Arteriovenous differences across human adipose and forearm tissues after overnight fast

Enhanced interleukin-6 in human adipose tissue vein after sprint exercise: Results from a pilot study

BACKGROUND

Low-volume sprint exercise is likely to reduce body fat. Interleukin (IL-6) may mediate this by increasing adipose tissue (AT) lipolysis. Therefore, the exchange of AT IL-6 and glycerol, a marker of lipolysis, was examined in 10 healthy subjects performing three 30-s all-out sprints.

METHODS

Blood samples were obtained from brachial artery (a) and a superficial subcutaneous vein (v) on the anterior abdominal wall up to 9 min after the last sprint and analysed for IL-6 and glycerol.

RESULTS

Arterial IL-6 increased 2-fold from rest to last sprint. AT venous IL-6 increased 15-fold from 0.4 ± 0.4 at rest to 7.0 ± 4 pg × mL-1 (p < 0.0001) and AT v-a difference increased 45-fold from 0.12 ± 0.3 to 6.0 ± 5 pg x mL-1 (p < 0.0001) 9 min after last sprint. Arterial glycerol increased 2.5-fold from rest to 9 min postsprint 1 (p < 0.0001) and was maintained during the exercise period. AT venous and v-a difference of glycerol increased 2-fold from rest to 9 min postsprint 1 (p < 0.0001 and p = 0.01, respectively), decreased until 18 min postsprint 2 (p < 0.001 and p < 0.0001), and then increased again until 9 min after last sprint (both p < 0.01).

CONCLUSIONS

The concurrent increase in venous IL-6 and glycerol in AT after last sprint is consistent with an IL-6 induced lipolysis in AT. Glycerol data also indicated an initial increase in lipolysis after sprint 1 that was unrelated to IL-6. Increased IL-6 in adipose tissue may, therefore, complement other sprint exercise-induced lipolytic agents.

Non-invasive Assessments of Adipose Tissue Metabolism In Vitro

Adipose tissue engineering is a diverse area of research where the developed tissues can be used to study normal adipose tissue functions, create disease models in vitro, and replace soft tissue defects in vivo. Increasing attention has been focused on the highly specialized metabolic pathways that regulate energy storage and release in adipose tissues which affect local and systemic outcomes. Non-invasive, dynamic measurement systems are useful to track these metabolic pathways in the same tissue model over time to evaluate long term cell growth, differentiation, and development within tissue engineering constructs. This approach reduces costs and time in comparison to more traditional destructive methods such as biochemical and immunochemistry assays and proteomics assessments. Towards this goal, this review will focus on important metabolic functions of adipose tissues and strategies to evaluate them with non-invasive in vitro methods. Current non-invasive methods, such as measuring key metabolic markers and endogenous contrast imaging will be explored.

Contrast-enhanced ultrasound assessment of impaired adipose tissue and muscle perfusion in insulin-resistant mice

BACKGROUND

In diabetes mellitus, reduced perfusion and capillary surface area in skeletal muscle, which is a major glucose storage site, contribute to abnormal glucose homeostasis. Using contrast-enhanced ultrasound, we investigated whether abdominal adipose tissue perfusion is abnormal in insulin resistance and correlates with glycemic control.

METHODS AND RESULTS

Contrast-enhanced ultrasound perfusion imaging of abdominal adipose tissue and skeletal muscle was performed in obese insulin resistance (db/db) mice at 11 to 12 or 14 to 16 weeks of age and in control lean mice. Time-intensity data were analyzed to quantify microvascular blood flow (MBF) and capillary blood volume (CBV). Blood glucose response for 1 hour was measured after insulin challenge (1 U/kg, IP). Compared with control mice, db/db mice at 11 to 12 and 14 to 16 weeks had a higher glucose concentration area under the curve after insulin (11.8±2.8, 20.6±4.3, and 28.4±5.9 mg·min/dL [×1000], respectively; P=0.0002) and also had lower adipose MBF (0.094±0.038, 0.035±0.010, and 0.023±0.01 mL/min per gram; P=0.0002) and CBV (1.6±0.6, 1.0±0.3, and 0.5±0.1 mL/100 g; P=0.0017). The glucose area under the curve correlated in a nonlinear fashion with both adipose and skeletal muscle MBF and CBV. There were significant linear correlations between adipose and muscle MBF (r=0.81) and CBV (r=0.66). Adipocyte cell volume on histology was 25-fold higher in 14- to 16-week db/db versus control mice.

CONCLUSIONS

Abnormal adipose MBF and CBV in insulin resistance can be detected by contrast-enhanced ultrasound and correlates with the degree of impairment in glucose storage. Abnormalities in adipose tissue and muscle seem to be coupled. Impaired adipose tissue perfusion is in part explained by an increase in adipocyte size without proportional vascular response.

The role of acetic acid on glucose uptake and blood flow rates in the skeletal muscle in humans with impaired glucose tolerance

BACKGROUND/OBJECTIVES

Previous studies support the glucose-lowering effect of vinegar. However, the effect of vinegar on muscle glucose metabolism and endothelial function has not been studied in humans. This open, randomized, crossover, placebo-controlled study aims to investigate the effects of vinegar on muscle glucose metabolism, endothelial function and circulating lipid levels in subjects with impaired glucose tolerance (IGT) using the arteriovenous difference technique.

SUBJECTS/METHODS

Eight subjects with IGT (4 males, age 46±10 years, body mass index 30±5) were randomised to consume 0.50 mmol vinegar (6% acetic acid) or placebo before a mixed meal. Plasma samples were taken for 300 min from the radial artery and the forearm vein for measurements of glucose, insulin, triglycerides, non-esterified fatty acids (NEFAs) and glycerol. Muscle blood flow was measured with strain gauge plethysmography. Glucose flux was calculated as the arteriovenous difference of glucose multiplied by the blood flow rates.

RESULTS

Vinegar compared with placebo: (1) decreased arterial plasma insulin (Poverall<0.001; P75 min=0.014, β=-42), (2) increased forearm blood flow (Poverall<0.001; P240 min=0.011, β=1.53; P300 min=0.023, β=1.37), (3) increased muscle glucose uptake (Poverall<0.001; P60 min=0.029, β=2.78) and (4) decreased arterial plasma triglycerides (Poverall=0.005; P240 min<0.001, β=-344; P300 min<0.001, β=-373), without changing NEFA and glycerol.

CONCLUSIONS

In individuals with IGT, vinegar ingestion before a mixed meal results in an enhancement of muscle blood flow, an improvement of glucose uptake by the forearm muscle and a reduction of postprandial hyperinsulinaemia and hypertriglyceridaemia. From this point of view, vinegar may be considered beneficial for improving insulin resistance and metabolic abnormalities in the atherogenic prediabetic state.

Comprehensive review on lactate metabolism in human health

Metabolic pathways involved in lactate metabolism are important to understand the physiological response to exercise and the pathogenesis of prevalent diseases such as diabetes and cancer. Monocarboxylate transporters are being investigated as potential targets for diagnosis and therapy of these and other disorders. Glucose and alanine produce pyruvate which is reduced to lactate by lactate dehydrogenase in the cytoplasm without oxygen consumption. Lactate removal takes place via its oxidation to pyruvate by lactate dehydrogenase. Pyruvate may be either oxidized to carbon dioxide producing energy or transformed into glucose. Pyruvate oxidation requires oxygen supply and the cooperation of pyruvate dehydrogenase, the tricarboxylic acid cycle, and the mitochondrial respiratory chain. Enzymes of the gluconeogenesis pathway sequentially convert pyruvate into glucose. Congenital or acquired deficiency on gluconeogenesis or pyruvate oxidation, including tissue hypoxia, may induce lactate accumulation. Both obese individuals and patients with diabetes show elevated plasma lactate concentration compared to healthy subjects, but there is no conclusive evidence of hyperlactatemia causing insulin resistance. Available evidence suggests an association between defective mitochondrial oxidative capacity in the pancreatic β-cells and diminished insulin secretion that may trigger the development of diabetes in patients already affected with insulin resistance. Several mutations in the mitochondrial DNA are associated with diabetes mellitus, although the pathogenesis remains unsettled. Mitochondrial DNA mutations have been detected in a number of human cancers. d-lactate is a lactate enantiomer normally formed during glycolysis. Excess d-lactate is generated in diabetes, particularly during diabetic ketoacidosis. d-lactic acidosis is typically associated with small bowel resection.

Adipose tissue oxygenation: Effects on metabolic function

With the increasing prevalence of obesity there is a concomitant increase in white adipose tissue dysfunction, with the tissue moving toward a proinflammatory phenotype. Adipose tissue hypoxia has been proposed as a key underlying mechanism triggering tissue dysfunction but data from human, in vivo studies, to support this hypothesis is limited. Human adipose tissue oxygenation has been investigated by direct assessment of tissue oxygen tension (pO2) or by expression of hypoxia-sensitive genes/protein in lean and obese subjects but findings are inconsistent. An obvious read-out of hypoxia is the effect on intermediary metabolism, and we have investigated the functional consequences, in terms of a "metabolic signature" of human adipose tissue hypoxia in vivo. Here, we discuss the different approaches used and the importance of integrative physiological techniques to try and elucidate what defines adipose tissue hypoxia in humans.

Metabolic signatures of human adipose tissue hypoxia in obesity

Adipose tissue (AT) hypoxia has been proposed as the cause of obesity-related AT dysfunction, moving the tissue toward a proinflammatory phenotype. In humans, AT oxygenation has been assessed by expression of hypoxia-sensitive genes or direct assessment of O₂ tension; the obvious read out of hypoxia, effects on intermediary metabolism, has not been investigated. We used tissue-specific venous catheterization of subcutaneous abdominal AT in humans to investigate oxygen-related metabolic processes, searching for metabolic signatures relating to hypoxia in obesity. O₂ delivery to AT was reduced in obesity (P < 0.05). However, O₂ consumption was low (<30% of resting forearm skeletal muscle [SM], P < 0.001); this was not related to obesity. AT primarily oxidized glucose, as demonstrated by a respiratory quotient close to 1.0 (higher than SM, P < 0.05). AT was a net producer of lactate, but there was an inverse relationship in venous outflow between lactate-to-pyruvate ratio (a marker of cytosolic redox state) and BMI, suggesting that AT is glycolytic but obese AT is not hypoxic. Although delivery of O₂ to the obese AT is reduced, O₂ consumption is low, and metabolic signatures of human AT do not support the notion of a hypoxic state in obesity.

Insulin-sensitizing effects on muscle and adipose tissue after dietary fiber intake in men and women with metabolic syndrome

CONTEXT

Dietary fibers have been associated with a reduced incidence of type 2 diabetes mellitus in epidemiological studies; however, the precise mechanisms are unknown.

OBJECTIVE

The objective of the study was to evaluate the efficacy and site of action of an insoluble dietary fiber derived from maize (HAM-RS2) in improving insulin resistance in subjects at increased risk of type 2 diabetes mellitus.

DESIGN

This study was a randomized, controlled crossover, dietary intervention study.

SETTING

The study was conducted at the Centre for Diabetes, Endocrinology, and Research, Royal Surrey County Hospital, Guildford, United Kingdom.

PARTICIPANTS

Fifteen men and women with insulin resistance participated in the study.

INTERVENTION

The intervention included 40 g/d HAM-RS2 compared with a matched placebo for 8 wk.

MAIN OUTCOME MEASURES

After each supplement, participants underwent a two-step hyperinsulinemic-euglycemic clamp study with the addition of glucose tracers; a meal tolerance test; arteriovenous sampling across forearm muscle tissue; and a sc adipose tissue biopsy for assessment of gene expression.

RESULTS

There was enhanced uptake of glucose into the forearm muscle measured by arteriovenous sampling (65 ± 15% increase after resistant starch; P < 0.001). Adipose tissue function was also affected, with enhanced fatty acid suppression after HAM-RS2 treatment and an increase in gene expression for hormone sensitive lipase (P = 0.005), perilipin (P = 0.011), lipoprotein lipase (P = 0.014), and adipose triglyceride lipase (P = 0.03) in biopsy samples. There was no effect on the insulin sensitivity of hepatic glucose production or plasma lipids after HAM-RS2.

CONCLUSION

HAM-RS2 improved peripheral but not hepatic insulin resistance and requires further study as an intervention in patients with or at risk for type 2 diabetes.

Multiorgan insulin sensitivity in lean and obese subjects

OBJECTIVE

To provide a comprehensive assessment of multiorgan insulin sensitivity in lean and obese subjects with normal glucose tolerance.

RESEARCH DESIGN AND METHODS

The hyperinsulinemic-euglycemic clamp procedure with stable isotopically labeled tracer infusions was performed in 40 obese (BMI 36.2 ± 0.6 kg/m(2), mean ± SEM) and 26 lean (22.5 ± 0.3 kg/m(2)) subjects with normal glucose tolerance. Insulin was infused at different rates to achieve low, medium, and high physiological plasma concentrations.

RESULTS

In obese subjects, palmitate and glucose R(a) in plasma decreased with increasing plasma insulin concentrations. The decrease in endogenous glucose R(a) was greater during low-, medium-, and high-dose insulin infusions (69 ± 2, 74 ± 2, and 90 ± 2%) than the suppression of palmitate R(a) (52 ± 4, 68 ± 1, and 79 ± 1%). Insulin-mediated increase in glucose disposal ranged from 24 ± 5% at low to 253 ± 19% at high physiological insulin concentrations. The suppression of palmitate R(a) and glucose R(a) were greater in lean than obese subjects during low-dose insulin infusion but were the same in both groups during high-dose insulin infusion, whereas stimulation of glucose R(d) was greater in lean than obese subjects across the entire physiological range of plasma insulin.

CONCLUSIONS

Endogenous glucose production and adipose tissue lipolytic rate are both very sensitive to small increases in circulating insulin, whereas stimulation of muscle glucose uptake is minimal until high physiological plasma insulin concentrations are reached. Hyperinsulinemia within the normal physiological range can compensate for both liver and adipose tissue insulin resistance, but not skeletal muscle insulin resistance, in obese people who have normal glucose tolerance.

Metabolic characteristics of human subcutaneous abdominal adipose tissue after overnight fast

Subcutaneous abdominal adipose tissue is one of the largest fat depots and contributes the major proportion of circulating nonesterified fatty acids (NEFA). Little is known about aspects of human adipose tissue metabolism in vivo other than lipolysis. Here we collated data from 331 experiments in 255 healthy volunteers over a 23-year period, in which subcutaneous abdominal adipose tissue metabolism was studied by measurements of arterio-venous differences after an overnight fast. NEFA and glycerol were released in a ratio of 2.7:1, different (P < 0.001) from the value of 3.0 that would indicate no fatty acid re-esterification. Fatty acid re-esterification was 10.2 ± 1.4%. Extraction of triacylglycerol (TG) (fractional extraction 5.7 ± 0.4%) indicated intravascular lipolysis by lipoprotein lipase, and this contributed 21 ± 3% of the glycerol released. Glucose uptake (fractional extraction 2.6 ± 0.3%) was partitioned around 20-25% for provision of glycerol 3-phosphate and 30% into lactate production. There was release of lactate and pyruvate, with extraction of the ketone bodies 3-hydroxybutyrate and acetoacetate, although these were small numerically compared with TG and glucose uptake. NEFA release (expressed per 100 g tissue) correlated inversely with measures of fat mass (e.g., with BMI, r(s) = -0.24, P < 0.001). We examined within-person variability. Systemic NEFA concentrations, NEFA release, fatty acid re-esterification, and adipose tissue blood flow were all more consistent within than between individuals. This picture of human adipose tissue metabolism in the fasted state should contribute to a greater understanding of adipose tissue physiology and pathophysiology.

Arterio-venous differences in peripheral blood mononuclear cells across human adipose tissue and the effect of adrenaline infusion

Recent evidence indicates that adipose tissue macrophages and lymphocytes have a major role in the pathophysiology of obesity. The arterio-venous (A-V) difference technique has been used very effectively to understand adipose tissue metabolism in humans in vivo, and we set out to investigate whether it is possible to apply and use this technique to determine A-V differences for peripheral blood mononuclear cells (PBMCs) across human adipose tissue. We used flow-cytometric analysis of arterial blood and venous blood draining upper- (abdominal) and lower-body (femoral) adipose tissue depots in middle-aged volunteers (age 45±8 years, BMI 25.9±4.1 kg m(-2)). We determined A-V differences for various PBMCs. In basal conditions, there was evidence of modest retention of some PBMCs in adipose tissue, whereas the infusion of low-dose (physiological) adrenaline led to a marked release of many PBMCs (with little evidence of depot-specific differences). In addition to the demonstration that this approach is technically feasible, these results also indicate that physiological stimuli that change adrenaline concentrations and/or adipose tissue blood flow (such as physical activity) provoke the release of PBMCs from femoral and abdominal adipose depots.

Regulation of Adipose Tissue Metabolism in Humans: Analysis of Responses to the Hyperinsulinemic-Euglycemic Clamp Experiment

The suppression of lipolysis is one of the key metabolic responses of the adipose tissue during hyperinsulinemia. The failure to respond and resulting increase in plasma fatty acids could contribute to the development of insulin resistance and perturbations in the fuel homeostasis in the whole body. In this study, a mechanistic, computational model of adipose tissue metabolism in vivo has been enhanced to simulate the physiological responses during hyperinsulinemic-euglycemic clamp experiment in humans. The model incorporates metabolic intermediates and pathways that are important in the fed state. In addition, it takes into account the heterogeneity of triose phosphate pools (glycolytic vs. glyceroneogenic), within the adipose tissue. The model can simulate not only steady-state responses at different insulin levels, but also concentration dynamics of major metabolites in the adipose tissue venous blood in accord with the in vivo data. Simulations indicate that (1) regulation of lipoprotein lipase (LPL) reaction is important when the intracellular lipolysis is suppressed by insulin; (2) intracellular diglyceride levels can affect the regulatory mechanisms; and (3) glyceroneogenesis is the dominant pathway for glycerol-3-phosphate synthesis even in the presence of increased glucose uptake by the adipose tissue. Reduced redox and increased phosphorylation states provide a favorable milieu for glyceroneogenesis in response to insulin. A parameter sensitivity analysis predicts that insulin-stimulated glucose uptake would be more severely affected by impairment of GLUT4 translocation and glycolysis than by impairment of glycogen synthesis and pyruvate oxidation. Finally, simulations predict metabolic responses to altered expression of phosphoenolpyruvate carboxykinase (PEP-CK). Specifically, the increase in the rate of re-esterification of fatty acids observed experimentally with the overexpression of PEPCK in the adipose tissue would be accompanied by the up-regulation of acyl Co-A synthase.

Regulation of human metabolism by hypoxia-inducible factor

The hypoxia-inducible factor (HIF) family of transcription factors directs a coordinated cellular response to hypoxia that includes the transcriptional regulation of a number of metabolic enzymes. Chuvash polycythemia (CP) is an autosomal recessive human disorder in which the regulatory degradation of HIF is impaired, resulting in elevated levels of HIF at normal oxygen tensions. Apart from the polycythemia, CP patients have marked abnormalities of cardiopulmonary function. No studies of integrated metabolic function have been reported. Here we describe the response of these patients to a series of metabolic stresses: exercise of a large muscle mass on a cycle ergometer, exercise of a small muscle mass (calf muscle) which allowed noninvasive in vivo assessments of muscle metabolism using (31)P magnetic resonance spectroscopy, and a standard meal tolerance test. During exercise, CP patients had early and marked phosphocreatine depletion and acidosis in skeletal muscle, greater accumulation of lactate in blood, and reduced maximum exercise capacities. Muscle biopsy specimens from CP patients showed elevated levels of transcript for pyruvate dehydrogenase kinase, phosphofructokinase, and muscle pyruvate kinase. In cell culture, a range of experimental manipulations have been used to study the effects of HIF on cellular metabolism. However, these approaches provide no potential to investigate integrated responses at the level of the whole organism. Although CP is relatively subtle disorder, our study now reveals a striking regulatory role for HIF on metabolism during exercise in humans. These findings have significant implications for the development of therapeutic approaches targeting the HIF pathway.

A computational model of adipose tissue metabolism: evidence for intracellular compartmentation and differential activation of lipases

Regulation of lipolysis in adipose tissue is critical to whole body fuel homeostasis and to the development of insulin resistance. Due to the challenging nature of laboratory investigations of regulatory mechanisms in adipose tissue, mathematical models could provide a valuable adjunct to such experimental work. We have developed a computational model to analyze key components of adipose tissue metabolism in vivo in human in the fasting state. The various key components included triglyceride-fatty acid cycling, regulation of lipolytic reactions, and glyceroneogenesis. The model, consisting of spatially lumped blood and cellular compartments, included essential transport processes and biochemical reactions. Concentration dynamics for major substrates were described by mass balance equations. Model equations were solved numerically to simulate dynamic responses to intravenous epinephrine infusion. Model simulations were compared with the corresponding experimental measurements of the arteriovenous difference across the abdominal subcutaneous fat bed in humans. The model can simulate physiological responses arising from the different expression levels of lipases. Key findings of this study are as follows: (1) Distinguishing the active metabolic subdomain ( approximately 3% of total tissue volume) is critical for simulating data. (2) During epinephrine infusion, lipases are differentially activated such that diglyceride breakdown is approximately four times faster than triglyceride breakdown. (3) Glyceroneogenesis contributes more to glycerol-3-phosphate synthesis during epinephrine infusion when pyruvate oxidation is inhibited by a high acetyl-CoA/free-CoA ratio.

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.

Endocrine role of the renin-angiotensin system in human adipose tissue and muscle: effect of beta-adrenergic stimulation

The renin-angiotensin system has been implicated in obesity-related hypertension and insulin resistance. We examined whether locally produced components of the renin-angiotensin system in adipose tissue and skeletal muscle play an endocrine role in vivo in humans. Furthermore, the effects of beta-adrenergic stimulation on plasma concentrations and tissue release of renin-angiotensin system components were investigated. Systemic renin-angiotensin system components and arteriovenous differences of angiotensin II (Ang II) and angiotensinogen (AGT) across abdominal subcutaneous adipose tissue and skeletal muscle were assessed in combination with measurements of tissue blood flow before and during systemic beta-adrenergic stimulation in 13 lean and 10 obese subjects. Basal plasma Ang II and AGT concentrations were not significantly different between lean and obese subjects. Ang II concentrations were increased in obese compared with lean subjects during beta-adrenergic stimulation (12.6+/-1.5 versus 8.1+/-1.0 pmol/L; P=0.04), whereas AGT concentrations remained unchanged. Plasma renin activity increased to a similar extent in lean and obese subjects during beta-adrenergic stimulation (both P<0.01). No net Ang II release across adipose tissue and skeletal muscle could be detected in both groups of subjects. However, AGT was released from adipose tissue and muscle during beta-adrenergic stimulation in obese subjects (both P<0.05). In conclusion, locally produced Ang II in adipose tissue and skeletal muscle exerts no endocrine role in lean and obese subjects. In contrast, AGT is released from adipose tissue and muscle in obese subjects during beta-adrenergic stimulation, which may contribute to the increased plasma Ang II concentrations during beta-adrenergic stimulation in obese subjects.

Adipose tissue metabolism, diabetes and vascular disease--lessons from in vivo studies

There is an epidemic of obesity, insulin resistance and cardiovascular disease. Adipose tissue plays a major metabolic role and produces hormones with important physiological effects. In vitro studies remove regulatory factors, such as blood flow, making results difficult to interpret, and animal studies cannot necessarily be extrapolated to humans. Fortunately, adipose tissue can be studied in vivo with microdialysis, adipose tissue vein cannulation, measurement of blood flow using 133Xenon washout, stable isotope tracers and biopsies. In vivo studies have shown that adipose tissue is an efficient buffer against the postprandial flux of non-esterified fatty acids (NEFA) in the circulation, protecting other tissues. When there is excess adipose tissue, this buffering effect may be impaired. The postprandial blood flow response is also reduced, potentially causing an atherogenic lipid profile and atheroma. A systems biology approach, combining in vivo techniques with genomics, proteomics and metabolomics, will clarify links between adipose tissue and vascular disease.

Changes in plasma lipids and lipoproteins following 10-days of prolonged walking: influence of age and relationship to physical activity level

The aim of the present study was to examine (1) the influence of 10 days of prolonged walking on plasma total-cholesterol (TC), high-density lipoprotein cholesterol (HDL-C) and low-density lipoprotein cholesterol (LDL-C) concentrations, (2) the effect of age on any changes in plasma cholesterol and lipoprotein concentration, and (3) whether any changes in cholesterol and lipoprotein concentrations are correlated to the participants' physical activity level (PAL). Seventeen male participants were divided into two groups according to their age. The nine participants in group 1 constituted the younger group (age 24 +/- 3 (SD) years), whereas eight older participants were in group 2 (age 56 +/- 3 years). Both groups completed 10 consecutive days of high-intensity hill walking. Mean (range) daily distances and ascent were 21 km (10-35 km) and 1,160 m (800-2,540 m), respectively. Identical distances and ascents were covered by each group. For each participant, PAL was calculated from energy expenditure, assessed by the doubly-labeled water technique, divided by the individual's basal metabolic rate. Venous blood was sampled immediately prior to, and following, the 10 consecutive days of walking. Following these 10 days, the older group showed a greater decrease in both TC (-25 +/- 11% vs. -10 +/- 11%; P < 0.05) and LDL-C (-26 +/- 12% vs. -4 +/- 13%; P < 0.05) when compared with the young. Likewise, the older group showed a greater increase in HDL-C (38 +/- 15%; P < 0.05), after the 10 days, whereas no significant change was evident in the younger group. In the older participants, there were strong positive relationships between PAL and the decreases in TC (r = 0.79, P < 0.05) and LDL-C (r = 0.74, P < 0.05). Conversely, in the younger group there were strong negative relationships between PAL and the decreases in TC (r = -0.74, P < 0.05) and LDL-C (r = -0.86, P < 0.01). These correlations persisted when changes in lipid concentrations were corrected for changes in plasma volume. These data suggest an 'age-dependant' threshold for PAL, rather than a specific exercise intensity or duration, may be critical for inducing favourable changes in HDL-C, LDL-C and TC.

β-ADRENERGICRECEPTORS ANDREGULATION OFENERGYEXPENDITURE: A Family Affair

The family of adrenergic receptors (ARs) expressed in adipocytes includes three sibling betaARs and two alphaAR cousins. Together they profoundly influence the mobilization of stored fatty acids, secretion of fat-cell derived hormones, and the specialized process of nonshivering thermogenesis in brown adipose tissue. The two types of fat cells that compose adipose tissue, brown and white, are structurally and functionally distinct. Studies on the mechanisms by which individual betaAR regulates these cell-specific functions have recently uncovered new signal transduction cascades involved in processes traditionally ascribed to adenylyl cyclase/cAMP/protein kinase A system. They illustrate how betaAR signaling can orchestrate a coordinated set of intracellular responses for fine control of metabolic balance.

Physiological, metabolic, and performance implications of a prolonged hill walk: influence of energy intake

We aimed to examine the effects of different energy intakes on a range of responses that are relevant to the safety of hill walkers. In a balanced design, 16 men completed a strenuous self-paced mountainous hill walk over 21 km, under either a low-energy (2.6 MJ; 616 kcal) intake (LEI) or high-energy (12.7 MJ; 3,019 kcal) intake (HEI) condition. During the hill walk, rectal temperatures were measured continuously, and blood samples for the analysis of metabolites and hormones were drawn before breakfast and immediately after the walk. Subjects also completed a battery of performance tests that included muscular strength, reaction times, flexibility, balance, and kinesthetic differentiation tests. During the LEI, mean blood glucose concentrations leveled off at the low-middle range of normoglycemia, whereas, on the HEI, they were significantly elevated compared with the LEI. The maintained blood glucose concentrations, during the LEI, were probably mediated via the marked fat mobilization, reflected by a two- to fivefold increase in nonesterified fatty acids, 3-hydroxybutyrate, and glycerol concentrations. The LEI group showed significantly slower one- and two-finger reaction time, had an impaired ability to balance, and were compromised in their ability to maintain body temperature, when compared with the HEI group. The modestly impaired performance (particularly with respect to balance) and thermoregulation during the LEI condition may increase susceptibility to both fatigue and injury during the pursuit of recreational activity outdoors.

Regional fat metabolism in human splanchnic and adipose tissues; the effect of exercise

This study was conducted to investigate the role of splanchnic and adipose tissue in the regulation of fatty acid (FA) metabolism at rest, during 1 h of semi-recumbent cycle exercise at 60 % of maximal power output and 3 h of recovery. In six post-absorptive healthy volunteers catheters were placed in a radial artery, hepatic vein and a subcutaneous vein on the anterior abdominal wall. Whole body, and regional splanchnic and adipose tissue FA metabolism were measured by a constant infusion of the stable isotopes [U-(13)C]palmitate and [(2)H(5)]glycerol and according to Fick's principle. The whole body rate of extracellular FA reesterification was similar at rest and during exercise (approximately 290 micromol min(-1)) and increased during recovery to a plateau of 390 micromol min(-1). FA and triacylglycerol (TAG) uptake by adipose tissue was undetectable, but a constant but small glycerol uptake of approximately 25 nmol (100 g)(-1) min(-1) was observed. From the FA taken up by the splanchnic area, 13 % was oxidized, 5-11 % converted to ketone bodies, and approximately 35 % incorporated in TAG released both at rest and at the third hour of recovery from exercise. Splanchnic FA reesterification could account for 51 % and 58 % of whole body extracellular FA reesterification, of which half was accounted for by TAG released from the splanchnic area, at rest and in recovery, respectively. In conclusion, in the post-absorptive state, adipose tissue contributes very little to extracellular FA reesterification and splanchnic reesterification can account for 50-60 %, implying that FA reesterification in other tissues is important. The extracellular FA reesterification rate does not change with exercise but is higher during recovery. Furthermore, the uptake of glycerol by adipose tissue indicates that adipose tissue can metabolize glycerol.

Energy balance, metabolism, hydration, and performance during strenuous hill walking: the effect of age

We aimed to examine the effect of age on energy balance, metabolism, hydration, and performance during 10 days of strenuous hill walking. Seventeen male subjects were divided into two groups according to their age. The nine subjects in group 1 constituted the younger group (age 24 +/- 3 yr), whereas eight older subjects were in group 2 (age 56 +/- 3 yr). Both groups completed 10 consecutive days of high-intensity hill walking. Mean (range) daily walking distances and ascent were 21 km (10-35 km) and 1,160 m (800-2,540 m), respectively. Energy intake was calculated from weighed food intake, and energy expenditure was measured by the doubly labeled water method. Blood and urine were sampled on alternative days to determine any changes in metabolism and hydration during the 10 days. Subjects also completed a battery of tests that included muscular strength (handgrip), jump performance, cognitive processing time, and flexibility. The younger group remained hydrated, whereas the older group became progressively dehydrated, indicated by a near twofold increase in urine osmolality concentration on day 11. This increased urine osmolality in the older group was highly correlated with impairment in vertical-jump performance (r = -0.86; P < 0.05) and decreased cognitive processing time (r = 0.79; P < 0.05). Despite energy expenditure of approximately 21 MJ/day, body mass was well maintained in both groups. Both groups displayed a marked increase in fat mobilization, reflected in significantly lowered prewalk insulin concentrations and elevated postwalk glycerol and nonesterified fatty acid concentrations. Despite the dehydration and impaired performance in the older group, blood glucose concentrations were well maintained in both groups, probably mediated via the increased mobilization of fat.

Whole body and leg acetate kinetics at rest, during exercise and recovery in humans

We have used a constant [1,2-(13)C]acetate infusion (0.12 micromol x min(-1) x kg( 1)) for 2 h at rest, followed by 2 h of one-legged knee-extensor exercise at 65% of leg maximal workload, and 3 h of recovery in six post-absorptive volunteers to quantify whole-body and leg acetate kinetics and determine whether the whole-body acetate correction factor can be used to correct leg substrate oxidation. The acetate whole-body rate of appearance (R(a)) was not significantly different at rest, during exercise or during recovery (365-415 micromol x min(-1)). The leg net acetate uptake was similar at rest and during recovery (approximately 10 micromol x min(-1)), but increased approximately 5-fold with exercise. At rest the leg acetate uptake (approximately 15 micromol x min(-1)) and release (approximately 5 micromol x min(-1)) accounted for 4 and 1.5 % of whole-body acetate disposal (R(d)) and R(a), respectively. When the leg acetate kinetics were extrapolated to the total body skeletal muscle mass, then skeletal muscle accounted for approximately 16 and approximately 6% of acetate R(d) and R(a). With exercise, leg acetate uptake increased approximately 6-fold, whereas leg acetate release increased 9-fold compared with rest. Whole-body acetate carbon recovery increased with time of infusion at rest and during recovery from 21% after 1.5 h of infusion to 45% in recovery after 7 h of infusion. Leg and whole-body acetate carbon recovery were similar under resting conditions, both before and after exercise. During exercise whole-body acetate carbon recovery was approximately 75%, however, acetate carbon recovery of the active leg was substantially higher (approximately 100%). It is concluded that inactive skeletal muscle plays a minor role in acetate turnover. However, active skeletal muscle enhances several-fold acetate uptake and subsequent oxidation, as well as release and its contribution to whole-body acetate turnover. Furthermore, under resting conditions the whole-body acetate correction factor can be used to correct for leg, skeletal muscle, substrate oxidation, but not during exercise.

Metabolic and appetite responses to prolonged walking under three isoenergetic diets

The effects of three isoenergetic diets on metabolic and appetite responses to prolonged intermittent walking were investigated. Eight men undertook three 450-min walks at intensities varying between 25-30 and 50-55% of maximal O2 uptake. In a balanced design, the subjects were given breakfast, snacks, and lunch containing total carbohydrate (CHO), protein (P), and fat (F) in the following amounts (g/70 kg body mass): mixed diet, 302 CHO, 50 P, 84 F; high-CHO diet, 438 CHO, 46 P, 35 F; high-fat diet, 63 CHO, 44 P, 196 F. Substrate balance was calculated by indirect calorimetry over the 450-min exercise period. Blood samples were taken before exercise and every 45 min during the exercise period. The high-fat diet resulted in a negative total CHO balance (-140 +/- 1 g) and a lower negative fat balance (-110 +/- 33 g) than the other two diets (P < 0.05). Plasma glucagon, nonesterified fatty acids, glycerol, and 3-hydroxybutyrate were higher with the high-fat diet (P < 0.05 vs. high CHO), whereas plasma insulin was lower after high fat (P < 0.05 vs. mixed and high CHO). Subjective ratings of fatigue and appetite showed no differences between the three trials. Although diet influenced the degree of total CHO and fat oxidation, fat was the main source of energy in all trials.

Lipolysis in human adipose tissue during exercise: comparison of microdialysis and a-v measurements

Subcutaneous adipose tissue lipolysis was studied in vivo by Fick's arteriovenous (a-v) principle using either calculated (microdialysis) or directly measured (catheterization) adipose tissue venous glycerol concentration. We compared results during steady-state (rest and prolonged continuous exercise), as well as during non-steady-state (onset of exercise and early exercise) experimental settings. Fourteen healthy women [age: 74 +/- 1 (SE) yr] were studied at rest and during 60-min continuous bicycling at 60% of peak O(2) uptake. Calculated and measured subcutaneous abdominal adipose tissue venous glycerol concentrations increased substantially from rest to exercise but were similar both at rest and during later stages of exercise. In contrast, during the initial approximately 40 min of exercise, calculated glycerol concentration was significantly lower (approximately 40%) than measured adipose tissue venous glycerol concentration. Despite several methodological limitations inherent to both techniques, the results strongly suggest that microdialysis and catheterization provide similar estimates of subcutaneous adipose tissue lipolysis in steady-state experimental settings like rest and continuous prolonged exercise. However, during shorter periods of exercise (<40 min), the results from the two techniques may differ quantitatively in the studied subjects. Caution should, therefore, be taken when lipolysis is evaluated, based on results obtained by the two techniques under non-steady-state conditions.

Subcutaneous abdominal adipose tissue lipolysis during exercise determined by arteriovenous measurements in older women

OBJECTIVES

To characterize the lipolytic response in the subcutaneous abdominal adipose tissue in older women to endurance exercise.

DESIGN

Cross-sectional exercise study.

SETTING

Exercise laboratory, Copenhagen, Denmark.

PARTICIPANTS

Seven healthy, older women (mean age +/- standard error = 75 +/- 2 years); weight: 67.8 +/- 4.9 kg; body fat: 40 +/- 3; maximal oxygen uptake (VO2max): 1.43 +/- 0.07 L.min 1).

MEASUREMENTS

Body composition (dual energy x-ray absorptiometry (DEXA)), maximal oxygen uptake (VO2max, maximal cycling test), lipolytic response to exercise (arterial and adipose tissue venous catheterization at rest and during 60 minutes of continuous cycling at a load corresponding to 60 of VO2max), adipose tissue blood flow (ATBF) (133Xenon (133Xe) washout), oxygen consumption and respiratory exchange ratio during exercise (indirect calorimetry), whole blood glycerol, plasma nonesterified fatty acids (NEFA), lactate, glucose, epinephrine, norepinephrine, insulin, serum growth hormone, and hematocrit.

RESULTS

Glycerol and NEFA mobilization rates increased by 250 and 180, respectively, from rest to exercise. This was achieved primarily by an increase in veno-arterial differences, because ATBF did not increase significantly. NEFA:glycerol mobilization ratio was about two at rest and remained at that level during exercise, indicating significant local reesterification in both conditions. After an initial decrease, arterial plasma NEFA concentration increased significantly, by 26, indicating that NEFA delivery exceeded muscle uptake.

CONCLUSIONS

Older women are capable of prompt and substantial increase in subcutaneous abdominal adipose tissue glycerol and NEFA mobilization rates in response to moderate acute endurance exercise. The lipolytic response matches skeletal muscle NEFA uptake, and decreased ability to mobilize fat during exercise is therefore not likely to cause increased fat mass with advancing age.

Macronutrient metabolism of adipose tissue at rest and during exercise: a methodological viewpoint

The metabolism of white adipose tissue is regulated by many factors, including hormones and substrates delivered in the blood, the activity of the autonomic nervous system and the rate of flow of blood through the tissue. An integrated view of adipose tissue metabolism can only be gained, therefore, from studies in vivo. Of the various techniques available for studying adipose tissue metabolism in vivo, the measurement of arterio-venous differences offers some unique possibilities. In human subjects this technique has been performed mostly by catheterization of the venous drainage of the subcutaneous abdominal depot. Studies using this technique indicate that adipose tissue has an active pattern of metabolism, responding rapidly to meal ingestion by suppressing the release of non-esterified fatty acids, or to exercise with an increase in fat mobilization. Adipose tissue blood flow may also change rapidly in these situations; for instance, it increases markedly after a meal, potentially increasing the delivery of triacylglycerol to the enzyme lipoprotein lipase (EC 3.1.1.34) for hydrolysis. During exercise, there is evidence that adipose tissue blood flow does not increase sufficiently to allow delivery of all the fatty acids released into the systemic circulation. The various adipose tissue depots have their own characteristic metabolic properties, although in human subjects these are difficult to study with the arterio-venous difference technique. A combination of tracer infusion with selective catheterization allows measurements of leg, splanchnic and non-splanchnic upper-body fat mobilization and triacylglycerol clearance. Development of such techniques may open up new possibilities in the future for obtaining an integrated picture of adipose tissue function and its depot-specific variations.

Effects of oxytocin on the IGF-axis and some gastrointestinal hormones in ad libitum fed and food-restricted female rats

The aims of this study were to investigate if administration of oxytocin to ad libitum fed and food-restricted female rats affects weight gain, body fatness, the IGF-axis, and some vagally mediated gastrointestinal hormones, such as gastrin, cholecystokinin (CCK) and somatostatin. Ad libitum fed and food-restricted (receiving 70% of the food intake of the ad libitum fed group) female rats were injected subcutaneously, once a day, for 10 days, with saline (control) or oxytocin (1 mg kg-1 bodyweight). The animals were killed 5 days after the last injection. Oxytocin-treated food-restricted females had more body fat and lower plasma levels of IGF-I, IGFBP-1 and IGFBP-3 compared with saline-treated counterparts. Oxytocin-treated ad libitum fed rats also had lower plasma levels of IGFBP-1 but contained less body fat, compared with saline-treated counterparts. There was no effect of oxytocin treatment on body weight or weight gain in either of the feeding groups. Except for gastrin, which was lower, there was no effect of oxytocin on the gastrointestinal hormones studied. The results indicate that oxytocin treatment influences fat deposition and the IGF-axis in female rats, but that the results are dependent on the nutritional status of the animal.

Regulation of body weight in humans

The mechanisms involved in body weight regulation in humans include genetic, physiological, and behavioral factors. Stability of body weight and body composition requires that energy intake matches energy expenditure and that nutrient balance is achieved. Human obesity is usually associated with high rates of energy expenditure. In adult individuals, protein and carbohydrate stores vary relatively little, whereas adipose tissue mass may change markedly. A feedback regulatory loop with three distinct steps has been recently identified in rodents: 1) a sensor that monitors the size of adipose tissue mass is represented by the amount of leptin synthesized by adipose cells (a protein encoded by the ob gene) which determines the plasma leptin levels; 2) hypothalamic centers, with specific leptin receptors, which receive and integrate the intensity of the signal; and 3) effector systems that influence the two determinants of energy balance, i.e., energy intake and energy expenditure. With the exception of a few very rare cases, the majority of obese human subjects have high plasma leptin levels that are related to the size of their adipose tissue mass. However, the expected regulatory responses (reduction in food intake and increase in energy expenditure) are not observed in obese individuals. Thus obese humans are resistant to the effect of endogenous leptin, despite unaltered hypothalamic leptin receptors. Whether defects in the leptin signaling cascade play a role in the development of human obesity is a field of great actual interest that needs further research. Present evidences suggest that genetic and environmental factors influence eating behavior of people prone to obesity and that diets that are high in fat or energy dense undermine body weight regulation by promoting an overconsumption of energy relative to need.

Does exercise intensity or diet influence lactic acid accumulation in breast milk?

PURPOSE

This study examined the relationships among diet, exercise intensity, and breast milk composition in lactating women.

METHODS

Twelve lactating women were randomly assigned to either a high (N = 6; 5.03 g carbohydrate (CHO) x kg body mass (BM)(-1)) or moderate (N = 6; 3.89 g CHO x kg BM(-1)) carbohydrate diet. Milk and blood samples were collected before and after a nonexercise session (control) and maximal, lactic acid-threshold (LAT), and 20% below the LAT (LAT-20) intensities.

RESULTS

The 30-min exercise LAT bout was more stressful than the 30-min LAT-20 bout (rating of perceived exertion (RPE) = 15 vs 12, respectively, P < 0.05). Milk LA was significantly higher at 0 min following maximal exercise in the high and moderate CHO groups (1.27+/-0.56 and 1.52+/-0.49 mM, respectively) and following LAT exercise (0.19+/-0.16 and 0.25+/-0.12 mM, respectively), when compared with the control session (0.08+/-0.03 and 0.09+/-0.05 mM, respectively). This was not observed following the LAT-20 exercise in the high and moderate CHO groups (0.11+/-0.04 and 0.12+/-0.08 mM, respectively). Elevated milk LA persisted in the 30-min collection point after maximal exercise only. There was no significant effect of dietary treatment on milk or blood LA at any of the collection points.

CONCLUSIONS

In lactating women whose caloric needs are being met: 1) dietary CHO intake, within a practical range, does not influence LA levels in breast milk at rest or after exercise; 2) LA appearance in the milk is a function of exercise intensity; and 3) moderate intensity exercise (RPE = 12) will not increase breast milk LA levels.

Effects of a sympathetic activation by a lower body negative pressure on glucose and lipid metabolism

The effects of a sympathetic activation elicited by a lower body negative pressure (LBNP) (at -15 mmHg for 75 min) were assessed in 7 healthy subjects on two occasions: (i) in post-absorptive conditions, and (ii) during glucose infusion (22.2 mumol kg-1 min-1). LBNP increased plasma norepinephrine concentration and heart rate. It did not alter whole-body glucose metabolism (measured with [6,6-2H]glucose) and glycerol turnover (measured with [1,1,2,3,3-2H]glycerol). Interstitial glycerol concentrations were monitored with microdialysis in subcutaneous adipose tissue and in skeletal muscle. LBNP increased dialysate glycerol concentrations in muscle by 16% (P < 0.03) but not in adipose tissue in post-absorptive conditions, and by 37% in adipose tissue (P < 0.05) but not in muscle during glucose infusion. These results indicate that an LBNP-induced sympathetic activation (i) does not increase endogenous glucose production, and (ii) induces only a slight stimulation of lipolysis in adipose tissue during glucose infusion.

Regional acetate kinetics and oxidation in human volunteers

We have used a 3-h primed continuous infusion of [1,2-13C]acetate in five fasted (24 h) volunteers to quantify splanchnic and leg acetate metabolism (protocol 1). Fractional extraction of acetate by both tissues was high ( approximately 70%), and simultaneous uptake and release of acetate were observed. Labeled carbon recovery in CO2 was 37.9 +/- 2.3% at the whole body level, 37.7 +/- 1.5% across the splanchnic bed, and 37.3 +/- 2.9% across the leg. Furthermore, we calculated whole body labeled carbon recovery during 15 h of [1, 2-13C]acetate infusion in three volunteers (protocol 2). Whole body acetate carbon recovery in CO2 was significantly higher (66.7 +/- 4. 5%) after 15 h of tracer infusion than after 3 h. We conclude that acetate is rapidly taken up by the leg and splanchnic tissues and that the percent recovery of CO2 from the oxidation of acetate is heavily dependent on the length of acetate tracer infusion. In the postabsorptive state, labeled carbon recovery from acetate across the leg and the splanchnic region is similar to the whole body CO2 recovery.

Metabolite levels in human skeletal muscle and adipose tissue studied with microdialysis at low perfusion flow

To identify a perfusion flow at which the interstitial fluid completely equilibrates with the microdialysis perfusion fluid, a protocol with successively lower perfusion flows was used. A colloid was included in the perfusion fluid to make sampling possible at the lowest perfusion flows. At 0.16 microliter/min, the measured metabolites had reached a complete equilibration in both tissues, and the measured concentrations of glucose, glycerol, and urea were in good agreement with expected tissue-specific levels. The glucose concentration in adipose tissue (4.98 +/- 0.14 mM) was equal to that of plasma (5.07 +/- 0.07 mM), whereas the concentration in muscle (4.41 +/- 0.11 mM) was lower than in plasma and adipose tissue (P < 0.001). The concentration of lactate was higher (P < 0.001) in muscle (2.39 +/- 0.22 mM) than in adipose tissue (1.30 +/- 0.12 mM), whereas the glycerol concentration in adipose tissue (233 +/- 19.7 microM) was higher (P < 0.001) than in muscle (40.8 +/- 3.0 microM) and in plasma (68.7 +/- 3.97 microM). The concentration of urea was equal in the two tissues. Overall, the study indicates that microdialysis at a low perfusion flow may be a tool to continuously monitor tissue interstitial concentrations.

The effect of triacylglycerol-fatty acid positional distribution on postprandial metabolism in subcutaneous adipose tissue

We hypothesized that fatty acids at the sn-2 position of chylomicron triacylglycerol are preferentially released into the venous plasma (rather than being taken up and stored in the adipocytes) after hydrolysis by lipoprotein lipase (EC 3.1.1.34) in adipose tissue. Arteriovenous differences across adipose tissue were studied in eight healthy subjects on two occasions for 6 h after ingestion of different structured triacylglycerols rich in palmitic acid either at the sn-2 or the sn-1,3 positions. In particular the specific fatty acids making up lipoprotein fractions and plasma non-esterified fatty acids were analysed. After the different meals there were no differences between either postprandial arterialized or venous plasma metabolite concentrations. Chylomicron triacylglycerol extraction in adipose tissue was the same following the two types of fat. There was no difference between the specific fatty acid composition of the postprandial non-esterified fatty acid release from adipose tissue after ingestion of the two triacylglycerols, indicating that there was no preferential release of a saturated fatty acid at the sn-2 position.

Periprandial systemic and regional lipase activity in normal humans

An assay for plasma lipoprotein lipase activity was used without prior injection of heparin to study arteriovenous differences of lipases across skeletal muscle and adipose tissue of normal male volunteers. Lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL) activities and triglyceride?concentrations were measured in arterial plasma and in venous effluent plasma from forearm skeletal muscle and subcutaneous abdominal adipose tissue, in the postabsorptive state and after a mixed meal. Triglyceride clearance by the tissues was greater across adipose tissue than across muscle. There were no arteriovenous differences for HTGL activity. In the postabsorptive state skeletal muscle released LPL activity, but adipose tissue did not. Postprandially the arterial LPL and HTGL activities did not change. LPL activity in adipose tissue venous effluent rose, whereas that in muscle venous effluent decreased. These results show that the release of LPL from subcutaneous adipose and forearm tissues is regulated differently, reflecting in vivo differences in LPL regulation at the tissue level.

Interstitial fluid concentrations of glycerol, glucose, and amino acids in human quadricep muscle and adipose tissue. Evidence for significant lipolysis in skeletal muscle

To determine the relationship between circulating metabolic fuels and their local concentrations in peripheral tissues we measured glycerol, glucose, and amino acids by microdialysis in muscle and adipose interstitium of 10 fasted, nonobese human subjects during (a) baseline, (b) euglycemic hyperinsulinemia (3 mU/kg per min for 3 h) and, (c) local norepinephrine reuptake blockade (NOR). At baseline, interstitial glycerol was strikingly higher (P < 0.0001) in muscle (3710 microM) and adipose tissue (2760 microM) compared with plasma (87 microM), whereas interstitial glucose (muscle 3.3, fat 3.6 mM) was lower (P < 0.01) than plasma levels (4.8 mM). Taurine, glutamine, and alanine levels were higher in muscle than in adipose or plasma (P < 0.05). Euglycemic hyperinsulinemia did not affect interstitial glucose, but induced a fall in plasma glycerol and amino acids paralleled by similar changes in the interstitium of both tissues. Local NOR provoked a fivefold increase in glycerol (P < 0.001) and twofold increase in norepinephrine (P < 0.01) in both muscle and adipose tissues. To conclude, interstitial substrate levels in human skeletal muscle and adipose tissue differ substantially from those in the circulation and this disparity is most pronounced for glycerol which is raised in muscle as well as adipose tissue. In muscle, insulin suppressed and NOR increased interstitial glycerol concentrations. Our data suggest unexpectedly high rates of intramuscular lipolysis in humans that may play an important role in fuel metabolism.

Venous blood gas analysis for evaluation of blood circulation of the hand during continuous axillary block

Computer analysis of blood gas measurements was used to evaluate the effect of a continuous axillary block on the circulation of the hand of a patient suffering from regional circulatory insufficiency due to ligation of the brachial artery. Venous blood samples drawn from the cephalic vein were analysed at 0, 1, 2.5 and 18 hours after the blockade. Improved circulation of the hand was indicated by decreased arterio-venous oxygen difference and increased venous oxygen partial pressure following the blockade. The advantage in using the extended blood gas analysis is the possibility of estimating the main factors influencing tissue oxygenation: oxygen capacity, oxygen partial pressure and the haemoglobin oxygen affinity.

Adipose tissue metabolism in humans determined by vein catheterization and microdialysis techniques

A technique for catheterization of a vein draining abdominal subcutaneous tissue and a microdialysis technique that allows measurements of intercellular water concentrations in adipose tissue in humans have recently been described. In the present study, we compare the two techniques during an oral glucose load. In addition a technique using microdialysis for measurement of tissue oxygen and carbon dioxide tensions is described. Microdialysis and vein catheterization were performed in the same region on the abdomen, and the subcutaneous adipose tissue blood flow was measured by the local 133Xe washout method. The results show that subcutaneous adipose tissue gas tensions are on level with gas tensions measured in abdominal venous blood. Comparison of metabolite concentrations measured in the venous blood and venous blood concentrations calculated from microdialysis data shows that there is good agreement between the concentrations obtained by the two techniques with respect to glucose and glycerol, whereas lactate concentrations are very different. With regard to substrate fluxes calculated by Fick's principle, the catheterization technique is probably the most reliable, considering the numerous assumptions on which calculations of venous concentrations from microdialysis data are based. Advantages and disadvantages of the two techniques are discussed.

Adipose tissue metabolism in obesity: lipase action in vivo before and after a mixed meal

Physiological actions of insulin include suppression of fat mobilization from adipose tissue and activation of adipose tissue lipoprotein lipase. Here, we report measurements of adipose tissue hormone-sensitive lipase (HSL) and lipoprotein lipase (LPL) action in vivo in 10 normal and eight obese subjects, with the latter group having varying degrees of glucose intolerance. HSL and LPL actions (per gram of adipose tissue) were similar in the two groups, after an overnight fast. In the normal subjects, HSL action was suppressed after a meal (by 75% +/- 6% between 60 to 300 minutes, P less than .01), and the action of LPL was increased (clearance of circulating triacylglycerol [TAG] increased by 140% +/- 57% at 300 minutes, P less than .05). Despite hyperinsulinemia, these responses were blunted in the obese subjects (P less than .05 for each change being less than in normal group). The adipose tissue of the obese subjects showed continued nonesterified fatty acid (NEFA) release at a time when NEFA mobilization was completely suppressed in the normal group. Both impaired suppression of HSL and low fractional retention of fatty acids for reesterification within the adipose tissue contributed to this abnormal NEFA release. Impaired activation of LPL was associated with a greater absolute increase in plasma TAG concentration postprandially in the obese. In obese subjects, adipose tissue HSL and LPL fail to respond to immunoreactive insulin postprandially, which may be an important maladaptation in terms of lipoprotein metabolism and risk of coronary heart disease.

Systemic and regional acetate kinetics in dogs

Little is known about the sites of production and uptake of acetate in nonruminants. We placed blood sampling catheters in the femoral artery and in the femoral, portal, hepatic, and renal veins of mongrel dogs (n = 11). The animals were studied in the conscious state 2 wk later during a primed continuous infusion of [1-14C]acetate. Systemic acetate turnover, oxidation, and clearance were determined, as well as regional uptake and release, by measuring 14CO2 excretion as well as plasma concentration and specific activity at the five sampling sites. Results showed systemic acetate turnover was 8.8 +/- 1.9 mumol.kg-1.min-1, approximating 5% of energy expenditure in dogs. Simultaneous uptake and release of acetate was demonstrated in intestine, liver, kidney, and hindlimb. The intestine was the greatest contributor to acetate production, whereas the liver was the most important site of uptake. Plasma acetate oxidation was 77 +/- 4% of turnover. Both systemic clearance (129 +/- 22 ml.kg-1.min-1) and tissue fractional extraction (68-85%) were many times greater than values reported for glucose, free fatty acids, lactate, or amino acids. In conclusion, most tissues simultaneously take up and release acetate in dogs. This may represent a mechanism for interorgan transport of energy, especially under conditions of caloric deprivation.

Glycerol and lactate uptake in human forearm

Arteriovenous differences for lactate and glycerol reported across the human forearm are inconsistent in direction and magnitude. Such inconsistency could represent the effects of differing forearm compositions. The hypothesis was tested by examination of 37 studies of forearm arteriovenous differences for lactate, glycerol, glucose, and oxygen (only measured in 25 studies) in 23 normal subjects after overnight fast. In 16 studies, glycerol was taken up rather than released by the forearm, and in 12 of these the arteriovenous difference was greater than could be accounted for by analytical variation. The arteriovenous differences for glycerol and lactate were positively correlated (r = .44, P less than .01). The hypothesis that glycerol and lactate uptake might reflect a more "oxidative" forearm was not borne out, since neither glycerol nor lactate arteriovenous differences correlated with that for oxygen, although oxygen and glucose arteriovenous differences and fluxes were correlated (fluxes: r = .60, P less than .01). The arteriovenous difference for glycerol was positively related to body mass index, arguing against a variable contribution from fat. The hypothesis that the direction of glycerol and lactate exchange would reflect the forearm composition was not borne out by the analysis of repeated studies on the same individual, which showed that the variation within subjects was not significantly less than that between subjects. Therefore, we conclude that in approximately 40% of studies in normal subjects after an overnight fast, the forearm will show glycerol uptake, although we have been unable to identify any physiological reason for this phenomenon. Peripheral glycerol uptake has implications for studies in which glycerol release is taken as a measure of lipolysis.

Metabolic responses of forearm and adipose tissues to acute ethanol ingestion

Although excess ethanol consumption is often considered to lead to adiposity, the metabolic routes by which this might occur are not clear. We have investigated some metabolic consequences of acute ethanol ingestion by measuring arteriovenous differences across forearm muscle and subcutaneous adipose tissue for 6 hours after ingestion of 47.5 g ethanol, in seven normal subjects fasted overnight. The expected systemic effects of ethanol ingestion were observed: slight lowering of the plasma glucose concentration, depression of plasma nonesterified fatty acid (NEFA) concentrations, and elevation of the blood lactate/pyruvate and 3-hydroxybutyrate/acetoacetate ratios. There was a marked reduction in blood total ketone bodies in relation to plasma NEFA concentrations. However, the only major change observed in peripheral tissue metabolism was an increased uptake of acetate into forearm muscle, equivalent, in whole-body terms, to only 3% of the ethanol load. Adipose tissue appeared to show a reduced cytoplasmic state in that it exported an increased ratio of lactate to pyruvate after ethanol ingestion. However, this reduced state did not lead to increased fatty acid reesterification within adipose tissue. No mechanism was clearly identified whereby ethanol ingestion might lead to net deposition of triacylglycerol in adipose tissue.