Estimating whole body intermuscular adipose tissue from single cross-sectional magnetic resonance images.

Intermuscular adipose tissue (IMAT), a novel fat depot linked with metabolic abnormalities, has been measured by whole body MRI. The cross-sectional slice location with the strongest relation to total body IMAT volume has not been established. The aim was to determine the predictive value of each slice location and which slice locations provide the best estimates of whole body IMAT. MRI quantified total adipose tissue of which IMAT, defined as adipose tissue visible within the boundary of the muscle fascia, is a subcomponent. Single-slice IMAT areas were calculated for the calf, thigh, buttock, waist, shoulders, upper arm, and forearm locations in a sample of healthy adult women, African-American [n = 39; body mass index (BMI) 28.5 +/- 5.4 kg/m2; 41.8 +/- 14.8 yr], Asian (n = 21; BMI 21.6 +/- 3.2 kg/m2; 40.9 +/- 16.3 yr), and Caucasian (n = 43; BMI 25.6 +/- 5.3 kg/m2; 43.2 +/- 15.3 yr), and Caucasian men (n = 39; BMI 27.1 +/- 3.8 kg/m2; 45.2 +/- 14.6 yr) and used to estimate total IMAT groups using multiple-regression equations. Midthigh was the best, or near best, single predictor in all groups with adjusted R2 ranging from 0.49 to 0.84. Adding a second and third slice further increased R2 and reduced the error of the estimate. Menopausal status and degree of obesity did not affect the location of the best single slice. The contributions of other slice locations varied by sex and race, but additional slices improved predictions. For group studies, it may be more cost-effective to estimate IMAT based on one or more slices than to acquire and segment for each subject the numerous images necessary to quantify whole body IMAT.     

Secreted proteins from adipose tissue and skeletal muscle - adipokines, myokines and adipose/muscle cross-talk

White adipose tissue and skeletal muscle are the largest organs in the body and both are composed of distinct cell types. The signature cell of adipose tissue is the adipocyte while myocytes are the defining cell of skeletal muscle. White adipocytes are major secretory cells and this is increasingly apparent also for myocytes. Both cells secrete a range of bioactive proteins, generally termed adipokines in the case of adipocytes and myokines for muscle cells. There has, however, been some confusion over nomenclature and we suggest that the name myokine is restricted to a protein that is secreted from myocytes, while the term adipokine should be used to describe all proteins secreted from any type of adipocyte (white, brown or brite). These definitions specifically exclude proteins secreted from other cells within adipose tissue and muscle, including macrophages. There is some commonality between the myokines and adipokines in that both groups include inflammation-related proteins - for example, IL-6, Il-8 and MCP-1. Adipokines and myokines appear to be involved in local autocrine/paracrine interactions within adipose tissue and muscle, respectively. They are also involved in an endocrine cross-talk with other tissues, including between adipose tissue and skeletal muscle, and this may be bi-directional. For example, IL-6, secreted from myocytes may stimulate lipolysis in adipose tissue, while adipocyte-derived IL-6 may induce insulin resistance in muscle.     

Cloning and characterization of microRNAs from porcine skeletal muscle and adipose tissue

MicroRNAs (miRNAs) are an abundant class of small regulatory RNAs that regulate the stability and translation of cognate mRNAs. Although an increasing number of porcine miRNAs has recently been identified, the full repertoire of miRNAs in pig remains to be elucidated. To identify porcine miRNAs potentially involved in myogenesis and adipogenesis, we constructed small RNA cDNA libraries from skeletal muscle and adipose tissue and identified 89 distinct miRNAs that are conserved in pig, of which 15 were new. Expression analysis of all newly identified and selected known porcine miRNAs revealed that some miRNAs were enriched in a tissue-specific manner, whereas others were expressed ubiquitously in the porcine tissues examined. Our results expand the number of known porcine miRNAs and provide useful information for further investigating the biological functions of miRNAs associated with growth and development of skeletal muscle or adipose tissue in pig.     

Inhibition of the reversible deactivation of chicken adipose tissue hormone-sensitive lipase by heat-stable proteins from adipose tissue and skeletal muscle.

The reversible deactivation of chicken adipose tissue hormone-sensitive lipase is catalyzed by a lipase phosphatase. Heat-stable protein preparations from rat epididymal fat pads, chicken adipose tissue, and rabbit skeletal muscle inhibited lipase phosphatase activity. Phosphatase inhibitor preparations from rat adipose tissue did not inhibit the protein kinase-catalyzed activation of hormone-sensitive lipase, whereas inhibitor preparations from rabbit skeletal muscle were contaminated with protein kinase inhibitor.     

Intermuscular adipose tissue-free skeletal muscle mass: estimation by dual-energy X-ray absorptiometry in adults.

Skeletal muscle (SM) is a large and physiologically important compartment. Adipose tissue is found interspersed between and within SM groups and is referred to as intermuscular adipose tissue (IMAT). The study objective was to develop prediction models linking appendicular lean soft tissue (ALST) estimates by dual-energy X-ray absorptiometry (DXA) with whole body IMAT-free SM quantified by magnetic resonance imaging. ALST and total-body IMAT-free SM were evaluated in 270 healthy adults [body mass index (BMI) of <35 kg/m(2)]. The SM prediction models were then validated by the leave-one-out method and by application in a new group of subjects who varied in SM mass [anorexia nervosa (AN), n = 23; recreational athletes, n = 16; patients with acromegaly, n = 7]. ALST alone was highly correlated with whole body IMAT-free SM [model 1: R(2) = 0.96, standard error (SE) = 1.46 kg, P < 0.001]; age (model 2: R(2) = 0.97, SE = 1.38 kg, P < 0.001) and sex and race (model 3: R(2) = 0.97, SE = 1.06 kg, both P < 0.001) added significantly to the prediction models. All three models validated in the athletes and patients with acromegaly but significantly (P < 0.01-0.001) over-predicted SM in the AN group as a whole. However, model 1 was validated in AN patients with BMIs in the model-development group range (n = 11; BMI of >16 kg/m(2)) but not in those with a BMI of <16 kg/m(2) (n = 12). The DXA-based models are accurate for predicting IMAT-free SM in selected populations and thus provide a new opportunity for quantifying SM in physiological and epidemiological investigations.     

Robust estimation of physiological cross-sectional area and geometric reconstruction for human skeletal muscle

Understanding muscle architecture is crucial to determining the mechanical function of muscle during body movements, because architectural parameters directly correspond to muscle performance. Accurate parameters are thus essential for reliable simulation. Human cadaveric muscle specimen data provides the anatomical detail needed for in-depth understanding of muscle and accurate parameter estimation. However, as muscle generally has non-uniform architecture, parameter estimation, specifically, physiological cross-sectional area (PCSA), is rarely straightforward. To deal effectively with this non-uniformity, we propose a geometric approach in which a polygon is sought to best approximate the cross-sectional area of each fascicle by accounting for its three-dimensional trajectory and arrangement in the muscle. Those polygons are then aggregated to determine PCSA and volume of muscle. Experiments are run using both synthetic data and muscle specimen data. From comparison of PCSA using synthetic data, we conclude that the proposed method enhances the robustness of PCSA estimation against variation in muscle architecture. Furthermore, we suggest reconstruction methods to extract 3D muscle geometry directly from fascicle data and estimated parameters using the level set method.     

Assessment of transcapillary glucose exchange in human skeletal muscle and adipose tissue

We studied the kinetics of glucose exchange between plasma and interstitial fluid (ISF) in human skeletal muscle and adipose tissue under fasting conditions. Five normal human subjects received an intravenous [6,6-2H2]glucose infusion in a prime-continuous fashion. During the tracer infusion, the open-flow microperfusion technique was employed to frequently sample ISF from quadriceps muscle and subcutaneous adipose tissue. The tracer glucose kinetics observed in muscle and adipose tissue ISF were found to be well described by a capillary-tissue exchange model. As a measure of transcapillary glucose exchange efficiency, the 95% equilibrium time was calculated from the identified model parameters. This time constant was similar for skeletal muscle and adipose tissue (28.6 +/- 3.2 vs. 26.8 +/- 3.6 min; P = 0.60). Furthermore, we found that the (total) interstitial glucose concentration was significantly lower (P < 0.01) in muscle (3.32 +/- 0.46 mmol/l) and adipose tissue (3.51 +/- 0.17 mmol/l) compared with arterialized plasma levels (5.56 +/- 0.13 mmol/l). Thus the observed gradients and dynamic relationships between plasma and ISF glucose in muscle and adipose tissue provide evidence that transcapillary exchange of glucose is limited in these two tissues under fasting conditions.     

Glucose-fatty acid interaction in skeletal muscle and adipose tissue in insulin resistance

Insulin resistance (IR) is the result of long-lasting positive energy balance and the imbalance between the uptake of energy rich substrates (glucose, lipids) and energy output. The defects in the metabolism of glucose in IR and type 2 diabetes are closely associated with the disturbances in the metabolism of lipids. In this review, we have summarized the evidence indicating that one of the important mechanisms underlying the development of IR is the impaired ability of skeletal muscle to oxidize fatty acids as a consequence of elevated glucose oxidation in the situation of hyperglycemia and hyperinsulinemia and the impaired ability to switch easily between glucose and fat oxidation in response to homeostatic signals. The decreased fat oxidation results into the accumulation of intermediates of fatty acid metabolism that are supposed to interfere with the insulin signaling cascade and in consequence negatively influence the glucose utilization. Pathologically elevated fatty acid concentration in serum is now accepted as an important risk factor leading to IR. Adipose tissue plays a crucial role in the regulation of fatty acid homeostasis. The adipose tissue may be the primary site where the early metabolic disturbances leading to the development of IR take place and the development of IR in other tissues follows. In this review we present recent evidence of mutual interaction between skeletal muscle and adipose tissue in the establishment of IR and type 2 diabetes.     

Lactate release from adipose tissue and skeletal muscle in vivo: defective insulin regulation in insulin-resistant obese women

To study the local tissue lactate production in the normal state and its possible disturbances in insulin resistance, rates of lactate release from adipose tissue (AT) and skeletal muscle (SM) were compared postabsorptively and during a hyperinsulinemic euglycemic clamp in 11 healthy nonobese and 11 insulin-resistant obese women. A combination of microdialysis, to measure interstitial lactate, and the 133Xe clearance technique, to determine local blood flow, were used. In the controls, local blood flow increased by 40% in SM (P<0.05) and remained unchanged in AT, whereas the interstitial-plasma difference in lactate doubled in AT (P<0.005) and was unaffected in SM during hyperinsulinemia. In the obese, blood flow and interstitial-plasma difference in lactate remained unchanged in both tissues during hyperinsulinemia. The lactate release (micromol100 g-1min-1) was 1.17+/-0.22 in SM and 0.43+/-0.11 in AT among the controls (P<0.01) and 0.86+/-0.23 in SM and 0.83+/-0.25 in AT among the obese women in the postabsorptive state. During insulin infusion, lactate release in the controls increased to 1.92+/-0.26 in SM (P<0.005) and to 1.14+/-0.22 in AT (P<0.005) but remained unchanged in the obese women. It is concluded that AT and SM are both significant sources of lactate release postabsorptively, and AT is at least as responsive to insulin as SM. The ability to increase lactate release in response to insulin is impaired in AT and SM in insulin-resistant obese women, involving defective insulin regulation of both tissue lactate metabolism and local blood flow.     

Potassium efflux from single skinned skeletal muscle fibers.

The efflux of 42K from single, skinned (sarcolemma removed) skeletal muscle fibers has been determined. Isotope washout curves are kinetically complex and can be fit as the sum of three exponentials, including a fast component (k = 0.25 s-1) with a pool size equivalent to 91% of the fiber volume, an intermediate component (k = 0.08 s-1) equivalent to 6% of the fiber volume, and a slow component (k = 0.008 s-1) equivalent to 0.5% of fiber volume. Only the intermediate kinetic component is significantly affected by pretreatment of fibers with detergent. Efflux curves from detergent-treated fibers could be fit as the sum of two exponentials with coefficients and rate constants comparable to those of the fast and slow component of washout of untreated controls. Similarly the washout of [14C]sucrose can be described as the sum of two exponentials. We conclude that the intermediate component of 42K washout results from the movement of ions from a membrane bound space within the skinned fiber. Because of its relative volume, the sarcoplasmic reticulum seems to be a reasonable choice as a structural correlate for this component. Our estimate of the potassium permeability for the sarcoplasmic reticulum (SR) based on the efflux data is 10(-7) cm/s. This value is less than previous estimates from isolated preparations.     

Expression of GLUT4 glucose transporter protein in adipose tissue and skeletal muscle from streptozotocin-induced diabetic pregnant rats.

Depletion of GLUT4, the primary glucose transporter protein in adipose tissue and skeletal muscle, is reported to contribute to insulin resistance in pregnancy or diabetes. To examine this phenomenon, the expression of GLUT4 protein was assessed by Western blotting in streptozotocin-induced diabetic pregnant rats. In adipose tissue, relative to control, it was decreased by 30% in the normal pregnant group (p<0.001), by 37% in the diabetic nonpregnant group (p<0.01) and by 65% in the diabetic pregnant group (p<0.001). On the other hand, no significant variation was evident among the groups in skeletal muscle. To assess the mechanisms responsible for depletion of GLUT4 protein in adipose tissue, we quantitated levels of GLUT4 mRNA with a RNase protection assay. It was decreased by 44% in the normal pregnant group (p<0.05) and by 55% in the diabetic pregnant group (p<0.05), but not altered in the diabetic nonpregnant group. These results suggest that the depletion of GLUT4 protein in adipose tissue is a factor contributing to insulin resistance in pregnancy or diabetes, especially when the two states exist in combination.     

Growth hormone-mediated breakdown of body fat: effects of GH on lipases in adipose tissue and skeletal muscle of old rats fed different diets.

Lipid storage and breakdown is mainly controlled by lipoprotein lipase and hormone-sensitive lipase. The aim of this work was to elucidate whether growth hormone mediated loss of adipose tissue involves a concerted action on tissue lipases, and to what degree such events are modulated by dietary regimen. Twelve-month-old rats fed first a high-fat diet or a low-fat diet for 14 weeks were injected with saline or growth hormone (4 mg/kg/d) for four days or three weeks in different combinations with either high- or low-fat diets. In adipose tissue, growth hormone generally inhibited lipoprotein lipase and also attenuated the inhibiting effect of insulin on hormone-sensitive lipase activity. Growth hormone treatment combined with restricted high-fat feeding reduced the activity of both lipases in adipose tissue and stimulated hormone-sensitive lipase in muscle. Generally, plasma levels of free fatty acids, glycerol and cholesterol were reduced by growth hormone, and in combination with restricted high-fat feeding, triglyceride levels improved too. We conclude that growth hormone inhibits lipid storage in adipose tissue by reducing both lipoprotein lipase activity and insulin's inhibitory action on hormone-sensitive lipase. We also propose that growth hormone's effects on tissue lipases and blood lipids are modulated by dietary regimen.     

ATP-sensitive potassium channels participate in glucose uptake in skeletal muscle and adipose tissue

ATP-sensitive potassium (K(ATP)) channels are known to be critical in the control of both insulin and glucagon secretion, the major hormones in the maintenance of glucose homeostasis. The involvement of K(ATP) channels in glucose uptake in the target tissues of insulin, however, is not known. We show here that Kir6.2(-/-) mice lacking Kir6.2, the pore-forming subunit of these channels, have no K(ATP) channel activity in their skeletal muscles. A 2-deoxy-[(3)H]glucose uptake experiment in vivo showed that the basal and insulin-stimulated glucose uptake in skeletal muscles and adipose tissues of Kir6.2(-/-) mice is enhanced compared with that in wild-type (WT) mice. In addition, in vitro measurement of glucose uptake indicates that disruption of the channel increases the basal glucose uptake in Kir6.2(-/-) extensor digitorum longus and the insulin-stimulated glucose uptake in Kir6.2(-/-) soleus muscle. In contrast, glucose uptake in adipose tissue, measured in vitro, was similar in Kir6.2(-/-) and WT mice, suggesting that the increase in glucose uptake in Kir6.2(-/-) adipocytes is mediated by altered extracellular hormonal or neuronal signals altered by disruption of the K(ATP) channels.     

Tissue-specific expression and regulation of GSK-3 in human skeletal muscle and adipose tissue

Glycogen synthase kinase-3 (GSK-3) is a ubiquitous kinase implicated in both insulin action and adipogenesis. To determine how these multiple roles may relate to insulin resistance, we studied the regulation of GSK-3 protein expression and phosphorylation in skeletal muscle and isolated adipocytes from nonobese healthy control (HC), obese control (OC), and obese type 2 diabetic (OT2D) subjects. At baseline there were no differences in the GSK-3 protein expression in adipocytes. OC subjects underwent a 6-mo caloric restriction resulting in a 7% decrease in body mass index (BMI) and a 21% improvement in insulin-stimulated whole body glucose disposal rate (GDR). GSK-3alpha and GSK-3beta expression decreased in adipocytes (P < 0.05), whereas GSK-3alpha protein expression increased in skeletal muscle (P < 0.05). OT2D subjects were treated with troglitazone or metformin for 3-4 mo. After troglitazone treatment GDR improved (P < 0.05) despite an increase in BMI (P < 0.05), whereas metformin had no significant effect on GDR. There was no significant change in GSK-3 expression in adipocytes following troglitazone, whereas both GSK-3alpha and -beta were decreased in skeletal muscle (P < 0.05). Metformin treatment had no significant impact on GSK-3 protein expression in either adipocytes or skeletal muscle. Neither treatment influenced GSK-3 serine phosphorylation in skeletal muscle or adipocytes. These results suggest that there is tissue specificity for the regulation of GSK-3 in humans. In skeletal muscle GSK-3 plays a role in control of metabolism and insulin action, whereas the function in adipose tissue is less clear.     

Alterations of mitochondrial protein metabolism in liver, brown adipose tissue and skeletal muscle. During cold-acclimation.

Incorporation of L-[U-14C] leucine into liver, brown adipose tissue and skeletal muscle mitochondrial proteins was determined in vivo and in vitro during cold-acclimation. Major alterations in mitochondrial protein metabolism were observed in brown adipose tissue and skeletal muscle but not in liver. Immediate cold-exposure is accompanied by an inhibition of the in vivo incorporation of L-[U-14C] leucine into mitochondrial proteins of all tissues. However, during cold-acclimation the incorporation of leucine increases markedly in brown adipose tissue, continues to decrease in skeletal muscle, nut does not change appreciably in the liver. Because increased incorporation of L-[U-14C]-leucine into brown adipose tissue mitochondrial proteins was observed both in vivo and in vitro, it can be concluded that the mitochondrial protein-synthesizing system of this tissue is directly affected by the acclimation process. The observed changes in mitochondrial protein metabolism of brown adipose tissue and skeletal muscle might be responsible for the development of several morphological and biochemical alterations that characterize the establishment in these tissues of the cold-acclimated state.     

Different effects of IGF-I on insulin-stimulated glucose uptake in adipose tissue and skeletal muscle

The effect of insulin-like growth factor I (IGF-I) on insulin-stimulated glucose uptake was studied in adipose and muscle tissues of hypophysectomized female rats. IGF-I was given as a subcutaneous infusion via osmotic minipumps for 6 or 20 days. All hypophysectomized rats received L-thyroxine and cortisol replacement therapy. IGF-I treatment increased body weight gain but had no effect on serum glucose or free fatty acid levels. Serum insulin and C-peptide concentrations decreased. Basal and insulin-stimulated glucose incorporation into lipids was reduced in adipose tissue segments and isolated adipocytes from the IGF-I-treated rats. In contrast, insulin treatment of hypophysectomized rats for 7 days increased basal and insulin-stimulated glucose incorporation into lipids in isolated adipocytes. Pretreatment of isolated adipocytes in vitro with IGF-I increased basal and insulin-stimulated glucose incorporation into lipids. These results indicate that the effect of IGF-I on lipogenesis in adipose tissue is not direct but via decreased serum insulin levels, which reduce the capacity of adipocytes to metabolize glucose. Isoproterenol-stimulated lipolysis, but not basal lipolysis, was enhanced in adipocytes from IGF-I-treated animals. In the soleus muscle, the glycogen content and insulin-stimulated glucose incorporation into glycogen were increased in IGF-I-treated rats. In summary, IGF-I has opposite effects on glucose uptake in adipose tissue and skeletal muscle, findings which at least partly explain previous reports of reduced body fat mass, increased body cell mass, and increased insulin responsiveness after IGF-I treatment.     

Measurement of interstitial albumin in human skeletal muscle and adipose tissue by open-flow microperfusion

The absolute concentration of albumin was measured in the interstitial fluid of subcutaneous adipose tissue and skeletal muscle in six healthy volunteers by combining the method of open-flow microperfusion and the no-net-flux calibration technique. By use of open-flow microperfusion, four macroscopically perforated double lumen catheters were inserted into the tissue regions of interest and constantly perfused. Across the macroscopic perforations of the catheters interstitial fluid was partially recovered in the perfusion fluid. Catheters were perfused with five solutions, each containing different concentrations of albumin. Absolute interstitial albumin concentrations were calculated by applying linear regression analysis to perfusate vs. sampled albumin concentration (no-net-flux calibration technique). Interstitial albumin concentrations were significantly lower (P < 0.0001) in adipose tissue (7.36 g/l; r = 0.99, P < 0.0003; range: 4.3-10.7 g/l) and in skeletal muscle (13.25 g/l; r = 0.99, P < 0.0012; range: 9.7 to 15.7 g/l) compared with the serum concentration (48.9 +/- 0.7 g/l, mean +/- SE, n = 6; range: 46.4-50.4 g/l). Furthermore, interstitial albumin concentrations were significantly higher in skeletal muscle compared with adipose tissue (P < 0.01). The study indicates that open-flow microperfusion allows stable sampling of macromolecules from the interstitial space of peripheral tissue compartments. Moreover, the present data report for the first time in healthy humans in vivo the true albumin concentrations of interstitial fluid of adipose tissue and skeletal muscle.