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Biomedical subjects

A Marette

Publications and source records attributed to A Marette.

At least 19 recordsLinked to original sources
PubMed2006

Overproduction of intestinal lipoprotein containing apolipoprotein B-48 in Psammomys obesus: impact of dietary n-3 fatty acids.

AIMS/HYPOTHESIS: Emerging evidence underscores the important role of the small intestine in the pathogenesis of dyslipidaemia in insulin resistance and type 2 diabetes. We therefore tested the hypothesis that n-3 fatty acids improve the various events governing intra-enterocyte lipid transport in Psammomys obesus gerbils, a model of nutritionally induced metabolic syndrome. MATERIALS AND METHODS: Experiments were carried out on Psammomys obesus gerbils that were assigned to an isocaloric control diet and a diet rich in fish oil for 6 weeks. RESULTS: Increased dietary intake of fish oil lowered body weight and improved hyperglycaemia and hyperinsulinaemia. It simultaneously decreased de novo intestinal lipogenesis and lipid esterification of the major lipid classes, e.g. triglycerides, phospholipids and cholesteryl esters, particularly in insulin-resistant and diabetic animals. Accordingly, lessened activity of monoacylglycerol and diacylglycerol acyltransferase was recorded. As assessed in cultured jejunal explants incubated with either [(14)C]-oleic acid or [(35)S]-methionine, fish oil feeding resulted in diminished triglyceride-rich lipoprotein assembly and apolipoprotein (apo) B-48 biogenesis, respectively. The mechanisms did not involve apo B-48 transcription or alter the gene expression and activity of the critical microsomal triglyceride transfer protein. Rather, the suppressed production of apo B-48 by n-3 fatty acids was associated with intracellular proteasome-mediated posttranslational downregulation in insulin-resistant and diabetic animals. CONCLUSIONS/INTERPRETATION: Our data highlight the beneficial impact of n-3 fatty acids on adverse effects of the metabolic syndrome and emphasise their influence on intestinal lipid transport, an effect which may limit postprandial lipaemia and the risk of atherosclerosis.

Acyltransferases↗
PubMed2001

Amino acid and insulin signaling via the mTOR/p70 S6 kinase pathway. A negative feedback mechanism leading to insulin resistance in skeletal muscle cells.

Amino acids have emerged as potent modulators of the mTOR/p70 S6 kinase pathway. The involvement of this pathway in the regulation of insulin-stimulated glucose transport was investigated in the present study. Acute exposure (1 h) to a balanced mixture of amino acids reduced insulin-stimulated glucose transport by as much as 55% in L6 muscle cells. The effect of amino acids was fully prevented by the specific mTOR inhibitor rapamycin. Time course analysis of insulin receptor substrate 1 (IRS-1)-associated phosphatidylinositol (PI) 3-kinase activity revealed that incubation with amino acids speeds up its time-dependent deactivation, leading to a dramatic suppression (-70%) of its activity after 30 min of insulin stimulation as compared with its maximal activation (5 min of stimulation). This accelerated deactivation of PI 3-kinase activity in amino acid-treated cells was associated with a concomitant and sustained increase in the phosphorylation of p70 S6 kinase. In marked contrast, inhibition of mTOR by rapamycin maintained PI 3-kinase maximally activated for up to 30 min. The marked inhibition of insulin-mediated PI 3-kinase activity by amino acids was linked to a rapamycin-sensitive increase in serine/threonine phosphorylation of IRS-1 and a decreased binding of the p85 subunit of PI 3-kinase to IRS-1. Furthermore, amino acids were required for the degradation of IRS-1 during long term insulin treatment. These results identify the mTOR/p70 S6 kinase signaling pathway as a novel modulator of insulin-stimulated glucose transport in skeletal muscle cells.

Amino Acids↗
PubMed2001

Targeted disruption of inducible nitric oxide synthase protects against obesity-linked insulin resistance in muscle.

Inducible nitric oxide synthase (iNOS) is induced by inflammatory cytokines in skeletal muscle and fat. It has been proposed that chronic iNOS induction may cause muscle insulin resistance. Here we show that iNOS expression is increased in muscle and fat of genetic and dietary models of obesity. Moreover, mice in which the gene encoding iNOS was disrupted (Nos2-/- mice) are protected from high-fat-induced insulin resistance. Whereas both wild-type and Nos2-/- mice developed obesity on the high-fat diet, obese Nos2-/- mice exhibited improved glucose tolerance, normal insulin sensitivity in vivo and normal insulin-stimulated glucose uptake in muscles. iNOS induction in obese wild-type mice was associated with impairments in phosphatidylinositol 3-kinase and Akt activation by insulin in muscle. These defects were fully prevented in obese Nos2-/- mice. These findings provide genetic evidence that iNOS is involved in the development of muscle insulin resistance in diet-induced obesity.

Animals↗
PubMed2001

Effects of insulin on regional blood flow and glucose uptake in Wistar and Sprague-Dawley rats.

The euglycemic-hyperinsulinemic clamp technique in conscious Sprague-Dawley and Wistar rats chronically instrumented with intravascular catheters and pulsed Doppler flow probes was used to examine insulin's actions on regional blood flow and glucose metabolism. The effect of insulin on in vivo and in vitro glucose utilization in individual muscles was estimated using [3H]-2-deoxy-D-glucose. We found that in both strains, insulin (4, 32, and 64 mU x kg(-1) x min(-1)) causes similar cardiovascular changes characterized by slight increases in blood pressure (at high dose), vasodilation in renal and hindquarter vascular beds, and vasoconstriction (at high dose) in the superior mesenteric vascular bed. However, at the lowest dose of insulin tested, we found a smaller insulin sensitivity index and a lower insulin-stimulated in vivo glucose uptake in extensor digitorum longus (EDL) muscles of Wistar versus Sprague-Dawley rats. Higher insulin-stimulated glucose transport activity was found in isolated soleus muscle, while greater basal glucose transport was noted in isolated EDL muscle from Sprague-Dawley versus Wistar rats. These results provide further evidence for an insulin blood flow-regulatory effect and suggest that strain characteristics (differences in muscle perfusion, hindquarter composition, or fiber insulin sensitivity) constitute a major determinant in the variation in whole-body insulin sensitivity.

Animals↗
PubMed2001

Prevention of skeletal muscle insulin resistance by dietary cod protein in high fat-fed rats.

In the present study, we tested the hypothesis that fish protein may represent a key constituent of fish with glucoregulatory activity. Three groups of rats were fed a high-fat diet in which the protein source was casein, fish (cod) protein, or soy protein; these groups were compared with a group of chow-fed controls. High-fat feeding led to severe whole body and skeletal muscle insulin resistance in casein- or soy protein-fed rats, as assessed by the euglycemic clamp technique coupled with measurements of 2-deoxy-D-[(3)H]glucose uptake rates by individual tissues. However, feeding cod protein fully prevented the development of insulin resistance in high fat-fed rats. These animals exhibited higher rates of insulin-mediated muscle glucose disposal that were comparable to those of chow-fed rats. The beneficial effects of cod protein occurred without any reductions in body weight gain, adipose tissue accretion, or expression of tumor necrosis factor-alpha in fat and muscle. Moreover, L6 myocytes exposed to cod protein-derived amino acids showed greater rates of insulin-stimulated glucose uptake compared with cells incubated with casein- or soy protein-derived amino acids. These data demonstrate that feeding cod protein prevents obesity-induced muscle insulin resistance in high fat-fed obese rats at least in part through a direct action of amino acids on insulin-stimulated glucose uptake in skeletal muscle cells.

Adipose Tissue↗
PubMed2001

Lipopolysaccharide-induced diaphragmatic contractile dysfunction and sarcolemmal injury in mice lacking the neuronal nitric oxide synthase.

In this study we evaluated the role of the neuronal nitric oxide synthase (nNOS) in lipopolysaccharide (LPS)-induced diaphragmatic contractile dysfunction and sarcolemmal injury. Wild-type (WT) mice or mice deficient in the nNOS gene (nNOS(-/-)) were injected with either saline (control) or Escherichia coli LPS (LPS groups) and sacrificed 12 h later. The diaphragm was then examined for NOS expression, NOS activity, and in-vitro contractility. We also assessed sarcolemmal injury in isolated muscle strips under resting condition and after 3 min of artificial stimulations. In WT mice, LPS injection reduced maximum force to about 75% of that of control animals and raised total NOS activity significantly due to the induction of the iNOS isoform. Although muscle fiber injury was minimal under resting condition, the percentage of injured fibers in control and LPS-injected mice approached 27% and 40% of total fibers, respectively, in response to artificial stimulation. By comparison, LPS injection in nNOS(-/-) mice elicited a worsening of muscle contractility (maximum force < 60% of control animals) but elicited degrees of sarcolemmal injury similar to those observed in the WT animals. In addition, muscle NOS activity and iNOS protein level in nNOS(-/-) mice injected with LPS reached about 10% and 60% of that of WT animals, respectively (p < 0.05 compared with WT animals). Protein level of endothelial NOS isoform in the diaphragm was not altered by LPS injection in either WT or nNOS(-/-) animals. We conclude that nNOS plays a protective role in attenuating the negative influence of sepsis on diaphragmatic contractility but is not involved in the pathogenesis of sepsis-induced sarcolemmal injury.

Animals↗
PubMed2001

Defective insulin-induced GLUT4 translocation in skeletal muscle of high fat-fed rats is associated with alterations in both Akt/protein kinase B and atypical protein kinase C (zeta/lambda) activities.

The cellular mechanism by which high-fat feeding induces skeletal muscle insulin resistance was investigated in the present study. Insulin-stimulated glucose transport was impaired ( approximately 40-60%) in muscles of high fat-fed rats. Muscle GLUT4 expression was significantly lower in these animals ( approximately 40%, P < 0.05) but only in type IIa-enriched muscle. Insulin stimulated the translocation of GLUT4 to both the plasma membrane and the transverse (T)-tubules in chow-fed rats. In marked contrast, GLUT4 translocation was completely abrogated in the muscle of insulin-stimulated high fat-fed rats. High-fat feeding markedly decreased insulin receptor substrate (IRS)-1-associated phosphatidylinositol (PI) 3-kinase activity but not insulin-induced tyrosine phosphorylation of the insulin receptor and IRS proteins in muscle. Impairment of PI 3-kinase function was associated with defective Akt/protein kinase B kinase activity (-40%, P < 0.01) in insulin-stimulated muscle of high fat-fed rats, despite unaltered phosphorylation (Ser473/Thr308) of the enzyme. Interestingly, basal activity of atypical protein kinase C (aPKC) was elevated in muscle of high fat-fed rats compared with chow-fed controls. Whereas insulin induced a twofold increase in aPKC kinase activity in the muscle of chow-fed rats, the hormone failed to further increase the kinase activity in high fat-fed rat muscle. In conclusion, it was found that GLUT4 translocation to both the plasma membrane and the T-tubules is impaired in the muscle of high fat-fed rats. We identified PI 3-kinase as the first step of the insulin signaling pathway to be impaired by high-fat feeding, and this was associated with alterations in both Akt and aPKC kinase activities.

Animals↗
PubMed2000

Mechanism of impaired nitric oxide synthase activity in skeletal muscle of streptozotocin-induced diabetic rats.

AIMS/HYPOTHESIS: The aims of our study were to investigate whether nitric oxide synthase (NOS) activity is impaired in skeletal muscle of insulin-deficient [Type I (insulin-dependent)] diabetic rats and if the case, to determine the mechanism of NOS dysregulation in this disorder. METHODS: Rats were rendered diabetic by streptozotocin injection (65 mg/kg, i.v.) and NOS activity and expression in gastrocnemius muscles were studied 1, 2, 3 or 4 weeks after diabetes induction. RESULTS: The diabetic state was associated with a progressive reduction (down to 42 % of control values after 4 weeks) in muscle NOS activity compared with control rats. Using reverse transcriptase-polymerase chain reaction, we could not detect statistically significant changes in the expression of either neuronal NOS (nNOS) or endothelial NOS (eNOS) mRNAs in diabetic muscle. The contents of nNOS and eNOS protein were, however, progressively reduced in muscle homogenates of diabetic rats and these alterations were prevented by insulin treatment. Subcellular fractionation of skeletal muscle showed that both nNOS and eNOS proteins are mainly localised to the plasma membrane with lower abundance in T-tubules and not detectable in sarcoplasmic reticulum-enriched fractions. After 1 week of diabetes, eNOS protein content was decreased only in the plasma membrane whereas nNOS protein abundance was not affected at this time. Neither the expression nor the interaction of caveolin-1 and caveolin-3 with NOS enzymes was found to be altered in muscle of diabetic rats. CONCLUSION/INTERPRETATION: These results show that skeletal muscle NOS activity is impaired during the progression of insulin-deficient diabetes and reduced NOS activity is associated with a decreased abundance of both nNOS and eNOS proteins, which appears to involve post-transcriptional mechanisms.

Animals↗
PubMed2000

Nitric oxide: a new player in the modulation of energy metabolism.

Nitric oxide (NO) is a key messenger molecule in several cell types. NO formation is catalyzed by a family of NO synthases (NOS) that use L-arginine as a substrate. Rat adipose tissue expresses the inducible, macrophage-type, nitric oxide (NO) synthase isoform (iNOS). Systemic administration of the bacterial endotoxin lipopolysaccharide (LPS) markedly increases the expression and activity of iNOS in both white and brown adipose tissues, as well as in skeletal muscle. iNOS induction can be reproduced in vitro by treatment of cultured white or brown adipocytes or L6 myocytes with LPS and inflammatory cytokines (TNFalpha, IFNgamma). The physiological role of NO in adipose tissues and skeletal muscle is still obscure. Recent evidence suggests that NO may be implicated in the regulation of energy metabolism. Using both pharmacological and genetic models of iNOS invalidation, we have recently begun to uncover a role for NO in the modulation of glucose transport and lipoprotein hydrolysis. These studies support the emerging concept that NO may fulfill the dual role of modulating energy metabolism in both physiological and pathological conditions as well as contributing to local immune defense during inflammatory processes.

Adipose Tissue↗
PubMed2000

Gender differences in hemodynamic responses to endothelin-1.

We investigated whether gender affects the pressor and constrictor effects of endothelin-1 (ET-1) in anesthetized rats. Baseline values for arterial blood pressure (mmHg) and hindquarter resistance (HQR, mmHg/ml/min/100 g), respectively, were similar in male (96.0 +/- 3.1 and 13.9 +/- 0.6) and female (103.1 +/- 5.6 and 12.8 +/- 2.3) rats. ET-1 (1 and 3 microg/kg) produced rapid and transient falls in arterial blood pressure and HQR, followed by long-lasting increases in blood pressure and HQR. The initial ET-1-induced hypotension and vasodilation were similar in both groups. However, the pressor and hindquarter vasoconstrictor effects were significantly higher (p < 0.05) in male than female rats. Thus, the increases in blood pressure and HQR to ET-1 (3 microg/kg) were attenuated in female (+15.3 +/- 3.4 mmHg and +3.7 +/- 1.6 mmHg/ml/min/100 g) compared to male (+24.9 +/- 2.3 mmHg and +8.0 +/- 1.6 mmHg/ml/ min/100 g) rats. These results indicate that there is a gender difference in the vasoconstrictor, but not the vasodilator effects of ET-1. We suggest that the lower ET-1-induced hypertension and hindlimb vasoconstrictions observed in female rats may be responsible for the lower risk of developing coronary heart disease in premenopausal women.

Animals↗
PubMed2000

Cod and soy proteins compared with casein improve glucose tolerance and insulin sensitivity in rats.

The aim of the present study was to determine the effects of feeding various dietary proteins on insulin sensitivity and glucose tolerance in rats. Male Wistar rats were fed for 28 days with isoenergetic diets containing either casein, soy protein, or cod protein. Cod protein-fed and soy protein-fed rats had lower fasting plasma glucose and insulin concentrations compared with casein-fed animals. After intravenous glucose bolus, cod protein- and soy protein-fed rats induced lower incremental areas under glucose curves compared with casein-fed animals. Improved peripheral insulin sensitivity was confirmed by higher glucose disposal rates in cod protein- and soy protein-fed rats (15.2 +/- 0.3 and 13.9 +/- 0.6 mg. kg(-1). min(-1), respectively) compared with casein-fed animals (6.5 +/- 0.7 mg. kg(-1). min(-1), P < 0.05). Moreover, test meal experiments revealed that, in the postprandial state, the lower plasma insulin concentrations in cod protein- and soy protein-fed animals could be also due to decreased pancreatic insulin release and increased hepatic insulin removal. In conclusion, the metabolic responses to three common dietary proteins indicate that cod and soy proteins, when compared with casein, improve fasting glucose tolerance and peripheral insulin sensitivity in rats.

Animals↗
PubMed2000

Abundance and subcellular distribution of MCT1 and MCT4 in heart and fast-twitch skeletal muscles.

The expression of two monocarboxylate transporters (MCTs) was examined in muscle and heart. MCT1 and MCT4 proteins are coexpressed in rat skeletal muscles, but only MCT1 is expressed in rat hearts. Among six rat fast-twitch muscles (red and white gastrocnemius, plantaris, extensor digitorum longus, red and white tibialis anterior) there was an inverse relationship between MCT1 and MCT4 (r = -0.94). MCT1 protein was correlated with MCT1 mRNA (r = 0.94). There was no relationship between MCT4 mRNA and MCT4 protein. MCT1 (r = -0.97) and MCT4 (r = 0.88) protein contents were correlated with percent fast-twitch glycolytic fiber. When normalized for their mRNAs, MCT1 but not MCT4 was still correlated with the percent fast-twitch glycolytic fiber composition of rat muscles (r = -0.98). MCT1 and MCT4 were also measured in plasma membranes (PM), triads (TR), T tubules (TT), sarcoplasmic reticulum (SR), and intracellular membranes (IM). There was an intracellular pool of MCT4 but not of MCT1. The MCT1 subcellular distribution was as follows: PM (100%) > TR (31.6%) > SR (15%) = TT (14%) > IM (1.7%). The MCT4 subcellular distribution was considerably different [PM (100%) > TR (66.5%) > TT (36%) = SR (43%) > IM (24%)]. These studies have shown that 1) the mechanisms regulating the expression of MCT1 (transcriptional and posttranscriptional) and MCT4 (posttranscriptional) are different and 2) differences in MCT1 and MCT4 expression among muscles, as well as in their subcellular locations, suggest that they may have different roles in muscle.

Animals↗
PubMed2000

Cytokines and endotoxin induce cytokine receptors in skeletal muscle.

Proinflammatory cytokines are important factors in the regulation of diverse aspects of skeletal muscle function; however, the muscle cytokine receptors mediating these functions are uncharacterized. Binding kinetics (dissociation constant = 39+/-4.7 x 10(-9) M, maximal binding = 3.5+/-0.23 x 10(-12) mol/mg membrane protein) of muscle tumor necrosis factor (TNF) receptors were obtained. Skeletal muscle was found to express mRNAs encoding interleukin-1 type I and II receptors, interleukin-6 receptor (IL-6R), and interferon-gamma receptor by RT-PCR, but these receptors were below limits of detection of ligand-binding assay (> or =1 fmol binding sites/mg protein). Twenty-four hours after intraperitoneal administration of endotoxin to rats, TNF receptor type II (TNFRII) and IL-6R mRNA were increased in skeletal muscle (P<0.05). In cultured L6 cells, the expression of mRNA encoding TNFRII and IL-6R receptors was induced by TNF-alpha, and all six cytokine receptor mRNA were induced by a mixture of TNF-alpha, IFN-gamma, and endotoxin (P<0.05). This suggests that the low level of cytokine receptor expression is complemented by a capacity for receptor induction, providing a clear mechanism for amplification of cytokine responses at the muscle level.

Animals↗
PubMed2000

Effect of metformin on the vascular and glucose metabolic actions of insulin in hypertensive rats.

We investigated the long-term effect of metformin treatment on blood pressure, insulin sensitivity, and vascular responses to insulin in conscious spontaneously hypertensive rats (SHR). The rats were instrumented with intravascular catheters and pulsed Doppler flow probes to measure blood pressure, heart rate, and blood flow. Insulin sensitivity was assessed by the euglycemic hyperinsulinemic clamp technique. Two groups of SHR received metformin (100 or 300 mg x kg(-1) x day(-1)) for 3 wk while another group of SHR and a group of Wistar Kyoto (WKY) rats were left untreated. We found that vasodilation of skeletal muscle and renal vasculatures by insulin is impaired in SHR. Moreover, a reduced insulin sensitivity was detected in vivo and in vitro in isolated soleus and extensor digitorum longus muscles from SHR compared with WKY rats. Three weeks of treatment with metformin improves the whole-body insulin-mediated glucose disposal in SHR but has no blood pressure-lowering effect and no influence on vascular responses to insulin (4 mU x kg(-1) x min(-1)). An improvement in insulin-mediated glucose transport activity was detected in isolated muscles from metformin-treated SHR, but in the absence of insulin no changes in basal glucose transport activity were observed. It is suggested that part of the beneficial effect of metformin on insulin resistance results from a potentiation of the hormone-stimulating effect on glucose transport in peripheral tissues (mainly skeletal muscle). The results argue against a significant antihypertensive or vascular effect of metformin in SHR.

Animals↗
PubMed2000

Sustained activation of insulin receptors internalized in GLUT4 vesicles of insulin-stimulated skeletal muscle.

Exposure of target cells to insulin results in the formation of ligand receptor complexes on the cell surface and their subsequent internalization into the endosomal apparatus. A current view is that endocytosis of the insulin receptor (IR) kinase results in its rapid deactivation and sorting of the IR back to the cell surface or to late endocytic compartments. We report herein that, in skeletal muscle, in vivo stimulation with insulin induced a rapid internalization of the IR to an insulin-sensitive GLUT4-enriched intracellular membrane fraction. After 30 min of stimulation, IR content and tyrosine phosphorylation were increased by three and nine times in that fraction, respectively, compared with unstimulated muscles. In vitro autophosphorylation assays revealed that the kinase activity of internalized IRs was markedly augmented (eight to nine times) by insulin. In marked contrast with hepatic endosomes or adipocyte low-density microsomes, no IR tyrosine dephosphorylation activity was observed in GLUT4-enriched vesicles isolated from skeletal muscle. The activated IR was recovered in immunopurified GLUT4 vesicles after insulin stimulation. Insulin also increased tyrosine-phosphorylated insulin receptor substrate 1 and phosphatidylinositol 3-kinase adapter (p85) subunit contents in the intracellular membrane fraction, but these signaling molecules were not directly associated with GLUT4 vesicles. These results show that, in skeletal muscle, the activated IR reaches a GLUT4-enriched compartment where its activity is apparently sustained. We propose that compartmentalization of activated IRs to GLUT4 vesicles may play a role in sustaining insulin signaling at this locus in skeletal muscle.

Animals↗