivity was not affected by histamine receptor blockade in the sham trials. These results indicate that insulin sensitivity following exercise is blunted by H1- and H2-receptor blockade and suggest that postexercise H1- and H2-receptor-mediated skeletal muscle vasodilatation benefits glucose regulation in healthy humans.

Advances in the development of AMPK-activating compounds

BACKGROUND

AMP-activated protein kinase (AMPK) is an energy sensing enzyme that controls glucose and lipid metabolism.

OBJECTIVE

This review summarizes the present data on AMPK as a pharmacologic target for the treatment of metabolic disorders.

METHODS

The mechanisms governing AMPK activity and how this enzyme controls different metabolic pathways are reviewed briefly, and details about the effect that AMPK activators have on glucose metabolism are provided.

CONCLUSION

Evidence obtained using the AMPK-activating compound 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR) suggests that AMPK promotes glucose transport into skeletal muscles and that this enzyme inhibits hepatic glucose production. AICAR also induces fatty acid oxidation in muscle and inhibits cholesterol synthesis in the liver. The metabolic effects of AICAR on glucose and lipid metabolism indicate that AMPK may be a good pharmacologic target for the treatment of type 2 diabetes and hypercholesterolemia. Novel AMPK-specific compounds are allowing researchers to examine whether this enzyme is a useful pharmacologic target for the treatment of human disease and whether chronic activation of AMPK will be safe.

Current understanding of increased insulin sensitivity after exercise - emerging candidates

Exercise counteracts insulin resistance and improves glucose homeostasis in many ways. Apart from increasing muscle glucose uptake quickly, exercise also clearly increases muscle insulin sensitivity in the post-exercise period. This review will focus on the mechanisms responsible for this increased insulin sensitivity. It is believed that increased sarcolemmal content of the glucose transporter GLUT4 can explain the phenomenon to some extent. Surprisingly no improvement in the proximal insulin signalling pathway is observed at the level of the insulin receptor, IRS1, PI3K or Akt. Recently more distal signalling component in the insulin signalling pathway such as aPKC, Rac1, TBC1D4 and TBC1D1 have been described. These are all affected by both insulin and exercise which means that they are likely converging points in promoting GLUT4 translocation and therefore possible candidates for regulating insulin sensitivity after exercise. Whereas TBC1D1 does not appear to regulate insulin sensitivity after exercise, correlative evidence in contrast suggests TBC1D4 to be a relevant candidate. Little is known about aPKC and Rac1 in relation to insulin sensitivity after exercise. Besides mechanisms involved in signalling to GLUT4 translocation, factors influencing the trans-sarcolemmal glucose concentration gradient might also be important. With regard to the interstitial glucose concentration microvascular perfusion is particular relevant as correlative evidence supports a connection between insulin sensitivity and microvascular perfusion. Thus, there are new candidates at several levels which collectively might explain the phenomenon.

The physiological regulation of glucose flux into muscle in vivo

Skeletal muscle glucose uptake increases dramatically in response to physical exercise. Moreover, skeletal muscle comprises the vast majority of insulin-sensitive tissue and is a site of dysregulation in the insulin-resistant state. The biochemical and histological composition of the muscle is well defined in a variety of species. However, the functional consequences of muscle biochemical and histological adaptations to physiological and pathophysiological conditions are not well understood. The physiological regulation of muscle glucose uptake is complex. Sites involved in the regulation of muscle glucose uptake are defined by a three-step process consisting of: (1) delivery of glucose to muscle, (2) transport of glucose into the muscle by GLUT4 and (3) phosphorylation of glucose within the muscle by a hexokinase (HK). Muscle blood flow, capillary recruitment and extracellular matrix characteristics determine glucose movement from the blood to the interstitium. Plasma membrane GLUT4 content determines glucose transport into the cell. Muscle HK activity, cellular HK compartmentalization and the concentration of the HK inhibitor glucose 6-phosphate determine the capacity to phosphorylate glucose. Phosphorylation of glucose is irreversible in muscle; therefore, with this reaction, glucose is trapped and the uptake process is complete. Emphasis has been placed on the role of the glucose transport step for glucose influx into muscle with the past assertion that membrane transport is rate limiting. More recent research definitively shows that the distributed control paradigm more accurately defines the regulation of muscle glucose uptake as each of the three steps that define this process are important sites of flux control.

Local histamine H(1-) and H(2)-receptor blockade reduces postexercise skeletal muscle interstitial glucose concentrations in humans

Elevated blood flow can potentially influence skeletal muscle glucose uptake, but the impact of postexercise hyperemia on glucose availability to skeletal muscle remains unknown. Because postexercise hyperemia is mediated by histamine H(1)- and H(2)-receptors, we tested the hypothesis that postexercise interstitial glucose concentrations would be lower in the presence of combined H1- and H2-receptor blockade. To this end, 4 microdialysis probes were inserted into the vastus lateralis muscle of 14 healthy subjects (21-27 years old) immediately after 60 min of either upright cycling at 60% peak oxygen uptake (exercise, n = 7) or quiet rest (sham, n = 7). Microdialysis probes were perfused with a modified Ringer's solution containing 3 mmol L(-1) glucose, 5 mmol L(-1) ethanol, and [6-3H] glucose (200 disintegrations·min-1 microL(-1)). Two sites (blockade) received both H1- and H2-receptor antagonists (1 mmol L(-1) pyrilamine and 3 mmol L-1 cimetidine) and 2 sites (control) did not receive antagonists. Ethanol outflow/inflow ratios (an inverse surrogate of local blood flow) were higher in blockade sites than in control sites following exercise (p < 0.05), whereas blockade had no effect on ethanol outflow/inflow ratios following sham (p = 0.80). Consistent with our hypothesis, during 3 of the 5 dialysate collection periods, interstitial glucose concentrations were lower in blockade sites vs. control sites following exercise (p < 0.05), whereas blockade had no effect on interstitial glucose concentrations following sham (p = 0.79). These findings indicate that local H1- and H2-receptor activation modulates skeletal muscle interstitial glucose levels during recovery from exercise in humans and suggest that the availability of glucose to skeletal muscle is enhanced by postexercise hyperemia.

Novel method for measuring active tension generation by C2C12 myotube using UV-crosslinked collagen film

We have developed a novel method for measuring active tension generated by cultured myotubes using UV-crosslinked collagen film. Skeletal myoblasts cell line C2C12 or human primary skeletal myoblasts were seeded onto a thin (35 microm) collagen film strip, on which they proliferated and upon induction of differentiation they formed multinucleated myotubes. The collagen film-myotube complex contracted upon electric pulse stimulation which could be observed by light microscope. When collagen film-myotube complex were attached to force transducer, active tension generation was observed upon electric pulse stimulation. Measurement of active tension was possible for multiple times for more than 1 month with the same batch of collagen film-myotube complex. Active tension generation capability of C2C12 myotubes increased with progression of differentiation, reaching maximal value 6 days after induction of differentiation. Using this method, we measured the effect of artificial exercise induced by electric pulse on active tension generation capability of C2C12 myotubes. When the electric pulses of 1 Hz were continuously applied to induce artificial exercise, the active tension augmentation was observed. After 1 week of artificial exercise, the active tension reached approximately 10x of that before the exercise. The increased active tension is attributable to the formation of the sarcomere structure within the myotubes and an increased amount of myotubes on the collagen film. The increased amount of myotubes is possibly due to the suppressed atrophy of myotubes by enhanced expression of Bcl-2.

Four grams of glucose

Four grams of glucose circulates in the blood of a person weighing 70 kg. This glucose is critical for normal function in many cell types. In accordance with the importance of these 4 g of glucose, a sophisticated control system is in place to maintain blood glucose constant. Our focus has been on the mechanisms by which the flux of glucose from liver to blood and from blood to skeletal muscle is regulated. The body has a remarkable capacity to satisfy the nutritional need for glucose, while still maintaining blood glucose homeostasis. The essential role of glucagon and insulin and the importance of distributed control of glucose fluxes are highlighted in this review. With regard to the latter, studies are presented that show how regulation of muscle glucose uptake is regulated by glucose delivery to muscle, glucose transport into muscle, and glucose phosphorylation within muscle.

Glucose directly links to lipid metabolism through high affinity interaction with peroxisome proliferator-activated receptor alpha

The pathophysiology of diabetes is characterized not only by elevated glucose but also elevated long chain fatty acid levels. We show for the first time that the peroxisome proliferator-activated receptor-alpha (PPARalpha) binds glucose and glucose metabolites with high affinity, resulting in significantly altered PPARalpha secondary structure. Glucose decreased PPARalpha interaction with fatty acid metabolites and steroid receptor coactivator-1 while increasing PPARalpha interaction with DNA. Concomitantly, glucose increased PPARalpha interaction with steroid receptor coactivator-1, DNA binding, and transactivation of beta-oxidation pathways in the presence of activating ligands. Heterodimerization of PPARalpha to the retinoid X receptor-alpha resulted in even larger increases in transactivation with the addition of glucose. These data suggest that PPARalpha is responsible for maintaining energy homeostasis through a concentration-dependent regulation of both lipids and sugars and that hyperglycemic injury mediated by PPARalpha occurs not only indirectly through elevated long chain fatty acid levels but also through direct action of glucose on PPARalpha.

Glucose kinetics and exercise tolerance in mice lacking the GLUT4 glucose transporter

The absence of GLUT4 severely impairs basal glucose uptake in vivo, but does not alter glucose homeostasis or circulating insulin. Glucose uptake in isolated contracting skeletal muscle (MGU) is also impaired by the absence of GLUT4, and onset of muscle fatigue is hastened. Whether the body can compensate and preserve glucose homeostasis during exercise, as it does in the basal state, is unknown. One aim was to test the effectiveness of glucoregulatory compensation for the absence of GLUT4 in vivo. The absence of GLUT4 was also used to further define the role of hexokinase (HK) II, which catalyses glucose phosphorylation after it is transported in the cell. HK II increases MGU during exercise, as well as exercise endurance. In the absence of GLUT4, HK II expression will not affect MGU. A second aim was to test whether, in the absence of GLUT4, HK II retains its ability to increase exercise endurance. Wild-type (WT), GLUT4 null (GLUT4(-/-)), and GLUT4 null overexpressing HK II (GLUT4(-/-)HK(Tg)) mice were studied using a catheterized mouse model that allows blood sampling and isotope infusions during treadmill exercise. The impaired capacity of working muscle to take up glucose in GLUT4(-/-) is partially offset by an exaggerated increase in the glucagon: insulin ratio, increased liver glucose production, hyperglycaemia, and a greater capillary density in order to increase the delivery of glucose to the exercising muscle of GLUT4(-/-). Hearts of GLUT4(-/-) also exhibited a compensatory increase in HK II expression and a paradoxical increase in glucose uptake. Exercise tolerance was reduced in GLUT4(-/-) compared to WT. As expected, MGU in GLUT4(-/-)HK(Tg) was the same as in GLUT4(-/-). However, HK II overexpression retained its ability to increase exercise endurance. In conclusion, unlike the basal state where glucose homeostasis is preserved, hyperglycaemia results during exercise in GLUT4(-/-) due to a robust stimulation of liver glucose release in the face of severe impairments in MGU. Finally, studies in GLUT4(-/-)HK(Tg) show that HK II improves exercise tolerance, independent of its effects on MGU.

Multi-scale computational model of fuel homeostasis during exercise: effect of hormonal control

A mathematical model of the whole-body metabolism is developed to predict fuel homeostasis during exercise by using hormonal control over cellular metabolic processes. The whole body model is composed of seven tissue compartments: brain, heart, liver, GI (gastrointestinal) tract, skeletal muscle, adipose tissue, and "other tissues". Each tissue compartment is described by dynamic mass balances and major cellular metabolic reactions. The glucagon-insulin controller is incorporated into the whole body model to predict hormonal changes during exercise. Moderate [150 W power output at 60% of peak oxygen consumption (VO(2max))] exercise for 60 min was implemented by increasing ATP utilization rates in heart and skeletal muscle. Arterial epinephrine level was given as an input function, which directly affects heart and skeletal muscle metabolism and indirectly other tissues via glucagon-insulin controller. Model simulations were validated with experimental data from human exercise studies. The exercise induced changes in hormonal signals modulated metabolic flux rates of different tissues in a coordinated way to achieve glucose homeostasis, demonstrating the efficacy of hormonal control over cellular metabolic processes. From experimental measurements of whole body glucose balance and arterial substrate concentrations, this model could predict the dynamic changes of hepatic glycogenolysis and gluconeogenesis, which are not easy to measure experimentally, suggesting the higher contribution of glycogenolysis ( approximately 75%). In addition, it could provide dynamic information on the relative contribution of carbohydrates and lipids for fuel oxidation in skeletal muscle. Model simulations indicate that external fuel supplies from other tissue/organ systems to skeletal muscle become important for prolonged exercise emphasizing the significance of interaction among tissues. In conclusion, this model can be used as a valuable complement to experimental studies due to its ability to predict what is difficult to measure directly, and usefulness to provide information about dynamic behaviors.

Baseline and stress-induced plasma corticosterone concentrations of mice selectively bred for high voluntary wheel running

The hypothalamic-pituitary-adrenal (HPA) axis is important in regulating energy metabolism and in mediating responses to stressors, including increasing energy availability during physical exercise. In addition, glucocorticoids act directly on the central nervous system and influence behavior, including locomotor activity. To explore potential changes in the HPA axis as animals evolve higher voluntary activity levels, we characterized plasma corticosterone (CORT) concentrations and adrenal mass in four replicate lines of house mice that had been selectively bred for high voluntary wheel running (HR lines) for 34 generations and in four nonselected control (C) lines. We determined CORT concentrations under baseline conditions and immediately after exposure to a novel stressor (40 min of physical restraint) in mice that were housed without access to wheels. Resting daytime CORT concentrations were approximately twice as high in HR as in C mice for both sexes. Physical restraint increased CORT to similar concentrations in HR and C mice; consequently, the proportional response to restraint was smaller in HR than in C animals. Adrenal mass did not significantly differ between HR and C mice. Females had significantly higher baseline and postrestraint CORT concentrations and significantly larger adrenal glands than males in both HR and C lines. Replicate lines showed significant variation in body mass, length, baseline CORT concentrations, and postrestraint CORT concentrations in one or both sexes. Among lines, both body mass and length were significantly negatively correlated with baseline CORT concentrations, suggesting that CORT suppresses growth. Our results suggest that selection for increased locomotor activity has caused correlated changes in the HPA axis, resulting in higher baseline CORT concentrations and, possibly, reduced stress responsiveness and a lower growth rate.

AMP-activated protein kinase: Role in metabolism and therapeutic implications

AMP-activated protein kinase (AMPK) is an enzyme that works as a fuel gauge which becomes activated in situations of energy consumption. AMPK functions to restore cellular ATP levels by modifying diverse metabolic and cellular pathways. In the skeletal muscle, AMPK is activated during exercise and is involved in contraction-stimulated glucose transport and fatty acid oxidation. In the heart, AMPK activity increases during ischaemia and functions to sustain ATP, cardiac function and myocardial viability. In the liver, AMPK inhibits the production of glucose, cholesterol and triglycerides and stimulates fatty acid oxidation. Recent studies have shown that AMPK is involved in the mechanism of action of metformin and thiazolidinediones, and the adipocytokines leptin and adiponectin. These data, along with evidence that pharmacological activation of AMPK in vivo improves blood glucose homeostasis, cholesterol concentrations and blood pressure in insulin-resistant rodents, make this enzyme an attractive pharmacological target for the treatment of type 2 diabetes, ischaemic heart disease and other metabolic diseases.

Metabolic and genetic regulation of cardiac energy substrate preference

Proper heart function relies on high efficiency of energy conversion. Mitochondrial oxygen-dependent processes transfer most of the chemical energy from metabolic substrates into ATP. Healthy myocardium uses mainly fatty acids as its major energy source, with little contribution of glucose. However, lactate, ketone bodies, amino acids or even acetate can be oxidized under certain circumstances. A complex interplay exists between various substrates responding to energy needs and substrate availability. The relative substrate concentration is the prime factor defining preference and utilization rate. Allosteric enzyme regulation and protein phosphorylation cascades, partially controlled by hormones such as insulin, modulate the concentration effect; together they provide short-term adjustments of cardiac energy metabolism. The expression of metabolic machinery genes is also dynamically regulated in response to developmental and (patho)physiological conditions, leading to long-term adjustments. Specific nuclear receptor transcription factors and co-activators regulate the expression of these genes. These include peroxisome proliferator-activated receptors and their nuclear receptor co-activator, estrogen-related receptor and hypoxia-inducible transcription factor 1. Increasing glucose and reducing fatty acid oxidation by metabolic regulation is already a target for effective drugs used in ischemic heart disease and heart failure. Interaction with genetic factors that control energy metabolism could provide even more powerful pharmacological tools.

Ribose versus dextrose supplementation, association with rowing performance: a double-blind study

OBJECTIVE

It has been hypothesized that ribose supplementation rapidly replenishes adenosine triphosphate stores and thereby improves exercise performance. We compared the effects of ribose versus dextrose on rowing performance.

DESIGN

Double-blind randomized trial.

SETTING

Rowing team training area of large midwestern university.

PARTICIPANTS

Thirty-one women collegiate rowers.

INTERVENTIONS

We studied the effects of ribose versus dextrose supplementation (10 g each in 8 oz water) for 8 weeks before and after practice and 2000-m time trials. OUTCOME MEASUREMENTS AND RESULTS: In the time trials, the dextrose group showed significantly more improvement at 8 weeks than the ribose group (median, 15.2 vs. 5.2 s; P = 0.031).

CONCLUSIONS

We doubt ribose impaired, and hypothesize dextrose enhanced, rowing performance. Further research is needed to define what role, if any, dextrose and ribose play as athletic supplements.

Skeletal muscle glucose uptake during exercise: how is it regulated?

The increase in skeletal muscle glucose uptake during exercise results from a coordinated increase in rates of glucose delivery (higher capillary perfusion), surface membrane glucose transport, and intracellular substrate flux through glycolysis. The mechanism behind the movement of GLUT4 to surface membranes and the subsequent increase in transport by muscle contractions is largely unresolved, but it is likely to occur through intracellular signaling involving Ca(2+)-calmodulin-dependent protein kinase, 5'-AMP-activated protein kinase, and possibly protein kinase C.

Effects of endurance exercise-training on single-fiber contractile properties of insulin-treated streptozotocin-induced diabetic rats

The purpose of this study was to characterize the contractile properties of individual skinned muscle fibers from insulin-treated streptozotocin-induced diabetic rats after an endurance exercise training program. We hypothesized that single-fiber contractile function would decrease in the diabetic sedentary rats and that endurance exercise would preserve the function. In the study, 28 rats were assigned to either a nondiabetic sedentary, a nondiabetic exercise, a diabetic sedentary, or a diabetic exercise group. Rats in the diabetic groups received subcutaneous intermediate-lasting insulin daily. The exercise-trained rats ran on a treadmill at a moderate intensity for 60 min, five times per week. After 12 wk, the extensor digitorum longus and soleus muscles were dissected. Single-fiber diameter, Ca2+-activated peak force, specific tension, activation threshold, and pCa50as well as the myosin heavy chain isoform expression (MHC) were determined. We found that in MHC type II fibers from extensor digitorum longus muscle, diameters were significantly smaller from diabetic sedentary rats compared with nondiabetic sedentary rats ( P < 0.001). Among the nondiabetic rats, fiber diameters were smaller with exercise ( P = 0.038). The absolute force-generating capacity of single fibers was lower in muscles from diabetic rats. There was greater specific tension (force normalized to cross-sectional area) by fibers from the rats that followed an endurance exercise program compared with sedentary. From the results, we conclude that alterations in the properties of contractile proteins are not implicated in the decrease in strength associated with diabetes and that endurance-exercise training does not prevent or increase muscle weakness in diabetic rats.

Interstitial glucose in skeletal muscle of diabetic patients during an oral glucose tolerance test

AIM

The presence of a transcapillary arterial-interstitial gradient for glucose (AIG(glu)) in skeletal muscle may be interpreted as a consequence of intact cellular glucose uptake. We hypothesized that the AIG(glu) decreases in Type 2 diabetes mellitus as a consequence of insulin resistance, whereas it remains intact in Type 1 diabetes.

METHODS

Glucose concentrations were measured in serum and interstitial space fluid of skeletal muscle during an oral glucose tolerance test (OGTT) in patients with Type 1 and Type 2 diabetes and in young and middle-aged healthy volunteers, using microdialysis.

RESULTS

The area under the curve for glucose in serum (AUC(SE)) was higher than in interstitial space fluid of skeletal muscle (AUC(MU)) in healthy young (AUC(SE) = 1147 +/- 332 vs. AUC(MU) = 633 +/- 257 mM/min/ml; P = 0.006), healthy middle-aged volunteers (AUC(SE) = 1406 +/- 186 vs. AUC(MU) = 1048 +/- 229 mM/min/ml; P = 0.001) and in Type 1 diabetic patients (AUC(SE) = 2273 +/- 486 vs. AUC(MU) = 1655 +/- 178 mM/min/ml; P = 0.003). In contrast, in Type 2 diabetic patients AUC(SE) (2908 +/- 1023 mM/min/ml) was not significantly different from AUC(MU) (2610 +/- 722 mM/min/ml; P = NS).

CONCLUSION

The present data indicate that AIG(glu) is compromised in Type 2 diabetes in contrast to Type 1 diabetes where it appears to be normal. Because no changes in muscle blood flow were detected, insulin resistance appears to be the main cause for the observed decreased AIG(glu) in skeletal muscle in Type 2 diabetic patients.

Distributed control of glucose uptake by working muscles of conscious mice: roles of transport and phosphorylation

Muscle glucose uptake (MGU) is determined by glucose delivery, transport, and phosphorylation. C57Bl/6J mice overexpressing GLUT4, hexokinase II (HK II), or both were used to determine the barriers to MGU. A carotid artery and jugular vein were catheterized for arterial blood sampling and venous infusions. Experiments were conducted in conscious mice approximately 7 days after surgery. 2-Deoxy-[3H]glucose was administered during rest or treadmill exercise to calculate glucose concentration-dependent (Rg) and -independent (Kg) indexes of MGU. Compared with wild-type controls, GLUT4-overexpressing mice had lowered fasting glycemia (165 +/- 6 vs. 115 +/- 6 mg/dl) and increased Rg by 230 and 166% in the gastrocnemius and superficial vastus lateralis (SVL) muscles under sedentary conditions. GLUT4 overexpression was not able to augment exercise-stimulated Rg or Kg. Whereas HK II overexpression had no effect on fasting glycemia (170 +/- 6 mg/dl) or sedentary Rg, it increased exercise-stimulated Rg by 82, 60, and 169% in soleus, gastrocnemius, and SVL muscles, respectively. Combined GLUT4 and HK II overexpression lowered fasting glycemia (106 +/- 6 mg/dl), increased nonesterified fatty acids, and increased sedentary Rg. Combined GLUT4 and HK II overexpression did not enhance exercise-stimulated Rg compared with HK II-overexpressing mice because of the reduced glucose concentration. GLUT4 combined with HK II overexpression resulted in a marked increase in exercise-stimulated Kg. In conclusion, control of MGU shifts from membrane transport at rest to phosphorylation during exercise. Glucose transport is not normally a significant barrier during exercise. However, when the phosphorylation barrier is lowered by HK II overexpression, glucose transport becomes a key site of control for regulating MGU during exercise.

Hexokinase II partial knockout impairs exercise-stimulated glucose uptake in oxidative muscles of mice

Muscle glucose uptake (MGU) is distributively controlled by three serial steps: delivery of glucose to the muscle membrane, transport across the muscle membrane, and intracellular phosphorylation to glucose 6-phosphate by hexokinase (HK). During states of high glucose fluxes such as moderate exercise, the HK activity is of increased importance, since augmented muscle perfusion increases glucose delivery, and increased GLUT4 at the cell membrane increases glucose transport. Because HK II overexpression augments exercise-stimulated MGU, it was hypothesized that a reduction in HK II activity would impair exercise-stimulated MGU and that the magnitude of this impairment would be greatest in tissues with the largest glucose requirement. To this end, mice with a HK II partial knockout (HK+/-) were compared with their wild-type control (WT) littermates during either sedentary or moderate exercise periods. Rg, an index of glucose metabolism, was measured using 2-deoxy-[3H]glucose. No differences in glucose metabolism were detected between sedentary groups. The increase in Rg due to exercise was impaired in the highly oxidative heart and soleus muscles of HK+/- compared with WT mice (7 +/- 10 vs. 29 +/- 9 and 8 +/- 3 vs. 25 +/- 7 micromol. 100 g-1. min-1, respectively). However, the increase in Rg due to exercise was not altered in gastrocnemius and superficial vastus lateralis muscles in HK+/- and WT mice (8 +/- 2 vs. 12 +/- 3 and 5 +/- 2 vs. 8 +/- 2 micromol. 100 g-1. min-1, respectively). In conclusion, MGU is impaired by reductions in HK activity during exercise, a physiological condition characterized by high glucose flux. This impairment is critically dependent on the tissue's glucose metabolic rate and correlates with tissue oxidative capacity.

Pro- and macroglycogenolysis in skeletal muscle during maximal treadmill exercise

The purpose was to investigate the degradation of proglycogen and macroglycogen in skeletal muscle during intense exercise. Ten Standardbred trotters performed a maximal treadmill exercise test comprising a warm-up period, an exercise period, starting at 7 m/s with increments of 1 m/s every 60 s until the onset of fatigue (mean +/- s.d. 246 +/- 32 s) and a walking recovery period. Muscle biopsies were taken at rest, immediately after exercise and 15 min postexercise. The exercise caused a marked anaerobic metabolism as shown by the decrease in both muscle ATP and creatine phosphate and increase in muscle lactate. Free muscle glucose increased immediately postexercise and a further increase was noted 15 min later. There was a significant decrease (P<0.05) in proglycogen (57.1 +/- 22.2 mmol/kg dw) and macroglycogen (63.0 +/- 65.5 mmol/kg dw) during exercise. The proglycogen concentration tended to increase 15 min after exercise (19.9 +/- 27.3 mmol/kg dw; P = 0.06). The results from this study demonstrate that both proglycogen and macroglycogen contribute equally to glycogenolysis during intense exercise and suggest that glycogen resynthesis starts in the proglycogen pool.

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.

Surgery and intensive care procedures affect the target site distribution of piperacillin

OBJECTIVE

Therapeutic failure of antibiotic therapy has been ascribed to pharmacokinetic alterations in compromised patient populations. The present study, therefore, aimed at examining the influences of cardiac surgery and intensive care procedures on the postoperative target site distribution of piperacillin. For this purpose, the penetration of piperacillin to the interstitial space fluid, the relevant target site for most bacterial infections, was compared between patients after aortic valve replacement and healthy volunteers.

DESIGN

Comparative study in two study populations.

SETTING

The intensive care unit and research ward of a university hospital.

PATIENTS

The study population included six otherwise healthy patients scheduled to undergo aortic valve replacement and a control group of six healthy male volunteers.

INTERVENTIONS

After the administration of a single i.v. infusion of 4.0 g piperacillin, free piperacillin concentrations were measured in the interstitium of skeletal muscle and subcutaneous tissue by in vivo microdialysis and in venous serum. Piperacillin concentrations were assayed with reversed phase high-performance liquid chromatography.

MEASUREMENTS AND MAIN RESULTS

Interstitial piperacillin concentrations in muscle and subcutaneous adipose tissue were significantly lower in patients compared with volunteers with the area under the curve for the interstitium/area under the curve for serum concentration ratios ranging from 0.25 to 0.27 and from 0.43 to 1.22 in patients and volunteers, respectively (p < .05 between groups). The terminal elimination half-life was markedly prolonged in patients, leading to a concomitant increase in t > minimal inhibitory concentration (MIC) values, the relevant surrogate for therapeutic success of therapy with beta-lactam antibiotics, for strains with MIC50 <4 microg/mL. For strains with MIC50 >20 microl/mL, however, inadequate target site concentrations were attained in the patient population.

CONCLUSIONS

During the postoperative and intensive care periods, target site concentrations of piperacillin are markedly altered and decreased. This may also be true for other antibiotic agents and may have clinical implications in that current dosing guidelines may result in inadequate target site concentrations for high-MIC strains. Conceivably, this could lead to therapeutic failure in some patients.