High-fat diet-induced muscle insulin resistance: relationship to visceral fat mass

It has been variously hypothesized that the insulin resistance induced in rodents by a high-fat diet is due to increased visceral fat accumulation, to an increase in muscle triglyceride (TG) content, or to an effect of diet composition. In this study we used a number of interventions: fish oil, leptin, caloric restriction, and shorter duration of fat feeding, to try to disassociate an increase in visceral fat from muscle insulin resistance. Substituting fish oil (18% of calories) for corn oil in the high-fat diet partially protected against both the increase in visceral fat and muscle insulin resistance without affecting muscle TG content. Injections of leptin during the last 4 days of a 4-wk period on the high-fat diet partially reversed the increase in visceral fat and the muscle insulin resistance, while completely normalizing muscle TG. Restricting intake of the high-fat diet to 75% of ad libitum completely prevented the increase in visceral fat and muscle insulin resistance. Maximally insulin-stimulated glucose transport was negatively correlated with visceral fat mass (P < 0.001) in both the soleus and epitrochlearis muscles and with muscle TG concentration in the soleus (P < 0.05) but not in the epitrochlearis. Thus we were unable to dissociate the increase in visceral fat from muscle insulin resistance using a variety of approaches. These results support the hypothesis that an increase in visceral fat is associated with development of muscle insulin resistance.     

Combined effects of troglitazone and muscle contraction on insulin sensitization in Balb-c mouse muscle

Thiazolidinediones, represented by troglitazone, are insulin-sensitizing agents with proven efficacy for the treatment of type 2 diabetes. Exercise is also recommended for patients with type 2 diabetes because it both stimulates glucose uptake directly and it increases insulin sensitivity following exercise. The purpose of this study was to investigate the effects of troglitazone combined with exercise on 2-deoxyglucose (2DG) uptake in both the epitrochlearis and soleus muscle of Balb-c mice. Acute, 1-h treatment with troglitazone (10 or 20 microM), in the presence or absence of insulin, had no effect on 2DG uptake in either muscle. Chronic treatment with troglitazone by feeding enhanced the insulin sensitivity and responsiveness of 2DG uptake primarily in the epitrochlearis. Direct electrical stimulation of in situ muscle was used to model exercise while the contralateral muscle was used as the unexercised control. This model mimicked exercise in that glycogen was depleted, immediate 2DG uptake was enhanced, and there was a post-exercise increase in insulin sensitivity. Troglitazone feeding had no effect on 2DG uptake in the soleus when measured immediately after electrical stimultion. However, 2DG uptake in the unstimulated epitrochlearis from troglitazone-fed mice was elevated when measured immediately after removal such that no additional effects of the electrical stimulation were measured. We found that the insulin-sensitizing effect of troglitazone was not additive to the insulin-sensitizing effect of exercise, which suggests that troglitazone and exercise share similar pathways. A unique finding in this study was the differential response to troglitazone between the epitrochlearis (fast twitch) and the soleus (slow twitch) muscle types. Possible mechanisms are discussed.     

Association of resistin with visceral fat and muscle insulin resistance

Maturing Sprague-Dawley (S-D) rats develop obesity and skeletal muscle insulin resistance. To investigate the relationship between fat mass and insulin responses, we performed surgical removal of the epididymal and retroperitoneal depots of visceral adipose tissue (VF) or sham surgery (SHAM) in male rats aged 4 months. At sacrifice, 30 days later, the mass of visceral fat was 48% lower (p<0.05) in VF- compared to SHAM, while subcutaneous fat was essentially unchanged. VF- animals displayed increased insulin responses in isolated strips of skeletal muscle. Insulin-stimulated glucose transport was increased 28% in soleus muscle (p<0.05), with a trend toward a 31% increase in extensor digitorum longus muscle (p=0.058). Glucose tolerance was not significantly affected by surgical fat removal. In VF- animals, serum resistin was reduced 26% (p<0.05) and serum adiponectin was reduced 30% (p<0.05), with trends for reductions in IL-4 (58% reduction, p=0.084) and IL-6 (56% reduction, p=0.123). TNF-alpha, leptin and free fatty acids (NEFAs) were unchanged. We conclude that in maturing S-D rats, increased visceral adiposity leads to an increase in systemic release in resistin and possibly interleukins. Elevation of circulating cytokines may play a role in the development of muscle insulin resistance.     

New imaging techniques of fat, muscle and liver within the context of determining insulin sensitivity

Body fat distribution and ectopic fat deposition are important determinants of insulin sensitivity. Fat deposition in muscle and the liver, in particular, has been found to impair insulin signalling in these insulin-sensitive tissues. Thus, exact quantification of fat content may help to distinguish between different sites of insulin resistance. Increased fat deposition in the visceral compartment compared with the subcutaneous depot also represents an important factor leading to insulin resistance. Recent data clearly showed that visceral fat is a strong determinant of liver fat content. Exact quantification of fat distribution by magnetic resonance imaging and magnetic resonance spectroscopy may help to define distinct 'fat-distribution phenotypes'. This may allow a search for new candidate genes for type 2 diabetes mellitus and identify, at an early stage, individuals at risk for decline in insulin sensitivity.     

Quercetin can reduce insulin resistance without decreasing adipose tissue and skeletal muscle fat accumulation

Quercetin exhibits a wide range of biological functions. The first aim of the present work was to analyze the effects of quercetin on fat accumulation in adipose tissue and glycemic control in rats. Any potential involvement of muscle fatty acid oxidation in its effect on glycemic control was also assessed. Animals were fed a high-fat high-sucrose diet either supplemented with quercetin (30 mg/kg body weight/day), or not supplemented, for 6 weeks. One week before killing, a glucose tolerance test was carried out. Muscle triacylglycerol content, serum glucose, insulin, fructosamine and free fatty acids were measured, and homeostatic model assessment for insulin resistance (HOMA-IR) was calculated. The activities of lipogenic enzymes and lipoprotein lipase in adipose tissue, carnitine palmitoyl transferase-1b (CPT-1b) and citrate synthase in skeletal muscle, and the expression of several genes, ACO, CD36, CPT-1b, PPAR-α, PGC-1α, UCP₃, TFAM and COX-2 in skeletal muscle were analyzed. Quercetin caused no significant reduction in body weight or adipose tissue sizes. However, fructosamine, basal glucose and insulin, and consequently HOMA-IR, were significantly reduced by quercetin. No changes were observed in the activity of lipogenic enzymes and lipoprotein lipase. Muscle triacylglycerol content was similar in both experimental groups. The expression of ACO, CD36, CPT-1b, PPAR-α, PGC-1α, UCP₃, TFAM and COX-2 remained unchanged. It can be concluded that quercetin is more effective as an anti-diabetic than as an anti-obesity biomolecule. The improvement in insulin resistance induced by this flavonoid is not mediated by a delipidating effect in skeletal muscle.     

Increased insulin mediated glucose metabolism in fat cells from I versus C57BL mice

The purpose of this study was to determine whether adipocytes from I strain mice, which are characterized by a greater in vivo glucose tolerance than most other strains, had a higher capacity to utilize glucose in response to physiological concentrations of insulin. Using C57BL mice as a control strain, we examined the effect of insulin on glucose metabolism in epididymal and inguinal adipocytes from 2-month-old male mice. Body weight was only slightly less (7%) for the I mice than for the C57BL mice, but fat pad sizes were 60 and 20% less for epididymal and inguinal depots, respectively, in the I mice. Fat cell size was also smaller in epididymal adipocytes from the I mice than from the C57BL mice. Fat cell size of inguinal adipocytes was similar in the two strains. Without insulin the rates of [U-14C]glucose incorporation into CO2 or lipids were twofold higher in cells from the I mice than in those from the C57BL mice. Maximal insulin concentration (2.5 nM) increased glucose metabolism by 140 and 500% in epididymal and inguinal adipose cells, respectively, in the I mice versus 30 and 50% in the C57BL mice. The maximal effect of insulin was reached at a much higher insulin concentration in the I mice than in the C57BL mice. The activity of fatty acid synthetase was four- to sixfold higher in fat cells from I than in those from C57BL mice. These results demonstrate an increased insulin responsiveness of glucose metabolism in fat cells from the I mice related to an increased lipogenic capacity. Furthermore, they show that adipose tissue in mice exhibits significant regional differences in terms of insulin responsiveness of glucose metabolism.     

Topiramate treatment causes skeletal muscle insulin sensitization and increased Acrp30 secretion in high-fat-fed male Wistar rats

We show that Topiramate (TPM) treatment normalizes whole body insulin sensitivity in high-fat diet (HFD)-fed male Wistar rats. Thus drug treatment markedly lowered glucose and insulin levels during glucose tolerance tests and caused increased insulin sensitization in adipose and muscle tissues as assessed by euglycemic clamp studies. The insulin-stimulated glucose disposal rate increased twofold (indicating enhanced muscle insulin sensitivity), and suppression of circulating FFAs increased by 200 to 300%, consistent with increased adipose tissue insulin sensitivity. There were no effects of TPM on hepatic insulin sensitivity in these TPM-treated HFD-fed rats. In addition, TPM administration resulted in a three- to fourfold increase in circulating levels of total and high-molecular-weight (HMW) adiponectin (Acrp30). Western blot analysis revealed normal AMPK (Thr(172)) phosphorylation in liver with a twofold increased phospho-AMPK in skeletal muscle in TPM-treated rats. In conclusion, 1) TPM treatment prevents overall insulin resistance in HFD male Wistar rats; 2) drug treatment improved insulin sensitivity in skeletal muscle and adipose tissue associated with enhanced AMPK phosphorylation; and 3) the tissue "specific" effects are associated with increased serum levels of adiponectin, particularly the HMW component.     

Rate of lumbar paravertebral muscle fat infiltration versus spinal degeneration in asymptomatic populations: an age-aggregated cross-sectional simulation study

Background

The spinal column including its vertebrae and disks has been well examined and extensively reported in relation to age-aggregated degeneration. In contrast, paravertebral muscles are poorly represented in describing normative degeneration. Increasing evidence points to the importance of paravertebral muscle quality in low back health, and their potential as a modifiable factor in low back pain (LBP). Studies examining normative decline of paravertebral muscles are needed to advance the field’s etiological understanding. With a novel approach and based on published data, we establish and compare decline rates of imaging features for degeneration of lumbar vertebrae and disks, versus fatty infiltration in paravertebral muscles in asymptomatic adults.

Methods

Our cross-sectional simulation study examined age-aggregated data from three published studies who reported on asymptomatic adults spanning 18–60 years. Prevalence rates of imaging degenerative features of the spinal column were examined via logistic regression and compared with percentage fatty infiltration in erector spinae, multifidus and psoas using synthetic data and Monte Carlo simulation with 10,000 endpoint-specific regression iterations. General linear regression models were employed to estimate marginal effects of age reported as a one-year change rate (with 95 % confidence intervals) for comparisons between all reported spinal features.

Results

Declines in multifidus (0.24 & 0.11 %/year), erector spinae (0.13 & 0.07 %/year), and psoas (0.04 %/year) occur at similarly slow rates to disk protrusion (0.25 %/year), annular fissure (0.15 %/year), and spondylolisthesis (0.29 %/year). Multifidus showed a trend for faster decline than erector spinae, particularly in men. Of the features examined, disk signal loss declined fastest, and psoas muscle the slowest.

Conclusions

Degeneration of lumbar paravertebral muscles occurs slowly in asymptomatic adults, with a tendency to be most pronounced in multifidus. Rate of decline of spinal structures represents a novel variable that warrants inclusion as a known feature of the expected degenerative cascade, and to provide a basis for comparison to diseases of the spine in research and clinical practice. Concurrent examination of spinal features using advanced imaging to improve muscle analysis would be a strong addition to the field.

     

Fish oil prevents high-saturated fat diet-induced impairments in adiponectin and insulin response in rodent soleus muscle

High saturated fatty acid (SFA) diets contribute to the development of insulin resistance, whereas fish oil-derived n-3 polyunsaturated fatty acids (PUFA) increase the secretion of adiponectin (Ad), an adipocyte-derived protein that stimulates fatty acid oxidation (FAO) and improves skeletal muscle insulin response. We sought to determine whether fish oil could prevent and/or restore high SFA diet-induced impairments in Ad and insulin response in soleus muscle. Sprague-Dawley rats were fed 1) a low-fat control diet (CON group), 2) high-SFA diet (SFA group), or 3) high SFA with n-3 PUFA diet (SFA/n-3 PUFA group). At 4 wk, CON and SFA/n-3 PUFA animals were terminated, and SFA animals were either terminated or fed SFA or SFA/n-3 PUFA for an additional 2 or 4 wk. The effect of diet on Ad-stimulated FAO, insulin-stimulated glucose transport, and expression of Ad, insulin and inflammatory signaling proteins was determined in the soleus muscle. Ad stimulated FAO in CON and 4 wk SFA/n-3 PUFA (+36%, +39%, respectively P ≤ 0.05) only. Insulin increased glucose transport in CON, 4 wk SFA/n-3 PUFA, and 4 wk SFA + 4 wk SFA/n-3 PUFA (+82%, +33%, +25%, respectively P ≤ 0.05); this effect was lost in all other groups. TLR4 expression was increased with 4 wk of SFA feeding (+24%, P ≤ 0.05), and this was prevented in 4 wk SFA/n-3 PUFA. Suppressor of cytokine signaling-3 expression was increased in SFA and SFA/n-3 PUFA (+33 and +18%, respectively, P ≤ 0.05). Our results demonstrate that fish oil can prevent high SFA diet-induced impairments in both Ad and insulin response in soleus muscle.     

Fat in liver/muscle correlates more strongly with insulin sensitivity in rats than abdominal fat

Intrahepatic or intramuscular lipid (IHL/IML) content has been reported to be correlated with insulin resistance. Visceral fat has also been shown to be associated with insulin resistance. Thus, we investigated whether IHL/IML or visceral fat content is more closely associated with insulin resistance. Twenty Sprague-Dawley rats were divided into two groups based on regular chow diet (RCD) or high-fat diet (HFD; 40% fat). The insulin-sensitivity index (ISI) was determined by euglycemic glucose clamp study, the amount of visceral fat by computed tomography (CT), and the IHL/IML content by magnetic resonance spectroscopy (MRS). Weight, food, and water intake, physical activity, energy expenditure, lipid profile, adiponectin, and high-sensitivity C-reactive protein (hsCRP) levels were measured. At the study end point, visceral fat, and the IHL/IML content were higher in the HFD group than in the RCD group. The IHL/IML content was more highly correlated with ISI than was visceral fat amount. Stronger correlations were also found between adiponectin or hsCRP level and IML/IHL content than visceral fat, especially in the HFD group. Furthermore, the IHL/IML content was significantly associated with the ISI in the multiple regression models but visceral fat was not. There was clear discrimination between RCD and HFD groups in scatter plots of IML/IHL against the ISI, but substantial overlap in that of visceral fat against the ISI. This result suggests that IHL/IML contents are closely related with insulin resistance or atherosclerosis and is a better metabolic index of insulin sensitivity than the visceral fat.     

Histopathological changes in rat pancreas and skeletal muscle associated with high fat diet induced insulin resistance

The effects of a high fat diet on the development of diabetes mellitus, insulin resistance and secretion have been widely investigated. We investigated the effects of a high fat diet on the pancreas and skeletal muscle of normal rats to explore diet-induced insulin resistance mechanisms. Forty-four male Wistar rats were divided into six groups: a control group fed standard chow, a group fed a 45% fat diet and a group fed a 60% fat diet for 3 weeks to measure acute effects; an additional three groups were fed the same diet regimens for 8 weeks to measure chronic effects. The morphological effects of the two high fat diets were examined by light microscopy. Insulin in pancreatic islets was detected using immunohistochemistry. The homeostasis model assessment of insulin resistance index and insulin staining intensity in islets increased significantly with acute administration of high fat diets, whereas staining intensity decreased with chronic administration of the 45% fat diet. Islet areas increased significantly with chronic administration. High fat diet administration led to islet degeneration, interlobular adipocyte accumulation and vacuolization in the pancreatic tissue, as well as degeneration and lipid droplet accumulation in the skeletal muscle tissue. Vacuolization in the pancreas and lipid droplets in skeletal muscle tissue increased significantly with chronic high fat diet administration. We suggest that the glucolipotoxic effects of high fat diet administration depend on the ratio of saturated to unsaturated fatty acid content in the diet and to the total fat content of the diet.     

The growing importance of fat in regenerative medicine

A recent publication by Michael Longaker and colleagues represents a landmark for the use of adipose tissue as a source of cells for tissue regeneration. The authors investigated the ability of adipose tissue-derived cells (ADCs) to regenerate critical size calvarial (superior portion of the skull) defects in mice by using a novel osteoconducive apatite-coated Poly-lactic-co-glycolic acid (PLGA) scaffold for cell delivery. Direct comparison of this osteogenic ability was performed with bone marrow stromal cells and juvenile calvarial-derived osteoblasts.     

Stretch-induced calcium sensitization of rat lymphatic smooth muscle

The relationships between smooth muscle calcium and isometric tension generation to spontaneous lymphatic pump activity and its modulation by stretch equivalent from 0 to approximately 6 cmH2O were investigated. Excised preparations of the rat thoracic duct were mounted on a wire myograph and loaded with the calcium-sensitive fluorochrome indo-1. Calcium-dependent fluorescence and isometric force were simultaneously recorded. The thoracic duct segments developed spontaneous rhythmic contractile activity. Each contraction was preceded by an increase in intracellular calcium. When the vessels were normalized and stabilized at a preload equal to 3 cmH2O, the peak generation in tension occurred 0.70 +/- 0.11 s after that of calcium. Incremental stretch enhanced the frequency of the phasic activity and amplitude of isometric force generation but not the basal calcium level or the amplitude of the calcium transient. These findings suggest that stretch enhances lymphatic pump activity by increasing the pacemaker activity and the calcium sensitivity of the contractile apparatus.     

Hydroid defenses against predators: the importance of secondary metabolites versus nematocysts

Marine hydroids are commonly thought to be defended by stinging organelles called nematocysts that penetrate predator tissues and inject proteinaceous venoms, but not all hydroids possess these nematocysts. Although an increasing number of bioactive secondary metabolites have been isolated from marine hydroids, ecological roles of these compounds are poorly known. To test the hypothesis that nematocysts and noxious secondary metabolites represent alternative defenses against predation, we examined hydroids from North Carolina, United States for: (1) the palatability of whole polyps before and after nematocysts had been deactivated; (2) the palatability of their chemical extracts; and (3) their nutritional value in terms of organic content, protein content, and levels of refractory structural material (chitin). All hydroids were avoided by a generalist predator, the pinfish Lagodon rhomboides, compared with palatable control foods. Two of these (Halocordyle disticha and Tubularia crocea) became palatable after being treated with potassium chloride to discharge their nematocysts, suggesting that these species rely on nematocysts for defenses against predators. Chemical extracts from nematocyst-defended species had no effect on fish feeding. The four species that remained unpalatable after nematocysts had been discharged (Corydendrium parasiticum, Eudendrium carneum, Hydractinia symbiolongicarpus, Tridentata marginata) possessed chemical extracts that deterred feeding by pinfish. We have isolated and characterized the structures of the deterrent metabolites in two of these species. We found no differences in nutritional content or levels of chitin between nematocyst-defended and chemically defended species, and no evidence that either of these played a role in the rejection of hydroids as prey. Our results suggest that, among hydroids, chemical defenses may be at least as common as nematocyst-based defenses and that the two may represent largely alternative defensive strategies. The four hydroid species with deterrent extracts represent four families and both sub-orders of hydroids, suggesting that chemical defenses in this group may be widespread and have multiple origins. Received: 25 May 1999 / Accepted: 1 February 2000     

Enhanced intestinal lymph formation during fat absorption: the importance of triglyceride hydrolysis.

The effect of intraduodenal administration of fats was studied in the rat to define the mechanisms responsible for the substantial increase in intestinal lymph flow and protein transport which follows fat ingestion. Triglyceride in the intestinal lumen, protected from hydrolysis, does not appear to enhance intestinal lymph production. Giving both long- and medium-chain fatty acids, however, causes intestinal lymph flow and protein transport to increase in a manner similar to that found after giving triglyceride which is allowed to undergo hydrolysis. Bile by itself does not seem to be responsible for the phenomenon.