Effect of noradrenaline on glycerol turnover and lipolysis in the whole body and subcutaneous adipose tissue in humans in vivo

A randomized, double-blind trial comparing the effect of two blood pressure targets on global brain metabolism after out-of-hospital cardiac arrest

PURPOSE

This study aimed to assess the effect of different blood pressure levels on global cerebral metabolism in comatose patients resuscitated from out-of-hospital cardiac arrest (OHCA).

METHODS

In a double-blinded trial, we randomly assigned 60 comatose patients following OHCA to low (63 mmHg) or high (77 mmHg) mean arterial blood pressure (MAP). The trial was a sub-study in the Blood Pressure and Oxygenation Targets after Out-of-Hospital Cardiac Arrest-trial (BOX). Global cerebral metabolism utilizing jugular bulb microdialysis (JBM) and cerebral oxygenation (rSO₂) was monitored continuously for 96 h. The lactate-to-pyruvate (LP) ratio is a marker of cellular redox status and increases during deficient oxygen delivery (ischemia, hypoxia) and mitochondrial dysfunction. The primary outcome was to compare time-averaged means of cerebral energy metabolites between MAP groups during post-resuscitation care. Secondary outcomes included metabolic patterns of cerebral ischemia, rSO₂, plasma neuron-specific enolase level at 48 h and neurological outcome at hospital discharge (cerebral performance category).

RESULTS

We found a clear separation in MAP between the groups (15 mmHg, p < 0.001). Cerebral biochemical variables were not significantly different between MAP groups (LPR low MAP 19 (16-31) vs. high MAP 23 (16-33), p = 0.64). However, the LP ratio remained high (> 16) in both groups during the first 30 h. During the first 24 h, cerebral lactate > 2.5 mM, pyruvate levels > 110 µM, LP ratio > 30, and glycerol > 260 µM were highly predictive for poor neurological outcome and death with AUC 0.80. The median (IQR) rSO₂ during the first 48 h was 69.5% (62.0-75.0%) in the low MAP group and 69.0% (61.3-75.5%) in the high MAP group, p = 0.16.

CONCLUSIONS

Among comatose patients resuscitated from OHCA, targeting a higher MAP 180 min after ROSC did not significantly improve cerebral energy metabolism within 96 h of post-resuscitation care. Patients with a poor clinical outcome exhibited significantly worse biochemical patterns, probably illustrating that insufficient tissue oxygenation and recirculation during the initial hours after ROSC were essential factors determining neurological outcome.

Critical Appraisal of Bidirectional Relationship between Periodontitis and Hyperlipidemia

Periodontal disease and hyperlipidemia are both multifactorial disease with a high prevalence Worldwide. Cross-sectional and longitudinal prospective clinical studies show some evidence for a bidirectional relationship. Periodontitis and hyperlipidemia share some common risk factors and there exist a mechanistic link between both. Studies have found a positive response to periodontal therapy among hyperlipidemic patients, and statin use by hyperlipidemic patients has shown to influence the periodontal health. However, in spite of the rising prevalence of both diseases, many people remain unaware of their association with each other. Hence, this article summarizes the cyclic relationship between periodontal disease and hyperlipidemia.

Beyond thermoregulation: metabolic function of cetacean blubber in migrating bowhead and beluga whales

The processes of lipid deposition and utilization, via the gene leptin (Lep), are poorly understood in taxa with varying degrees of adipose storage. This study examines how these systems may have adapted in marine aquatic environments inhabited by cetaceans. Bowhead (Balaena mysticetus) and beluga whales (Delphinapterus leucas) are ideal study animals-they possess large subcutaneous adipose stores (blubber) and undergo bi-annual migrations concurrent with variations in food availability. To answer long-standing questions regarding how (or if) energy and lipid utilization adapted to aquatic stressors, we quantified variations in gene transcripts critical to lipid metabolism related to season, age, and blubber depth. We predicted leptin tertiary structure conservation and assessed inter-specific variations in Lep transcript numbers between bowheads and other mammals. Our study is the first to identify seasonal and age-related variations in Lep and lipolysis in these cetaceans. While Lep transcripts and protein oscillate with season in adult bowheads reminiscent of hibernating mammals, transcript levels reach up to 10 times higher in bowheads than any other mammal. Data from immature bowheads are consistent with the hypothesis that short baleen inhibits efficient feeding. Lipolysis transcripts also indicate young Fall bowheads and those sampled during Spring months limit energy utilization. These novel data from rarely examined species expand the existing knowledge and offer unique insight into how the regulation of Lep and lipolysis has adapted to permit seasonal deposition and maintain vital blubber stores.

Leptin in teleost fishes: an argument for comparative study

All organisms face tradeoffs with regard to how limited energy resources should be invested. When is it most favorable to grow, to reproduce, how much lipid should be allocated to storage in preparation for a period of limited resources (e.g., winter), instead of being used for growth or maturation? These are a few of the high consequence fitness "decisions" that represent the balance between energy acquisition and allocation. Indeed, for animals to make favorable decisions about when to grow, eat, or reproduce, they must integrate signals among the systems responsible for energy acquisition, storage, and demand. We make the argument that leptin signaling is a likely candidate for an integrating system. Great progress has been made understanding the leptin system in mammals, however our understanding in fishes has been hampered by difficulty in cloning fish orthologs of mammalian proteins and (we assert), underutilization of the comparative approach.

Effect of beta-adrenergic stimulation on whole-body and abdominal subcutaneous adipose tissue lipolysis in lean and obese men

AIMS/HYPOTHESIS

Obesity is characterised by increased triacylglycerol storage in adipose tissue. There is in vitro evidence for a blunted beta-adrenergically mediated lipolytic response in abdominal subcutaneous adipose tissue (SAT) of obese individuals and evidence for this at the whole-body level in vivo. We hypothesised that the beta-adrenergically mediated effect on lipolysis in abdominal SAT is also impaired in vivo in obese humans.

METHODS

We investigated whole-body and abdominal SAT glycerol metabolism in vivo during 3 h and 6 h [2H5]glycerol infusions. Arterio-venous concentration differences were measured in 13 lean and ten obese men after an overnight fast and during intravenous infusion of the non-selective beta-adrenergic agonist isoprenaline [20 ng (kg fat free mass)(-1) min(-1)].

RESULTS

Lean and obese participants showed comparable fasting glycerol uptake by SAT (9.7+/-3.4 vs 9.3+/-2.5% of total release, p=0.92). Furthermore, obese participants showed an increased whole-body beta-adrenergically mediated lipolytic response versus lean participants. However, their fasting lipolysis was blunted [glycerol rate of appearance: 7.3+/-0.6 vs 13.1+/-0.9 micromol (kg fat mass)(-1) min(-1), p<0.01], as was the beta-adrenergically mediated lipolytic response per unit SAT [Delta total glycerol release: 140+/-71 vs 394+/-112 nmol (100 g tissue)(-1) min(-1), p<0.05] compared with lean participants. Net triacylglycerol flux tended to increase in obese compared with lean participants during beta-adrenergic stimulation [Delta net triacylglycerol flux: 75+/-32 vs 16+/-11 nmol (100 g tissue)(-1) min(-1), p=0.06].

CONCLUSIONS/INTERPRETATION

We demonstrated in vivo that beta-adrenergically mediated lipolytic response is impaired systematically and in abdominal SAT of obese versus lean men. This may be important in the development or maintenance of increased triacylglycerol stores and obesity.

Catecholamine regulation of local lactate production in vivo in skeletal muscle and adipose tissue: role of -adrenoreceptor subtypes

CONTEXT

The regulation of lactate production in skeletal muscle (SM) and adipose tissue (AT) is not fully elucidated.

OBJECTIVE

Our objective was to investigate the catecholamine-mediated regulation of lactate production and blood flow in SM and AT in healthy, normal-weight subjects by using microdialysis.

METHODS

First, lactate levels in SM and AT were measured during an iv norepinephrine infusion (n = 11). Local blood flow was determined with the 133Xe-clearance technique. Second, muscle lactate was measured during hypoglycemia and endogenous epinephrine stimulation (n = 12). Third, SM was perfused with selective beta(1-3)-adrenoreceptor agonists in situ (n = 8). Local blood flow was measured with the ethanol perfusion technique.

RESULTS

In response to iv norepinephrine, the fractional release of lactate (difference between tissue and arterial lactate) increased by 40% in SM (P = 0.001), whereas remaining unchanged in AT. Blood flow decreased by 40% in SM (P < 0.005) and increased by 50% in AT (P < 0.05). In response to hypoglycemia, epinephrine increased 10-fold, and the fractional release of lactate in SM doubled (P < 0.0001). The blood flow remained unchanged. The beta2-agonist, terbutaline, caused a marked concentration-dependent increase of muscle lactate and blood flow (P < 0.0001). The beta(1)-agonist, dobutamine, induced a discrete increase of muscle lactate (P < 0.0001), and the blood flow remained unchanged. The beta3-agonist, CPG 12177, did not affect muscle lactate or blood flow.

CONCLUSIONS

Catecholamines stimulate lactate production in SM, but not in AT. In SM, the beta2-adrenoreceptor is the most important beta-adrenergic receptor subtype in the regulation of lactate production.

Substantial working muscle glycerol turnover during two-legged cycle ergometry

We combined tracer and arteriovenous (a-v) balance techniques to evaluate the effects of exercise and endurance training on leg triacylglyceride turnover as assessed by glycerol exchange. Measurements on an exercising leg were taken to be a surrogate for working skeletal muscle. Eight men completed 9 wk of endurance training [5 days/wk, 1 h/day, 75% peak oxygen consumption (Vo(2peak))], with leg glycerol turnover determined during two pretraining trials [45 and 65% Vo(2peak) (45% Pre and 65% Pre, respectively)] and two posttraining trials [65% of pretraining Vo(2peak) (ABT) and 65% of posttraining Vo(2peak) (RLT)] using [(2)H(5)]glycerol infusion, femoral a-v sampling, and measurement of leg blood flow. Endurance training increased Vo(2peak) by 15% (45.2 +/- 1.2 to 52.0 +/- 1.8 mlxkg(-1)xmin(-1), P < 0.05). At rest, there was tracer-measured leg glycerol uptake (41 +/- 8 and 52 +/- 15 micromol/min for pre- and posttraining, respectively) even in the presence of small, but significant, net leg glycerol release (-68 +/- 19 and -50 +/- 13 micromol/min, respectively; P < 0.05 vs. zero). Furthermore, while there was no significant net leg glycerol exchange during any of the exercise bouts, there was substantial tracer-measured leg glycerol turnover during exercise (i.e., simultaneous leg muscle uptake and leg release) (uptake, release: 45% Pre, 194 +/- 41, 214 +/- 33; 65% Pre, 217 +/- 79, 201 +/- 84; ABT, 275 +/- 76, 312 +/- 87; RLT, 282 +/- 83, 424 +/- 75 micromol/min; all P < 0.05 vs. corresponding rest). Leg glycerol turnover was unaffected by exercise intensity or endurance training. In summary, simultaneous leg glycerol uptake and release (indicative of leg triacylglyceride turnover) occurs despite small or negligible net leg glycerol exchange, and furthermore, leg glycerol turnover can be substantially augmented during exercise.

Effect of Caralluma fimbriata extract on appetite, food intake and anthropometry in adult Indian men and women

Caralluma fimbriata is an edible cactus, used by tribal Indians to suppress hunger and enhance endurance. The effect of Caralluma extract was assessed in overweight individuals by a placebo controlled randomized trial. Fifty adult men and women (25-60 years) with a body mass index (BMI) greater than 25 kg/m2 were randomly assigned into a placebo or experimental group; the latter received 1 g of Caralluma extract per day for 60 days. All subjects were given standard advice regarding a weight reducing diet and physical activity. At the end of 30 and 60 days of intervention, blood glucose and lipids, anthropometric measurements, dietary intake and assessment of appetite was performed. Waist circumference and hunger levels over the observation period showed a significant decline in the experimental group when compared to the placebo group. While there was a trend towards a greater decrease in body weight, body mass index, hip circumference, body fat and energy intake between assessment time points in the experimental group, these were not significantly different between experimental and placebo groups. Caralluma extract appears to suppress appetite, and reduce waist circumference when compared to placebo over a 2 month period.

The origins of cachexia in acute and chronic inflammatory diseases

The term cachexia originates from the Greek root kakos hexis, which translates into "bad condition," recognized for centuries as a progressive deterioration of body habitus. Cachexia is commonly associated with a number of disease states, including acute inflammatory processes associated with critical illness and chronic inflammatory diseases, such as cancer, congestive heart failure, chronic obstructive pulmonary disease, and human immunodeficiency virus infection. Cachexia is responsible for the deaths of 10%-22% of all patients with cancer and approximately 15% of the trauma deaths that occur from sepsis-induced organ dysfunction and malnutrition days to weeks after the initial traumatic event. The abnormalities associated with cachexia include anorexia, weight loss, a preferential loss of somatic muscle and fat mass, altered hepatic glucose and lipid metabolism, and anemia. Anorexia alone cannot fully explain the development of cachexia; metabolic alterations in carbohydrate, lipid, and protein metabolism contribute to the severe tissue losses. Despite significant advances in our understanding of specific disease processes, the mechanisms leading to cachexia remain unclear and multifactorial. Although complex, increasing evidence from both animal models and clinical studies suggests that an inflammatory response, mediated in part by a dysregulated production of proinflammatory cytokines, plays a role in the genesis of cachexia, associated with both critical illness and chronic inflammatory diseases. These cytokines are further thought to induce an acute phase protein response (APR) and produce the alterations in lipid and carbohydrate metabolism identified as crucial markers of acute inflammation in states of malignancy and critical illness. Although much is still unknown about the etiology of cachexia, there is growing appreciation that cachexia represents the endproduct of an inappropriate interplay between multiple cytokines, neuropeptides, classic stress hormones, and intermediary substrate metabolism.

A multicompartmental model of in vivo adipose tissue glycerol kinetics and capillary permeability in lean and obese humans

Lipolysis of adipose tissue triglycerides releases glycerol. Twenty-four volunteers, of whom 6 were obese and 13 were women, received a primed-constant infusion of 2H5-glycerol for 120 min during postabsorptive steady-state conditions. Arterial, abdominal venous, and interstitial (microdialysis) samples were taken, and a four-compartment model was applied to assess subcutaneous abdominal adipose tissue glycerol kinetics. Adipose tissue blood flow was measured using 133Xe washout. Venous glycerol concentrations (median 230 micromol/l [interquartile range 210-268]) were consistently greater than those of arterial blood (69.1 micromol/l [56.5-85.5]), while glycerol isotopic enrichments (tracer-to-tracee ratio) were greater in arterial blood (8.34% [7.44-10.1]) than venous blood (2.34% [1.71-2.69], P < 0.01). Microdialysate glycerol enrichment was 1.44% (1.11-1.79), indicating incomplete permeability of glycerol between capillary blood and interstitium. Calculated interstitial glycerol concentrations were between 270 micromol/l (256-350) and 332 micromol/l (281-371) (examining different boundary conditions). The calculated capillary diffusion capacity (ps) was between 2.21 ml . 100 g tissue(-1) . min(-1) (1.31-3.13) and 3.09 ml . 100 g tissue(-1) . min(-1) (1.52-4.90) and correlated inversely with adiposity (Rs< or = -0.45, P < 0.05). Our results support previous estimates of interstitial glycerol concentration within adipose tissue and reveal capillary diffusion capacity is reduced in obesity.

Major differences in noradrenaline action on lipolysis and blood flow rates in skeletal muscle and adipose tissue in vivo

AIMS/HYPOTHESIS

The regulation of skeletal muscle lipolysis is not fully understood. In the present study, the effects of systemic and local noradrenaline administration on lipolysis and blood flow rates in skeletal muscle and adipose tissue were studied in vivo.

METHODS

First, circulating noradrenaline levels were raised tenfold by a continuous i.v. infusion (n=12). Glycerol levels (an index of lipolysis) were measured in m. gastrocnemius and in abdominal adipose tissue using microdialysis. Local blood flow was determined with the (133)Xe clearance technique and whole-body lipolysis rates assessed with a stable glycerol isotope technique ([(2)H(5)] glycerol). Second, interstitial glycerol levels in m. gastrocnemius, m. vastus and adipose tissue were measured by microdialysis during local perfusion with noradrenaline (10(-8)-10(-6) mol/l) (n=10). Local blood flow was monitored with the ethanol perfusion technique.

RESULTS

With regard to systemic noradrenergic stimulation, no change in fractional release of glycerol (difference between tissue and arterial glycerol) was seen in skeletal muscle. In adipose tissue it transiently increased twofold (p<0.0001), and the rate of appearance of glycerol in plasma showed the same kinetic pattern. Blood flow was reduced by 40% in skeletal muscle (p<0.005) and increased by 50% in adipose tissue (p<0.05). After noradrenaline stimulation in situ, a discrete elevation of skeletal muscle glycerol was registered only at the highest concentration of noradrenaline (10(-6) mol/l) (p<0.05). Adipose tissue glycerol doubled already at the lowest concentration (10(-8) mol/l) (p<0.05). In skeletal muscle a decrease in blood flow was seen at the highest noradrenaline concentrations (p<0.05).

CONCLUSIONS/INTERPRETATION

Lipolysis and blood flow rates are regulated differently in adipose tissue and skeletal muscle. Adipose tissue displays a high, but transient (tachyphylaxia) sensitivity to noradrenaline, leading to stimulation of both lipolysis and blood flow rates. In skeletal muscle, physiological concentrations of noradrenaline decrease blood flow but have no stimulatory effect on lipolysis rates.

Physiological regulation of NEFA availability: lipolysis pathway

Plasma NEFA are an important energy substrate and, furthermore, play a key role in the induction of insulin resistance in the body. The availability of NEFA is determined predominantly by their mobilization from adipose tissue triacylglycerol stores by the process of lipolysis. Adipose tissue lipolysis in man is regulated by a number of hormonal and paracrine and/or autocrine signals. The main hormonal signals may be represented by catecholamines, insulin, growth hormone, natriuretic peptides and some adipocytokines. The absolute levels and relative importance and contribution of these signals vary in different physiological situations, with diet and physical exercise being the main physiological variables that affect the hormonal signalling. Thus, modulations in hormonal signals induce an increase in NEFA mobilization in the post-absorptive state and during an acute bout of exercise, and suppress NEFA mobilization in the postprandial state. In addition, hormonal regulation is modified by long-term interventions in energy balance, such as dietary restriction and/or physical training, and is disturbed in some pathological states, such as obesity or diabetes. The question that remains is whether disturbances in lipolysis regulation in obese and diabetic subjects may be 'corrected' by the long-term interventions in diet and physical activity.

Carnitine palmitoyltransferase-1 (CPT-1) activity stimulation by cerulenin via sympathetic nervous system activation overrides cerulenin's peripheral effect

To clarify the paradoxic effects of cerulenin, namely its in vitro inhibitory effects on fat catabolism and its in vivo reduction of fat mass, we studied the in vivo and in vitro effects of cerulenin on carnitine palmitoyltransferase-1 (CPT-1) activity, the rate-limiting enzyme of fatty acid oxidation. A single ip injection of cerulenin significantly reduced body weight and increased core temperature without significantly reducing food intake. In situ hybridization study revealed that a single injection of cerulenin did not affect the expression of orexigenic neuropeptide mRNA. Cerulenin's effect on CPT-1 activity was biphasic in the liver and muscle: early suppression during the first 1 h and late stimulation in the 3-5 h after ip treatment. In vitro cerulenin treatment reduced CPT-1 activity, which was overcome by cotreating with catecholamine. Intracerebroventricular injection of cerulenin increased CPT-1 activity significantly in soleus muscle, and this effect was sustained for up to 3 h. Pretreatment with alpha-methyl-p-tyrosine inhibited the cerulenin-induced increase in core temperature and the late-phase stimulating effect of cerulenin on CPT-1 activity. In adrenalectomized mice, cerulenin also increased the activity. In vivo cerulenin treatment enhanced muscle CPT-1 activity in monosodium glutamate-treated arcuate nucleus lesioned mice but not in gold thioglucose-treated ventromedial hypothalamus lesioned mice. These findings suggest that cerulenin-induced late-phase stimulating effects on CPT-1 activity and energy expenditure is mediated by the activation of innervated sympathetic nervous system neurons through the firing of undefined neurons of the ventromedial hypothalamus, rather than the arcuate nucleus.

Fat metabolism in exercise - with special reference to training and growth hormone administration

Despite abundance of fat, exclusive dependency on fat oxidation can only sustain a metabolic rate corresponding to 50-60% of VO(2max) in humans. This puzzling finding has been subject to intense research for many years. Lately, it has gained renewed interest as a consequence of increased obesity and physical inactivity imposed by Western lifestyle. Why are humans so poor at metabolizing fat? Can fat metabolism be manipulated by exercise, training, diet and hormones? And why is fat stored in specialized adipose tissue and not just as lipid droplets inside muscle cells? In the present review, human fat metabolism is discussed in relation to how human fat metabolism is designed. Limitations in this design are explored and examples of different designs for fat metabolism from animal physiology are included to illustrate these limitations. Various means of manipulating fat metabolism are discussed with special emphasis on exercise, training, growth hormone (GH) physiology and GH administration. It is concluded that fat stores, non-esterified fatty acids (NEFAs) availability and enzymes for fat oxidation can be increased substantially. However, it is almost impossible to increase fat oxidation during endurance exercise at higher intensities. It seems that, for some reason, the human being is far from optimally designed for fat oxidation during exercise. Acute GH administration has several unexpected effects on fat and carbohydrate metabolism during aerobic exercise, and future research in this area is likely to provide valuable information with respect to GH physiology and the regulation of fat and carbohydrate metabolism during aerobic exercise.

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.

Regulation of dietary fatty acid entrapment in subcutaneous adipose tissue and skeletal muscle

Using stable isotopic labeling of dietary fatty acids in conjunction with arteriovenous difference measurements, we have assessed the regulation of lipoprotein lipase-derived fatty acid entrapment in subcutaneous adipose tissue and forearm muscle in healthy subjects in the postprandial state. Eight volunteers fasted overnight and were then given a mixed meal containing [ 1-(13)C]palmitic acid and [1-(13)C]oleic acid. At baseline and for 6 h after the meal, blood samples were obtained from an arterialized hand vein and veins draining subcutaneous abdominal adipose tissue and forearm muscle, and arteriovenous differences were calculated. Entrapment of labeled fatty acids released by circulating triacylglycerol hydrolysis was close to 100% at 60 min, decreasing to 10-30% by 360 min. Entrapment of labeled fatty acids in forearm muscle was >100% and did not change with time. This study shows that entrapment of dietary fatty acids in adipose tissue in the postprandial period is a highly regulated process (varying with time) and that this can be studied in humans using stable isotope- labeled fatty acids in combination with measurement of appropriate arteriovenous differences. Also, fatty acid trapping in skeletal muscle is fundamentally different from that in adipose tissue, in that all the fatty acids released by lipoprotein lipase in skeletal muscle are taken up by the tissue.

Preferential stimulation of abdominal subcutaneous lipolysis after prednisolone exposure in humans

OBJECTIVE

The role of cortisol in the regulation of lipolysis is not clear. This study was undertaken to explore whether a standard dose of prednisolone for 1 week would influence lipolysis in abdominal and femoral tissue.

RESEARCH METHODS AND PROCEDURES

We used the microdialysis technique, the forearm technique, and indirect calorimetry, in the fasting state, after 1 week of treatment with prednisolone (30 mg daily) or placebo. Eight healthy young men (age: 25 +/- 3 years; height: 181 +/- 1 cm; body mass index [BMI]: 23.3 +/- 0.7 kg/m(2)) were studied.

RESULTS

Treatment with prednisolone induced insulin resistance (Homeostasis Model Assessment index: placebo vs. prednisolone: 7.15 +/- 1.63 vs. 17.00 +/- 14.26, p = 0.03), hyperinsulinemia (p = 0.01), and hyperglucagonemia (p = 0.001), whereas growth hormone concentrations were unaffected. Abdominal adipose tissue interstitial glycerol was increased during treatment with prednisolone in the face of significant hyperinsulinemia, although it barely reached statistical significance (p = 0.06). At the femoral adipose tissue depot, no difference in lipolysis was found. Arterial and venous free fatty acids (FFA) were comparable in the two situations, whereas the arteriovenous difference across the forearm was significantly decreased during treatment with prednisolone, indicating increased uptake, or decreased release of FFA. Energy expenditure (p = 0.3), respiratory quotient (p = 0.9), glucose oxidation (p = 0.9), lipid oxidation (p = 1.0), and protein oxidation (p = 0.1) were unaltered on the 2 study days.

DISCUSSION

Short-term treatment with a standard dose of corticosteroids induces increased abdominal adipose tissue lipolysis, as well as hyperinsulinemia, hyperglucagonemia, and insulin resistance.

Norepinephrine spillover from human adipose tissue before and after a 72-hour fast

Adipose tissue lipolysis is at least in part stimulated by the sympathetic nervous system (SNS). Although there is a generalized decrease in SNS activity with fasting, the rate of lipolysis during fasting increases. The aim of this study was to determine whether there is an association between activation of sympathetic nerves innervating adipose tissue and the increase in lipolysis seen during fasting in humans. We used the isotope dilution technique to measure regional norepinephrine spillover from abdominal sc adipose tissue from seven healthy subjects before and after a 72-h fast. Our results showed a significant increase in adipose tissue spillover of norepinephrine (mean +/- SEM, 0.40 +/- 0.09 vs. 1.08 +/- 0.18 pmol.100 g(-1).min(-1), P < 0.05) and arterial norepinephrine concentrations (0.92 +/- 0.10 vs. 1.23 +/- 0.08 nmol.liter(-1), P < 0.05) after the fast with no significant change in total body norepinephrine spillover, forearm norepinephrine spillover, epinephrine concentrations, or energy expenditure. We show for the first time, in humans, a selective regional increase in adipose tissue norepinephrine spillover in response to a 72-h fast and suggest that the SNS may play a greater role in the regulation of lipid metabolism during fasting than previously thought.

Comparing energy expenditure data among individuals differing in body size and composition: statistical and physiological considerations

Acute and chronic diseases are frequently characterized by alterations in energy metabolism that influence nutritional requirements and clinical care. Knowledge of the effect of disease on daily energy expenditure and its components is fundamental to understanding the impact of the disease process on energy balance. To obtain this information, energy expenditure data are often compared between healthy and diseased individuals. This review focuses on the statistical and physiological issues related to comparing energy expenditure data among individuals who differ in body size and composition.

Effects of acipimox on the lipolysis rate in subcutaneous adipose tissue of obese subjects

BACKGROUND

Acipimox is a hypolipidaemic agent reducing serum concentrations of triglycerides and non-esterified fatty acids. Acipimox may reduce triglyceride synthesis by decreasing non-esterified fatty acid availability from adipocytes, but this effect has yet to be demonstrated in vivo. Lipolysis after acipimox treatment was examined in subcutaneous adipose tissue of severely obese subjects with associated metabolic disorders.

METHODS

The microdialysis technique was performed in abdominal subcutaneous adipose tissue of eight hyperinsulinaemic subjects. After oral treatment with acipimox, glycerol concentration was determined as an index of lipolysis rate. Blood flow was assessed by the ethanol escape technique. The rates of release of glycerol from human adipose tissue maximally stimulated by norepinephrine were also investigated in the presence of acipimox. Eight weight- and age-matched subjects served as a control group.

RESULTS

Under acipimox treatment, basal glycerol release decreased in subcutaneous adipose tissue, whereas no effect was observed on blood flow. In stimulated adipose tissue acipimox showed no effect.

CONCLUSION

In the present study basal glycerol outflow from adipose tissue was inhibited by acipimox. The anti-lipolytic action of the agent may diminish elevated plasma concentrations of free fatty acids in subjects with severe obesity.

Oxidation of [(13)C]glycerol ingested along with glucose during prolonged exercise

The respective oxidation of glycerol and glucose (0.36 g/kg each) ingested simultaneously immediately before exercise (120 min at 68 +/- 2% maximal oxygen uptake) was measured in six subjects using (13)C labeling. Indirect respiratory calorimetry corrected for protein and glycerol oxidation was used to evaluate the effect of glucose + glycerol ingestion on the oxidation of glucose and fat. Over the last 80 min of exercise, 10.0 +/- 0.8 g of exogenous glycerol were oxidized (43% of the load), while exogenous glucose oxidation was 21% higher (12.1 +/- 0.7 g or 52% of the load). However, because the energy potential of glycerol is 18% higher than that of glucose (4.57 vs. 3.87 kcal/g), the contribution of both exogenous substrates to the energy yield was similar (4.0-4.1%). Total glucose and fat oxidation were similar in the placebo (144.4 +/- 13.0 and 60.5 +/- 4.2 g, respectively) and the glucose + glycerol (135.2 +/- 12.0 and 59.4 +/- 6.5 g, respectively) trials, whereas endogenous glucose oxidation was significantly lower than in the placebo trial (123.7 +/- 11.7 vs. 144.4 +/- 13.0 g). These results indicate that exogenous glycerol can be oxidized during prolonged exercise, presumably following conversion into glucose in the liver, although direct oxidation in peripheral tissues cannot be ruled out.

Sir David Cuthbertson Medal Lecture. Regulation of lipid metabolism in adipose tissue

Adipose tissue is a major source of metabolic fuel. This metabolic fuel is stored in the form of triacylglycerol. Lipolysis of triacylglycerol yields non-esterified fatty acids and glycerol. In human subjects in vivo studies of the regulation of lipid metabolism in adipose tissue have been difficult because of the heterogeneous nature of the tissue and lack of a vascular pedicle. In the last decade the methodology of study of adipose tissue has improved with the advent of the anterior abdominal wall adipose tissue preparation technique and microdialysis. These techniques have demonstrated that lipid metabolism in adipose tissue is finely coordinated during feeding and fasting cycles, in order to provide metabolic fuel when required. Lipolysis takes place both in extracellular and intracellular space. The extracellular lipolysis is regulated by lipoprotein lipase and the intracellular lipolysis is regulated by hormone-sensitive lipase. In pathophysiological conditions such as trauma, sepsis and starvation profound changes are induced in the regulation of lipid metabolism. The increased mobilization of lipid fuel is brought about by the differential actions of various counter-regulatory hormones on adipose tissue blood flow and adipose tissue lipolysis through lipoprotein lipase and hormone-sensitive lipase, resulting in increased availability of non-esterified fatty acids as a source of fuel. In recent years, it has been demonstrated that adipose tissue produces various cytokines and these cytokines can have paracrine and endocrine effects. It would appear that adipose tissue has the ability to regulate lipid metabolism locally as well as at distant sites such as liver, muscle and brain. In future, it is likely that the mechanisms that lead to the secondary effects of lipid metabolism on atheroma, immunity and carcinogenesis will be demonstrated.

Functional glycerol kinase activity and the possibility of a major role for glyceroneogenesis in mammalian skeletal muscle

According to textbook descriptions of glycerol metabolism, liver and kidney are the only tissues that express significant glycerol kinase activity. Thus esterification of fatty acids to triglycerides in peripheral tissues such as skeletal muscle and adipose tissue is presumed to be dependent on the synthesis of glycerol-3-phosphate from glucose. This report describes exciting new data indicating that, although low, the glycerol kinase activity of skeletal muscle is functional. Interestingly, the results also suggest that neither glycerol nor glucose is the major substrate for the synthesis of muscle triglyceride glycerol. Rather, glyceroneogenesis, the synthesis of glycerol-3-phosphate from lactate, may play an as yet under-appreciated, but quantitatively important, role.

Effect of local sympathetic blockade on forearm blood flow and glucose uptake during hypoglycemia

During hypoglycemia, hepatic glucose production increases and peripheral glucose utilization decreases. Systemic beta-adrenergic blockade during hypoglycemia increases peripheral glucose utilization. To explore the local effects of increased alpha- and beta-adrenergic activity on skeletal muscle glucose utilization, we measured arterial and venous plasma glucose concentrations, forearm blood flow (FBF), and forearm glucose uptake (FGU) during a hyperinsulinemic (40 mU/m2/min) stepped-hypoglycemic clamp with intrabrachial artery infusion of saline, phentolamine, propranolol, or combined phentolamine and propranolol. A control study was also performed with a euglycemic clamp and intraarterial saline. During hypoglycemia with saline and phentolamine, there were significant increases in FBF (130% +/- 38% and 180% +/- 35%, respectively) and FGU (120% +/- 51% and 230% +/- 150%, respectively). During hypoglycemia with propranolol and phentolamine + propranolol, FBF remained constant. FGU during hypoglycemia with propranolol was not different versus hypoglycemia with saline. No differences were found in these studies for forearm lactate output (FLO) or venous free fatty acid concentrations. These results demonstrate that local, as opposed to systemic, blockade during hypoglycemia does not alter peripheral glucose utilization.

Use of stable isotopes to study carbohydrate and fat metabolism at the whole-body level

The present review discusses the advantages and limitations of using stable-isotope tracers to assess carbohydrate and fat metabolism at the whole-body level. One advantage of stable- (v. radioactive-) isotope tracers is the relative ease with which the location of a label within a molecule can be determined using selected-ion-monitoring GC-mass spectrometry (SIM-GC-MS). This technique minimizes potential problems due to label recycling, allows the use of multiple-labelled compounds simultaneously (e.g. to quantify glucose cycling), and perhaps most importantly, has led to the development of unique stable-isotope methods for, for example, quantifying gluconeogenesis. However, the limited sensitivity of SIM-GC-MS sometimes requires that relatively large amounts of a stable-isotope tracer be used, thus increasing cost and potentially altering metabolism. At least theoretically, stable- (or radioactive-) isotope tracers can also be used in conjunction with indirect calorimetry to estimate utilization of muscle glycogen or triacylglycerol stores, thus potentially circumventing the need to obtain muscle biopsies. These calculations, however, require certain critical assumptions, which if incorrect could lead to major errors in the values obtained. Despite such limitations, stable-isotope tracers provide a powerful and sometimes unique tool for investigating carbohydrate and fat metabolism at the whole-body level. With continuing advances in availability, instrumentation and methods, it is likely that stable-isotope tracers will become increasingly important in the immediate future.

Glycerol production and utilization measured using stable isotopes

The rate of appearance of glycerol in the systemic circulation is determined from the enrichment of arterial blood glycerol when labelled glycerol is infused intravenously. This value provides a good measure of whole-body lipolysis during fasting, except that arterial infusion and venous sampling, if feasible, would probably give a higher more-accurate value. Lipolysis occurs primarily in adipose tissue, although other tissues contribute, notably muscle. Measurement is based on the difference in the enrichment of the glycerol entering and leaving the tissue. Lipolysis is underestimated by the extent to which glycerol released by lipolysis does not enter the systemic circulation, as occurs when lipolysis takes place in the non-hepatic tissue of the splanchnic bed. Glycerol released into the systemic circulation is utilized mainly by liver, although kidney and muscle are also major users of glycerol. Measurement of glycerol utilization is based on the amount of labelled glycerol taken up by the tissues. Other tissues probably utilize glycerol to a smaller extent, but in total this represents a significant amount. Most glycerol taken up by liver is converted to glucose. Glucose is probably the major source of glycerol-3-phosphate used in the esterification of fatty acids by adipose tissue.

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.

Blood glycerol is an important precursor for intramuscular triacylglycerol synthesis

The utilization of blood glycerol and glucose as precursors for intramuscular triglyceride synthesis was examined in rats using an intravenous infusion of [2-(14)C]glycerol and [6-(3)H]glucose or [6-(14)C]glucose. In 24-h fasted rats, more glycerol than glucose was incorporated into intramuscular triglyceride glycerol in soleus (69 +/- 23 versus 4 +/- 1 nmol/micromol triglyceride/h, respectively, p = 0.02 glycerol versus glucose) and in gastrocnemius (25 +/- 5 versus 9 +/- 2 nmol/micromol triglyceride/h, respectively, p = 0.02). Blood glucose was utilized more than blood glycerol for triglyceride glycerol synthesis in quadriceps. In fed rats, the blood glycerol incorporation rates (4 +/- 2, 8 +/- 3, and 9 +/- 3 nmol/micromol triglyceride/h) were similar (p > 0.3) to those of glucose (5 +/- 2, 8 +/- 2, and 5 +/- 2 nmol/micromol triglyceride/h for quadriceps, gastrocnemius, and soleus muscle, respectively). Glucose incorporation into intramuscular triglycerides was less with [6-(3)H]glucose than with [6-(14)C]glucose, suggesting an indirect pathway for glucose carbon entry into muscle triglyceride. The isotopic equilibrium between plasma and intramuscular free glycerol ([U-(13)C]glycerol) was complete in quadriceps and gastrocnemius, but not soleus, within 2 h after beginning the tracer infusion. We conclude that blood glycerol is a direct and important precursor for muscle triglyceride synthesis in rats, confirming the presence of functionally important amounts of glycerol kinase in skeletal muscle.

Substrate metabolism when subjects are fed carbohydrate during exercise

This study determined the effect of carbohydrate ingestion during exercise on the lipolytic rate, glucose disappearance from plasma (Rd Glc), and fat oxidation. Six moderately trained men cycled for 2 h on four separate occasions. During two trials, they were fed a high-glycemic carbohydrate meal during exercise at 30 min (0.8 g/kg), 60 min (0.4 g/kg), and 90 min (0.4 g/kg); once during low-intensity exercise [25% peak oxygen consumption (VO2 peak)] and once during moderate-intensity exercise (68% VO2 peak). During two additional trials, the subjects remained fasted (12-14 h) throughout exercise at each intensity. After 55 min of low-intensity exercise in fed subjects, hyperglycemia (30% increase) and a threefold elevation in plasma insulin concentration (P < 0.05) were associated with a 22% suppression of lipolysis compared with when subjects were fasted (5.2 +/- 0.5 vs. 6.7 +/- 1.2 micromol. kg-1. min-1, P < 0.05), but fat oxidation was not different from fasted levels at this time. Fat oxidation when subjects were fed carbohydrate was not reduced below fasting levels until 80-90 min of exercise, and lipolysis was in excess of fat oxidation at this time. The reduction in fat oxidation corresponded in time with the increase in Rd Glc. During moderate-intensity exercise, the very small elevation in plasma insulin concentration (approximately 3 microU/ml; P < 0.05) during the second hour of exercise when subjects were fed vs. when they were fasted slightly attenuated lipolysis (P < 0.05) but did not increase Rd Glc or suppress fat oxidation. These findings indicate that despite a suppression of lipolysis after carbohydrate ingestion during exercise, the lipolytic rate remained in excess and thus did not limit fat oxidation. Under these conditions, a reduction in fat oxidation was associated in time with an increase in glucose uptake.

Glycerol and nonesterified fatty acid metabolism in human muscle and adipose tissue in vivo

To determine the relationship between glycerol and nonesterified fatty acid (NEFA) release from adipose tissue, and to test whether forearm muscle and abdominal adipose tissue are capable of extracting these two lipolytic products from the circulation, 13 male subjects were studied after an overnight fast during combined infusion of radiolabeled palmitate and glycerol. Blood samples were taken from a radial artery, a deep forearm vein, and a superficial abdominal vein before and during a 2-h infusion of glucose at approximately 7 mg. kg-1. min-1. The ratio of the appearance rates of total NEFA to glycerol was approximately 3/1 during the baseline period but decreased to 1.3/1 during glucose infusion. There was significant extraction of both glycerol and NEFA by forearm muscle. In contrast, there was no apparent uptake of glycerol by adipose tissue. Adipose tissue NEFA uptake was undetectable during the baseline period but became significant during glucose infusion. These data indicate that there is very little to no in situ reesterification of NEFA in adipose tissue after an overnight fast. During glucose infusion, there was apparently a relative increase in the fraction of glycerol derived from the action of lipoprotein lipase and an increase in reesterification in situ.

Inhibition of lipolysis reduces beta1-adrenoceptor-mediated thermogenesis in man

The purpose of the study was to investigate whether the increase in energy expenditure and lipid oxidation during beta1-adrenergic stimulation is caused by the concomitant increase in lipolysis. Twelve healthy male subjects participated in three trials: no-LIP/-, inhibition of lipolysis by pretreatment with acipimox followed by saline infusion; -/BETA, no pretreatment, with dobutamine infusion to stimulate beta1-adrenoceptors; and no-LIP/BETA, pretreatment with acipimox followed by dobutamine infusion. Inhibition of lipolysis did not affect baseline energy expenditure, but decreased lipid oxidation and increased carbohydrate oxidation. Energy expenditure and lipid oxidation increased significantly during beta1-adrenergic stimulation, but this increase was significantly smaller when lipolysis was inhibited ([baseline v infusion period] energy expenditure: -/BETA, 5.15 +/- 0.16 v 6.11 +/- 0.26 kJ/min, P < .001; no-LIP/BETA, 5.28 +/- 0.17 v 5.71 +/- 0.19 kJ/min, P < .01; lipid oxidation: -/BETA, 0.059 +/- 0.004 v 0.073 +/- 0.006 g/min, P < .01; no-LIP/BETA, 0.034 +/- 0.005 v 0.039 +/- 0.006 g/min, P < .05). Baseline plasma glycerol and nonesterified fatty acid (NEFA) concentrations decreased after inhibition of lipolysis. Glycerol and NEFA increased significantly during beta1-adrenergic stimulation alone (glycerol, 65.0 +/- 5.3 v 117.0 +/- 10.9 micromol/L; NEFA, 362 +/- 24 v 954 /- 89 micromol/L; both P < .001). Concomitant administration of acipimox prevented a substantial part of the increase in lipolysis during beta1-adrenergic stimulation, but the increase in plasma glycerol and NEFA remained significant (glycerol, 40.4 +/- 2.2 v 44.8 +/- 2.2 micromol/L; NEFA, 118 +/- 18 v 160 +/- 19 micromol/L; both P < .05). In conclusion, a reduced availability of plasma NEFA was associated with a reduced increase in energy expenditure and lipid oxidation during beta1-adrenergic stimulation in man.

Regulation of fatty acid delivery in vivo

Adipose tissue triacylglycerol (TG) constitutes by far the largest energy store in the body. In order for this TG to be used as a substrate for oxidative metabolism, it has to be exported from adipose tissue and transported to the tissues where it will be used. Following hydrolysis of stored TG, non-esterified fatty acids (NEFA) leave the adipocyte and enter the plasma. Unlike tissues such as skeletal muscle which extract plasma NEFA, in adipose tissue the flow of fatty acids across the cell membrane is bi-directional, outward in times of net fat mobilization such as fasting and exercise, and inward during the postprandial period. Factors regulating NEFA delivery in vivo include hormonal and nervous stimulation of lipolysis, and a variety of factors, local and systemic, which oppose this by suppressing lipolysis. Adipose tissue blood flow (ATBF) is also important. ATBF is increased in states of fat mobilization and fat deposition, although there is evidence that during strenuous exercise the increase in ATBF is not sufficient for export of all the NEFA made available from lipolysis. There are well-documented regional variations in lipolysis. The intra-abdominal depots appear to have the highest rates of TG turnover, the subcutaneous abdominal an intermediate rate, and the gluteal-femoral depots to have relatively sluggish turnover. However, much of the evidence for this derives from studies of isolated adipocytes, and confirmation in vivo is much needed. There are links between abdominal fat deposition and risk of cardiovascular disease which may be mediated through increased fatty acid delivery from abdominal fat depots. The ability of exercise specifically to decrease intra-abdominal fat stores may be yet another health benefit of regular exercise.

Lipolytic suppression following carbohydrate ingestion limits fat oxidation during exercise

This study determined if the suppression of lipolysis after preexercise carbohydrate ingestion reduces fat oxidation during exercise. Six healthy, active men cycled 60 min at 44 +/- 2% peak oxygen consumption, exactly 1 h after ingesting 0.8 g/kg of glucose (Glc) or fructose (Fru) or after an overnight fast (Fast). The mean plasma insulin concentration during the 50 min before exercise was different among Fast, Fru, and Glc (8 +/- 1, 17 +/- 1, and 38 +/- 5 microU/ml, respectively; P < 0.05). After 25 min of exercise, whole body lipolysis was 6.9 +/- 0.2, 4.3 +/- 0.3, and 3.2 +/- 0.5 micromol x kg(-1) x min(-1) and fat oxidation was 6.1 +/- 0.2, 4.2 +/- 0.5, and 3.1 +/- 0.3 micromol x kg(-1) x min(-1) during Fast, Fru, and Glc, respectively (all P < 0.05). During Fast, fat oxidation was less than lipolysis (P < 0.05), whereas fat oxidation approximately equaled lipolysis during Fru and Glc. In an additional trial, the same subjects ingested glucose (0.8 g/kg) 1 h before exercise and lipolysis was simultaneously increased by infusing Intralipid and heparin throughout the resting and exercise periods (Glc+Lipid). This elevation of lipolysis during Glc+Lipid increased fat oxidation 30% above Glc (4.0 +/- 0.4 vs. 3.1 +/- 0.3 micromol x kg(-1) x min(-1); P < 0.05), confirming that lipolysis limited fat oxidation. In summary, small elevations in plasma insulin before exercise suppressed lipolysis during exercise to the point at which it equaled and appeared to limit fat oxidation.

Desensitization of human adipose tissue to adrenaline stimulation studied by microdialysis

1. Desensitization of fat cell lipolysis to catecholamine exposure has been studied extensively in vitro but only to a small extent in human adipose tissue in vivo. 2. We measured interstitial glycerol concentrations by microdialysis in subcutaneous, abdominal adipose tissue in healthy humans during intravenous adrenaline infusion for three 35 min periods with 30 min breaks in between. Local blood flow, interstitial adrenaline and arterial glycerol concentrations were also measured. Adrenaline was infused to result in either a high, a low and a high arterial concentration (5.8, 3.1 and 5.6 nM, respectively) or a low, a high and a low concentration (2.5, 4.6 and 2.6 nM, respectively) in order to examine both desensitization and the dose dependency of adipose tissue lipolysis to adrenaline. 3. Adipose tissue lipolysis was calculated and was found to vary directly with arterial adrenaline concentration. However, lipolytic responses to adrenaline decreased markedly during repeated stimulation at a given concentration. Further, arterial glycerol and free fatty acid concentrations varied directly with arterial adrenaline concentrations and showed reduced responses upon repeated exposure. 4. The increase in adipose tissue blood flow in response to adrenaline was also reduced by prior adrenaline exposure, but no consistent desensitization could be demonstrated for whole-body energy expenditure, blood pressure and heart rate. 5. In the basal state, arterial plasma and interstitial adrenaline concentrations did not differ. During perturbations of arterial adrenaline concentrations, changes in interstitial concentrations were highly reproducible but smaller than changes in arterial concentrations. 6. In conclusion, in vivo adrenaline-mediated adipose tissue lipolysis and blood flow increments are desensitized by prior adrenaline exposure.

Modified hormonal effects on fat metabolism after severe head injury in children

Previous studies within our research group have indicated that the hormonal influences on whole body energy expenditure may be modified in severely head-injured children. The aim of this study was to examine plasma concentrations of nonesterified fatty acids (NEFA) and the hormonal and metabolic mediators which influence these to determine whether there is a similarly modified effect on fat metabolism. A total of 64 serial measurements were made in 21 fasting severely head-injured children aged 2-15 y (Glasgow Coma Score < or = 8) who were receiving neurointensive care. Circulating NEFA, ketone bodies, and lactate concentrations were analyzed using microenzymatic or electrochemical techniques. Plasma concentrations of adrenaline and insulin were measured using radioenzymatic and RIA techniques, respectively. Net fat oxidation rates were determined using indirect calorimetry. Plasma NEFA concentrations showed a significant positive relationship with both net fat oxidation rates (p = 0.02) and log ketone body concentrations (p = 0.008), indicating that NEFA concentrations were significantly related with utilization. When compared with reference values for normal resting adults, 59 (92%) adrenaline measurements were elevated, whereas only 8 (12%) NEFA values lay above the reference range. Surprisingly, between children, there was a significant negative relationship between NEFA and adrenaline concentrations, even after allowing for the effects of insulin and lactate (p = 0.015). Both plasma NEFA and adrenaline concentrations were significantly related with Glasgo Coma Score (p = 0.04, p = 0.007, respectively), the most severely injured children having the lowest NEFA and highest adrenaline concentrations. The mechanisms underlying these metabolic changes may be related to the severity of head injury and may involve changes in triglyceride/NEFA cycling and/or peripheral effects on adrenergic receptors. If children are to be treated effectively after trauma, it is important to discover the mechanism of these changes which must reflect a fundamental alteration in metabolism.

Effect of training on epinephrine-stimulated lipolysis determined by microdialysis in human adipose tissue

Trained humans (Tr) have a higher fat oxidation during submaximal physical work than sedentary humans (Sed). To investigate whether this reflects a higher adipose tissue lipolytic sensitivity to catecholamines, we infused epinephrine (0.3 nmol.kg-1.min-1) for 65 min in six athletes and six sedentary young men. Glycerol was measured in arterial blood, and intercellular glycerol concentrations in abdominal subcutaneous adipose tissue were measured by microdialysis. Adipose tissue blood flow was measured by 133Xe-washout technique. From these measurements adipose tissue lipolysis was calculated. During epinephrine infusion intercellular glycerol concentrations were lower, but adipose tissue blood flow was higher in trained compared with sedentary subjects (P < 0.05). Glycerol output from subcutaneous tissue (Tr: 604 +/- 322 nmol.100 g-1.min-1; Sed: 689 +/- 203; mean +/- SD) as well as arterial glycerol concentrations (Tr: 129 +/- 36 microM; Sed: 119 +/- 56) did not differ between groups. It is concluded that in intact subcutaneous adipose tissue epinephrine-stimulated blood flow is enhanced, whereas lipolytic sensitivity to epinephrine is the same in trained compared with untrained subjects.

A new correction factor for use in tracer estimations of plasma fatty acid oxidation

The purpose of this study was to acquire a new correction factor for use in tracer estimations of plasma fatty acid oxidation that would fully account for label fixation during the infusion of fatty acid tracers. Thus volunteers were infused with 13C-labeled fatty acids and [1-14C]acetate in the basal state, during hyperinsulinemia-hyperglycemia (clamp), and during 1 h of cycling exercise. The fractional recovery of acetate label (i.e., the acetate correction factor) was 0.56 +/- 0.02, 0.50 +/- 0.03, and 0.80 +/- 0.03 in the basal state and during the clamp and exercise, respectively. Isotopically determined plasma fatty acid oxidation rates (mumol.kg-1.min-1) were 1.7 +/- 0.2, 0.8 +/- 0.2, and 6.4 +/- 0.5 (no correction); 2.1 +/- 0.2, 1.0 +/- 0.2, and 6.7 +/- 0.5 (bicarbonate correction); and 3.1 +/- 0.2, 1.5 +/- 0.2, and 8.2 +/- 0.4 (acetate correction). We conclude that use of the acetate correction factor in place of the bicarbonate correction factor should improve the accuracy of isotopic measurements of plasma fatty acid oxidation, because it accounts for label fixation that might occur at any step between the entrance of labeled acetyl-CoA into the tricarboxylic acid cycle until the recovery of label in breath CO2.

Cholinoceptor-mediated effects on glycerol output from human adipose tissue using in situ microdialysis

1. Possible cholinoceptor-mediated effects on lipolysis were investigated in vivo in human subcutaneous adipose tissue of non-obese, non-smoking, healthy subjects, by use of microdialysis. Cholinomimetic and sympathomimetic agents were added to the in going dialysate solvent. 2. Addition of nicotine to the perfusion solvent caused a concentration-dependent reversible increase in the levels of glycerol in the dialysate (lipolysis index). The opposite effect (also concentration-dependent and reversible) was caused by the addition of carbachol. The maximum effects were 100% stimulation and 50% inhibition, respectively, by nicotine and carbachol. Neither nicotine nor carbachol stimulated nutritive blood flow in adipose tissue (as measured with an ethanol escape technique). 3. The nicotine effect in situ was concentration-dependently counteracted by the nicotinic cholinoceptor antagonist, mecamylamine. Likewise, the carbachol effect was concentration-dependently counteracted by the muscarinic cholinoceptor antagonist, atropine. 4. When adipose tissue was pretreated with phentolamine plus propranolol in order to obtain a complete alpha and beta-adrenoceptor blockade, the subsequent addition of nicotine or carbachol still induced an increase and decrease in dialysate glycerol levels (lipolytic or antilipolytic effects), respectively. When adipose tissue was pretreated with mecamylamine or atropine, the subsequent addition of acetylcholine caused a reversible decrease and increase, respectively, of the dialysate glycerol levels. 5. Nicotine and carbachol had no effects on glycerol release from human isolated subcutaneous fat cells that were incubated in vivo. 6. In conclusion, the data demonstrate a dual effect of the cholinoceptor system on glycerol output inhuman adipose tissue: stimulation through nicotinic receptors and inhibition through muscarinic receptors. These effects, which are not observed in vitro, are independent of the adrenergic system and the local blood flow and seem not to be mediated by a direct action on the fat cell.

Muscle and whole body metabolism after norepinephrine

The effect of an infusion of norepinephrine (0.42 nmol.kg-1.min-1) on energy metabolism in the whole body (using indirect calorimetry and the arteriovenous forearm catheterization techniques in eight healthy young male adults. The activity of the triglyceride-fatty acid cycle, which mainly operates in nonmuscular tissues, was also assessed by measuring glycerol turnover using [2H5]glycerol (to indicate lipolysis) and indirect calorimetry (to indicate net fat oxidation). Norepinephrine increased whole body oxygen consumption by almost 10% (P < 0.01), but the estimated oxygen consumption of muscles tended to decrease. Muscle blood flow (measured by 133Xe) and forearm blood flow (measured by strain-gauge plethysmography) were not significantly affected by norepinephrine, but the rate of uptake of nonesterified fatty acids and beta-hydroxybutyrate increased severalfold (P < 0.05), whereas that of glucose did not. The activity of the triglyceride-fatty acid cycle increased fourfold after norepinephrine administration, having a marginal effect on resting energy expenditure (approximately 1.5%) but accounting for approximately 15% of the increase in whole body energy expenditure. This study provides no evidence that skeletal muscle is an important site for norepinephrine-induced thermogenesis and suggests that an increase in the activity of the triglyceride-fatty acid cycle contributes to the norepinephrine-induced increase in energy expenditure of nonmuscular tissues.