Systemic nitric oxide synthase inhibition increases insulin sensitivity in man

Mechanisms of obesity-induced metabolic and vascular dysfunctions

Obesity has reached epidemic proportions and its prevalence is climbing. Obesity is characterized by hypertrophied adipocytes with a dysregulated adipokine secretion profile, increased recruitment of inflammatory cells, and impaired metabolic homeostasis that eventually results in the development of systemic insulin resistance, a phenotype of type 2 diabetes. Nitric oxide synthase (NOS) is an enzyme that converts L-arginine to nitric oxide (NO), which functions to maintain vascular and adipocyte homeostasis. Arginase is a ureohydrolase enzyme that competes with NOS for L-arginine. Arginase activity/expression is upregulated in obesity, which results in diminished bioavailability of NO, impairing both adipocyte and vascular endothelial cell function. Given the emerging role of NO in the regulation of adipocyte physiology and metabolic capacity, this review explores the interplay between arginase and NO, and their effect on the development of metabolic disorders, cardiovascular diseases, and mitochondrial dysfunction in obesity. A comprehensive understanding of the mechanisms involved in the development of obesity-induced metabolic and vascular dysfunction is necessary for the identification of more effective and tailored therapeutic avenues for their prevention and treatment.

Inhibition of nitric oxide release pre-slaughter increases post-mortem glycolysis and improves tenderness in ovine muscles

The aim of this experiment was to determine the effect of inhibiting the release of nitric oxide (NO) pre-slaughter in lambs on post-slaughter muscle metabolism and meat quality. Exercise was used as a positive control as NO is known to be released in skeletal muscle during exercise. Forty Border Leicester×Merino lambs were assigned to the treatments L-NAME (NO synthase inhibitor) infusion (0mg/kg vs. 30mg/kg, 135min pre-slaughter) and exercise (none vs. 15min immediately pre-slaughter). The inhibition of NO release using L-NAME reduced Warner-Bratzler shear force (WBSF) in the longissimus thoracis et lumborum (LTL) after 3days of ageing, while the Semimembranosous (SM) was unaffected. Inhibition of NO release with L-NAME resulted in altered glucose metabolism as indicated by reduced plasma glucose pre-slaughter particularly in exercised lambs, reduced LTL and SM glycogen of non-exercised lambs post-slaughter and increased SM lactate in exercised lambs post-slaughter. In conclusion, inhibition of NO Synthase with L-NAME pre-slaughter increases post-mortem glycolysis and improves tenderness in the loin muscle.

Nitrosative stress and pathogenesis of insulin resistance

Insulin resistance is a major causative factor for type 2 diabetes and is associated with increased risk of cardiovascular disease. Despite intense investigation for a number of years, molecular mechanisms underlying insulin resistance remain to be determined. Recently, chronic inflammation has been highlighted as a culprit for obesity-induced insulin resistance. Nonetheless, upstream regulators and downstream effectors of chronic inflammation in insulin resistance remain unclarified. Inducible nitric oxide synthase (iNOS), a mediator of inflammation, has emerged as an important player in insulin resistance. Obesity is associated with increased iNOS expression in insulin-sensitive tissues in rodents and humans. Inhibition of iNOS ameliorates obesity-induced insulin resistance. However, molecular mechanisms by which iNOS mediates insulin resistance remain largely unknown. Protein S-nitrosylation, a covalent attachment of NO moiety to thiol sulfhydryls, has emerged as a major mediator of a broad array of NO actions. S-nitrosylation is elevated in patients with type 2 diabetes, and increased S-nitrosylation of insulin signaling molecules, including insulin receptor, insulin receptor substrate-1, and Akt/PKB, has been shown in skeletal muscle of obese, diabetic mice. Akt/PKB is reversibly inactivated by S-nitrosylation. Based on these findings, S-nitrosylation has recently been proposed to play an important role in the pathogenesis of insulin resistance.

Insulin sensitivity and resistin expression in nitric oxide-deficient rats

AIMS/HYPOTHESIS

The aim of this study was to investigate changes in insulin sensitivity and expression of the gene encoding resistin (Retn) in adipocytes from long-term nitric oxide (NO)-deficient rats.

METHODS

Male Sprague-Dawley rats received [Formula: see text]-nitro-L: -arginine methyl ester (L-NAME 0.5 mg/ml) in their drinking water for 4 weeks, while control rats received plain drinking water. During the experimental period, changes in plasma glucose, insulin and C-peptide levels were measured. After administration of L-NAME for 4 weeks, insulin sensitivity was evaluated in vivo and in vitro. An insulin binding assay was also performed to determine the number and binding affinity of insulin receptors in adipocytes. Adipocyte Retn mRNA levels were examined using northern blotting.

RESULTS

Successful induction of NO deficiency was demonstrated by an increase in systemic blood pressure. No difference in plasma glucose levels was found between the two groups. Compared with the control rats, plasma insulin and C-peptide levels were significantly decreased in the NO-deficient rats, and insulin sensitivity was significantly increased. Insulin-stimulated glucose uptake and insulin binding capacity, but not binding affinity, were significantly increased in adipocytes isolated from NO-deficient rats. In addition, adipocyte Retn mRNA levels, but not plasma resistin levels, were significantly decreased in NO-deficient rats, and the Retn mRNA levels were negatively correlated with insulin sensitivity.

CONCLUSIONS/INTERPRETATION

Insulin sensitivity was increased in NO-deficient rats and this was associated with insulin binding capacity and downregulated Retn expression. These findings suggest that NO plays a regulatory role in metabolism. Dysregulation of NO production may result in the development of metabolic disorders.

Angiotensin II enhances insulin sensitivity in vitro and in vivo

The renin-angiotensin system plays a critical role in the pathogenesis of obesity, obesity-associated hypertension, and insulin resistance. However, the biological actions of angiotensin II (AII) on insulin sensitivity remain controversial. Because angiotensinogen and AII receptors are expressed on adipose tissue, we investigated the effect of AII on the insulin sensitivity of isolated rat adipocytes. The results of a receptor binding assay showed the maximal AII binding capacity of adipocytes to be 8.3 +/- 0.9 fmol/7 x 10(6) cells and the dissociation constant to be 2.72 +/- 0.11 nM. Substantial expression of both type 1 and 2 AII (AT1 and AT2) receptors was detected by RT-PCR. AII had no effect on basal glucose uptake, but significantly potentiated insulin-stimulated glucose uptake; this effect was abolished by the AT1 antagonist, losartan. In addition, AII did not alter the insulin binding capacity of adipocytes, but increased insulin-stimulated tyrosine phosphorylation of the insulin receptor beta-subunit, Akt phosphorylation, and translocation of glucose transporter 4 to the plasma membrane. AII potentiated insulin-stimulated glucose uptake through the AT1 receptor and by alteration of the intracellular signaling of insulin. Intraperitoneal injection of Sprague Dawley rats with AII increased insulin sensitivity in vivo. In conclusion, we have shown that AII enhances insulin sensitivity both in vitro and in vivo, suggesting that dysregulation of the insulin-sensitizing effect of AII may be involved in the development of insulin resistance.

S-nitrosylation-dependent inactivation of Akt/protein kinase B in insulin resistance

Inducible nitric-oxide synthase (iNOS) has been implicated in many human diseases including insulin resistance. However, how iNOS causes or exacerbates insulin resistance remains largely unknown. Protein S-nitrosylation is now recognized as a prototype of a redox-dependent, cGMP-independent signaling component that mediates a variety of actions of nitric oxide (NO). Here we describe the mechanism of inactivation of Akt/protein kinase B (PKB) in NO donor-treated cells and diabetic (db/db) mice. NO donors induced S-nitrosylation and inactivation of Akt/PKB in vitro and in intact cells. The inhibitory effects of NO donor were independent of phosphatidylinositol 3-kinase and cGMP. In contrast, the concomitant presence of oxidative stress accelerated S-nitrosylation and inactivation of Akt/PKB. In vitro denitrosylation with reducing agent reactivated recombinant and cellular Akt/PKB from NO donor-treated cells. Mutated Akt1/PKBalpha (C224S), in which cysteine 224 was substituted by serine, was resistant to NO donor-induced S-nitrosylation and inactivation, indicating that cysteine 224 is a major S-nitrosylation acceptor site. In addition, S-nitrosylation of Akt/PKB was increased in skeletal muscle of diabetic (db/db) mice compared with wild-type mice. These data suggest that S-nitrosylation-mediated inactivation may contribute to the pathogenesis of iNOS- and/or oxidative stress-involved insulin resistance.

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.

Insulin sensitivity and big ET-1 conversion to ET-1 after ETA- or ETB-receptor blockade in humans

Cardiovascular diseases are characterized by insulin resistance and elevated endothelin (ET)-1 levels. Furthermore, ET-1 induces insulin resistance. To elucidate this mechanism, six healthy subjects were studied during a hyperinsulinemic euglycemic clamp during infusion of (the ET-1 precursor) big ET-1 alone or after ET(A)- or ET(B)-receptor blockade. Insulin levels rose after big ET-1 with or without the ET(B) antagonist BQ-788 (P < 0.05) but were unchanged after the ET(A) antagonist BQ-123 + big ET-1. Infused glucose divided by insulin fell after big ET-1 with or without BQ-788 (P < 0.05). Insulin and infused glucose divided by insulin values were normalized by ET(A) blockade. Mean arterial blood pressure rose during big ET-1 with or without BQ-788 (P < 0.001) but was unchanged after BQ-123. Skeletal muscle, splanchnic, and renal blood flow responses to big ET-1 were abolished by BQ-123. ET-1 levels rose after big ET-1 (P < 0.01) in a similar way after BQ-123 or BQ-788, despite higher elimination capacity after ET(A) blockade. In conclusion, ET-1-induced reduction in insulin sensitivity and clearance as well as splanchnic and renal vasoconstriction are ET(A) mediated. ET(A)-receptor stimulation seems to inhibit the conversion of big ET-1 to ET-1.

Inhibition of nitric oxide synthesis attenuates insulin-mediated sympathetic activation in rats

BACKGROUND AND OBJECTIVES

Infusion of insulin produces sympathoexcitation, nitric oxide (NO) generation and NO-mediated vasodilation. Because central nervous system NO may inhibit sympathetic outflow, the present study was designed to determine whether NO synthase blockade would enhance insulin-mediated sympathetic activation. We additionally aimed to determine whether augmented sympathoexcitation and reduced NO-mediated vasodilation, during combined NO synthase blockade and hyperinsulinemia, would result in a blood pressure increase.

DESIGN AND METHODS

We infused vehicle (Control; n = 7) or insulin (10 mU/min) in anaesthetized rats receiving either no pretreatment (Insulin; n = 7) or after pretreatment with the NO blocker, NG-monomethyl-L-arginine (L-NMMA-insulin; 0.25 mg/kg per min; n = 7), while measuring mean arterial pressure (MAP), heart rate and lumbar sympathetic nerve activity (SNA) during euglycemic clamp. An additional control group received L-NMMA (L-NMMA; n = 7).

RESULTS

Insulin rats had large SNA increases (190 +/- 22% from 100% baseline), contrasting with small increases in the Control (136 +/- 10%) and L-NMMA (135 +/- 20%) groups. Unexpectedly, NO blockade abolished insulin-induced SNA increases in the L-NMMA-insulin group (96 +/- 12%). In agreement with the SNA findings, Insulin rats had heart rate increases while no heart rate changes were observed in the L-NMMA-insulin, Control, or L-NMMA groups. In addition, there was an unexpected was a lack of MAP increase in L-NMMA-insulin rats. MAP also did not change in the Control, L-NMMA or Insulin groups.

CONCLUSIONS

These findings suggest that NO is necessary for insulin to exert its sympathoexcitatory effects, and that insulin-induced NO release may play a role in activating increases in lumbar SNA.

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.

Lack of effect of sodium nitroprusside on insulin-mediated blood flow and glucose disposal in the elderly

Insulin increases skeletal muscle blood flow in healthy young subjects by a nitric oxide (NO)-dependent mechanism. Impairment of this mechanism may contribute to the insulin resistance of normal aging, a state characterized by reduced endothelial production of NO, an attenuated effect of insulin on skeletal muscle blood flow, and resistance to insulin-mediated glucose uptake (IMGU). We tested the hypothesis that the NO donor sodium nitroprusside (SNP) would augment insulin-mediated vasodilation and thus increase IMGU in healthy elderly subjects. Experiments were performed with young (n = 9; age, 25 +/- 1 years; body mass index [BMI], 24 +/- 1 kg/m2) and old (n = 10; age, 78 +/- 2 years; BMI, 25 +/- 1 kg/m2) healthy subjects. Each group underwent two studies in random order. In one study (control), insulin was infused using the euglycemic clamp protocol for 240 minutes at a rate of 40 mU/m2/min (young) and 34 mU/m2/min (old). In the other study (SNP), SNP was coinfused with insulin from 120 to 240 minutes. At regular intervals in each study, blood samples were obtained and calf blood flow was measured using venous occlusion plethysmography. Glucose and insulin values were similar in control and SNP studies in both age groups. In the young, SNP had no effect on blood flow to the calf, but its action in calf resistance vessels augmented insulin-mediated vasodilation, since incremental calf vascular conductance was greater during SNP infusion (control v SNP, 0.027 +/- 0.002 v 0.040 +/- 0.008 mL/100 mL/min/mm Hg, P< .0001). However, SNP had no effect on insulin-mediated glucose disposal. In the elderly, SNP reduced the blood flow to the calf, but this was countered by its effect on calf resistance vessels such that vascular conductance was unaffected (control v SNP, 0.012 +/- 0.003 v 0.011 +/- 0.003 mL/100 mL/min/mm Hg, P = nonsignificant [NS]). Steady-state (180 to 240 minutes) glucose disposal (control v SNP, 7.47 +/- 0.47 v 6.54 +/- 0.56 mg/kg/min, P < .01) rates were significantly lower during SNP infusion. In summary, systemic infusion of SNP did not increase insulin-mediated glucose disposal in either young or old subjects. Thus, the present findings do not support the concept that increasing NO availability will enhance glucose disposal in either age group. However, because the incremental increases in IMGU during SNP infusion paralleled the changes in blood supply to the calf rather than calf vascular conductance, any potential benefits on NO delivery in elderly subjects may have been offset by the direct or reflex effects of systemic hypotension. Other stimuli to NO production that do not cause hypotension must be tested before this therapeutic strategy can be considered as a potential means for enhancing the metabolic actions of insulin in the elderly.