A single bout of exercise induces beta-adrenergic desensitization in human adipose tissue

This study was designed to assess whether physiological activation of the sympathetic nervous system induced by exercise changes adipose tissue responsiveness to catecholamines in humans. Lipid mobilization in abdominal subcutaneous adipose tissue was studied with the use of a microdialysis method in 11 nontrained men (age: 22. 3 +/- 1.5 yr; body mass index: 23.0 +/- 1.6). Adipose tissue adrenergic sensitivity was explored with norepinephrine, dobutamine (beta(1)-agonist), or terbutaline (beta(2)-agonist) perfused during 30 min through probes before and after 60-min exercise (50% of the maximal aerobic power). The increase in extracellular glycerol concentration during infusion was significantly lower after the exercise when compared with the increase observed before the exercise (P < 0.05, P < 0.02, and P < 0.01, respectively, for norepinephrine, dobutamine, and terbutaline). In a control experiment realized without exercise, no difference in norepinephrine-induced glycerol increase between the two infusions was observed. To assess the involvement of catecholamines in the blunted beta-adrenergic-induced lipolytic response after exercise, adipose tissue adrenergic sensitivity was explored with two 60-min infusions of norepinephrine or epinephrine separated by a 60-min interval. With both catecholamines, the increase in glycerol was significantly lower during the second infusion (P < 0.05). The findings suggest that aerobic exercise, which increased adrenergic activity, induces a desensitization in beta(1)- and beta(2)-adrenergic lipolytic pathways in human subcutaneous adipose tissue.     

Endurance training and GH administration in elderly women: effects on abdominal adipose tissue lipolysis

In the present study, the effect of endurance training alone and endurance training combined with recombinant human growth hormone (rhGH) administration on subcutaneous abdominal adipose tissue lipolysis was investigated. Sixteen healthy women [age 75 +/- 2 yr (mean +/- SE)] underwent a 12-wk endurance training program on a cycle ergometer. rhGH was administered in a randomized, double-blinded, placebo-controlled design in addition to the training program. Subcutaneous abdominal adipose tissue lipolysis was estimated by means of microdialysis combined with measurements of subcutaneous abdominal adipose tissue blood flow (ATBF; (133)Xe washout). Whole body fat oxidation was estimated simultaneously by indirect calorimetry. Before and after completion of the training program, measurements were performed both at rest and during 60 min of continuous cycling at a workload corresponding to 60% of pretraining peak oxygen uptake. Endurance training alone did not affect subcutaneous abdominal adipose tissue lipolysis either at rest or during exercise, as reflected by identical levels of interstitial adipose tissue glycerol, subcutaneous abdominal ATBF, and plasma nonesterified fatty acids before and after completion of the training program. Similarly, no effect on subcutaneous abdominal adipose tissue lipolysis was observed when combining endurance training with rhGH administration. However, in both the placebo and the GH groups, fat oxidation was significantly increased during exercise performed at the same absolute workload after completion of the training program. We conclude that the changed lipid metabolism during exercise observed after endurance training alone or after endurance training combined with rhGH administration is not due to alterations in subcutaneous abdominal adipose tissue metabolism in elderly women.     

Atrial natriuretic peptide stimulates lipid mobilization during repeated bouts of endurance exercise

Atrial natriuretic peptide (ANP) controls lipolysis in human adipocytes. Lipid mobilization is increased during repeated bouts of exercise, but the underlying mechanisms involved in this process have not yet been delineated. The relative involvement of catecholamine- and ANP-dependent pathways in the control of lipid mobilization during repeated bouts of exercise was thus investigated in subcutaneous adipose tissue (SCAT) by microdialysis. The study was performed in healthy males. Subjects performed two 45-min exercise bouts (E1 and E2) at 50% of their maximal oxygen uptake separated by a 60-min rest period. Extracellular glycerol concentration (EGC), reflecting SCAT lipolysis, was measured in a control probe perfused with Ringer solution and in two other probes perfused with either Ringer plus phentolamine (alpha(1/2)-AR antagonist) or Ringer plus both phentolamine and propranolol (beta-AR antagonist). Plasma epinephrine, plasma glycerol, and EGC were 1.7-, 1.6-, and 1.2-fold higher in E2 than in E1, respectively. Phentolamine potentiated exercise-induced EGC increase during E2 only. Propranolol reduced the lipolytic rate during both E1 and E2 compared with the probe with phentolamine. Plasma ANP concentration increased more during E2 than during E1 and was correlated with the increase in EGC in the probe containing phentolamine plus propranolol. The results suggest that ANP is involved in the control of lipolysis during exercise and that it contributes to stimulation of lipolysis during repeated bouts of exercise.     

Gender-specific response to interstitial angiotensin II in human white adipose tissue.

Angiotensin II is synthesized locally in various tissues. However, the role of interstitial angiotensin II in the regulation of regional metabolism and tissue perfusion has not as yet been clearly defined. We characterized the effect of interstially applied angiotensin II in abdominal subcutaneous adipose tissue of young, normal-weight, healthy men (n = 8) and women (n = 6) using the microdialysis technique. Adipose tissue was perfused with 0.01, 0.1, and 1 micro M angiotensin II. Dialysate concentrations of ethanol, glycerol, glucose, and lactate were measured to assess changes in blood flow (ethanol dilution technique), lipolysis, and glycolysis, respectively. Baseline ethanol ratio and dialysate lactate were both significantly higher, whereas dialysate glucose was significantly lower in men vs. women. In men, ethanol ratio and dialysate glucose, lactate and glycerol did not change significantly during perfusion with angiotensin II. In women, however, angiotensin II induced a significant increase in ethanol ratio and dialysate lactate and a decrease in dialysate glucose close to values found for men and this response was almost maximal at the lowest angiotensin II concentration used. Dialysate glycerol did not change significantly. We conclude that baseline blood flow and glucose supply and metabolism is significantly higher in women than in men. In men, interstitial Ang II has only a minimal effect on adipose tissue blood flow and metabolism. In women, however, a high physiological concentration of interstitial angiotensin II can reduce blood flow down to values found in men. This is associated with an impaired glucose supply and metabolism. Additionally, Ang II inhibits lipolysis.     

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.     

In vivo lipolysis in adipose tissue from two anatomical locations measured by microdialysis

In this study, the difference in lipolytic response in inguinal subcutaneous and epididymal adipose tissues of male Sprague-Dawley rats was assessed in vivo by microdialysis. Probes were perfused with Ringer solution in which increasing concentrations of isoproterenol (10(-7) - 10(-4) mol/L) were added. Glycerol release, expressed as extracellular glycerol concentration, was used as lipolytic index. The effect of isoproterenol on local blood flow was investigated using the ethanol technique. No differences were found in the interstitial glycerol concentration between both adipose tissues under basal conditions. When isoproterenol was perfused, a dose-response increase in glycerol production was induced in both tissues. Interstitial glycerol concentration from epididymal adipose tissue was higher than that of inguinal subcutaneous depot at all isoproterenol concentrations. No vasodilatory effect of isoproterenol was found. These results suggest that epididymal adipose tissue is more responsive in vivo to beta-adrenergic lipolysis stimulation than is subcutaneous fat pad from the inguinal region.     

Factors controlling fat mobilization from human subcutaneous adipose tissue during exercise

To investigate possible factors that limit fat utilization during exercise, arteriovenous differences of plasma nonesterified fatty acids (NEFA) and glycerol were measured across the subcutaneous adipose tissue of the anterior abdominal wall in nine subjects who exercised for 60 min at 50-70% of their maximal O2 consumption. The large gradient of NEFA concentration from adipose tissue venous to arterial plasma increased throughout the exercise period. Maximal plasma NEFA concentrations in adipose venous drainage were reached postexercise (median 3,800 mumol/l), with a median NEFA-to-albumin molar ratio of 5.7. Fractional reesterification of fatty acids within the tissue (assessed from the ratio of NEFA to glycerol release) was 20-30% in the basal state and declined during exercise. After exercise there was apparently negative reesterification, implying release of NEFA retained in adipose tissue during exercise. Although these findings challenge current views on the regulation of NEFA release, they are in agreement with the concept of supply of fatty acids from adipose tissue as the major factor limiting fat oxidation during sustained exercise.     

Extracellular cyclic AMP and adenosine appearance in adipose tissue of Sus scrofa: effects of exercise

Cyclic AMP (cAMP) appears extracellularly in a variety of tissues including brain, liver, and kidney; whether it appears in adipose tissue and responds to physiological perturbation is unknown. The purpose of this study was to examine adipose tissue extracellular cAMP appearance and metabolism in situ and in vitro in physiologically challenged animals. Littermate swine were either sedentary or exercise trained on a treadmill for 3 months and subjected to acute exercise on experiment day. In situ, microdialysis probes in subcutaneous back fat were perfused before, during, and after animals performed 20 mins of acute exercise, and dialysate was analyzed for cAMP and adenosine. In vitro, isolated adipocytes were hormonally stimulated to provoke cAMP synthesis and efflux, and plasma membrane phosphodiesterase and 5'-nucleotidase activities were measured. Extracellular cAMP and adenosine levels in adipose tissue of sedentary swine averaged 5.2 +/- 1.7 and 863 +/- 278 nM, respectively. Exercise training tended to increase extracellular cAMP (11.3 +/- 1.7 nM) and reduce extracellular adenosine (438 +/- 303 nM), although neither change was statistically significant. Acute exercise caused a significant 3-fold and 16-fold increase in extracellular cAMP and adenosine, respectively, compared to rest. These changes occurred despite a 2- to 3-fold increase in adipose tissue blood flow during acute exercise. In vitro, cAMP efflux from exercise-trained swine was 42% greater than that from adipocytes of sedentary swine, yet adipocyte plasma membranes from exercise-trained and sedentary swine did not differ in maximal phosphodiesterase and 5'-nucleotidase activities. We conclude that cAMP appears extracellularly in swine adipose tissue and that the levels of extracellular cAMP and adenosine in intact swine adipose tissue are influenced by both acute and chronic exercise. The subsequent impact of the changes in these biochemicals on local cellular metabolism and growth remains to be determined.     

Lack of alpha(2)-adrenergic antilipolytic effect during exercise in subcutaneous adipose tissue of trained men.

The aim of this study was to investigate the involvement of the antilipolytic alpha(2)-adrenergic receptor pathway in the regulation of lipolysis during exercise in subcutaneous abdominal adipose tissue (SCAAT). Seven trained men and 15 untrained men were studied. With the use of microdialysis, the extracellular glycerol concentration was measured in SCAAT at rest and during 60 min of exercise at 50% of maximal oxygen consumption. One microdialysis probe was perfused with Ringer solution; the other was supplemented with phentolamine (alpha(2)-adrenergic receptor antagonist). No differences in baseline extracellular or plasma glycerol concentrations were found between the two groups. The exercise-induced extracellular and plasma glycerol increase was higher in trained compared with untrained subjects (P < 0.05). Addition of phentolamine to the perfusate enhanced the exercise-induced response of extracellular glycerol in untrained subjects but not in trained subjects. The exercise-induced increase in plasma norepinephrine and epinephrine concentrations and the decrease in plasma insulin were not different in the two groups. These in vivo findings demonstrate higher exercise-induced lipolysis in trained compared with untrained subjects and show that, in trained subjects, the alpha(2)-mediated antilipolytic action is not involved in the regulation of lipolysis in SCAAT during exercise.     

Whole body and abdominal lipolytic sensitivity to epinephrine is suppressed in upper body obese women

We measured whole body and regional lipolytic and adipose tissue blood flow (ATBF) sensitivity to epinephrine in 8 lean [body mass index (BMI): 21 +/- 1 kg/m(2)] and 10 upper body obese (UBO) women (BMI: 38 +/- 1 kg/m(2); waist circumference >100 cm). All subjects underwent a four-stage epinephrine infusion (0.00125, 0.005, 0.0125, and 0.025 microgram. kg fat-free mass(-1). min(-1)) plus pancreatic hormonal clamp. Whole body free fatty acid (FFA) and glycerol rates of appearance (R(a)) in plasma were determined by stable isotope tracer methodology. Abdominal and femoral subcutaneous adipose tissue lipolytic activity was determined by microdialysis and (133)Xe clearance methods. Basal whole body FFA R(a) and glycerol R(a) were both greater (P < 0.05) in obese (449 +/- 31 and 220 +/- 12 micromol/min, respectively) compared with lean subjects (323 +/- 44 and 167 +/- 21 micromol/min, respectively). Epinephrine infusion significantly increased FFA R(a) and glycerol R(a) in lean (71 +/- 21 and 122 +/- 52%, respectively; P < 0.05) but not obese subjects (7 +/- 6 and 39 +/- 10%, respectively; P = not significant). In addition, lipolytic and ATBF sensitivity to epinephrine was blunted in abdominal but not femoral subcutaneous adipose tissue of obese compared with lean subjects. We conclude that whole body lipolytic sensitivity to epinephrine is blunted in women with UBO because of decreased sensitivity in upper body but not lower body subcutaneous adipose tissue.     

Studies of phosphodiesterase effects on adipose tissue metabolism in obese subjects by the microdialysis technique.

The effect of non-selective (theophylline) inhibition of cyclic AMP breakdown on norepinephrine stimulated lipolysis rate was investigated in subcutaneous adipose tissue of obese subjects. In addition, changes in interstitial glucose and lactate concentration were assessed by means of the microdialysis technique. The interaction of endogenous released insulin and theophylline on adipocyte metabolism was determined. Theophylline and norepinephrine alone increased glycerol outflow significantly. When both agents were perfused in combination, interstitial glycerol concentration increased further. The enhanced glycerol level due to theophylline application was slightly decreased by insulin. In the presence of theophylline, extracellular glucose concentration increased, in contrast to the catecholamine. Norepinephrine decreased interstitial glucose level. When both drugs were added in combination, the level of interstitial glucose increased to about 1 mM, greater than with theophylline alone. With each intervention, lactate was synthesized. Local adipose tissue blood flow was increased by theophylline and theophylline plus norepinephrine. In conclusion, post-receptor mechanisms increased norepinephrine maximal stimulated lipolysis rate in subcutaneous adipose tissue. Glucose uptake was inhibited by the non-specific inhibitor of phosphodiesterase. The effect of insulin on inhibition of lipolysis was modest but sustained in the presence of high theophylline (10(-4) M) concentration. Phosphodiesterase activity may be relatively low in obese subjects in comparison with lean subjects. In lean subjects theophylline caused a transient reversal of the antilipolytic effect of insulin.     

Effect of training on insulin sensitivity of glucose uptake and lipolysis in human adipose tissue

Training increases insulin sensitivity of both whole body and muscle in humans. To investigate whether training also increases insulin sensitivity of adipose tissue, we performed a three-step hyperinsulinemic, euglycemic clamp in eight endurance-trained (T) and eight sedentary (S) young men [insulin infusion rates: 10,000 (step I), 20,000 (step II), and 150,000 (step III) microU x min(-1) x m(-2)]. Glucose and glycerol concentrations were measured in arterial blood and also by microdialysis in interstitial fluid in periumbilical, subcutaneous adipose tissue and in quadriceps femoris muscle (glucose only). Adipose tissue blood flow was measured by (133)Xe washout. In the basal state, adipose tissue blood flow tended to be higher in T compared with S subjects, and in both groups blood flow was constant during the clamp. The change from basal in arterial-interstitial glucose concentration difference was increased in T during the clamp but not in S subjects in both adipose tissue and muscle [adipose tissue: step I (n = 8), 0.48 +/- 0.18 mM (T), 0.23 +/- 0.11 mM (S); step II (n = 8), 0.19 +/- 0.09 (T), -0.09 +/- 0.24 (S); step III (n = 5), 0.47 +/- 0.24 (T), 0.06 +/- 0.28 (S); (T: P < 0.001, S: P > 0.05); muscle: step I (n = 4), 1. 40 +/- 0.46 (T), 0.31 +/- 0.21 (S); step II (n = 4), 1.14 +/- 0.54 (T), -0.08 +/- 0.14 (S); step III (n = 4), 1.23 +/- 0.34 (T), 0.24 +/- 0.09 (S); (T: P < 0.01, S: P > 0.05)]. Interstitial glycerol concentration decreased faster in T than in S subjects [half-time: T, 44 +/- 9 min (n = 7); S, 102 +/- 23 min (n = 5); P < 0.05]. In conclusion, training enhances insulin sensitivity of glucose uptake in subcutaneous adipose tissue and in skeletal muscle. Furthermore, interstitial glycerol data suggest that training also increases insulin sensitivity of lipolysis in subcutaneous adipose tissue. Insulin per se does not influence subcutaneous adipose tissue blood flow.     

In Vivo Response to α1‐Adrenoreceptor Stimulation in Human White Adipose Tissue

Objective: Recent studies in rats suggest an important effect of α₁‐adrenoreceptor stimulation on glucose uptake in white adipocytes. It is not known if α₁‐adrenoreceptor stimulation elicits similar metabolic effects in humans. Research Methods and Procedures: Three microdialysis catheters in abdominal subcutaneous adipose tissue were perfused with 0.00, 0.01, 0.10, 1.00, and 10.00 μM isoproterenol, phenylephrine, or phenylephrine plus 100 μM propranolol. Dialysate concentrations of ethanol, glycerol, glucose, and lactate were measured for estimating blood flow (ethanol‐dilution technique), lipolysis, and glycolysis, respectively. Results: Phenylephrine, with or without propranolol, did not elicit a change in ethanol ratio. In contrast, the ethanol ratio decreased markedly with isoproterenol. Dialysate glucose concentration decreased with phenylephrine with and without propranolol and increased with isoproterenol. Phenylephrine caused a dose‐dependent increase in dialysate glycerol concentration, with a maximal effect similar to that of isoproterenol. The effect was attenuated with propranolol. Discussion: Our findings suggest that α₁‐adrenoreceptor stimulation by phenylephrine increases glucose uptake and metabolism in human abdominal adipose tissue. Furthermore, phenylephrine elicits a marked increase in lipolytic activity in white adipose tissue through β‐adrenoreceptor activation.     

Fat metabolism and acute resistance exercise in trained men.

The purpose of this study was to investigate the effect of acute resistance exercise (RE) on lipolysis within adipose tissue and subsequent substrate oxidation to better understand how RE may contribute to improvements in body composition. Lipolysis and blood flow were measured in abdominal subcutaneous adipose tissue via microdialysis before, during, and for 5 h following whole body RE as well as on a nonexercise control day (C) in eight young (24 +/- 0.7 yr), active (>3 RE session/wk for at least 2 yr) male participants. Fat oxidation was measured immediately before and after RE via indirect calorimetry for 45 min. Dialysate glycerol concentration (an index of lipolysis) was higher during (RE: 200.4 +/- 38.6 vs. C: 112.4 +/- 13.1 micromol/l, 78% difference; P = 0.02) and immediately following RE (RE: 184 +/- 41 vs. C: 105 + 14.6 micromol/l, 75% difference; P = 0.03) compared with the same time period on the C day. Energy expenditure was elevated in the 45 min after RE compared with the same time period on the C day (RE: 104.4 +/- 6.0 vs. C: 94.5 +/- 4.0 kcal/h, 10.5% difference; P = 0.03). Respiratory exchange ratio was lower (RE: 0.71 +/- 0.004 vs. C: 0.85 +/- .03, 16.5% difference; P = 0.004) and fat oxidation was higher (RE: 10.2 +/- 0.8 vs. C: 5.0 +/- 1.0 g/h, 105% difference; P = 0.004) following RE compared with the same time period on the C day. Therefore, the mechanism behind RE contributing to improved body composition is in part due to enhanced abdominal subcutaneous adipose tissue lipolysis and improved whole body fat oxidation and energy expenditure in response to RE.     

Are blood flow and lipolysis in subcutaneous adipose tissue influenced by contractions in adjacent muscles in humans?

Aerobic exercise increases whole body adipose tissue lipolysis, but is lipolysis higher in subcutaneous adipose tissue (SCAT) adjacent to contracting muscles than in SCAT adjacent to resting muscles? Ten healthy, overnight-fasted males performed one-legged knee extension exercise at 25% of maximal workload (W(max)) for 30 min followed by exercise at 55% W(max) for 120 min with the other leg and finally exercised at 85% W(max) for 30 min with the first leg. Subjects rested for 30 min between exercise periods. Femoral SCAT blood flow was estimated from washout of (133)Xe, and lipolysis was calculated from femoral SCAT interstitial and arterial glycerol concentrations and blood flow. In general, blood flow and lipolysis were higher in femoral SCAT adjacent to contracting than adjacent to resting muscle (time 15-30 min; blood flow: 25% W(max) 6.6 +/- 1.0 vs. 3.9 +/- 0.8 ml x 100 g(-1) x min(-1), P < 0.05; 55% W(max) 7.3 +/- 0.6 vs. 5.0 +/- 0.6 ml x 100 g(-1) x min(-1), P < 0.05; 85% W(max) 6.6 +/- 1.3 vs. 5.9 +/- 0.7 ml x 100 g(-1) x min(-1), P > 0.05; lipolysis: 25% W(max) 102 +/- 19 vs. 55 +/- 14 nmol x 100 g(-1) x min(-1), P = 0.06; 55% W(max) 86 +/- 11 vs. 50 +/- 20 nmol x 100 g(-1) x min(-1), P > 0.05; 85% W(max) 88 +/- 31 vs. -9 +/- 25 nmol x 100 g(-1) x min(-1), P < 0.05). In conclusion, blood flow and lipolysis are generally higher in SCAT adjacent to contracting than adjacent to resting muscle irrespective of exercise intensity. Thus specific exercises can induce "spot lipolysis" in adipose tissue.     

Metabolic alterations in adipose tissue during the early phase of experimental endotoxemia in humans

Adipose tissue plays an important role in energy homeostasis; however, there is only little knowledge about its metabolic activity during critical illness or sepsis. We assessed adipose tissue metabolic activity and local blood flow during experimental endotoxemia in otherwise healthy humans. In a prospective, placebo controlled and randomized experiment we measured changes in lactate, glycerol, and pyruvate concentrations in microdialysate samples of femoral adipose tissue after an intravenous bolus of lipopolysaccharide (LPS, 4 ng/kg). Intravenous endotoxin caused an early and constant increase in interstitial pyruvate, while formation of lactate in adipose tissue was not affected. In contrast, lactate levels in serum were elevated significantly after 90 min (p<0.05) and likewise, serum glycerol concentrations rose 90 min after LPS treatment (p<0.05) and 60 min earlier than in adipose tissue. Subcutaneous adipose tissue blood perfusion increased 2-fold while there was a strong decline in skin blood flow. Pyruvate accumulation in subcutaneous adipose tissue is an early marker of endotoxemia. While adipose tissue is a major source of serum glycerol and lactate in humans during physiological conditions, it contributes only little to increased serum lactate and glycerol levels during endotoxemia.     

Influence of lipolysis and fatty acid availability on fuel selection during exercise

The aim of the present study was to investigate the influence of substrate availability on fuel selection during exercise. Eight endurance-trained male cyclists performed 90-min exercise at 70 % of their maximal oxygen uptake in a cross-over design, either in rested condition (CON) or the day after 2-h exercise practised at 70 % of maximal oxygen uptake (EX). Subjects were given a sucrose load (0.75 g kg^?1 body weight) 45 min after the beginning of the 90-min exercise test. Lipolysis was measured in subcutaneous abdominal adipose tissue (SCAT) by microdialysis and substrate oxidation by indirect calorimetry. Lipid oxidation increased during exercise and tended to decrease during sucrose ingestion in both conditions. Lipid oxidation was higher during the whole experimental period in the EX group (p?=?0.004). Interestingly, fuel selection, assessed by the change in respiratory exchange ratio (RER), was increased in the EX session (p?=?0.002). This was paralleled by a higher rate of SCAT lipolysis reflected by dialysate glycerol, plasma glycerol, and fatty acids (FA) levels (p?<?0.001). Of note, we observed a significant relationship between whole-body fat oxidation and dialysate glycerol in both sessions (r ²?=?0.33, p?=?0.02). In conclusion, this study highlights the limiting role of lipolysis and plasma FA availability to whole-body fat oxidation during exercise in endurance-trained subjects. This study shows that adipose tissue lipolysis is a determinant of fuel selection during exercise in healthy subjects.     

Effect of carbohydrate ingestion on adipose tissue lipolysis during long-lasting exercise in trained men

To study whethersucrose administration acts on lipid mobilization during prolongedexercise, we used subcutaneous abdominal adipose tissue microdialysisin eight well-trained subjects submitted at random to two 100-minexercises (50% maximal aerobic power) on separate days. After 50 minof exercise, the subjects ingested either a sucrose solution (0.75 g/kgbody wt) or water. By using a microdialysis probe, dialysate wasobtained every 10 min from the subjects at rest, during exercise, andduring a 30-min recovery period. During exercise without sucrose,plasma and dialysate glycerol increased significantly. With sucrose,the response was significantly lower for dialysate glycerol(P < 0.05). Plasma free fatty acidlevel was lower after sucrose than after water ingestion(P < 0.05). With water ingestion,plasma catecholamines increased significantly, whereas insulin fell(P < 0.05). With sucrose ingestion,the epinephrine response was blunted, whereas the insulin level wassignificantly increased. In conclusion, the use of adipose tissuemicrodialysis directly supports a lower lipid mobilization duringexercise when sucrose is supplied, which confirms that the availabilityof carbohydrate influences lipid mobilization.

     

Adipose tissue extracts plasma ammonia after sprint exercise in women and men.

This study evaluates a possible contribution of adipose tissue to the elimination of plasma ammonia (NH(3)) after high-intensity sprint exercise. In 14 healthy men and women, repeated blood samples for plasma NH(3) analyses were obtained from brachial artery and from a subcutaneous abdominal vein before and after three repeated 30-s cycle sprints separated by 20 min of recovery. Biopsies from subcutaneous abdominal adipose tissue were obtained and analyzed for glutamine and glutamate content. After exercise, both arterial and abdominal venous plasma NH(3) concentrations were lower in women than in men (P < 0.01 and P < 0.001, respectively). All postexercise measurements showed sex-independent positive arterio-subcutaneous abdominal venous plasma NH(3) concentration differences (a-v(abd)), indicating a net uptake of NH(3) from blood to adipose tissue. However, the fractional extraction (a-v(abd)/a) of NH(3) was higher in women than in men (P < 0.05). The glutamine-to-glutamate ratio in adipose tissue was increased after the second and third bout of sprint exercise (2.2 +/- 0.7 and 1.6 +/- 0.8, respectively) compared with the value at rest (1.2 +/- 0.6), suggesting a reaction of the extracted NH(3) with glutamate resulting in its conversion to glutamine. Adipose tissue may thus play an important physiological role in eliminating plasma NH(3) and thereby reducing the risk of NH(3) intoxication after high-intensity exercise.     

Adipose tissue lipolysis is increased during a repeated bout of aerobic exercise.

The goal of the study was to examine whether lipid mobilization from adipose tissue undergoes changes during repeated bouts of prolonged aerobic exercise. Microdialysis of the subcutaneous adipose tissue was used for the assessment of lipolysis; glycerol concentration was measured in the dialysate leaving the adipose tissue. Seven male subjects performed two repeated bouts of 60-min exercise at 50% of their maximal aerobic power, separated by a 60-min recovery period. The exercise-induced increases in extracellular glycerol concentrations in adipose tissue and in plasma glycerol concentrations were significantly higher during the second exercise bout compared with the first (P < 0.05). The responses of plasma nonesterified fatty acids and plasma epinephrine were higher during the second exercise bout, whereas the response of norepinephrine was unchanged and that of growth hormone lower. Plasma insulin levels were lower during the second exercise bout. The results suggest that adipose tissue lipolysis during aerobic exercise of moderate intensity is enhanced when an exercise bout is preceded by exercise of the same intensity and duration performed 1 h before. This response pattern is associated with an increase in the exercise-induced rise of epinephrine and with lower plasma insulin values during the repeated exercise bout.