Fuel oxidation in skeletal muscle is increased by nitric oxide/cGMP--evidence for involvement of cGMP-dependent protein kinase

Proteomics and bioinformatics analyses based on two-dimensional electrophoresis and LC-MS/MS for the primary characterization of protein changes in chicken breast meat from divergent farming systems: Organic versus antibiotic-free

Proteomics is a key analytical method in meat research thanks to its potential in investigating the proteins at interplay in post-mortem muscles. This study aimed to characterize for the first time the differences in early post-mortem muscle proteomes of chickens raised under two farming systems: organic versus antibiotic-free. Forty post-mortem Pectoralis major muscle samples from two chicken strains (Ross 308 versus Ranger Classic) reared under organic versus antibiotic-free farming systems were characterized and compared using two-dimensional electrophoresis and LC-MS/MS mass spectrometry. Within antibiotic-free and organic farming systems, 14 and 16 proteins were differentially abundant between Ross 308 and Ranger Classic, respectively. Within Ross 308 and Ranger Classic chicken strains, 12 and 18 proteins were differentially abundant between organic and antibiotic-free, respectively. Bioinformatics was applied to investigate the molecular pathways at interplay, which highlighted the key role of muscle structure and energy metabolism. Antibiotic-free and organic farming systems were found to significantly impact the muscle proteome of chicken breast meat. This paper further proposes a primary list of putative protein biomarkers that can be used for chicken meat or farming system authenticity.

Dietary supplementation with L-leucine reduces nitric oxide synthesis by endothelial cells of rats

This study tested the hypothesis that elevated L-leucine concentrations in plasma reduce nitric oxide (NO) synthesis by endothelial cells (ECs) and affect adiposity in obese rats. Beginning at four weeks of age, male Sprague-Dawley rats were fed a casein-based low-fat (LF) or high-fat (HF) diet for 15 weeks. Thereafter, rats in the LF and HF groups were assigned randomly into one of two subgroups (n = 8/subgroup) and received drinking water containing either 1.02% L-alanine (isonitrogenous control) or 1.5% L-leucine for 12 weeks. The energy expenditure of the rats was determined at weeks 0, 6, and 11 of the supplementation period. At the end of the study, an oral glucose tolerance test was performed on all the rats immediately before being euthanized for the collection of tissues. HF feeding reduced (P < 0.001) NO synthesis in ECs by 21% and whole-body insulin sensitivity by 19% but increased (P < 0.001) glutamine:fructose-6-phosphate transaminase (GFAT) activity in ECs by 42%. Oral administration of L-leucine decreased (P < 0.05) NO synthesis in ECs by 14%, increased (P < 0.05) GFAT activity in ECs by 35%, and reduced (P < 0.05) whole-body insulin sensitivity by 14% in rats fed the LF diet but had no effect (P > 0.05) on these variables in rats fed the HF diet. L-Leucine supplementation did not affect (P > 0.05) weight gain, tissue masses (including white adipose tissue, brown adipose tissue, and skeletal muscle), or antioxidative capacity (indicated by ratios of glutathione/glutathione disulfide) in LF- or HF-fed rats and did not worsen (P > 0.05) adiposity, whole-body insulin sensitivity, or metabolic profiles in the plasma of obese rats. These results indicate that high concentrations of L-leucine promote glucosamine synthesis and impair NO production by ECs, possibly contributing to an increased risk of cardiovascular disease in diet-induced obese rats.

Metabolic effects of L-citrulline in type 2 diabetes

The prevalence of type 2 diabetes (T2D) is increasing worldwide. Decreased nitric oxide (NO) bioavailability is involved in the pathophysiology of T2D and its complications. L-citrulline (Cit), a precursor of NO production, has been suggested as a novel therapeutic agent for T2D. Available data from human and animal studies indicate that Cit supplementation in T2D increases circulating levels of Cit and L-arginine while decreasing circulating glucose and free fatty acids and improving dyslipidemia. The underlying mechanisms for these beneficial effects of Cit include increased insulin secretion from the pancreatic β cells, increased glucose uptake by the skeletal muscle, as well as increased lipolysis and β-oxidation, and decreased glyceroneogenesis in the adipose tissue. Thus, Cit has antihyperglycemic, antidyslipidemic, and antioxidant effects and has the potential to be used as a new therapeutic agent in the management of T2D. This review summarizes available literature from human and animal studies to explore the effects of Cit on metabolic parameters in T2D. It also discusses the possible mechanisms underlying Cit-induced improved metabolic parameters in T2D.

l-Arginine supplementation regulates energy-substrate metabolism in skeletal muscle and adipose tissue of diet-induced obese rats

Dietary supplementation with l-arginine has been reported to reduce white fat mass in diet-induced obese rats and in obese humans. This study was conducted to test the hypothesis that the arginine treatment regulates glucose and fatty acid metabolism in insulin-sensitive tissues. Male Sprague–Dawley rats (4-week-old) were fed either low- or high-fat diets for 15 weeks ( n = 16/diet). Thereafter, lean or obese rats were fed their respective diets and received drinking water containing either 1.51% l-arginine-HCl or 2.55% alanine (isonitrogenous control) ( n = 8/treatment group). After 12 weeks of treatment, rats were euthanized and tissue samples were collected for biochemical assays. High-fat feeding increased the size of adipocytes isolated from retroperitoneal (RP) adipose tissue, while arginine treatment reduced their size. The total number of adipocytes in the adipose tissue did not differ among the four groups of rats. Glucose oxidation in extensor digitorum longus (EDL) muscle, soleus muscle, and RP adipose tissue were reduced in response to high-fat feeding. On the contrary, oleic acid oxidation in RP adipose tissue was enhanced in rats fed the high-fat diet. Arginine treatment stimulated both glucose and oleic acid oxidation in EDL and soleus muscles, while having no effect on glucose oxidation, oleic acid oxidation, or basal lipolysis per 106 adipocytes in RP adipose tissue. Collectively, these results indicate that oral supplementation with arginine to diet-induced obese rats promoted the oxidation of energy substrates in skeletal muscle, thereby reducing white fat in the body.

Bradykinin B2 Receptor Signaling Increases Glucose Uptake and Oxidation: Evidence and Open Questions

The Kinin B2 receptor (B2R) is classically involved in vasodilation and inflammatory responses. However, through the observation of hypoglycemic effects of Angiotensin-I-Converting Enzyme (ACE) inhibitors, this protein has been related to metabolic glucose modulation in physiological and pathophysiological contexts. Although several studies have evaluated this matter, the different methodologies and models employed, combined with the distinct target organs, results in a challenge to summarize and apply the knowledge in this field. Therefore, this review aims to compile human and animal data in order to provide a big picture about what is already known regarding B2R and glucose metabolism, as well to suggest pending investigation issues aiming at evaluating the role of B2R in relation to glucose metabolism in homeostatic situations and metabolic disturbances. The data indicate that B2R signaling is involved mainly in glucose uptake in skeletal muscle and adipose tissue, acting as a synergic player beside insulin. However, most data indicate that B2R induces increased glucose oxidation, instead of storage, via activation of a broad signaling cascade involving Nitric Oxide (NO) and cyclic-GMP dependent protein kinase (PKG). Additionally, we highlight that this modulation is impaired in metabolic disturbances such as diabetes and obesity, and we provide a hypothetic mechanism to explain this blockade in light of literature data provided for this review, as well as other authors.

miRNA-mRNA network regulation in the skeletal muscle fiber phenotype of chickens revealed by integrated analysis of miRNAome and transcriptome

Skeletal muscle fibers are primarily categorized into oxidative and glycolytic fibers, and the ratios of different myofiber types are important factors in determining livestock meat quality. However, the molecular mechanism for determining muscle fiber types in chickens was hardly understood. In this study, we used RNA sequencing to systematically compare mRNA and microRNA transcriptomes of the oxidative muscle sartorius (SART) and glycolytic muscle pectoralis major (PMM) of Chinese Qingyuan partridge chickens. Among the 44,705 identified mRNAs in the two types of muscles, 3,457 exhibited significantly different expression patterns, including 2,364 up-regulated and 1,093 down-regulated mRNAs in the SART. A total of 698 chicken miRNAs were identified, including 189 novel miRNAs, among which 67 differentially expressed miRNAs containing 42 up-regulated and 25 down-regulated miRNAs in the SART were identified. Furthermore, function enrichment showed that the differentially expressed mRNAs and miRNAs were involved in energy metabolism, muscle contraction, and calcium, peroxisome proliferator-activated receptor (PPAR), insulin and adipocytokine signaling. Using miRNA-mRNA integrated analysis, we identified several candidate miRNA-gene pairs that might affect muscle fiber performance, viz, gga-miR-499-5p/SOX6 and gga-miR-196-5p/CALM1, which were supported by target validation using the dual-luciferase reporter system. This study revealed a mass of candidate genes and miRNAs involved in muscle fiber type determination, which might help understand the molecular mechanism underlying meat quality traits in chickens.

The phosphodiesterase 5 inhibitor tadalafil regulates lipidic homeostasis in human skeletal muscle cell metabolism

PURPOSE

Tadalafil seems to ameliorate insulin resistance and glucose homeostasis in humans. We have previously reported that tadalafil targets human skeletal muscle cells with an insulin (I)-like effect. We aim to evaluate in human fetal skeletal muscle cells after tadalafil or I: (i) expression profile of I-regulated genes dedicated to cellular energy control, glycolitic activity or microtubule formation/vesicle transport, as GLUT4, PPARγ, HK2, IRS-1, KIF1C, and KIFAP3; (ii) GLUT4, Flotillin-1, and Caveolin-1 localization, all proteins involved in energy-dependent cell trafficking; (iii) activation of I-targeted paths, as IRS-1, PKB/AKT, mTOR, P70/S6K. Free fatty acids intracellular level was measured. Sildenafil or a cGMP synthetic analog were used for comparison; PDE5 and PDE11 gene expression was evaluated in human fetal skeletal muscle cells.

METHODS

RTq-PCR, PCR, western blot, free fatty acid assay commercial kit, and lipid stain non-fluorescent assay were used.

RESULTS

Tadalafil upregulated I-targeted investigated genes with the same temporal pattern as I (GLUT4, PPARγ, and IRS-1 at 3 h; HK2, KIF1C, KIFAP3 at 12 h), re-localized GLUT4 in cell sites positively immune-decorated for Caveolin-1 and Flotillin-1, suggesting the involvement of lipid rafts, induced specific residue phosphorylation of IRS-1/AKT/mTOR complex in association with free fatty acid de novo synthesis. Sildenafil or GMP analog did not affect GLUT4 trafficking or free fatty acid levels.

CONCLUSION

In human fetal skeletal muscle cells tadalafil likely favors energy storage by modulating lipid homeostasis via IRS-1-mediated mechanisms, involving activation of I-targeted genes and intracellular cascade related to metabolic control. Those data provide some biomolecular evidences explaining, in part, tadalafil-induced favorable control of human metabolism shown by clinical studies.

Molecular mechanisms of ROS production and oxidative stress in diabetes

Oxidative stress and chronic inflammation are known to be associated with the development of metabolic diseases, including diabetes. Oxidative stress, an imbalance between oxidative and antioxidative systems of cells and tissues, is a result of over production of oxidative-free radicals and associated reactive oxygen species (ROS). One outcome of excessive levels of ROS is the modification of the structure and function of cellular proteins and lipids, leading to cellular dysfunction including impaired energy metabolism, altered cell signalling and cell cycle control, impaired cell transport mechanisms and overall dysfunctional biological activity, immune activation and inflammation. Nutritional stress, such as that caused by excess high-fat and/or carbohydrate diets, promotes oxidative stress as evident by increased lipid peroxidation products, protein carbonylation and decreased antioxidant status. In obesity, chronic oxidative stress and associated inflammation are the underlying factors that lead to the development of pathologies such as insulin resistance, dysregulated pathways of metabolism, diabetes and cardiovascular disease through impaired signalling and metabolism resulting in dysfunction to insulin secretion, insulin action and immune responses. However, exercise may counter excessive levels of oxidative stress and thus improve metabolic and inflammatory outcomes. In the present article, we review the cellular and molecular origins and significance of ROS production, the molecular targets and responses describing how oxidative stress affects cell function including mechanisms of insulin secretion and action, from the point of view of possible application of novel diabetic therapies based on redox regulation.

Na,K-ATPase regulation in skeletal muscle

Skeletal muscle contains one of the largest and the most dynamic pools of Na,K-ATPase (NKA) in the body. Under resting conditions, NKA in skeletal muscle operates at only a fraction of maximal pumping capacity, but it can be markedly activated when demands for ion transport increase, such as during exercise or following food intake. Given the size, capacity, and dynamic range of the NKA pool in skeletal muscle, its tight regulation is essential to maintain whole body homeostasis as well as muscle function. To reconcile functional needs of systemic homeostasis with those of skeletal muscle, NKA is regulated in a coordinated manner by extrinsic stimuli, such as hormones and nerve-derived factors, as well as by local stimuli arising in skeletal muscle fibers, such as contractions and muscle energy status. These stimuli regulate NKA acutely by controlling its enzymatic activity and/or its distribution between the plasma membrane and the intracellular storage compartment. They also regulate NKA chronically by controlling NKA gene expression, thus determining total NKA content in skeletal muscle and its maximal pumping capacity. This review focuses on molecular mechanisms that underlie regulation of NKA in skeletal muscle by major extrinsic and local stimuli. Special emphasis is given to stimuli and mechanisms linking regulation of NKA and energy metabolism in skeletal muscle, such as insulin and the energy-sensing AMP-activated protein kinase. Finally, the recently uncovered roles for glutathionylation, nitric oxide, and extracellular K(+) in the regulation of NKA in skeletal muscle are highlighted.

Role of nitric oxide in skeletal muscle glucose uptake during exercise

Nitric oxide is produced within skeletal muscle fibres and has various functions in skeletal muscle. There is evidence that NO may be essential for normal increases in skeletal muscle glucose uptake during contraction/exercise. Although there have been some discrepant results, it has been consistently demonstrated that inhibition of NO synthase (NOS) attenuates the increase in skeletal muscle glucose uptake during contraction in mouse and rat muscle ex vivo, during in situ contraction in rats and during exercise in humans. The NO-mediated increase in skeletal muscle glucose uptake during contraction/exercise is probably due to the modulation of intramuscular signalling that ultimately increases glucose transporter 4 (GLUT4) translocation and is, surprisingly, independent of blood flow. In this review, we discuss the evidence for and against a role of NO in regulating skeletal muscle glucose uptake during contraction/exercise and outline the possible mechanism(s) involved. Emerging findings regarding the role of neuronal NOS mu (nNOSμ) in this process are also discussed.

Cyclic mononucleotides modulate potassium and calcium flux responses to H2O2 in Arabidopsis roots

Cyclic mononucleotides are messengers in plant stress responses. Here we show that hydrogen peroxide (H2O2) induces rapid net K(+)-efflux and Ca(2+)-influx in Arabidopsis roots. Pre-treatment with either 10μM cAMP or cGMP for 1 or 24h does significantly reduce net K(+)-leakage and Ca(2+)-influx, and in the case of the K(+)-fluxes, the cell permeant cyclic mononucleotides are more effective. We also examined the effect of 10μM of the cell permeant 8-Br-cGMP on the Arabidopsis microsomal proteome and noted a specific increase in proteins with a role in stress responses and ion transport, suggesting that cGMP is sufficient to directly and/or indirectly induce complex adaptive changes to cellular stresses induced by H2O2.

Counteracting neuronal nitric oxide synthase proteasomal degradation improves glucose transport in insulin-resistant skeletal muscle from Zucker fa/fa rats

AIMS/HYPOTHESIS

Insulin-mediated glucose transport and utilisation are decreased in skeletal muscle from type 2 diabetic and glucose-intolerant individuals because of alterations in insulin receptor signalling, GLUT4 translocation to the plasma membrane and microvascular blood flow. Catalytic activity of the muscle-specific isoform of neuronal nitric oxide synthase (nNOS) also participates in the regulation of glucose transport and appears to be decreased in a relevant animal model of drastic insulin resistance, the obese Zucker fa/fa rat. Our objective was to determine the molecular mechanisms involved in this defect.

METHODS

Isolated rat muscles and primary cultures of myocytes were used for western blot analysis of protein expression, immunohistochemistry, glucose uptake measurements and GLUT4 translocation assays.

RESULTS

nNOS expression was reduced in skeletal muscle from fa/fa rats. This was caused by increased ubiquitination of the enzyme and subsequent degradation by the ubiquitin proteasome pathway. The degradation occurred through a greater interaction of nNOS with the chaperone heat-shock protein 70 and the co-chaperone, carboxyl terminus of Hsc70-interacting protein (CHIP). In addition, an alteration in nNOS sarcolemmal localisation was observed. We confirmed the implication of nNOS breakdown in defective insulin-induced glucose transport by demonstrating that blockade of proteasomal degradation or overexpression of nNOS improved basal and/or insulin-stimulated glucose uptake and GLUT4 translocation in primary cultures of insulin-resistant myocytes.

CONCLUSIONS/INTERPRETATION

Recovery of nNOS in insulin-resistant muscles should be considered a potential new approach to address insulin resistance.

Regulation of metabolism by cGMP

The second messenger cyclic guanosine monophosphate (cGMP) mediates the physiological effects of nitric oxide and natriuretic peptides in a broad spectrum of tissues and cells. So far, the major focus of research on cGMP lay on the cardiovascular system. Recent evidence suggests that cGMP also plays a major role in the regulation of cellular and whole-body metabolism. Here, we focus on the role of cGMP in adipose tissue. In addition, other organs important for the regulation of metabolism and their regulation by cGMP are discussed. Targeting the cGMP signaling pathway could be an exciting approach for the regulation of energy expenditure and the treatment of obesity.

Acute vagal stimulation attenuates cardiac metabolic response to β-adrenergic stress

The effects of vagal stimulation (VS) on cardiac energy substrate metabolism are unknown. We tested the hypothesis that acute VS alters the balance between free fatty acid (FFA) and carbohydrate oxidation and opposes the metabolic effects of β-adrenergic stimulation. A clinical-type selective stimulator of the vagal efferent fibres was connected to the intact right vagus in chronically instrumented dogs. VS was set to reduce heart rate by 30 beats min(-1), and the confounding effects of bradycardia were then eliminated by pacing the heart at 165 beats min(-1). [(3)H]Oleate and [(14)C]glucose were infused to measure FFA and glucose oxidation. The heart was subjected to β-adrenergic stress by infusing dobutamine at 5, 10 and 15 μg kg(-1) min(-1) before and during VS. VS did not significantly affect baseline cardiac performance, haemodynamics or myocardial metabolism. However, at peak dobutamine stress, VS attenuated the increase in left ventricular pressure-diameter area from 235.9 ± 72.8 to 167.3 ± 55.8%, and in cardiac oxygen consumption from 173.9 ± 23.3 to 127.89 ± 6.2% (both P < 0.05), and thus mechanical efficiency was not enhanced. The increase in glucose oxidation fell from 289.3 ± 55.5 to 131.1 ± 20.9% (P < 0.05), while FFA oxidation was not increased by β-adrenergic stress and fell below baseline during VS only at the lowest dose of dobutamine. The functional and in part the metabolic changes were reversed by 0.1 mg kg(-1) atropine i.v. Our data show that acute right VS does not affect baseline cardiac metabolism, but attenuates myocardial oxygen consumption and glucose oxidation in response to adrenergic stress, thus functioning as a cardio-selective antagonist to β-adrenergic activation.

Effects of neuronal nitric oxide synthase inhibition on microvascular and contractile function in skeletal muscle of aged rats

Advanced age is associated with derangements in skeletal muscle microvascular function during the transition from rest to contractions. We tested the hypothesis that, contrary to what was reported previously in young rats, selective neuronal nitric oxide (NO) synthase (nNOS) inhibition would result in attenuated or absent alterations in skeletal muscle microvascular oxygenation (Po(2)(mv)), which reflects the matching between muscle O(2) delivery and utilization, following the onset of contractions in old rats. Spinotrapezius muscle blood flow (radiolabeled microspheres), Po(2)(mv) (phosphorescence quenching), O(2) utilization (Vo(2); Fick calculation), and submaximal force production were measured at rest and following the onset of contractions in anesthetized old male Fischer 344 × Brown Norway rats (27 to 28 mo) pre- and postselective nNOS inhibition (2.1 μmol/kg S-methyl-l-thiocitrulline; SMTC). At rest, SMTC had no effects on muscle blood flow (P > 0.05) but reduced Vo(2) by ∼23% (P < 0.05), which elevated basal Po(2)(mv) by ∼18% (P < 0.05). During contractions, steady-state muscle blood flow, Vo(2), Po(2)(mv), and force production were not altered after SMTC (P > 0.05 for all). The overall Po(2)(mv) dynamics following onset of contractions was also unaffected by SMTC (mean response time: pre, 19.7 ± 1.5; and post, 20.0 ± 2.0 s; P > 0.05). These results indicate that the locus of nNOS-derived NO control in skeletal muscle depends on age and metabolic rate (i.e., rest vs. contractions). Alterations in nNOS-mediated regulation of contracting skeletal muscle microvascular function with aging may contribute to poor exercise capacity in this population.

Skeletal muscle nitric oxide signaling and exercise: a focus on glucose metabolism

Nitric oxide (NO) is an important vasodilator and regulator in the cardiovascular system, and this link was the subject of a Nobel prize in 1998. However, NO also plays many other regulatory roles, including thrombosis, immune function, neural activity, and gastrointestinal function. Low concentrations of NO are thought to have important signaling effects. In contrast, high concentrations of NO can interact with reactive oxygen species, causing damage to cells and cellular components. A less-recognized site of NO production is within skeletal muscle, where small increases are thought to have beneficial effects such as regulating glucose uptake and possibly blood flow, but higher levels of production are thought to lead to deleterious effects such as an association with insulin resistance. This review will discuss the role of NO in skeletal muscle during and following exercise, including in mitochondrial biogenesis, muscle efficiency, and blood flow with a particular focus on its potential role in regulating skeletal muscle glucose uptake during exercise.

Enhanced external counterpulsation improves peripheral artery function and glucose tolerance in subjects with abnormal glucose tolerance

Background: in coronary artery disease patients, enhanced external counterpulsation (EECP) improves peripheral arterial function and nitric oxide (NO) bioavailability, which have been implicated in the pathogenesis of abnormal glucose tolerance (AGT). We sought to evaluate the effects of EECP on outcomes of arterial function, glucose tolerance, and skeletal muscle morphology in subjects with AGT. Methods and Results: 18 subjects with AGT were randomly (2:1 ratio) assigned to receive either 7 wk (35 1-h sessions) of EECP ( n = 12) or 7 wk of standard care (control; n = 6). Peripheral vascular function, biochemical assays, glucose tolerance, and skeletal muscle morphology were evaluated before and after EECP or control. EECP increased normalized brachial artery (27%) and popliteal artery (52%) flow-mediated dilation. Plasma nitrite/nitrate (NOx) increased (30%) and 8-isoprostane-PGF-F2α, a marker of lipid peroxidation in the plasma, decreased (−23%). Fasting plasma glucose declined (−16.9 ± 5.4 mg/dl), and the homeostasis model assessment of insulin resistance (HOMA-IR) decreased (31%) following EECP. Plasma glucose 120 min after initiation of oral glucose tolerance testing decreased (−28.3 ± 7.3 mg/dl), and the whole body composite insulin sensitivity index (C-ISI) increased (21%). VEGF concentrations increased (75%), and vastus lateralis skeletal muscle biopsies demonstrated improvements in capillary density following EECP. No change was observed in cellular signaling pathways, but there was a significant increase GLUT-4 protein expression (47%) following EECP. Conclusions: our findings provide novel evidence that EECP has a beneficial effect on peripheral arterial function and glucose tolerance in subjects with AGT.

Overexpression of constitutively active PKG-I protects female, but not male mice from diet-induced obesity

Cyclic guanosine monophosphate (cGMP)-dependent protein kinase I (PKG-I) is a multifunctional protein. The direct effects of PKG-I activation on energy homeostasis and obesity development are not well understood. Herein, we generated transgenic mice with expression of the constitutively active PKG-I in adipose tissue as well as in other tissues. Male and female PKG-I overexpressing mice were fed a low-fat (LF) or high-fat (HF) diet for 16 weeks. HF-fed female PKG-I transgenic mice had decreased body weight gain, lower percentage of body fat, and improved glucose tolerance compared to HF-fed wild-type (WT) controls. In contrast, male transgenic PKG-I mice were not resistant to the development of HF-diet-induced obesity, and exhibited similar levels of adiposity and glucose intolerance as HF-fed WT controls. Furthermore, we found that HF-fed female transgenic PKG-I mice had increased energy expenditure and cold-induced adaptive thermogenesis compared to HF-fed WT controls, which was associated with increased expression of uncoupling protein-1 (UCP1) in brown adipose tissue (BAT). In addition, the rates of lipolysis in white adipose tissue (WAT) were also increased in female transgenic PKG-I mice compared to WT controls due to increased phosphorylation of hormone-sensitive lipase (HSL). However, in male mice, adaptive thermogenesis or WAT lipolysis was similar between transgenic PKG-I mice and WT controls. Together, these data demonstrate sex differences in effects of PKG-I activation on the regulation of adipose tissue function and its contribution to diet induced obesity.

Downstream mechanisms of nitric oxide-mediated skeletal muscle glucose uptake during contraction

There is evidence that nitric oxide (NO) is required for the normal increases in skeletal muscle glucose uptake during contraction, but the mechanisms involved have not been elucidated. We examined whether NO regulates glucose uptake during skeletal muscle contractions via cGMP-dependent or cGMP-independent pathways. Isolated extensor digitorum longus (EDL) muscles from mice were stimulated to contract ex vivo, and potential NO signaling pathways were blocked by the addition of inhibitors to the incubation medium. Contraction increased (P < 0.05) NO synthase (NOS) activity (∼40%) and dichlorofluorescein (DCF) fluorescence (a marker of oxidant levels; ∼95%), which was prevented with a NOS inhibitor N(G)-monomethyl-L-arginine (L-NMMA), and antioxidants [nonspecific antioxidant, N-acetylcysteine (NAC); thiol-reducing agent, DTT], respectively. L-NMMA and NAC both attenuated glucose uptake during contraction by ∼50% (P < 0.05), and their effects were not additive. Neither the guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one, which prevents the formation of cGMP, the cGMP-dependent protein (PKG) inhibitor Rp-8-bromo-β-phenyl-1,N2-ethenoguanosine 3',5'-cyclic monophosphorothioate sodium salt nor white light, which breaks S-nitrosylated bonds, affects glucose uptake during contraction; however, DTT attenuated (P < 0.05) contraction-stimulated glucose uptake (by 70%). NOS inhibition and antioxidant treatment reduced contraction-stimulated increases in protein S-glutathionylation and tyrosine nitration (P < 0.05), without affecting AMPK or p38 MAPK phosphorylation. In conclusion, we provide evidence to suggest that NOS-derived oxidants regulate skeletal muscle glucose uptake during ex vivo contractions via a cGMP/PKG-, AMPK-, and p38 MAPK-independent pathway. In addition, it appears that NO and ROS may regulate skeletal muscle glucose uptake during contraction through a similar pathway.

Skeletal muscle glucose uptake during contraction is regulated by nitric oxide and ROS independently of AMPK

Reactive oxygen species (ROS) and nitric oxide (NO) have been implicated in the regulation of skeletal muscle glucose uptake during contraction, and there is evidence that they do so via interaction with AMP-activated protein kinase (AMPK). In this study, we tested the hypothesis that ROS and NO regulate skeletal muscle glucose uptake during contraction via an AMPK-independent mechanism. Isolated extensor digitorum longus (EDL) and soleus muscles from mice that expressed a muscle-specific kinase dead AMPKalpha2 isoform (AMPK-KD) and wild-type litter mates (WT) were stimulated to contract, and glucose uptake was measured in the presence or absence of the antioxidant N-acetyl-l-cysteine (NAC) or the nitric oxide synthase (NOS) inhibitor N(G)-monomethyl-l-arginine (l-NMMA). Contraction increased AMPKalpha2 activity in WT but not AMPK-KD EDL muscles. However, contraction increased glucose uptake in the EDL and soleus muscles of AMPK-KD and WT mice to a similar extent. In EDL muscles, NAC and l-NMMA prevented contraction-stimulated increases in oxidant levels (dichloroflourescein fluorescence) and NOS activity, respectively, and attenuated contraction-stimulated glucose uptake in both genotypes to a similar extent. In soleus muscles of AMPK-KD and WT mice, NAC prevented contraction-stimulated glucose uptake and l-NMMA had no effect. This is likely attributed to the relative lack of neuronal NOS in the soleus muscles compared with EDL muscles. Contraction increased AMPKalpha Thr(172) phosphorylation in EDL and soleus muscles of WT but not AMPK-KD mice, and this was not affected by NAC or l-NMMA treatment. In conclusion, ROS and NO are involved in regulating skeletal muscle glucose uptake during contraction via an AMPK-independent mechanism.

Glucose downregulation of PKG-I protein mediates increased thrombospondin1-dependent TGF-{beta} activity in vascular smooth muscle cells

Diabetes is a major predictor of in-stent restenosis, which is associated with fibroproliferative remodeling of the vascular wall due to increased transforming growth factor-beta (TGF-beta) action. It is well established that thrombospondin1 (TSP1) is a major regulator of TGF-beta activation in renal and cardiac complications of diabetes. However, the role of the TSP1-TGF-beta pathway in macrovascular diabetic complications, including restenosis, has not been addressed. In mesangial cells, high glucose concentrations depress protein kinase G (PKG) activity, but not PKG-I protein, thereby downregulating transcriptional repression of TSP1. Previously, we showed that high glucose downregulates PKG-I protein expression by vascular smooth muscle cells (VSMCs) through altered NADPH oxidase signaling. In the present study, we investigated whether high glucose regulation of PKG protein and activity in VSMCs similarly regulates TSP1 expression and downstream TGF-beta activity. These studies showed that high glucose stimulates both TSP1 expression and TGF-beta bioactivity in primary murine aortic smooth muscle cells (VSMCs). TSP1 is responsible for the increased TGF-beta bioactivity under high glucose conditions, because treatment with anti-TSP1 antibody, small interfering RNA-TSP1, or an inhibitory peptide blocked glucose-mediated increases in TGF-beta activity and extracellular matrix protein (fibronectin) expression. Overexpression of constitutively active PKG, but not the PKG-I protein, inhibited glucose-induced TSP1 expression and TGF-beta bioactivity, suggesting that PKG protein expression is insufficient to regulate TSP1 expression. Together, these data establish that glucose-mediated downregulation of PKG levels stimulates TSP1 expression and enhances TGF-beta activity and matrix protein expression, which can contribute to vascular remodeling in diabetes.

Skeletal muscle glucose uptake during exercise: a focus on reactive oxygen species and nitric oxide signaling

Like insulin, muscle contraction (in vitro or in situ) and exercise increase glucose uptake into skeletal muscle. However, the contraction/exercise pathway of glucose uptake in skeletal muscle is an independent pathway to that of insulin. Indeed, skeletal muscle glucose uptake is normal during exercise in those who suffer from insulin resistance and diabetes. Thus, the pathway of contraction-mediated glucose uptake into skeletal muscle provides an attractive potential target for pharmaceutical treatment and prevention of such conditions, especially as skeletal muscle is the major site of impaired glucose disposal in insulin resistance. The mechanisms regulating skeletal muscle glucose uptake during contraction have not been fully elucidated. Potential regulators include Ca(2+) (via CaMK's and/or CaMKK), AMPK, ROS, and NO signaling, with some redundancy likely to be evident within the system. In this review, we attempt to briefly synthesize current evidence regarding the potential mechanisms involved in regulating skeletal muscle glucose uptake during contraction, focusing on ROS and NO signaling. While reading this review, it will become clear that this is an evolving field of research and that much more work is required to elucidate the mechanism(s) regulating skeletal muscle glucose uptake during contraction.

Potential role of nitric oxide in contraction-stimulated glucose uptake and mitochondrial biogenesis in skeletal muscle

1. The present review discusses the potential role of nitric oxide (NO) in the: (i) regulation of skeletal muscle glucose uptake during exercise; and (ii) activation of mitochondrial biogenesis after exercise. 2. We have shown in humans that local infusion of an NO synthase inhibitor during exercise attenuates increases in skeletal muscle glucose uptake without affecting blood flow. Recent studies from our laboratory in rodents support these findings in humans, although rodent studies from other laboratories have yielded conflicting results. 3. There is clear evidence that NO increases mitochondrial biogenesis in non-contracting cells and that NO influences basal skeletal muscle mitochondrial biogenesis. However, there have been few studies examining the potential role of NO in the activation of mitochondrial biogenesis following an acute bout of exercise or in response to exercise training. Early indications are that NO is not involved in regulating the increase in mitochondrial biogenesis that occurs in response to exercise. 4. Exercise is considered the best prevention and treatment option for diabetes, but unfortunately many people with diabetes do not or cannot exercise regularly. Alternative therapies are therefore critical to effectively manage diabetes. If skeletal muscle NO is found to play an important role in regulating glucose uptake and/or mitochondrial biogenesis, pharmaceutical agents designed to mimic these effects of exercise may improve glycaemic control.

Modulation of glucose uptake in adipose tissue by nitric oxide-generating compounds

There is increasing evidence that endogenous nitric oxide (NO) influences adipogenesis, lipolysis and insulin-stimulated glucose uptake. We investigated the effect of NO released from S-nitrosoglutathione (GSNO) and S-nitroso-N-acetylpenicillamine (SNAP) on basal and insulin-stimulated glucose uptake in adipocytes of normoglycaemic and streptozotocin (STZ)-induced diabetic rats. GSNO and SNAP at 0.2,0.5, and 1 mM brought about a concentration-dependent increase in basal and insulin-stimulated 2-deoxyglucose uptake in adipocytes of normoglycaemic and STZ-induced diabetic rats. SNAP at 1.0 mM significantly elevated basal 2-deoxyglucose uptake (115.8+/-10.4% compared with GSNO at the same concentration (116.1+/-9.4%; P less than 0.05) in STZ-induced diabetic rats. Conversely, SNAP at concentrations of 10 mM and 20 mM significantly decreased basal 2-deoxyglucose uptake by 50.0+/-4.5% and 61.5+/-7.2% respectively in adipocytes of STZ-induced diabetic rats (P less than 0.05). GSNO at concentrations of 10 mM and 20 mM also significantly decreased basal 2-deoxyglucose uptake by 50.8+/-6.4% and 55.2+/-7.8% respectively in adipocytes of STZ-induced diabetic rats (P less than 0.05). These observations indicate that NO released from GSNO and SNAP at 1 mM or less stimulates basal and insulin-stimulated glucose uptake,and at concentrations of 10 mM and 20 mM inhibits basal glucose uptake. The additive effect of GSNO or SNAP, and insulin observed in this study could be due to different mechanisms and warrants further investigation.

Beta3-adrenoceptor agonist stimulation of the Na+, K+ -pump in rat skeletal muscle is mediated by beta2- rather than beta3-adrenoceptors

BACKGROUND AND PURPOSE

In cardiac muscle, BRL 37344, a selective beta3-adrenoceptor agonist, activates the Na+, K+ -pump via NO signalling. This study investigated whether BRL 37344 also activates the Na+, K+ -pump via beta3-adrenoceptors in skeletal muscle.

EXPERIMENTAL APPROACH

Isolated rat soleus muscles were incubated between 1 and 60 min in buffer. Intracellular Na+, K+ content and Na+, K+ -pump activity were measured using flame photometry and ouabain-suppressible 86Rb+ uptake, respectively. Additional muscles were mounted on force transducers and stimulated (60 Hz for 2 s) every 10 min.

KEY RESULTS

BRL 37344 (10(-8) -10(-5) M) induced a concentration- and time-dependent reduction in intracellular Na+, and increased ouabain-suppressible 86Rb+ uptake by up to 112%. BRL 37344-induced reductions in intracellular Na+ were blocked by the beta1/beta2-adrenoceptor antagonist, nadolol (10(-7) M), and the beta2-adrenoceptor antagonist, ICI 118,551 (10(-7) -10(-5) M), but not by beta3- or beta1-adrenoceptor antagonists, SR 59230A (10(-7) M) and CGP 20712A (10(-7) -10(-5) M), respectively. Another beta3-adrenoceptor agonist, CL 316,243, did not alter intracellular Na+. BRL 37344-induced reductions in intracellular Na+ were not blocked by L-NAME, an NOS inhibitor, or ODQ, a guanylyl cyclase inhibitor. The NO donors, SNP and SNAP, did not alter intracellular Na+. BRL 37344 rapidly recovered force in muscles depressed by high [K+]o, an effect that was blocked by nadolol, but not L-NAME.

CONCLUSIONS AND IMPLICATIONS

In rat soleus muscle, the beta3-adrenoceptor agonist BRL 37344 stimulated the Na+, K+ -pump via beta2-adrenoceptors. A more selective beta3-adrenoceptor agonist did not affect Na+, K+ homeostasis in skeletal muscle. NO did not seem to mediate Na+, K+ -pump stimulation in skeletal muscle.

Regulatory role for the arginine–nitric oxide pathway in metabolism of energy substrates

Nitric oxide (NO) is synthesized from L-arginine by NO synthase in virtually all cell types. Emerging evidence shows that NO regulates the metabolism of glucose, fatty acids and amino acids in mammals. As an oxidant, pathological levels of NO inhibit nearly all enzyme-catalyzed reactions through protein oxidation. However, as a signaling molecule, physiological levels of NO stimulate glucose uptake as well as glucose and fatty acid oxidation in skeletal muscle, heart, liver and adipose tissue; inhibit the synthesis of glucose, glycogen, and fat in target tissues (e.g., liver and adipose); and enhance lipolysis in adipocytes. Thus, an inhibition of NO synthesis causes hyperlipidemia and fat accretion in rats, whereas dietary arginine supplementation reduces fat mass in diabetic fatty rats. The putative underlying mechanisms may involve multiple cyclic guanosine-3',5'-monophosphate-dependent pathways. First, NO stimulates the phosphorylation of adenosine-3',5'-monophosphate-activated protein kinase, resulting in (1) a decreased level of malonyl-CoA via inhibition of acetyl-CoA carboxylase and activation of malonyl-CoA decarboxylase and (2) a decreased expression of genes related to lipogenesis and gluconeogenesis (glycerol-3-phosphate acyltransferase, sterol regulatory element binding protein-1c and phosphoenolpyruvate carboxykinase). Second, NO increases the phosphorylation of hormone-sensitive lipase and perilipins, leading to the translocation of the lipase to the neutral lipid droplets and, hence, the stimulation of lipolysis. Third, NO activates expression of peroxisome proliferator-activated receptor-gamma coactivator-1alpha, thereby enhancing mitochondrial biogenesis and oxidative phosphorylation. Fourth, NO increases blood flow to insulin-sensitive tissues, promoting substrate uptake and product removal via the circulation. Modulation of the arginine-NO pathway through dietary supplementation with L-arginine or L-citrulline may aid in the prevention and treatment of the metabolic syndrome in obese humans and companion animals, and in reducing unfavorable fat mass in animals of agricultural importance.

Does Nitric Oxide Regulate Skeletal Muscle Glucose Uptake during Exercise?

Although rodent studies are contradictory, there is accumulating evidence in humans suggesting that nitric oxide (NO) is involved in skeletal muscle glucose uptake during exercise. This brief review discusses this controversial area, including potential upstream regulators of skeletal muscle NO synthase (NOS).

Inhibition of endogenous nitric oxide production influences ovine hindlimb metabolism independently of insulin concentrations1

The hindlimb arteriovenous difference (AVD) model was used to determine whether 30 mg/ kg of the nitric oxide synthase (NOS) inhibitor L-NGnitroarginine methyl ester (hydrochloride; L-NAME) inhibited ovine NO synthesis and influenced muscle metabolism. Eight Border Leicester x Merino cross lambs (50 to 55 kg BW) were infused with saline (control) or saline containing L-NAME via an indwelling jugular vein catheter in a balanced randomized crossover design with 3 d between treatments. The abdominal aorta and deep femoral vein were catheterized for assessment of AVD of hind limb metabolism. Arterial hematocrit and insulin concentration and both arterial and venous concentrations of nitrate/nitrite (NOx), glucose, lactate, NEFA, and urea were determined. Infusion of L-NAME decreased arterial NOx concentrations (P = 0.049), indicating inhibition of systemic NO synthesis. Treatment had no effect on arterial (3.5 vs. 3.6 +/- 0.19 mmol/L for control and L-NAME lambs, respectively; P = 0.39) or venous (3.3 vs. 3.4 +/- 0.16 mmol/L, P = 0.55) plasma glucose concentrations or on glucose AVD (0.19 vs. 0.27 +/- 0.065 mmol/L, P = 0.20). There was an interaction (P = 0.038) between time and treatment, such that L-NAME initially increased the AVD of glucose (up to 180 m) divergent from control lambs. The response was then decreased before a possible inflection beyond 240 min. Infusion of L-NAME increased hindlimb venous NEFA (222 vs. 272 +/- 13.2 micromol/L, P = 0.007) and NEFA AVD (79.4 vs. -13.3 +/- 31.5 micromol/L, P = 0.018). These metabolic changes were independent of plasma insulin concentrations, which were not affected by L-NAME infusion (25.3 vs. 27.8 +/- 3.62 mU/L, P = 0.85). The increase in hindlimb lipolysis after L-NAME infusion does not seem to be due to increased lipolysis of plasma triacylglycerol because circulating arterial (155 vs. 142 +/- 20.8 micromol/L, P = 0.58), venous (154 vs. 140 +/- 20.5 micromol/L, P = 0.50), and AVD (1.0 vs. 2.9 +/- 3.17 micromol/L, P = 0.38) triacylglycerol concentrations were unaffected by L-NAME infusion. In conclusion, these data indicate that infusion of 30 mg of L-NAME/kg inhibits NO synthesis, which in turn influences fat and carbohydrate metabolism in the ovine hindlimb independently of plasma insulin concentrations.

Guanylyl cyclase is an ATP sensor coupling nitric oxide signaling to cell metabolism

Defending cellular integrity against disturbances in intracellular concentrations of ATP ([ATP](i)) is predicated on coordinating the selection of substrates and their flux through metabolic pathways (metabolic signaling), ATP transfer from sites of production to utilization (energetic signaling), and the regulation of processes consuming energy (cell signaling). Whereas NO and its receptor, soluble guanylyl cyclase (sGC), are emerging as key mediators coordinating ATP supply and demand, mechanisms coupling this pathway with metabolic and energetic signaling remain undefined. Here, we demonstrate that sGC is a nucleotide sensor whose responsiveness to NO is regulated by [ATP](i). Indeed, ATP inhibits purified sGC with a K(i) predicting >60% inhibition of NO signaling in cells maintaining physiological [nucleotide](i). ATP inhibits sGC by interacting with a regulatory site that prefers ATP > GTP. Moreover, alterations in [ATP](i), by permeabilization and nucleotide clamping or inhibition of mitochondrial ATP synthase, regulate NO signaling by sGC. Thus, [ATP](i) serves as a "gain control" for NO signaling by sGC. At homeostatic [ATP](i), NO activation of sGC is repressed, whereas insults that reduce [ATP](i,) derepress sGC and amplify responses to NO. Hence, sGC forms a key synapse integrating metabolic, energetic, and cell signaling, wherein ATP is the transmitter, allosteric inhibition the coupling mechanism, and regulated accumulation of cGMP the response.

Effect of NO synthase inhibition on myocardial metabolism during moderate ischemia

Nitric oxide (NO) is involved in the control of myocardial metabolism. In normoperfused myocardium, NO synthase inhibition shifts myocardial metabolism from free fatty acid (FFA) toward carbohydrate utilization. Ischemic myocardium is characterized by a similar shift toward preferential carbohydrate utilization, although NO synthesis is increased. The importance of NO for myocardial metabolism during ischemia has not been analyzed in detail. We therefore assessed the influence of NO synthase inhibition with N(G)-nitro-l-arginine (l-NNA) on myocardial metabolism during moderate ischemia in anesthetized pigs. In control animals, the increase in left ventricular pressure with l-NNA was mimicked by aortic constriction. Before ischemia, l-NNA decreased myocardial FFA consumption (MV(FFA); P < 0.05), while consumption of carbohydrate and O(2) (MVo(2)) remained constant. ATP equivalents [calculated with the assumption of complete oxidative substrate decomposition (ATP(eq))] decreased with l-NNA (P < 0.05), associated with a decrease of regional myocardial function (P < 0.05). In contrast, aortic constriction had no effect on MV(FFA), while MVo(2) increased (P < 0.05) and ATP(eq) and regional myocardial function remained constant. During ischemia, alterations in myocardial metabolism were similar in control and l-NNA-treated animals: MV(FFA) decreased (P < 0.05) and net lactate consumption was reversed to net lactate production (P < 0.05). Regional myocardial function was decreased (P < 0.05), although more markedly in animals receiving l-NNA (P < 0.05). We conclude that the efficiency of oxidative metabolism was impaired by l-NNA per se, paralleled by impaired regional myocardial function. During ischemia, l-NNA had no effect on myocardial substrate consumption, indicating that NO synthases were no longer effectively involved in the control of myocardial metabolism.

Role of nitric oxide in the regulation of substrate metabolism in heart failure

Solid experimental evidence indicates that nitric oxide (NO) inhibits oxygen utilization in vitro and in vivo. The role played by NO in cellular metabolism is likely extended to the control of substrate utilization. Studies performed in normal hearts show that NO inhibits glucose uptake and that a reduced synthesis of NO impairs free fatty acid consumption. Interestingly, we found also that myocardial free fatty acid utilization decreases while glucose consumption is enhanced in end stage heart failure, when cardiac NO production falls dramatically. This phenomenon led us to the hypothesis that the reduced synthesis of NO could be at least in part responsible for myocardial metabolic alterations occurring in severe heart failure. The present review mentions some of the seminal studies that defined the function of NO as metabolic modulator. A particular emphasis is put on available data suggesting a role for NO in the control of cardiac substrate utilization in normal and failing hearts.

A receptor-type guanylyl cyclase expression is regulated under circadian clock in peripheral tissues of the silk moth. Light-induced shifting of the expression rhythm and correlation with eclosion

The mechanisms by which the circadian clock controls behavior through regulating gene expression in peripheral tissues are largely unknown. Here we demonstrate that the expression of a receptor-type guanylyl cyclase (BmGC-I) from the silk moth Bombyx mori is regulated in the flight muscles in a circadian fashion. BmGC-I mRNA was expressed from the end of the light period through the middle of the dark period. BmGC-I protein expression and cGMP levels were high around the initiation of eclosion events at the beginning of the photoperiod. The rhythm of the BmGC-I and cGMP levels free-ran in constant light and synchronized to the environmental photoperiodic cycle. The circadian regulation of BmGC-I expression was also observed in the legs but not in other tissues examined. BmGC-I therefore represents a circadian output gene that regulates eclosion behavior.

Endothelial- and nitric oxide-dependent effects on oxidative metabolism of intact artery

Oxidative metabolism and its possible modulation by nitric oxide (NO) was examined in endothelial-intact and endothelial-denuded segments of porcine carotid arteries. Endothelial-intact arteries displayed appropriate NO-mediated vasorelaxation to acetylcholine (ACh). Endothelial-denuded arteries demonstrated absent vasorelaxation to ACh stimulation and depressed contractile responsiveness to K(+) depolarization, which was normalized by inhibition of NO synthesis by N(omega)-nitro-L-arginine methylester (L-NAME). Confirmation that carotid arteries continued to produce NO despite removal of the endothelium was indicated by detection of NO metabolites in the incubation medium bathing the arteries. O(2) consumption and the oxidation of glucose and fatty acid were depressed in endothelial-denuded arteries. Depression of O(2) consumption and glucose oxidation was completely reversed by treatment with L-NAME. We conclude that endogenous NO produced by non-endothelial vascular cells depresses contractility, O(2) consumption, and oxidation of energy substrates in vascular smooth muscle. The endothelium may play a role in oxidative metabolism of vascular smooth muscle possibly by modulating the effects of NO produced by other cells of the vessel wall, or by other factors.

Nitric oxide: biologic effects on muscle and role in muscle diseases

Nitric oxide is a ubiquitous cell-signaling molecule involved in regulation of numerous homeostatic mechanisms and in mediation of tissue injury. Nitric oxide influences contraction, blood flow, and metabolism, as well as myogenesis. Nitric oxide exerts its influence by activation of guanylate cyclase and nitrosylation of proteins, which include glyceraldehyde-3-phosphate dehydrogenase, the ryanodine receptor and actomyosin ATPase. Skeletal muscle expresses all three isoforms of the nitric oxide synthase, including a muscle-specific splice variant; expression of the isoforms is fiber-type specific and influenced by age and disease. Nitric oxide produced with certain systemic conditions and local inflammation is likely toxic to skeletal muscle, either directly or in reactions with oxygen-derived radicals. Although nitric oxide impacts on many functions in muscle, its effects are subtle, and much work remains to be done to determine its importance in the pathogenesis of muscle diseases.

Nitric oxide, reactive oxygen species, and skeletal muscle contraction

Nitric oxide (NO) derivatives and reactive oxygen species (ROS) modulate contractile function of respiratory and limb skeletal muscle. The intracellular processes regulated by NO and ROS remain enigmatic, however. Studies of reduced preparations have identified a number of regulatory proteins that exhibit altered function when exposed to exogenous NO or ROS donors ex vivo. The relative importance of these targets in the intact cell is not known and conflicting theories abound regarding the mechanism(s) whereby NO and ROS regulate contraction. This review article provides a personal perspective on the processes regulated by NO and ROS by addressing three major topics: 1) the regulatory mechanisms by which endogenous NO depresses force production, 2) the processes whereby endogenous ROS modulate contraction of unfatigued muscle, and 3) the site(s) of action and reversibility of ROS effects in muscle fatigue.

Regulation of mitochondrial respiration by nitric oxide inhibition of cytochrome c oxidase

Nitric oxide (NO) and its derivatives inhibit mitochondrial respiration by a variety of means. Nanomolar concentrations of NO immediately, specifically and reversibly inhibit cytochrome oxidase in competition with oxygen, in isolated cytochrome oxidase, mitochondria, nerve terminals, cultured cells and tissues. Higher concentrations of NO and its derivatives (peroxynitrite, nitrogen dioxide or nitrosothiols) can cause irreversible inhibition of the respiratory chain, uncoupling, permeability transition, and/or cell death. Isolated mitochondria, cultured cells, isolated tissues and animals in vivo display respiratory inhibition by endogenously produced NO from constitutive isoforms of NO synthase (NOS), which may be largely mediated by NO inhibition of cytochrome oxidase. Cultured cells expressing the inducible isoform of NOS (iNOS) can acutely and reversibly inhibit their own cellular respiration and that of co-incubated cells due to NO inhibition of cytochrome oxidase, but after longer-term incubation result in irreversible inhibition of cellular respiration due to NO or its derivatives. Thus the NO inhibition of cytochrome oxidase may be involved in the physiological and/or pathological regulation of respiration rate, and its affinity for oxygen.

Physiology of nitric oxide in skeletal muscle

In the past five years, skeletal muscle has emerged as a paradigm of "nitric oxide" (NO) function and redox-related signaling in biology. All major nitric oxide synthase (NOS) isoforms, including a muscle-specific splice variant of neuronal-type (n) NOS, are expressed in skeletal muscles of all mammals. Expression and localization of NOS isoforms are dependent on age and developmental stage, innervation and activity, history of exposure to cytokines and growth factors, and muscle fiber type and species. nNOS in particular may show a fast-twitch muscle predominance. Muscle NOS localization and activity are regulated by a number of protein-protein interactions and co- and/or posttranslational modifications. Subcellular compartmentalization of the NOSs enables distinct functions that are mediated by increases in cGMP and by S-nitrosylation of proteins such as the ryanodine receptor-calcium release channel. Skeletal muscle functions regulated by NO or related molecules include force production (excitation-contraction coupling), autoregulation of blood flow, myocyte differentiation, respiration, and glucose homeostasis. These studies provide new insights into fundamental aspects of muscle physiology, cell biology, ion channel physiology, calcium homeostasis, signal transduction, and the biochemistry of redox-related systems.

Nitric oxide as a competitive inhibitor of oxygen consumption in the mitochondrial respiratory chain

Nitric oxide (NO) and its derivatives inhibit mitochondrial respiration by various means. The author and others have shown that low (nanomolar) concentrations of NO immediately, specifically and reversibly inhibit cytochrome oxidase in competition with oxygen, in isolated cytochrome oxidase, mitochondria, nerve terminals, cultured cells and tissues. Primary astrocytes and a macrophage cell line, activated by cytokines and endotoxin to express the inducible isoform of NO synthase, strongly and reversibly inhibited their own respiration and that of co-incubated cells by this means. Primary aortic endothelial cells transiently inhibited their own respiration when NO production was acutely stimulated with bradykinin or ATP, and basal NO release increased the apparent Km for oxygen of respiration in these cells. Thus the NO inhibition of cytochrome oxidase may be involved in the physiological and/or pathological regulation of respiration rate and its affinity for oxygen.

Myocardial glucose uptake is regulated by nitric oxide via endothelial nitric oxide synthase in Langendorff mouse heart

Although the role of nitric oxide (NO) in the modulation of vascular tone has been studied and well understood, its potential role in the control of myocardial metabolism is only recently evident. Several lines of evidence indicate that NO regulates myocardial glucose metabolism; however, the details and mechanisms responsible are still unknown. The aim of this study was to further define the role of NO in the control of myocardial glucose metabolism and the nitric oxide synthase (NOS) isoform responsible using transgenic animals lacking endothelial NOS (ecNOS). In the present study, we examined the regulation of myocardial glucose uptake using isometrically contracting Langendorff-perfused hearts from normal mice (C57BL/6J), mice with defects in the expression of ecNOS [ecNOS (-/-)], and its heterozygote [ecNOS (+/-)], and wild-type mice [ecNOS (+/+)] (n=6, respectively). In hearts from normal mice, little myocardial glucose uptake was observed. This myocardial glucose uptake increased significantly in the presence of N(omega)-nitro-L-arginine methyl ester (L-NAME). Similarly, in the hearts from ecNOS (-/-), glucose uptake was much greater than in normal mice, whereas myocardial glucose uptake of ecNOS (+/-) and ecNOS (+/+) mice was not different from normal mice. In addition, myocardial glucose uptake of ecNOS (+/-) and ecNOS (+/+) mice increased significantly in the presence of L-NAME. At a workload of 800 g. beats/min, L-NAME increased glucose uptake from 0.1+/-0.1 to 3+/-0.4 microg/min x mg in ecNOS (+/-) mice and from 0.2+/-0.1 to 2.7+/-0.7 microg/min x mg in ecNOS (+/+) mice. Furthermore, in the hearts from ecNOS (-/-) mice, 8-bromoguanosine 3':5'-cyclic monophosphate (8-Br-cGMP), a cGMP analog or S-nitroso-N-acetylpenicillamine (SNAP), a NO donor essentially shut off glucose uptake, and in hearts from ecNOS (+/-) mice, 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ), an inhibitor of cGMP, increased the glucose uptake significantly. These results indicate clearly that cardiac NO production regulates myocardial glucose uptake via a cGMP-dependent mechanism and strongly suggest that ecNOS plays a pivotal role in this regulation. These findings may be important in the understanding of the pathogenesis of the diseases such as ischemic heart disease, heart failure, diabetes mellitus, hypertension, and hypercholesterolemia, in which NO synthesis is altered and substrate utilization by the heart changes.

Sodium nitroprusside increases human skeletal muscle blood flow, but does not change flow distribution or glucose uptake

1. The role of blood flow as a determinant of skeletal muscle glucose uptake is at present controversial and results of previous studies are confounded by possible direct effects of vasoactive agents on glucose uptake. Since increase in muscle blood flow can be due to increased flow velocity or recruitment of new capillaries, or both, it would be ideal to determine whether the vasoactive agent affects flow distribution or only increases the mean flow. 2. In the present study blood flow, flow distribution and glucose uptake were measured simultaneously in both legs of 10 healthy men (aged 29 +/- 1 years, body mass index 24 +/- 1 kg m-2) using positron emission tomography (PET) combined with [15O]H2O and [18F]fluoro-2-deoxy-D-glucose (FDG). The role of blood flow in muscle glucose uptake was studied by increasing blood flow in one leg with sodium nitroprusside (SNP) and measuring glucose uptake simultaneously in both legs during euglycaemic hyperinsulinaemia (insulin infusion 6 pmol kg-1 min-1). 3. SNP infusion increased skeletal muscle blood flow by 86 % (P < 0.01), but skeletal muscle flow distribution and insulin-stimulated glucose uptake (61.4 +/- 7. 5 vs. 67.0 +/- 7.5 micromol kg-1 min-1, control vs. SNP infused leg, not significant), as well as flow distribution between different tissues of the femoral region, remained unchanged. The effect of SNP infusion on blood flow and distribution were unchanged during infusion of physiological levels of insulin (duration, 150 min). 4. Despite a significant increase in mean blood flow induced by an intra-arterial infusion of SNP, glucose uptake and flow distribution remained unchanged in resting muscles of healthy subjects. These findings suggest that SNP, an endothelium-independent vasodilator, increases non-nutritive, but not nutritive flow or capillary recruitment.

Acute hyperglycemia depresses arteriolar NO formation in skeletal muscle

In the rat intestinal and cerebral microvasculatures, acute D-glucose hyperglycemia suppresses endothelium-dependent dilation to ACh without affecting endothelium-independent dilation to nitroprusside. This study determined whether acute hyperglycemia suppressed arteriolar wall nitric oxide concentration ([NO]) at rest or during ACh stimulation and inhibited nitroprusside-, ACh- or contraction-induced dilation of rat spinotrapezius arterioles. Vascular responses were measured before and after 1 h of topical 300 mg/100 ml D-glucose; arteriolar [NO] was measured with NO-sensitive microelectrodes. Arteriolar dilation to ACh was not significantly altered after superfusion of 300 mg/100 ml D-glucose. However, after hyperglycemia, arteriolar [NO] was not increased by ACh, compared with a 300 nM increase attained during normoglycemia. Arteriolar dilation to submaximal nitroprusside and muscle contractions was enhanced by hyperglycemia. These results indicated that in the rat spinotrapezius muscle, acute hyperglycemia suppressed arteriolar NO production while simultaneously augmenting vascular smooth muscle responsiveness to nitroprusside, presumably through cGMP-mediated mechanisms. In effect, this may have allowed ACh- and muscle contraction-induced vasodilation to be maintained during hyperglycemia despite an impaired NO system.

Nitric oxide and mitochondrial respiration

Nitric oxide (NO) and its derivative peroxynitrite (ONOO-) inhibit mitochondrial respiration by distinct mechanisms. Low (nanomolar) concentrations of NO specifically inhibit cytochrome oxidase in competition with oxygen, and this inhibition is fully reversible when NO is removed. Higher concentrations of NO can inhibit the other respiratory chain complexes, probably by nitrosylating or oxidising protein thiols and removing iron from the iron-sulphur centres. Peroxynitrite causes irreversible inhibition of mitochondrial respiration and damage to a variety of mitochondrial components via oxidising reactions. Thus peroxynitrite inhibits or damages mitochondrial complexes I, II, IV and V, aconitase, creatine kinase, the mitochondrial membrane, mitochondrial DNA, superoxide dismutase, and induces mitochondrial swelling, depolarisation, calcium release and permeability transition. The NO inhibition of cytochrome oxidase may be involved in the physiological regulation of respiration rate, as indicated by the finding that isolated cells producing NO can regulate cellular respiration by this means, and the finding that inhibition of NO synthase in vivo causes a stimulation of tissue and whole body oxygen consumption. The recent finding that mitochondria may contain a NO synthase and can produce significant amounts of NO to regulate their own respiration also suggests this regulation may be important for physiological regulation of energy metabolism. However, definitive evidence that NO regulation of mitochondrial respiration occurs in vivo is still missing, and interpretation is complicated by the fact that NO appears to affect tissue respiration by cGMP-dependent mechanisms. The NO inhibition of cytochrome oxidase may also be involved in the cytotoxicity of NO, and may cause increased oxygen radical production by mitochondria, which may in turn lead to the generation of peroxynitrite. Mitochondrial damage by peroxynitrite may mediate the cytotoxicity of NO, and may be involved in a variety of pathologies.

Nonpregnant sheep uterine type I and type III nitric oxide synthase expression is differentially regulated by estrogen

The aim of this study was to characterize the effect of estrogen on the expression of neuronal and endothelial isoforms of nitric oxide (NO) synthase (NOS) in myometrium, endometrium, and caruncle (nonglandular endometrium) in nonpregnant sheep. Twenty sheep were castrated during synchronized estrus (Days 14-16) and 4 days after surgery treated i.v. through the jugular with 100 microg/day of estradiol-17beta for 5 (n = 6) or 8 (n = 6) days or with vehicle (n = 8). Nitric oxide synthase mRNA was measured by ribonuclease protection assay, and NOS protein mass was measured by Western immunoblotting. Data were analyzed by ANOVA and Tukey's test. The three distinct uterine compartments studied contained the mRNA and protein for the neuronal (type I NOS) and the endothelial (type III NOS) isoforms of NOS. However, no inducible NOS was detected. Estrogen exhibited a differential effect on NOS expression in a tissue compartment- and NOS isoform-specific manner. In myometrium and caruncles, but not in endometrium, type I NOS mRNA and protein mass increased significantly (p < 0.05) after 5 or 8 days of estrogen. In contrast, type III NOS increased significantly in myometrium only after 8 days, whereas in endometrium and caruncles the increase was significant in the 5-day treatment group (p < 0.05). We conclude that the expression of type I NOS and type III NOS in the uterus are differentially regulated by estrogen. This differential regulation suggests that the NO produced within the uterus serves more than one physiological role. In myometrium it may be a uterorelaxant and regulate glucose utilization, and in endometrium and myometrium it may regulate blood flow.

Inactivation of aconitase and oxoglutarate dehydrogenase in skeletal muscle in vitro by superoxide anions and/or nitric oxide

Strips of rat soleus muscle were incubated in media containing a superoxide generating system and/or the nitric oxide donor sodium nitroprusside (SNP) before the maximal catalytic activities of aconitase, citrate synthase, and oxoglutarate dehydrogenase were measured. The maximal activities of aconitase and oxoglutarate dehydrogenase were both decreased by 25-30% by superoxide anions; however, only the maximal activity of aconitase was decreased, by approximately 50%, by incubation of muscles with SNP. Furthermore, when both superoxide and NO were present in the medium, aconitase activity was decreased by 70%. The maximal activity of citrate synthase was not affected by any of the treatments. This is the first time that superoxide anions or NO has been shown to inactivate aconitase and oxoglutarate dehydrogenase in skeletal muscle. It is suggested that these effects may be responsible for some alterations in skeletal muscle metabolism, and these possibilities are discussed.